effect of zinc addition on the properties of

EFFECT OF ZINC ADDITION ON THE PROPERTIES OF MAGNESIUM

ALLOYS

SAMIR SANI ABDULMALIK

A project report submitted in partial fulfillment of the

requirements for the award of the degree of

Master of Engineering

(Mechanical-Advanced Manufacturing Technology)

Faculty of Mechanical Engineering

University Technology Malaysia

JANUARY 2012

iv

To my mother for her tireless prayers

To Engr Isyaku Jibrin Sani for his Financial Support

v

ACKNOWLEDGEMENT

I would like to say thank you very much to my supervisor Assoc. Prof. Dr.

Mohd Hasbullah Bin Hj. Idris for his wonderful supervision style and encouragement

throughout the project work

My special regards also goes to my mother for her tireless prayers, and to

Engineer Jibring Isyaku Sani for his tremendous financial support

Finally I want to appreciate the effort of all those who have directly or

indirectly contributed to the successful completion of this project work, thank you all.

vi

ABSTRACT

Magnesium alloys are currently used in many structural applications. It is

believed that magnesium and its alloys may also find applications in biomedical

application. In this study, the effects of Zinc (Zn) addition on the properties of

magnesium (Mg) alloys, i.e. Mg–xZn (x = 2, 4, 6, 8, and 10) were investigated. Optical

microscopy, scanning Electron Microscope (SEM), tensile and Vickers hardness testing

were used for the characterization and evaluation of the microstructure and mechanical

properties of the alloys. Electrochemical corrosion measurement was also employed to

determine the corrosion resistance of the alloys. The results show that magnesium alloy

with 6 wt. % zinc content (denoted as Mg- 6Zn) shows good corrosion resistance and

mechanical properties).

vii

ABSTRAK

Pada masa ini aloi magnesium (Mg) telah digunakan dalam pelbagai aplikasi

struktur. Dipercayai bahawa magnesium dan aloinya telah digunakan dalam bidang

bioperubatan. Dalam kajian ini, kasan pertambahan zink (Zn) (2, 4, 6, 8 dan 10% berat)

tehadap sifat mekanikal dan kakisan aloi magnesium, Mg-xZn telah dikaji. Analisis

menggunakan mikroscop optik, Scanning Electron Mikroscopy (SEM),ujian ketegangan

dan kekerasan Vickers telah digunakan bagi pencirian dan penilaian mikrostructur dan

sifat mekanikal aloi yang dikaji. Ujian kakisan electrokimia juga telah digunakan untuk

menilai sifat rintangan kakisan aloi. Keputusan ujikaji menunjukan bahawa aloi

magnesium dengan kandungan 6% berat zink (diwakili dengan Mg-6Zn) memberikan

sifat kakaisan dan mekanikal yang baik.

viii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iv

ACKNOWLEDGMENT v

ABSTRACT vi

ABSTRAK vii

LIST OF CONTENTS viii

LIST OF TABLES xii

LIST OF FIGURES xiii

LIST OF APPENDICES xv

1 INTRODUCTION

1.1 Background 1

1.2 Statement of Problem 3

1.3 Objectives 3

1.4 Scopes 4

2 LITRETURE REVIEW

2.1 Overview of Biomaterials 5

2.1.1 Uses for Biomaterials 6

ix

2.1.1.1 Orthopedics 6

2.1.1.2 Cardiovascular Applications 7

2.1.1.3 Ophthalmic 7

2.1.1.4 Dental Applications 7

2.1.2 Types of Biomaterials 7

2.1.2.1 Metallic materials 8

2.1.2.2 Polymers 9

2.1.2.3 Ceramics 9

2.1.2.4 Composites 10

2.1.3 Natural Biomaterials 11

2.1.4 Application of Biomaterials 11

2.2 Natural Bone 12

2.2.1 Desirable Properties of Artificial Bone Material 13

2.2.1.1 Body Condition 13

2.2.1.2 Mechanical Properties 15

2.2.1.3 Corrosion Resistance 15

2.3 Conventional Metallic Materials Used For Medical Devices 16

2.3.1 Stainless steels 17

2.3.2 Cobalt-Base Alloys 17

2.3.3 Titanium and Titanium-Base Alloys 18

2.4 Magnesium 18

2.4.1 Properties of Pure Magnesium 19

2.4.2 Melting and casting of magnesium 20

2.4.2.1 Melting 20

x

2.4.2.2 Casting and working of magnesium 22

2.4.3 Magnesium Alloys 22

2.4.3.1 Common Alloying Elements 23

2.4.3.1.1 Aluminum 23

2.4.3.1.2 Calcium 23

2.4.3.1.3 Manganese 23

2.4.3.1.4 Rare Earths 24

2.4.3.1.5 Zinc 24

2.5 Zinc Metal 24

2.5.1 Zinc Biological role 24

2.6 Researched Biodegradable Magnesium Alloys 25

3 RESEARCH METHODOLOGY

3.1 Introduction 28

3.2 Research Design 30

3.2.1 Casting 30

3.2.2. Microstructural Characterization 33

3.2.3 Hardness Test 35

3.2.4 Tensile Test 37

3.2.5 Electrochemical Measurement 38

4 RESULTS AND DISCUSSION

4.1 Selection of optimum zinc addition 39

4.1.1 Nominal Composition Analysis 39

xi

4.1.2 Microstructural Characterization 40

4.1.3 Hardness Test 43

4.1.4 Tensile Test 44

4.1.5 Corrosion Electrochemical Test 46

5 CONCLUSION 47

REFERENCES 48

APENDIX A-C 51

xii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Example of Medical and Dental Material and their Applications 8

2.2 Example of Polymers used as Biomaterials 9

2.3 Example of Biomaterial Ceramics 10

2.4 Summary of the mechanical properties and porosity of human

bone 15

2.5 Raw Materials for Magnesium Production 19

4.1 Nominal chemical composition of the Mg-Zn alloys 39

4.2 The Tensile strength, Yield, and Elastic Modulus value

for Mg-Zn alloys 45

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Implant material requirements in orthopedic applications 6

2.2 Hip joint replacement 12

2.3 Details of the bone structure 13

2.4 Closed packed structure of pure magnesium 20

3.1 Flowchart showing the summary of research methodology 29

3.2 (a) Magnesium Ingot (b) Pure Zinc 30

3.3 (a) Mg-Zn Melting, (b) Pouring into steel mold,

(c) Designed Mold, (e) cast sample (f) the mold used 31

3.4 (a) Olympus BX60, (b) Philips XL 40, (c) Supra 35VP,

used for the characterization of the microstructure 34

3.5 (a) Matsuzawa DVK-2 used for the hardness testing

(b) location of the test on the sample 36

3.6 Instron universal tensile testing machine used in the tensile

testing of the samples 37

3.7 Electrochemical test (Parstat-2263) set up used for the corrosion

Measurement 38

4.1 Microstructure of the as cast (a) pure magnesium, (b) Mg-2Zn,

(c) Mg-4Zn, (d) Mg-6Zn, (e) Mg-8Zn, (f) Mg-10Zn 40

xiv

4.2 FE-SEM micrographs of (a) Mg-8Zn alloy,

(b) Mg-10Zn Alloy, (c) (Mg, Zn)-containing phase in the grain,

(d) (mg, Zn)-containing phase at the grain boundary 41

4.3 EDS analysis of the secondary phases (a) on the grain,

(b) at the grain boundary 42

4.4 The hardness value of Mg-Zn alloys as a function

of zinc addition 43

4.5 The Tensile strength value of Mg-Zn alloys as a function

of zinc addition. 45

4.6 Electrochemical polarization curves of Mg-Zn alloys

under investigation 46

xv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Compositional Analysis of the as- cast Samples 51

B The Stress/Strain Graphs for the alloys Samples 61

C Polarization Curves of the Samples 66

1

CHAPTER 1

INTRODUCTION

1.1 Background

Biomaterial implants are used as a replacement of a bone part or as a support in

the healing process. Replacement of a bone part requires implants to stay in the body

permanently, while support only requires that the implant remain in the body for a

shorter period. When permanent implant is used for a temporary application, additional

surgeries are required to remove these devices after the healing process. Thus, removal

process increases the patient grim and cost of health care. In contrast, biodegradable

materials require no additional surgeries for removal as they dissolve after the healing

process is complete. This also eliminates the complications associated with the long-

term presence of implants in the body. Finally, after these materials degrade within the

body, it is important that the body can metabolized the degradation products, and thus

are bioabsorbable.

The first materials to be used as commercial biodegradable and bioabsorbable

implant materials were polymers. The most commonly and earliest used absorbable

materials include polyglycolic acid (PGA), poly-lactic acid (PLA), and poly-dioxanone

(PDS). However, low mechanical properties and radiolucency are the limitation with

these materials. Applications of polymeric materials in load-bearing and tissue

2

supporting applications is severely restricts due to low strength, because the mechanical

needs of the body required a greater amount of material.

Metals due to their relatively high strength and fracture toughness possesses

desirable mechanical properties, however, most of the metals are biologically toxic.

Studies revealed that conventional implant, like cobalt, stainless, chromium, and nickel-

based alloys produce corrosion products, which are harmful to the human body [1] [2]

[3] [4].

Magnesium and its alloys are biodegradable metals and exhibit improved

mechanical properties and corrosion resistance. However most of the reported

biomedical magnesium alloys contain aluminum and/or rare earth (RE) elements. It is

well known that Al and rare earth elements are harmful to neurons, osteoblasts, and also

associated with dementia and could lead to hepatotoxicity. Consequently, Al and RE are

unsuitable alloying elements for biomedical magnesium materials, particularly when

they are above certain levels [5]

Pure magnesium was indicated as suitable candidate for temporary implant;

however, the major drawback of Mg is its low corrosion resistance which results to low

mechanical strength in the physiological environment. Alloying elements can be added

to increase the strength of pure Mg but alloying elements should be selected carefully to

maintain the Mg’s biocompatibility.

With the purpose of searching for suitable alloying elements for biomedical

magnesium alloys, researchers demonstrated that Calcium (Ca), Manganese (Mn), and

Zinc (Zn) could be appropriate candidates. Zinc is one of the essential elements in

human body that also provide mechanical strengthening in Mg-based alloys.

Zinc can improve the corrosion resistance and mechanical properties of

magnesium alloys, Zinc additions increase the strength of Mg-based alloys primarily

through precipitation strengthening. Furthermore, zinc is one of the most abundant

3

nutritionally essential elements in the human body, and has basic safety for biomedical

applications [6] [7].

1.2 Statement of Problem

The mechanical properties and corrosion resistance of magnesium alloys must

be sufficiently investigated for medical application. Magnesium is essential to human

metabolic functions and is the fourth most abundant cation in human body. In vitro

cytotoxicity of pure magnesium metal showed positive cell proliferation and viability

with no sign of growth inhibition. The fracture toughness of magnesium is greater than

that of ceramics, but pure magnesium corrodes too quickly in the physiological

environment (pH 7.4–7.6), losing mechanical integrity before tissue healing. In an effort

to maintain the mechanical integrity, and biocompatibility, more alloying compositions

are necessary.

1.3 Objectives

The objectives of this project are:

1. To establish optimum material composition Mg-Zn

2. To establish the effect of Zinc addition on the properties of Mg alloy as

biodegradable material

4

1.4 Scopes

This project was conducted within the following boundaries:

1. Mg-Zn alloys was prepared and cast using gravity die casting process

2. The effect of zinc addition was characterized and measured through:

(a) Microstructure observation

(b) Mechanical properties test, and

(c) Electrochemical corrosion tests

5

CHAPTER 2

LITRATURE REVIEW

2.1 Overview of Biomaterials

A biomaterial is any synthetic material that is used to replace or restore function

to a body tissue and is continuously or intermittently in contact with body fluids.

Exposure to body fluids usually implies that the biomaterial is placed within the interior

of the body, and this places several strict restrictions on materials that can be used as a

biomaterial [8].

Biomaterial must be biocompatible; it should not elicit an adverse response

from the body, and vice versa. Additionally, it should be nontoxic and noncarcinogenic.

These requirements eliminate many engineering materials that are available. Next, the

biomaterial should possess adequate physical and mechanical properties to serve as

augmentation or replacement of body tissues. For practical use, a biomaterial should be

amenable to being formed or machined into different shapes, have relatively low cost,

and be readily available. Various material requirements that must be met for successful

total joint replacement are listed in Figure 2.1 [9].

6

Figure 2.1: Implant material requirements in orthopedic applications. [9]

2.1.1 Uses for Biomaterials

Biomaterials are primarily used to replace hard or soft tissues that have become

destroyed or damaged through some pathological process. As a result of these

circumstances, it may be possible to remove the damaged tissue and replace it with some

suitable synthetic material [10]. Listed below are some common uses of biomaterials.

2.1.1.1 Orthopedics

Orthopedic implant devices are one of the most prominent application areas for

biomaterials. It has been possible to replace joints, such as the hip, knee, shoulder,

ankle, and elbow, and the pains resulted can be considerable, since the introduction of

anesthesia, antisepsis, and antibiotics. The relief of pain and restoration of mobility is

well known to hundreds of thousands of patients.

7

2.1.1.2 Cardiovascular Applications

Problems arose with heart valves and arteries can be successfully treated with

implants. The heart valves sometimes fails to either fully opening or fully closing,

meaning to say the valve is affected with disease, the diseased valve can be replaced

with a variety of substitutes.

2.1.1.3 Ophthalmic

The tissues of the eye suffer from several diseases, leading to reduced vision and

eventually, blindness. Cataracts, for example, cause cloudiness of the lens. This may be

replaced with a synthetic (polymer). As with intraocular lenses, biomaterials are used to

preserve and restore vision

2.1.1.4 Dental Applications

Within the mouth, both the tooth and supporting gum tissues can be readily

destroyed by bacterially controlled diseases. Teeth in their entirety and segments of teeth

both can be replaced and restored by a variety of materials.

2.1.2 Types of Biomaterials

In general, synthetic biomaterials used for implants can be categorized as:

metals, polymers, ceramics, and composites [8]

8

2.1.2.1 Metallic materials

Metallic materials are the most widely used for load-bearing implants. For

instance, some of the most common orthopedic surgeries involve the implantation of

metallic implants. These range from simple wires and screws to fracture fixation plates

and total joint prostheses (artificial joints) for hips, knees, shoulders, ankles, and so on.

In addition to orthopedics, metallic implants are used in maxillofacial surgery,

cardiovascular surgery, and as dental materials. Although many metals and alloys are

used for medical device applications, the most commonly employed are stainless steels,

commercially pure titanium and titanium alloys, and cobalt-base alloys (Table 2.1).

Table 2.1: Example of Medical and Dental Material and their Applications

9

2.1.2.2 Polymers

Polymers are used in medicine as biomaterials. Their applications range from

facial prostheses to tracheal tubes, from kidney and liver parts to heart components, and

from dentures to hip and knee joints. Polymeric materials are also used for medical

adhesives and sealants and for coatings that serve a variety of functions. Example of

polymer material is shown in Table 2.2.

Table 2.2: Example of Polymers used as Biomaterials

2.1.2.3 Ceramics

Traditionally, ceramics have seen widescale use as restorative materials in

dentistry. These include materials for crowns, cements, and dentures. However, their use

in other fields of biomedicine has not been as extensive, compared to metals and

polymers. For example, the poor fracture toughness of ceramics severely limits their use

for load-bearing applications. Some ceramic materials are used for joint replacement and

bone repair and augmentation as shown in the Table 2.3.

10

Table 2.3: Example of Biomaterial Ceramics

Ceramics and glasses Applications

Alumina Join replacement, dental implants

Zirconia Join replacement

Calcium phosphate Bone repair and augmentation, surface

coatings on metals

Bioactive glasses Bone replacement

Porcelain Dental restoration

Carbons Heart valves, percutaneous devices, dental

implants

2.1.2.4 Composites

Composites biomaterials are used in the field of dentistry as restorative

materials or dental cements. Although carbon-carbon and carbonreinforced polymer

composites are of great interest for bone repair and joint replacement because of their

low elastic modulus levels, these materials have not displayed a combination of

mechanical and biological properties appropriate to these applications. Composite

materials are, however, used extensively for prosthetic limbs, where their combination

of low density/weight and high strength make them ideal materials for such applications.

11

2.1.3 Natural Biomaterials

One of the advantages of using natural materials for implants is that they are

similar to materials familiar to the body. Natural materials do not usually offer the

problems of toxicity often faced by synthetic materials. Also, they may carry specific

protein binding sites and other biochemical signals that may assist in tissue healing or

integration. The problem with the natural materials is that they can be subjected to

immunogenicity, and their tendency to denature or decompose at temperatures below

their melting points. This severely limits their fabrication into implants of different sizes

and shapes. An example of a natural material is collagen, which exists mostly in fibril

form, has a characteristic triple-helix structure, and is the most prevalent protein in the

animal world [8].

2.1.4 Application of Biomaterials

Total joint replacement is widely regarded as the major achievement in

orthopedic surgery in the 20th century. Arthroplasty, or the creation of a new joint, is the

name given to the surgical treatment of degenerate joints aimed at the relief of pain and

the restoration of movement. This has been achieved by excision, interposition, and

replacement arthroplasty and by techniques that have been developed over

approximately 180 years. Hip arthroplasty generally requires that the upper femur (thigh

bone) be replaced and the mating pelvis (hip bone) area be replaced or resurfaced. As

shown in Figure 3, a typical hip prosthesis consists of the femoral stem, a femoral ball,

and a polymeric (ultrahigh molecular weight polyethylene, or (UHMWPE)) socket (cup)

with or without a metallic backing [9].

12

Figure 2.2: Hip joint replacement

2.2 Natural Bone

Bone is a composite type substance containing calcium, phosphate, magnesium

and collagen. Structurally bone is divided into five parts, namely:

1. Periostium

2. Compact bone

3. Spongy bone

4. Bone marrow, and

5. Epiphyseal plate

Bones are rigid and elastic in nature. Major percent of the bone is hydroxyapatite and

another small percent of carbonate is present in human bone.

13

Figure 2.3: Details of the bone structure [11]

Mineral substances in the bone like calcium, phosphate and magnesium make the bone

as rigid substance and collagen makes it as elastic substance. During bone development

stage mineral substance are converted into apatite minerals from crystallographicall

amorphous [11] [12].

2.2.1 Desirable Properties of Artificial Bone Material 2.2.1.1 Body Condition

Temperature conditions are not extreme for the materials used, body

temperatures being a little less than 38°C (98.6°F). However, the chemical physiological

environment and biomechanical environment can be extreme. For structural implants

used to repair the hip, it is estimated that the average nonactive person may place 1 to

14

2.5 × 106 cycles of stress on his or her hip in a year. For a person 20 to 30 years of age,

with a life expectancy of 70 to 80 years, that is the equivalent of approximately 108

cycles of loading in a lifetime. The actual loads and cycles are a function of the weight

and activity level of the person, but the need for longtime cyclic capability in fatigue is

obvious. Other applications in the body also impart many millions of fatigue cycles to

the device or component implanted.

In considering the parameters of materials for intracorporeal applications,

several factors are of major importance. It is generally agreed that the material must:

i. Be nontoxic and noncarcinogenic, cause little or no foreign-body reaction, and be

chemically stable and corrosion resistant. This is known as biocompatibility.

ii. Be able to endure large and variable stresses in the highly corrosive environment

of the human body

iii. Be able to be fabricated into intricate shapes and sizes

Many structural applications of materials in the body require that the

replacement material fit into a space perhaps only one-fourth the area of the part being

permanently or temporarily replaced or assisted. Consequently, the implant may have to

withstand loads up to 16 or more times that which the human bone must withstand. In

restorative dentistry, high compressive biting forces are combined with large

temperature changes and acidity to produce a challenging environment. It is clear that

there can be very great mechanical loading demands on biomaterials used for structural

purposes [13]

15

2.2.1.2 Mechanical Properties

Mechanical properties of artificial bone material should be similar to the natural

bone. Rejection of artificial implants due to mismatch in mechanical property between

natural bone and implant is known as biomechanical incompatibility.

Important mechanical properties are tensile strength, hardness and modulus of

elasticity. Artificial bone material considered for implant should have high strength and

low modulus of elasticity to match the property of natural bone [14].

Table 2.4: Summary of the mechanical properties and porosity of human bone [14]

Bone Compressive Strength (MPa)

Flexural Strength (MPa)

Tensile Strength (MPa)

Modulus (MPa)

Porosity (%)

Cortical Bone

130-180 135-193 50-150 12-18 5-13

Cancellous bone

4-12 NA (Not

available

1-5 0.1-0.5 30-90

2.2.1.3 Corrosion Resistance

Corrosion has been a major determining factor in the selection of materials for

use in the body environment. The first requirement for any material whether a

metal/alloy, ceramic, or polymer to be placed in the body is that it should be

biocompatible and not cause any adverse reaction in the body. The material must

withstand the body environment and not degrade to the point that it cannot function in

the body as intended. For example, metals used in the cardiovascular system must be

nonthrombogenic, and, in general, the more electronegative the metal with respect to

blood, the less thrombogenic the metal will be. For a material to be considered resistant

16

to corrosion in the body, its general corrosion rate usually must be less than, 0.01 mil/yr

(0.00025 mm/yr).

In vitro electrochemical measurements can be conducted in controlled

environments, and these techniques provide methods of determining the basic corrosion

reactions necessary for predicting the corrosion behavior of materials and for screening

and characterizing materials intended for use in surgical applications [15].

2.3 Conventional Metallic Materials Used For Medical Devices

Metals have been successfully used as biomaterials for many years. Besides

orthopedics, there are other markets for metallic implants and devices, including oral

and maxillofacial surgery (e.g., dental implants, craniofacial plates and screws) and

cardiovascular surgery (e.g., parts of pacemakers, defibrillators, and artificial hearts;

balloon catheters; valve replacements; stents; and aneurysm clips). Surgical instruments,

dental instruments, needles, staples, and implantable drug pump housings are also made

from metallic materials.

For structural applications in the body (e.g., implants for hip, knee, ankle,

shoulder, wrist, finger, or toe joints), the principal metals are stainless steels, cobalt-base

alloys, and titanium-base alloys. These metals are popular primarily because of their

ability to bear significant loads, withstand fatigue loading, and undergo plastic

deformation prior to failure. Other metals and alloys employed in implantable devices

include commercially pure titanium (CP-Ti), shape memory alloys (alloys based on the

nickel-titanium binary system), zirconium alloys, tantalum (and, to a lesser extent,

niobium), and precious metals and alloys [16].

17

2.3.1 Stainless steels

Stainless steels are iron-base alloys that contain a minimum of 10.5% Cr, the

amount needed to prevent the formation of rust in unpolluted atmospheres. Stainless

steels used for implants are suitable for close and prolonged contact with human tissue

(i.e., warm, saline conditions). Specific requirements for resistance to pitting and crevice

corrosion and the quantity and size of nonmetallic inclusions apply to implant-grade

stainless steels.

Austenitic stainless steels are popular for implant applications because they are

relatively inexpensive, they can be formed with common techniques, and their

mechanical properties can be controlled over a wide range for optimal strength and

ductility. Stainless steels for implants are wrought alloys (i.e., they are fabricated by

forging and machining). Austenitic stainless steels are not sufficiently corrosion resistant

for long-term use as an implant material. They find use as bone screws, bone plates,

intramedullary nails and rods, and other temporary fixation devices.

Recently, other stainless steels ‘nitrogen-strengthening stainless steel’ has been

developed and standardized that have increased corrosion resistance and improved

mechanical properties. Nitrogen-strengthened alloys are being used for bone plates, bone

screws, spinal fixation components, and other medical components. Nitrogen-

strengthened stainless steels have better crevice and pitting corrosion resistance.

2.3.2 Cobalt-Base Alloys

Cobalt-base alloys were first used in the 1930s. The Co-Cr-Mo alloy Vitallium

was used as a cast dental alloy and then adopted to orthopedic applications starting in the

1940s. The corrosion of cobalt-chromium alloys is more than an order of magnitude

greater than that of stainless steels, and they possess high mechanical property

18

capability. Although cobalt alloys were first used as cast components, wrought alloys

later came into use.

2.3.3 Titanium and Titanium-Base Alloys

Titanium and its alloys used for implant devices have been designed to have

excellent biocompatibility, with little or no reaction with tissue surrounding the implant.

Titanium derives it corrosion resistance from the stable oxide film that forms on its

surface, which can reform at body temperatures and in physiological fluids if damaged.

Increased use of titanium alloys as biomaterials is occurring due to their lower modulus,

superior biocompatibility, and enhanced corrosion resistance when compared to more

conventional stainless steels and cobaltbase alloys.

2.4 Magnesium

Magnesium always appears in nature in ionic form with the following electron

arrangement: 1S22S22P63S2.This arrangement is characterized by the low ionization

energies relative to the two most external electrons, which are at the 3S level. The low

standard reduction potential of magnesium is the reason why no metallic magnesium is

found in nature:

Mg2+ + 2e– = Mg E0 = –2.375 V

The raw materials for the production of magnesium come from different magnesium

sources. In all cases they will be accompanied by additional materials, depending on

their source.

19

Table 2.5: Raw Materials for Magnesium Production

Material Chemical formula

Magnsite MgCO3

Dolomite MgCO3·CaCO3

Bischofite MgCL2·6H2O

Carnallite MgCL2·KCL·6H2O

Serpentine 3MgO·2SiO2·2H2O

Sea water Mg2+(aq)

2.4.1 Properties of Pure Magnesium

Magnesium is classified as an alkaline earth metal. It is found in Group 3 of the

periodic table, and has the atomic properties as:

i. Element Symbol Mg

ii. Atomic Number 12

iii. Atomic Weight 24.3050

iv. Atomic Diameter 0.320 nm

v. Atomic Volume 14.0 cm3/mol

Lattice parameters of pure magnesium estimated at room temperature are: a = 0.32092

nm and c = 0.52105 nm. The c/a ratio is 1.6236 which is close to the ideal value of

1.633. Therefore, magnesium is considered as perfectly closed packed [17].

20

Figure 2.4: Closed packed structure of pure magnesium

The density of magnesium at 20°C is 1.738 g/cm3.At the melting point of 650°C

reduced to 1.65 g/cm3, on melting there is an expansion in volume of 4.2%. At higher

temperature volume diffusion is very important, especially at T > 0.6 Tm, where Tm is

the absolute melting point. At lower temperatures pipe diffusion, i.e., diffusion along

dislocation cores, may become more significant. Grain boundary diffusion plays a role

in polycrystals because the grain boundary acts as a low energy channel for the

movement of atoms.

The thermal conductivity of pure magnesium measured at elevated temperatures

decreases with increasing temperature. At very low temperatures the thermal

conductivity exhibits high values [17]

2.4.2 Melting and casting of magnesium

2.4.2.1 Melting

It is usual for magnesium to be melted in mild steel crucibles for both the

alloying and refining or cleaning stages before producing cast or wrought components.

21

Unlike aluminium and its alloys, the presence of an oxide film on molten magnesium

does not protect the metal from further oxidation. On the contrary, it accelerates this

process. Melting is complete at or below 650 °C and the rate of oxidation of the molten

metal surface increases rapidly with rise in temperature such that, above 850 °C, a

freshly exposed surface spontaneously bursts into flame. Consequently, suitable fluxes

or inert atmospheres must be used when handling molten magnesium and its alloys.

For many years, thinly fluid salt fluxes were used to protect molten magnesium

which were mixtures of chlorides such as MgCl2 with KCl or NaCl. However, the

presence of the chlorides often led to problems with corrosion when the cast alloys were

used in service [19].

Cover gases (e.g. SO2 or argon), or a mixture of an active gas diluted with CO2,

N2, replaced the used of the salt, also sulphur hexafluoride (SF6) was widely accepted as

the active gas because it is non-toxic, odourless, colourless, and effective at low

concentrations. But the disadvantage of sulphur hexafluoride SF6 is, however, relatively

expensive, and is now realised to be a particularly potent greenhouse gas with a so-

called Global Warming Potential (GWP) of 22, 000 to 23, 000 on a 100 year time

horizon.

As a result of that efforts are therefore being made to find other active gases

containing fluorine and one promising alternative is the organic compound HFC 134a

(1,1,1,2-tetrafluoroethane) that is readily available worldwide because of its use as a

refrigerant gas. It is also less expensive than SF6. HFC 134a has a GWP of only 1600,

and an estimated atmospheric lifetime of 13.6 years compared with 3, 200 years for SF6.

Moreover less is consumed on a daily basis so that the overall potential to reduce

greenhouse gas emissions is predicted to be 97% [19].

22

2.4.2.2 Casting and working of magnesium

Most magnesium components are now produced by high-pressure die casting

machine. Cold chamber machines are used for the largest castings and molten shot

weights of 10 kg or more can now be injected in less than 100 ms at pressures that may

be as high as 1500 bar. Hot chamber machines are used for most applications and are

more competitive for smaller sizes due to the shorter cycle times that are obtainable.

A reported disadvantage with high pressure die castings is that they may contain

relatively high levels of porosity. This restricts opportunities for using heat treatment to

improve their properties because exposure to high temperatures may cause the pores to

swell and form surface blisters.

Sand castings and low pressure permanent mould castings generally contain

less porosity and are used to produce components having more complicated shapes.

They can then be heat treated if the alloys respond to age hardening. With permanent

mould casting, turbulence can be minimized by introducing the molten metal into the

bottom of the mould cavity, under a controlled pressure, thereby allowing unidirectional

filling of the mould [19].

2.4.3 Magnesium Alloys

Magnesium is readily available commercially with purities exceeding 99.8%

but it is rarely used for engineering applications without being alloyed with other metals.

The fact that its atomic diameter (0.320 nm) is such that it enjoys favourable size factors

with a diverse range of solute elements.

23

2.4.3.1 Common Alloying Elements

2.4.3.1.1 Aluminum

Aluminum is the most commonly used alloying element, and the maximum

solubility is 11.5 at % (12.7 mass %) and alloys in excess of 6 mass % can be heat

treated. Aluminum improves strength, the optimum combination of strength and

ductility being observed at about 6%.Alloys is readily castable.

2.4.3.1.2 Calcium

Alloying with calcium is becoming more common in the development of

cheap creep resistant alloys. Ca can act as deoxidant in the melt or in subsequent heat

treatment. It improves the roll ability of sheet but >0.3 mass % can reduce the weld

ability.

2.4.3.1.3 Manganese

Manganese is usually not employed alone but with other elements, e.g., Al. It

reduces the solubility of iron and produces relatively innocuous compounds. It increases

the yield strength and improves salt water corrosion resistance of Mg-Al and Mg-Al-Zn

alloys. Binary alloys (M1A) are used in forgings or extruded bars. The maximum

amount of manganese is 1.5–2 mass %.

24

2.4.3.1.4 Rare Earths

Rare Earths are added to magnesium alloys to improve the high temperature

strength, and creep resistance; they are usually added as Mischmetal.

2.4.3.1.5 Zinc

Zinc is one of the commonest alloying additions. It is used in conjunction with

Al (e.g., AZ91 or with zirconium, thorium or rare earths) [18].

2.5 Zinc Metal

Zinc is, in some respects, chemically similar to magnesium, because its ion is of

similar size and its only common oxidation state is +2. Zinc is the 24th most abundant

element in the Earth's crust and has five stable isotopes [19].

2.5.1 Zinc Biological role

Zinc is an essential trace element, necessary for plants, animals, and

microorganisms. Zinc is found in nearly 100 specific enzymes (other sources say 300),

serves as structural ions in transcription factors and is stored and transferred in

metallothioneins. It is "typically the second most abundant transition metal in

organisms" after iron and it is the only metal which appears in all enzyme classes.

In proteins, Zn ions are often coordinated to the amino acid side chains of

aspartic acid, glutamic acid, cysteine and histidine. The theoretical and computational

25

description of this zinc binding in proteins (as well as that of other transition metals) is

difficult. There are 2–4 grams of zinc distributed throughout the human body. Most zinc

is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the

prostate and parts of the eye. Semen is particularly rich in zinc, which is a key factor in

prostate gland function and reproductive organ growth.

In humans, zinc plays "ubiquitous biological roles". It interacts with "a wide

range of organic ligands", and has roles in the metabolism of RNA and DNA, signal

transduction, and gene expression. It also regulates apoptosis. A 2006 study estimated

that about 10% of human proteins potentially bind zinc, in addition to hundreds which

transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found

2367 zinc-related proteins [21].

In the brain, zinc is stored in specific synaptic vesicles by glutamatergic neurons

and can "modulate brain excitability". It plays a key role in synaptic plasticity and so in

learning. However it has been called "the brain's dark horse" since it also can be a

neurotoxin, suggesting zinc homeostasis plays a critical role in normal functioning of the

brain and central nervous system [21].

2.6 Researched Biodegradable Magnesium Alloys

For the purpose of searching for suitable alloying elements for biomedical

magnesium alloys, researchers exploited the in vivo and the in vitro behavior of

magnesium alloys.

Yizao Wan [22] in his research work named preparation and characterization of

a new biomedical magnesium–calcium alloy. Demonstrated that 0.6 wt % calcium

content improved corrosion and mechanical properties of magnesium and the alloy Mg-

0.6Ca shows good potential as a new biomedical material.

Hui Du [23] researched on the effects of the addition of Zn element on the

properties of Mg–3Ca. He pointed out that the element Zn could improve both tensile

26

strength and elongation of Mg–3Ca alloys, and attributed that the presence of

Ca2Mg6Zn3 phase found in the alloy mainly contributes to these improvement.

Jun Wang [24] investigates the Microstructure and corrosion properties of as

sub-rapid solidification of Mg–Zn–Y–Nd alloy in dynamic simulated body fluid for

vascular stent application. The research shows that as sub-rapid solidification can

improve the corrosion resistance of Mg–Zn–Y–Nd alloy in dynamic SBF due to grain

refinement.

Yuncang Li [25] exploded the effects of calcium (Ca) and yttrium (Y) on the

microstructure, mechanical properties, corrosion behavior and biocompatibility of

magnesium (Mg) alloys. Results of the investigation indicated that Ca content can

enhances the compressive strength, elastic modulus and hardness of the Mg–Ca alloys,

but deteriorates the ductility, corrosion resistance and biocompatibility of the Mg–Ca

alloys. Also revealed that yttrium addition increases ductility; but decreases the

compressive strength, hardness, corrosion resistance and biocompatibility of the alloy

Mg–1Ca–1Y.

Erlin Zhang [26] research on the in vivo degradation of magnesium alloy

implant highlighted that rapid degradation of magnesium implant was observed in the

marrow channel than in the cortical bone. Also shown that the degradation of the

magnesium implant in the blood caused little change to blood composition but no

disorder to liver or kidneys

E. Aghion G. Levy [27] evaluated the effect of 0.4% Ca on the in vitro

corrosion behavior of Mg–1.2% Nd–0.5% Y–0.5% Zr in a simulated physiological

environment. He outlined that 0.4% Ca has a beneficial effect on the corrosion resistance

of the tested alloy, and attributed this to the effect of calcium, which reduces oxidation

in the molten condition and consequently improves the soundness of the obtained

casting, E. Aghion result also shown that the addition of calcium has a damaging effect

on the stress corrosion behavior in terms of reduced ultimate tensile strength and

27

ductility of the alloy, and this was mainly due to the embrittlement effect of calcium that

was generated by the formation and distribution of Mg2Ca phase at grain boundaries.

Zijian Li [28] in his work, research on the development of binary Mg-Ca alloys

for use as biodegradable materials reveals that controlled calcium content and processing

treatment can lead to the improvement in tensile strength and corrosion properties of the

alloy. Also highlighted that Mg-1Ca alloy show an acceptable biocompatibility as a new

kind of biodegradable implant material.

Yingwei Song [29] explores the in vitro corrosion behaviors of the

biodegradable AZ31 in simulated body fluid (SBF), pointing out that some protective

film layer was formed on the surface of AZ31 and had perfect biocompatibility.

X.N. GU [30] investigates the Corrosion fatigue behaviors of two biomedical

Mg alloys AZ91D and WE43 – In simulated body fluid. Demonstrated that die-cast

AZ91D alloy indicated a lower fatigue limit than that observed for extruded WE43 alloy

.

Yin Dongsong [31] demonstrated that Zinc content (3% wt) refined the

microstructure, and improved the mechanical properties of Mg-Mn alloy.

Shaoxiang Zhang [32] explored in vitro and in vivo potentials of Mg-Zn alloy

and point out the alloy shown suitable mechanical properties for implant application and

also good corrosion resistance.

28

CHAPTER 3

RESEARCH METHODOLOGY

3.1 Introduction

The experimental work in this project was to prepare the magnesium zinc alloys

and to study the effect of zinc addition on the microstructure, mechanical and corrosion

properties of Mg-Zn alloys. Figure 3.1.shown the general flowchart of the experimental

procedures.

29

Figure 3.1: Flowchart showing the summary of research methodology

Prepare

Mg-Zn

Prepare mold Gravity die mold

Melting at 7000C-7500C in mild steel crucible

Cast at 7300C into steel mold

Microstructure characterization

Mechanical Test Tensile/Hardness

test

Stop

Result/Discussion

Start

30

3.2 Research Design

3.2.1 Casting

Pure magnesium ingot (99.99 wt %) Figure 3.2(a) and, pure zinc ingot (99.995

wt. %), Figure 3.2(b), were used in this experiment. Melting process was carried out in a

high frequency induction furnace (inductothem) Figure 3.3a using a mild steel crucible

under the argon gas atmosphere. After magnesium metal was melted at about 6500C,

pure zinc ingot was added. After all these materials were melted completely and

superheat to around 7500C, the melt was then cast into a steel mold at 7300C Figure

3.3b. Figure 3.3(c, d and e) shows the schematic mold design, as cast sample, and the

original mold used in the casting process.

(a) (b)

Figure 3.2: (a) Magnesium Ingot (b) Pure Zinc

31

(a)

(b)

32

(c)

(d)

33

Figure 3.3: (a) Mg-Zn Melting, (b) Pouring into steel mold, (c) Designed Mold, (e) Cast

Sample, (f) the mold used

3.2.2. Microstructural Characterization

Characterization of the microstructure and phases of the Mg-Zn alloys was

conducted using optical microscope (Olympus BX60) and scanning electron microscope

(Philips XL 40) equipped with energy dispersive spectroscopy (EDS). And carls zeiss

supra 35VP (FE-SEM), Shown in figure 3.4.

34

(a)

(b)

35

(c)

Figure 3.4: (a) Olympus BX60, (b) Philips XL 40, (c) Supra 35VP, used for the

characterization of the microstructure

3.2.3 Hardness Test

The Vickers hardness tests were performed using Matsuzawa DVK-2 material

testing machine, at five different locations of the samples according to ASTM-E98-82

standard, Figure 3.5

36

(a)

(b)

Figure 3.5: (a) Matsuzawa DVK-2 used for the hardness testing (b) location of the test

on the sample

37

3.2.4 Tensile Test: The tests were conducted according to ASTM-A370 on an instron universal

testing machine with a tensile speed of 1 mm/min. The test sample has a gauge length of

25mm and thickness of 10mm. Extensometer was used to measure the elongation. Data

presented in this report were average value of 4 separated measurements.

Figure 3.6: Instron universal tensile testing machine used in the tensile testing of the

samples

38

3.2.5 Electrochemical Measurement:

The electrochemical test ware performed using a (Parstat-2263) test set up. A

saturated calomel electrode (KCL) and graphite electrode were used as the reference and

counter electrodes, respectively. Samples (working electrode) with a cross section of 1

cm2 was used, all the polarization curves were measured at a scan rate of 0.9 mV/s.

Figure 3.7: Electrochemical test (Parstat-2263) set up used for the corrosion

measurement

39

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Selection of Optimum Zinc addition

4.1.1 Nominal Composition Analysis

Chemical composition as-cast samples were determined by Energy Dispersive

Spectrometer (EDS) detector attached to Scanning Electron Microscopy (SEM), as

shown in Table 4.1.

Table 4.1: Nominal chemical composition of the Mg-Zn alloys (wt. %)

Composition

Zn

Mg

Balance

Sample 1 1.62 98.38 100 Sample 2 4.03 95.97 100 Sample 3 6.33 93.67 100 Sample 4 8.46 91.54 100 Sample 5 9.65 90.35 100

40

4.1.2 Microstructural Characterization

Optical microstructures of as cast Mg–Zn alloys were shown in figure 4.1. All

materials show nearly equiaxed grain structure. However, differences are noted among

these samples. Small and separated precipitates are observed within grains for alloys and

the width of the grain boundary becomes thicker as the content of zinc increases. In

addition, the grain boundaries are characterized by a discontinuous distribution of small

precipitates.

Figure 4.1: Microstructure of the as cast (a) pure magnesium, (b) Mg-2Zn,

(c) Mg-4Zn, (d) Mg-6Zn, (e) Mg-8Zn, (f) Mg-10Zn

41

In trying to point out what the precipitation in Figure 4.1(e) and (f) constitutes of, Figure

4.2(a) and (b) show the FE-SEM micrographs of Mg-8Zn, and Mg-10Zn alloys

respectively, and the EDS analysis conducted on the precipitation along the grain and

the grain boundary indicates that it is rich in zinc and small amount of magnesium

Figure 4.3(a) and (b) suggesting that the precipitation consist of zinc and magnesium.

Fine second phases with size of 1-2 μm can be seen in the Mg-Zn alloy in the grain and

in the grain boundary of the alloy Figure 4.2(c) and (d).

Figure 4.2: FE-SEM micrographs of (a) Mg-8Zn alloy, (b) Mg-10Zn Alloy, (c) (Mg,

Zn)-containing phase in the grain, (d) (mg, Zn)-containing phase at the grain boundary

42

Figure 4.3: EDS analysis of the secondary phases (a) on the grain, (b) at the grain

boundary

43

4.1.3 Hardness Test

With respect to pure magnesium in figure 4.4 the Vickers hardness values of Mg

xZn (2, 4, and 6) as a function of the %wt Zn follow an increasing pattern but slightly

drop at Mg-8Zn, and Mg-10Zn alloys. This slight drop in hardness value may be

attributed the formation of the secondary phases on these alloys Mg-8Zn, and Mg-10Zn.

Because the maximum solubility of zinc in magnesium is 6.2% wt and as highlighted by

Yin Dongsong [31] the excess Zinc reacts with Mg and form large amount of Mg, Zn

containing phases in the matrix and grain boundary. These phases segregate the matrix

and increase the number of crack sources. Therefore, the strength/hardness of the alloy

will not be improved.

Figure 4.4: The hardness value of Mg-Zn alloys as a function of zinc addition.

44

4.1.4 Tensile Test

The tensile strength of the alloy with respect to %wt Zn addition in figure 4.5

also shows an improving trend until 8%wt Zn where the value drop but rise again at

10%wt Zn. The value of the tensile test should follow similar trend as the hardness

graph, but this dissimilarity may be related to error or inconsistency in the experiment or

the test machine especially for Mg-10Zn, even though the student was very careful to

ensure consistency throughout the project. On the other hand the Yield strength value in

Table 4.2 shows an ascending pattern of strength for all the alloys. But some rise and fall

were observed in the value of the elastic modulus of the alloys. These phenomena of the

may be fully explain by the formation of secondary phase as has been shown in figure

4.1(e, f) and 4.2(a, b). As described by Yin Dongsong, excess Zn results in the formation

of second phases when reacts with magnesium and become sources of separation in the

matrix and the grain boundary, hence, the tensile strength of the alloy will drop.

Figure 4.5: The Tensile strength value of Mg-Zn alloys as a function of zinc addition.

105.57 134.82

139.84

108.28

159.57

020406080

100120140160180

2%Zn 4%Zn 6%Zn 8%Zn %10Zn

Tens

ile s

tren

ght (

MPa

)

wt% zn

Tensile

Mg-Zn alloys

45

Table 4.2: The Tensile strength, Yield, and Elastic Modulus value of Mg-Zn alloys

Alloys

Tensile Strength

(MPa)

Yield strength

(MPa)

Modulus

(GPa)

Mg-2Zn

105.57

54.19

35.94

Mg-4Zn

134.82

65.61

29.93

Mg-6Zn

139.84

83.43

48.66

Mg-8Zn

108.28

89.98

33.29

Mg-10Zn

159.57

128.80

41.55

46

4.1.5 Corrosion Electrochemical Test

Figure 4.6 shows the electrochemical polarization curves of the Mg-xZn alloys

(x = 2 - 10wt %). It could be seen that the cathodic polarization current of hydrogen

evolution reaction (-1.675V) on Mg-6Zn alloy sample was greater than that on Mg-Zn

(2, 4, 8, and 10) alloy samples, which indicated that the over potential of the cathodic

hydrogen evolution reaction was lower for Mg-6Zn alloy sample. Meaning to say Mg-

6Zn is less prone to corrosion compare to the other samples. According to Zijian Li [28]

generally, the cathodic polarization curves were assumed to represent the cathodic

hydrogen evolution through water reduction.

Figure 4.6: Electrochemical polarization curves of Mg-Zn alloys under investigation

47

CHAPTER 5

CONCLUSION

Developing optimum Mg-Zn binary alloy composition in terms of tracing the

effect of zinc addition in magnesium was the major objective of this project. So,

therefore it can conclude that:

i. Mg-6Zn alloy with the hardness value of 74.44HV, tensile strength of

139.88MPa, and modulus of 48.66GPa could be considered the optimum

composition based on this project work. It shows significant improvement in

respond to the zinc addition, virtually better than the other composition studied.

Likewise in terms of resistance to corrosion it shows higher potential then the

rest of the alloys.

ii. In comparison with the reported hardness and tensile strength of the hardest

natural bone (16-168HV, and 50-150), and due to the fact that implant material

should not be harder than the natural bone, it can be concluded that the

properties exhibits by Mg-6Zn are comparable.

48

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1672-6421(2009)01-043-05

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Chaoying Xie, Hairong Tao, Yan Zhang, Yaohua He, Yao Jiang, Yujun Bian

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51

APPENDIX A

Compositional Analysis of the as- cast Samples

Compositional Analysis of the as- cast Samples of Mg-2Zn

52

Compositional Analysis for the as- cast Sample Mg-2Zn

53


54


55


56


57


58


59


60


61

APPENDIX B

The Stress/Strain Graphs for the alloys Samples

The Stress/Strain Graphs for Mg-2Zn

62


63


64


65


66

APPENDIX C

Polarization Curves of the Samples

Polarization Curves of Mg-2Zn

Mg-2Zn

67

Polarization Curves for Mg-4Zn

Mg-4Zn

68


Mg-6Zn

69


Mg-8Zn

70


Mg-10Zn

effect of zinc addition on the properties of

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