mechanical properties and microstructure of new magnesium-lithium base...

E L S E V I E R Materials Science and Engineering A220 (1996) 69-77

M A T E R I A L S SCIENCE &

ENGINEERING l

Mechanical properties and microstructure of new magnesium-lithium base alloys

A. Sanschagrin a, R. Tremblay b,*, R. Angers b, D. Dllb6 b aInstitute of Magnesium Technology Inc., Sainte-Foy, Qudbec, GIP 4N7 Canada

bDepartment of Min#~g and Metallurgy, Laval University, Qudbec, GIK 7P4 Canada

Received 10 June 1996

Abstract

Magnesium-Iithium base a11oys containing aluminum, silicon and rare-earth additions have been prepared by melting and solidification in a low-carbon steel crucible, homogenized at 350°C and extruded at the same temperature. The distribution of alloying elements in homogenized specimens was determined and the microstructure of these alloys characterized. The mechanical properties were measured and compared with those of similar alloys.

Keywor&: Magnesium-lithium alloys; Scanning electron microscope; X-ray diffraction

1. Introduction

Magnesium and specially magnesium-lithium alloys, because of their superior stiffness-to-weight ratio, provide a promising matrix for composites in applications such as aerospace and aircraft structures as well as for structural components in ultra-lightweight com- munication systems [1-3]. These alloys also show high electrical and thermal conductivities. However their poor resistance to corrosion and their low creep strength have slowed their development and industrial use.

Recent works indicated that an important reinforce- ment of magnesium-lithium alloys is obtained with additions of SiC particulates or short fibers through the conventional melt route [4-6]. However, chemical reactions affecting the mechanical properties have been observed at the SiC/metal interface and these reactions must be eliminated to reduced to minimum [5-8]. All these facts have stimulated the research for suitable alloying elements that could both improve the mechanical properties of the matrix and reduce the overall interfaciaI reactions.

The influence of lithium on the microstructure and mechanical properties of magnesium alloys is well

* Corresponding author.

0921-5093196[$15.00 © 1996 --Elsevier Science S.A. All rights reserved PII S0921-5093(96)10460-3

known [9-13]. Solid solution of magnesium alloys with low lithium contents (the so-called ~ phase alloys) retains the hexagonal structure and shows a moderate strength and a low formability. The ductility is much improved with larger additions of lithium which bring about the formation of the centered cubic phase (the/3 phase) but the strength is then notably lowered. High lithium alloys containing the cubic phase exhibit lower creep properties. However a two-phase structure (~ + fl) constitutes an interesting compromise since it com- bines the moderate strength of the ~ phase with the excellent ductility of the fl phase. Also a superplastic behaviour was reported recently for Mg-Li alloys with such a two-phase structure [14,15].

Aluminum additions increase the tensile strength and hardness of magnesium-lithium alloys [9-11,13,16- 18]. It also improves the ductility at moderate concentrations and reduces corrosion problems. The Mg-Li-A1 alloys display strengthening by age harden- ing [17-20]. Aluminum additions could also intensify the reactions with SiC particles or fibers in composite materials [21].

Small additions of silicon in magnesium alloys moderately increase the ductility [22]. However, at high silicon concentrations, the brittle Mg2Si phase forms. Although this intermetallic phase increases the tensile strength and creep resistance, it reduces the ductility and corrosion resistance of the alloys [21,22]. Jackson

70 A. Sanschagrin et al. / Materials Science

Table I Composition of magnesium-lithium based alloys

Alloy Chemical composition (wt.%)

Mg Li A1 Si Ce La

Mg-Li 91.1 8.7 . . . . Mg-Li-A1 84.7 8.8 6.4 - - - - - - Mg-Li-AI-Si 83.7 8.2 6.9 1.1 - - - - Mg-Li-AI-Si-RE 79.I 8.2 6.8 1.2 2.7 1.8

[10] reported a significant improvement of the tensile strength of the /? (BCC) M g - l l L i alloy with silicon additions but the influence of this element on the two-phase structure of magnesium-l i th ium alloys is not known. Schemme and Hornbogen [23] indicated that the solubility of silicon in hexagonal phase is lower than 0.12 wt.%.

Small additions of rare-earth elements such as cerium, lanthanum, praseodymium, yttrium and neodymium usually provide an overall increase in the mechanical properties of magnesium alloys due to solid-solution strengthening and fine dispersion of intermetallic phases [24,25]. Little information was found about the influence of rare-earth elements on the mechanical properties or microstructure of magnes ium- lithium alloys. Wyss [13] indicated that 5.8% yttrium in two-phase magnes ium-l i th ium alloys cause a refine- ment of the microstructure of the ~ phase within which it partitions. However the solubility of yttrium appears to be very low because much of it is out of the solution. Additions of 1 -3% cerium to M g - L i - A 1 alloys only

Table 2 Chemical compositions of alloying elements and alloys (wt.%)

and Engineering A220 (t996) 69-77

moderately affected the mechanical properties [13]. In the present work, aluminum, silicon and mis-

chmetal were added to magnesium-l i th ium base alloy in order to study the effect of these elements on their microstructure and mechanical properties. The lithium content in magnesium was adjusted in order to produce an even proportion of cubic and hexagonal phases in the structure. Interfacial reactions between these molten alloys used with various reinforcing ceramics to form metal-matrix composites will be presented in a forth- coming article.

2. Experimental procedure

2.1. Specimen preparation

Magnesium-l i th ium base alloys were melted in a controlled atmosphere chamber by induction heating in low carbon steel crucibles. The compositions of these alloys are given in Table 1. They were selected in order to produce a two-phase structure containing equal properties of the c~ and /? phases.

A special procedure was used to introduce the alloying elements into the basic charge and in order to limit their oxidation and losses during melting. Alloying elements were inserted into holes previously drilled in 99.8% pure magnesium ingots. The highly reactive lithium rods were first cleaned and inserted in a magnesium piece under the protective atmosphere of a glove box. The piece was then placed into the crucible. The average mass of each batch was about 100 g.

Mg ingot Mg Zn Fe Mn Si Ca Cu Ni

99.8 0.08 0.07 0.02 0.01 0.002 0.001 0.001

A1 ingot A1 Fe Si Cu Ga V Ti Ni

99.8 0.1 0.046 0.02 0.014 0.008 0.003 0.00t

A1-25%Si ingot A1 Si Fe Ti Ga Mn Zn Zr

74.8 24.9 0.21 0.013 0.012 0.005 0.005 0.002

Li rods Li Ca C1 Fe K N Na Si

99.96

A. Sanschagrin et al. / Materials Science and Engineering A220 (1996) 69-77 7l

• 200 gm

Fig. 1. Microstructure of Mg-Li base alloys after homogenization at 350°C: (a) Mg-8.7Li, (b) Mg-8.SLi-6.4A1, (c) Mg-8.2Li-6.9AI-I.ISi and (d) Mg- 8.2Li- 6.8A1-1.2Si-4.SRE.

Aluminum and silicon used for alloying were taken from 99.8% pure ingots and pre-alloyed A1-25%Si ingots respectively. Their composition as well as the composition of lithium rods and mischmetal are given in Table 2.

Outgassing of the crucible and its content was con- ducted under vacuum at 250°C for 1.8 ks in order to remove most of the adsorbed oxygen and moisture. Argon was then introduced and the crucible heated to a temperature between 750 and 950°C depending upon the composition of each alloy. For example, prelimi- nary experiments indicated that silicon additions re- quired a higher melting temperature for a complete dissolution. The heating time was kept to a minimum, typically between 0.6 and 1.8 ks depending on the alloy, in order to reduce evaporation losses of magnesium and lithium.

The melt was stirred with a slowly rotating stainless steel helix mixer and the melt was allowed to solidify inside the crucible and cooled within about 3.6 ks. A strong adherence between the solidified alloy and

crucible, attributed to the presence of lithium, hin- dered demoulding. The ingots was released by machin- ing away the steel crucible. All magnesium-li thium alloys were heat-treated under argon at 350°C during 86 ks for a better homogenization of the elements.

The alloys were reheated and then extruded at 350°C using a graphite lubricant to produce 9.45 mm diameter rods. The extrusion speed and extrusion ratio were 0.4 cm s-1 and 16:1, respectively.

The cylindrical tensile test specimens were prepared according to the ASTM B-557 standard procedure. They were machined from the extruded rods with a gauge diameter of 6.3 mm and a gauge length of 25.4 ram. Tensile tests were carried out at room temperature and at a strain rate of 0.0017 s - 1. The hardness of the extruded specimens was measured with a Rock- well hardness tester, and then transformed into Brinell units, a direct Brinell hardness measurement being prohibited by the relatively small size of the specimens. Density of the alloys was determined by Archimede's method.

72 A. Sansehagrin et al. / Materials Science and Engineering A220 (1996) 69-77

Fig. 2. ton imaging of elements in Mg-8.SLi-6.4A1 alloy after homogenization: (a) magnesium, (b) lithium and (c) aluminum.

~ . . I I ~ '~ a~.-~

, - - - , . . . .

\ ]~,,~ .~

Fig. 3, Ion imaging of elements in Mg-8.2Li-6,gA1-1.1Si alloy after homogenization: (a) magnesium, (b) lithium and (c) aluminum.

A. Sansckagrin et al. / Materials Science and Engineering A220 (i996) 69 -77 73

2.2. Microstructural characterization 3. Results and discussion

The specimens microstructure was characterized by optical and scanning electron microscope (SEM). This characterization had to be performed very shortly after polishing or etching due to the particularly sensitive nature of specimens in ambient atmosphere. The crys- talline structure of the alloy phases was determined by X-ray diffraction (XRD) analysis. Chemical microanalysis of phases was carried out using an energy disper- sive X-ray spectrometer (EDS) with the SEM. The mapping of elements was performed using secondary ion mass spectroscopy (SIMS). Such analysis were carried out on a semi-quantitative basis because lithium, which is present in many phases, cannot be detected by EDS and also because quantitative analysis with SIMS remains very limited without appropriate standard alloys.

(b) !0 m

Fig. 4. Ion imaging of elements in Mg-8.2Li-6.gAt-l . lSi alloy after homogenization: (a) aluminum and (b) silicon.

3.1. Microstructure and composition of phases after homogenization

The microstructure of specimens after the homogenization treatment at 350°C is shown in Fig. 1. The SEM micrograph of the Mg-8.TLi specimen (Fig. l(a)) shows that it is composed of two phases. Small whitish grains consisting of the magnesium-rich c~ phase are embedded in the darker /~ phase containing more lithium. The homogenization treatment produced less angular (more round) ~ grains. The proportion of phases, evaluated from backscattered electron images, reveals that the volume proportion of ~ varies between 40 and 60% depending on the sampling area. This proportion is not very far from the estimations based on theoretical density and the equilibrium phase diagram [26]. Iron-rich inclusions, a few micrometers in diameter, were also observed.

The SEM micrograph of the Mg-8.8Li-6.4AI specimen is shown in Fig. l(b). The addition of aluminum to the magnesium-lithium base alloy deeply modified the microstructure which consists of grains exhibiting fine lamellae embedded in featureless grains containing relatively less aluminum. The A1Li phase is expected to appear in the microstructure according to the isother- mal ternary phase diagram [11,t6-20]. XRD analysis of these specimens confirms that A1Li and also LiMg A12 phases are present in minor quantities. Ion imaging with SIMS (Fig. 2) indicates that aluminum is found mostly in the magnesium-rich c~ phase leaving relatively less aluminum for the /7 phase which contains more lithium. These results confirm previous measurements by Wyss [13] on other Mg-Li-A1 alloys. A linear scanning of the ~-/~ interface using SIMS revealed a thin A1 and Li-rich zone at this interface. This observa- tion suggests that some AILi and/or LiMgA12 phases are located at the interface, although quantitative measurement of the lithium concentration cannot be made with this method. The coexistence of these phases was also reported by Wyss [13] for both single-phase and double-phase Mg-Li-A1 alloys. Minor amounts of small A1-Fe inclusions were also observed in these alloys.

The optical micrograph of Mg-8.2Li-6.gAI-I . ISi specimens (Fig. 1(c)) reveals dark and angular phases which were identified by XRD analysis as intermetallic Mg2Si. This prismatic phase is known to be relatively hard and brittle. Oxygen was detected in this phase and lithium could also be present but it cannot be analyzed by EDS. Cracks, sometimes partially opened, are visible in the brittle Mg2Si phase. It is likely that these cracks formed during solidification of the surrounding

74 A, Sanschagrin et aL / Materials Science and Engineer#~g A220 (1996) 69-77

, . , % , • ' ~ ¢ a " " =

N'" N : . . , .i ~ • ~ : : ' . - . ~ . , ~ - , • . a : , " . , " , = f : "';

~ g . ~ t : N .~.: : 5:l;,,2g'e~22,

' . ' _e ,~ . , , P , : J : 1 8

Fig. 5. Ion imaging of elements in Mg-8.2Li-6.SAI-I.2Si-4.5RE alloy after homogenization: (a) magnesium, (b) lithium, (c) aluminum and (d) silicon.

magnesium-lithium alloy and were infiltrated by a liquid phase. The relative distribution of the elements for homogenized specimens obtained by ion imaging is also shown in Figs. 3 and 4. These mappings indicate that more aluminum and silicon is found in the magnesium-rich phase showing lamellae. Neglecting the presence of lithium in order to obtain a rough analysis by EDS, 7%A1 and 1.7%Si were measured in the magnesium-rich phase whereas 3%At and 1.4%Si were measured in the /? phase containing more lithium. The volume proportion of the Mg2Si phase is less than t%.

The influence of mischmetal additions on the microstructure is illustrated by Fig. l(d) which shows the optical micrograph of the Mg-8.2Li-6.SAI-I .2Si- 4.5RE alloy. It was not possible, using EDS microanalysis to determine the presence of rare-earth etements in the matrix, their concentration being below the detec- tion limit. Rare-earth element appear to be concen- trated in small, elongated and angular structures well dispersed in the bulk. Assuming that the angular phases contain no lithium, the EDS microanalysis indicated that their compositions correspond to REAl3, REAl 4

and A1SiRE (with RE = Ce, La, Nd). XRD analysis confirmed the stoichiometry of the first two phases only. Prisms of the Mg2Si phase are also visible as well as few iron-rich inclusions. The volume proportion of these minor phases (RE-A1, Mg2Si and iron-rich inclusions) is approximately 15%. XRD analysis confirmed that the matrix presents a two-phase structure, some grains showing a fine lamellar structure. The ion images (Fig. 5) show that more aluminum and silicon are found in the magnesium-rich phase.

3.2. Microstructure of extruded specimens

The extrusion of specimens led to a texturing of their microstructure as shown in Fig. 6 for Mg-8.7Li rods. Recrystallization of elongated fl grains which contain more lithium occurred during extrusion. New equiaxial grains formed within the elongated grains of/? and are outlined by the magnesium-rich c~ phase (whitish phase) at grain boundaries. An identical re-crystallization occurred in /? grains of the Mg-8.SLi-6.4A1 and Mg- 8.2Li-6.gAl-l.lSi specimens. Lamellae were observed

A. Sanschagrin et al . / Materials Science and Engineer#~g A220 (1996) 69-77 75

~ extrusion direction ~ : - ~ : 2

1 L~.25%~ . . . .

#

300

250

13. 200

O 3 ( , 9

,e 150 03

'~ 100 o 1-

50

1 1 1 l

Mg-8.2Li-6.8A[-1.2Si-4.SRE

~ L i - 6 . 4 A I

i Mg-8 2Li-6 9A1-1 lSi

I t I t I

0 0 10 20 30 40 50 60 Strain (%)

Fig. 6. Microstructure of Mg-8.7Li extruded rods showing recrystallization in fl grains elongated during extrusion. Whitish zones correspond to the magnesium-rich c~ phase.

in the ~ grains of the Mg-8.SLi-6.4A1 specimens. In Mg-8.2Li-6.9AI-I . ISi specimens, Mg2Si inclusions were fragmented by the mechanical deformation and dispersed in the longitudinal direction (Fig. 7). The extruded specimens contained no visible porosity or open cracks.

3.3. Mechanical properties of extruded specimens

Typical tensile stress-strain curves are shown in Fig. 8 for the investigated alloys. Bulk modulus, average tensile strength, elongation and hardness are given in Table 3. Densities measured for the cast alloys are also given.

" " _e_x t rus lond [ rec t /on ._ _ . . - -~ : : -%-~ .

u

j - _ . ~

.

76 A. Sansehagrin et aL / Materials Science and Engineering A220 (I996) 69-77

Table 3 Mechanical and physical properties measured at 20°C

Alloy type E (OPa) Y,S.(0.2%) (MPa) U.T.S. (MPa) Hardness (BHN) Elongation (%) Density (gcm -3)

Mg-Li 40 93 132 36 52 1.51 Mg- Li-AI 39 184 239 53 33 1.53 Mg-Li-AI-Si 44 145 225 57 20 1.59 Mg-Li-A1-Si-RE 53 200 260 62 14 1.60

4. Conclusions

The combined influence of aluminum, silicon and rare-earth additions on the microstructure and me-

Tensile strength (MPa)

1 O0 200 300

Mg

Mg - Li [this work]

Mg 8-10Li 4-7A1 [10,13,171

~'x\\ '<

LA91 [12]

LA141 [121

Mg - Li - AI [this work]

Mg 11Li 1-4Si [10]

Mg - Li - AI - Si [this 'g'o rk]

Mg 9.8Li 2AI 2.95Ce [13]

Mg 10Li 1.77A1 1.ICe [13]

Mg- Li - AI- Si- RE [this work]

~ \ \ \ \ ' I

rain max

[NI

Iq

0 50 100 Elongation (%)

Fig. 9, Mechanical properties of the Mg-Li based alloys produced during the present work and compared with those already reported in literature.

chanical properties of magnesium-l i th ium based alloys was studied. The microstructure and elemental distribution in these alloys were examined and related to mechanical properties. This study confirms the strong influence of aluminum additions on the tensile strength and ductility of M g - L i alloys. This effect is likely the result of solution strengthening of the ~ and /3 phases although the contribution of the MgLi2A1 phase is not ruled out.

Additions of silicon to the M g - L i - A 1 alloy reduced the mechanical properties, The brittle Mg2Si phase likely contributed to the overall decrease in mechanical properties. It is suggested that the presence of silicon produces a different distribution of aluminum in the c~ and /3 phases which could affect the mechanical properties. More silicon and aluminum was found by ion imaging in the c~ phase (magnesium- rich) than in the/3 phase.

Mischmetal additions produced abundant and well dispersed precipitates of REAt3, REAl 4 and A1SiRE (with RE = Ce, La, Nd) in the matrix. The tensile strength of the M g - L i - A I - S i alloy was improved by rare-earth additions but at the expense of ductility.

Acknowledgements

The authors gratefully acknowledge the contribution of Dr A. Adnot of the Department of Chemical Engineering, Laval University, for surface analysis measurements and useful discussion. The authors also thank Mr Marco Savard for Ms technical work as undergraduate student.

References

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A. Sanschagrin et al./ Materials Science and Engineering A220 (t996) 69-77 77

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mechanical properties and microstructure of new magnesium-lithium base...

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