Develop an effective oxygen removal method for copper powder

Lei Zhaoa, Xuhu Zhanga, Taiqing Denga, Jun Jiangb,*

aAerospace Research Institute of Materials & Processing Technology, Beijing 100076, PR China

bDepartment of Mechanical Engineering, Imperial College London, London SW7 2AZ, United

Kingdom

*Corresponding author. E-mail address: jun.jiang@imperial.ac.uk

Abstract

At present, one of crucial limitations for the hot isostatically pressed (HIPed) Cu-3Ag-0.5Zr alloy,

which is used on the combustion chamber liner of aerospace engine, is the high oxygen content,

which easily results in the intergranular fracture under high temperature, pressure, liquid hydrogen

and oxygen environment during operation. In this study, a novel effective oxygen control method is

developed, for which vacuum degassing process is integrated with a flowing hydrogen reduction

reaction at an elevated temperature before HIP. For this technique, a container is designed with two

gas pipes for hydrogen inflow and outflow, so the hydrogen circulation can be established. Allowing

hydrogen to react effectively with oxygen, the oxygen content of HIPed alloy is found to drop

significantly from 140 ppm (raw powder) to 28 ppm, which is equivalent to the oxygen-free copper

and copper alloys. As a result of the reduction, no prior particle boundaries could be observed in the

low oxygen content material. Although the tensile strength of the materials with and without

employing this technique does not vary significantly, the ductility of low oxygen content material has

improved by about 70% at 500°C. This significant improvement of ductility is critical to ensure the

safety critical PM components.

Keywords: powder metallurgy; hot isostatic pressing; oxygen content; copper alloy; microstructure;

mechanical property

1

1. Introduction

Compared to conventional thermal-mechanical manufacturing process for high performance, safety

critical materials e.g. the rocket engines, aero-turbine engines, the powder metallurgy (PM) method

provides more uniform and finer grain distribution with minimum segregation, less internal defects,

higher material performance, near-net-shape or net-shape final geometry and lower processing cost

[1-5]. Nevertheless, the high oxygen content in the prealloyed powder in PM processing is of a great

challenge as it can result in considerable reduction in the ductility of formed parts [6-9]. At present, to

reduce the oxygen content of powder materials, there are two common methods adopted in the PM

process. First, inert gases are used during the atomization of powder formation process to protect

powder from reacting with oxygen [7, 10-13]. Second, chemical reduction reactions [14-17] are used

to extract oxygen from the powder and react with reduction agents such as hydrogen [16] or calcium

[17].

Comparing these two methods, the second one has attracted more attentions in the PM industry,

because it can not only remove oxygen from the powder formation process but also be applied to the

powder storage and transportation processes. For example, Berglund, et al. [16] proposed a novel

process for controlling oxygen content within stainless steel powder by using hydrogen, in that a

canister with a getter layer and filled powder was evacuated. This canister was subjected to a

hydrogen atmosphere at elevated temperatures. The hydrogen atoms diffused into the canister through

the walls and formed H2O when reacted with the oxygen of the powder. The H2O was then reacted

with the getter layer. Reacting with the getter materials, oxide is formed to transfer the oxygen from

the powder to the getter layer. Subsequently, the hydrogen atoms within the canister can be diffused

out through the canister walls again. Less than 100 ppm oxygen content was reported. Oh, at al. [14]

utilised a non-contact calcium pot to remove oxygen from the commercially pure Ti powder. The

calcium pot was heated up closing to its melting temperature to react with oxygen of the powder. The

oxygen concentration was reduced to 814 ppm from the initial stage of 2200 ppm. Despite that these

relatively complicated methods, a simpler, more efficient and cost-effective oxygen reduction process

is required for the PM industry.

2

One particular case in aerospace industry is the combustion chamber (MCC) liner of NASA’s space

shuttle and rocket main engines [18, 19], which are made of high strength and high thermal

conductive Cu-3Ag-0.5Zr alloy, also known as NARloy-Z, by the PM and hot isostatic pressing (HIP)

process. The high oxygen content within the PM/HIP formed MCC is a big structural integrity

challenge. The oxidation process preferentially occurs around the grain boundaries with the formation

of prioritized zirconium enriched precipitates (Zr as a getter for soluble oxygen in Cu-Ag-Zr alloy),

Cu2O and CuO [18, 20]. These grain boundaries react with hydrogen to form H2O around grain

boundaries within the propellants combustion (liquid hydrogen and oxygen) and liquid hydrogen

cooling environment. This can result in less traceable, sudden brittle intergranular fracture of MCC

parts, which is also commonly known as ‘hydrogen disease’, and may lead to a catastrophic disaster.

In this study, a novel and effective oxygen control approach for this Cu-3Ag-0.5Zr alloy is

proposed and its feasibility and effectiveness is denonstrated. The variation of oxygen contents,

mechanical performance, and microstructure have been obtained and compared.

2. Methodology

In this section, one of conventional and low cost PM/HIP process (rather than the advanced

atomizing integrated with HIP processing route with a full closed environment and no air contact) is

briefly introduced and then the novel oxygen control process is described. The typical stages in this

HIP process consist of powder preparation, classification and preservation, the design and fabrication

of the stainless steel container, filling the prealloyed powder into container, vacuum degassing and

sealing of container, HIP, and subsequent heat treatment [6-7]. It was noticed that three important

factors that even with careful control, the powder is inevitably contacted with air and oxidized before

container sealing in present PM/HIP case; second, once the powder is sealed within the vacuumed

containers, the oxygen content will not change; third, after removing the containers, the free surface

of HIPed parts is oxidized during the heat treatment, but can be easily removed. Thus, it was

identified that the vacuum degassing before sealing powder into container stage is the key stage which

determines the oxygen content of the final formed parts. Also, since the powder is contained within a

closed environment, a reduction reaction before vacuum degassing can be effectively taken place. For 3

Cu-3Ag-0.5Zr alloy, since its MCC liner is operated in high temperature and pressure hydrogen

environment, high purity hydrogen (99.999%) is selected as the reducing agent without adding other

impurities. The design of new containers and process are described as follows.

2.1. Container design

Fig. 1 (a) schematic diagram of newly designed stainless powder container with two pipes (unit of mm); (b)

image of this novel stainless powder container.

A novel stainless steel container for powder with two gas pipes was designed for the inflow and

outflow of hydrogen and vacuum degassing, as shown in the schematic diagram in Fig. 1 (a), for

which the conventional container only has one pipe for vacuum degassing used in the PM industry.

The stainless container with 60 mm internal diameter, 1 mm wall thickness and 150 mm length were

designed in this study. Two circular holes with the diameter of 5 mm were located at the centre of the

two ends as marked in Fig. 1 (a). These two holes were further connected by two stainless pipes with

5 mm internal diameter and 1 mm thickness for gas flowing. In order to avoid the powder loss during

the powder filling, hydrogen reduction reaction and vacuum degassing, two layers of stainless filters

with the mesh size of 25 μm (less than powder size of 50~200 μm, as detailed in the later section)

were placed at the two ends of the container.

2.2. Design of flowing hydrogen reduction and vacuum degassing

4

(b)(a)

150mm

In this novel design, the hydrogen reduction reaction and vacuum degassing were combined into

one process. This integrated process consists of four steps, namely, air tightness checking, hydrogen

reduction, hydrogen cleaning and vacuum degassing. The schematic flow diagram is shown in Fig. 2.

Fig. 2 Schematic diagram of flowing hydrogen reduction and vacuum degassing process: (a) air tightness

checking; (b) hydrogen reduction; (c) hydrogen cleaning; (d) vacuum degassing.

In this design, the air tightness of the container and the up and down two gas channels is checked

by a vacuum test. During this test, one end of the entire gas channel is sealed and another end is

connected with vacuum pump, as step 1 shown in Fig. 2 (a).

In step 2, as seen in Fig. 2 (b), the container is put into a furnace along the vertical direction.

Hydrogen (density of 0.09 kg/m3) flows into the container from the up gas channel to fully exhaust the

air (density of 1.293 kg/m3). During this process, the hydrogen reacts with oxygen on the powder

surface and forms H2O (density of 0.6 kg/m3). The extra hydrogen flowed out from the down gas

channel is ignited to avoid the hydrogen diffusing into the environment, which could lead to the

potential explosion.

As hydrogen occupies the entire container and needs to be removed in order to avoid the potential

risk of hydrogen-oxygen reaction at high temperature. In step 3, the container and gas channels are

turned upside down and the gas inlet is connected to argon gas of higher density (1.784 kg/m3) for

extracting the remaining hydrogen in the container, as shown in Fig. 2 (c).

After step 3, the inlet and outlet of gas channels are simultaneously connected to a vacuum pump

for degassing, as step 4 shown in Fig. 2 (d).

5

2.3. Experimental operation of flowing hydrogen reduction and vacuum degassing

The concept of the design is described in section 2.2. Detailed description of practical operation is

shown in this section. The prealloyed powder was filled into the designed stainless steel container, as

shown in Fig. 1 (b). Based on the proposed integrated design, the hydrogen reduction and vacuum

degassing, the air tightness checking, hydrogen reduction, hydrogen cleaning and vacuum degassing

were carried out in order as shown in Fig. 3 (a), (b), (c) and (d).

For step 1, as shown in Fig. 3 (a), hydrogen and argon cylinders were connected to a Y-type three-

ways valve with two switches for control of the flow of hydrogen and argon respectively. The left

outlet of this 3-ways valve was connected to the upside pipe of the container through a two-ways

valve (designated as 1# valve), which is used to ensure the container seal before step 4. The downside

pipe of container was connected to a vacuum pump also through a 2-ways valve (designated as 2#

valve) for the same reason. Heat-resistance rubber pipes were used for linking the entire gas channel.

During the air tightness checking process, the hydrogen and argon cylinders were closed (marked

as × in Fig. 3). The 3-ways, 1# and 2# valves were opened (marked as √ in Fig. 3). The vacuum of 10 -

3 Pa and holding time of 10 minutes without changing in vacuum were achieved, indicating good air

tightness in the entire gas channel. Then, the 3-ways valve controlling argon flowing was closed in

order to avoid the air re-entry.

The vacuum pump was replaced by a stainless steel pipe in order to ignite hydrogen. To avoid the

air re-entry during step 2, the stainless steel pipe is put under water as seen in Fig. 3 (b). The container

was placed in a 1100×600×500 mm box furnace as seen in Fig. 3 (b). This box furnace can reach the

maximum temperature of 1100°C with uniform and stable distribution as firebrick and asbestos

blanket were used. Then the hydrogen cylinder is opened such that bubbles in the water could be

observed. After approximately 2 minutes, the uniform and equal size bubbles on the water surface can

be seen, indicating the complete discharge of the air. Subsequently, this pipe was taken out and the

hydrogen tail was ignited. The container was heated to 650°C for effective reduction reaction to take

place. After reaching this specific temperature, 0.5 and 1 hour reduction reaction treatment was

carried out.6

For step 3, the container with two gas channels was turned upside down as revealed in Fig. 3(c).

The hydrogen switch on the 3-ways valve was closed and argon gas switch was opened. The pipe end

was put into water to seal after the hydrogen flame was completely extinguished. In order to avoid the

contact between the oxygen reduced powder and the air during the connection with a vacuum pump,

the 1# and 2# valves were closed.

Fig. 3 Experiment schematic diagrams of (a) air tightness checking, (b) hydrogen reduction, (c) hydrogen

cleaning and (d) vacuum degassing.

For step 4, two gas channels sealed by 1# and 2# valves (as point A and B marked in Fig. 3 (c))

were connected to a vacuum pump as shown in Fig. 3 (d).The vacuum pump and 1# and 2# valves

7

(a) Step 1: air tightness checking (b) Step 2: hydrogen reduction

(c) Step 3: hydrogen cleaning (d) Step 4: vacuum degassing

were turned on successively. The container was degassed at the temperature of 650°C with the

vacuum of 10-3 Pa maintained for 9 hours.

2.4. Hot Isostatic Pressing

After the step 4, the oxygen reduced prealloyed powder was sealed by hammer forging the inlet and

outlet of the vacuum container. The encapsulated powder was consolidated in the industrial HIP

equipment at the optimized temperature of 750°C, pressure of ≥120 MPa for 2.5 hours. For a

comparison, the HIPed alloy without the flowing hydrogen reduction is also obtained under the same

conditions.

The chemical composition of prealloyed powder, HIPed alloys with and without hydrogen

reduction was determined in this study. The oxygen content was measured by the inert gas fusion

analysis using a LECO ONH836 Oxygen/Nitrogen/Hydrogen elemental analyzer, and the element of

Ag, Zr, Fe and Ni was measured by a 725-ES inductively coupled plasma-atomic emission

spectrometer (ICP-AES), and the element of S was measured by a LECO CS844 infrared absorption

carbon sulfur analyzer, and the other impurity elements were measured by a 7700CE inductively

coupled plasma-mass spectrometer (ICP-MS).

In addition, some typical physical properties such as density (ρ), coefficient of linear expansion,

specific heat capacity (c) and coefficient of thermal conductivity (λ) of HIPed alloy with 1 hour

hydrogen reduction were also characterized in this study. The ρ was determined by Archimedes’

method using a Mettler Toledo balance (readability: 0.1 mg), and the coefficient of linear expansion

was confirmed by a NETZSCH DIL 402C dilatometer with a resolution of ΔL = 1.25nm, and the c

was determined by a traditional calorimetric method using a self-developed instrument with an

accuracy of ±5%, and the thermal diffusivity (a) was measured by a traditional laser pulse method

using a self-developed instrument with an accuracy of ±5%, and then the λ was calculated based on

the equation of “λ= a×c×ρ”.

Furthermore, in order to check the structural changes during the reduction of oxygen contents, the

crystalline structure of HIPed alloy with 1 hour hydrogen reduction and prealloyed powder was

examined by X-ray diffraction (XRD) using a Bruker AXS D8 X-ray diffractometer with Cu Ka 8

radiation. Additionally, in order to further investigate structural difference between the HIPed alloys

with and without hydrogen reduction, the microstructure of these HIPed alloys with 1 hour and

without hydrogen reduction was characterized by an optical microscope.

Finally, according to GB/T 228.1-2010 (Metallic materials-tensile testing-Part 1: method of test at

room temperature, Chinese national standard) [21] and GB/T 4338-1995 (Metallic material-tensile

testing at an elevated temperature, Chinese national standard) [22], the tensile test specimens of these

HIPed alloys with 1 hour and without hydrogen reduction were prepared and the tensile tests were

carried out at room temperature (RT) and 500 , respectively, by a SANS tensile testing machine.℃

The fracture surfaces of the failed specimens after tensile tests at room and high temperature were

characterized by a scanning electron microscope (SEM).

3. Results and Discussion

Fig. 4 SEM images of (a) particle morphology of inert gas atomized prealloyed powder and (b) typical

morphology of individual powder particles.

The Cu-3Ag-0.5Zr prealloyed powder was prepared by the inert gas atomization (high purity argon

with purity of 99.999% used in this study), then packaged in vacuum and stored in the argon

environment. The size ranges of prealloyed powder were carefully separated by difference sizes of

sifters (aperture of 25 and 198 μm). Fig. 4 (a) and (b) show the obtained particle morphology. It is

clear that the sizes of the powder are between 50~200 μm, and most of powder is spherical and a

small amounts of particles are long striped and irregular. Fig. 4 (b) further shows the typical surface

morphology of individual powder particle, indicating a slightly cracked surface may due to the rapid

9

(a) (b)

400 μm

Diameter of 66.9μm

Diameter of 193.1μm

Diameter of 141.1μm

20 μm

cooling of metal molten droplet during the atomising process. This type of surface morphology is

more easily oxidized.

The chemical composition of the prealloyed powder as shown in Table 1. It is seen that the oxygen

content of the prealloyed powder was 0.0140 (140 ppm), which is significantly higher than the level

of the wrought oxygen-free copper or copper alloy according to the typical material specifications,

such as ≤ 0.0030 in GB/T 5231-2012 standard (Designation and chemical composition of wrought

copper and copper alloys, Chinese national standard) [23] and ≤ 0.0010 in ASTM standard

designation for wrought copper alloys [24].

The chemical composition of HIPed Cu-3Ag-0.5Zr alloy after hydrogen reduction for 0.5 and 1

hours prior to HIP are also listed in Table 1. It is seen that the oxygen content corresponding to the

reduction time of 0.5 and 1 hour are 0.0084 (84 ppm) and 0.0028 (28 ppm), respectively, indicating

that the oxygen content decreases remarkably with the increasing time of hydrogen treatment. The

lowest value of oxygen content is comparable to the level of wrought oxygen-free copper and copper

alloy (≤ 30 ppm). In addition, the corresponding oxygen content of HIPed alloy without hydrogen

reduction was also determined as 0.0149 (149 ppm), which is obviously higher than 28 ppm and

shown in Table 1. These direct results indicate that this vacuum degassing integrated with a flowing

hydrogen reduction method is an effective way to control oxygen content of HIPed material.

10

Table 1 The chemical composition of designed Cu-3Ag-0.5Zr alloy, prealloyed powder and HIPed alloys and the oxygen content of copper alloys in national standards (wt. %)

Element Cu Ag Zr Bi Sb P As Ni Sn Pb Zn Fe S O

Designed Cu-Ag-Zr alloyBa

l． 2.8~3.2 0.4~0.6 ≤0.001 ≤0.002 ≤0.002 ≤0.002 ≤0.002 ≤0.002 ≤0.003 ≤0.003 ≤0.004 ≤0.004 ≤0.003

Prealloyed powderBa

l． 3.02 0.53 ＜0.0005 ＜0.001 ＜0.001 ＜0.001 0.0012 ＜0.001 ＜0.001 ＜0.001 0.0014 ＜0.001 0.0140

HIPed alloy without hydrogen

reduction

Ba

l． 2.94 0.44 ＜0.0005 ＜0.001 ＜0.001 ＜0.001 0.0018 ＜0.001 ＜0.001 ＜0.001 0.0020 ＜0.001 0.0149

HIPed alloy after hydrogen

reduction of 0.5h

Ba

l． 3.01 0.48 ＜0.0005 ＜0.001 ＜0.001 ＜0.001 0.0016 ＜0.001 ＜0.001 ＜0.001 0.0018 ＜0.001 0.0084

HIPed alloy after hydrogen

reduction of 1h

Ba

l． 3.01 0.44 ＜0.0005 ＜0.001 ＜0.001 ＜0.001 0.0015 ＜0.001 ＜0.001 ＜0.001 0.0015 ＜0.001 0.0028

GB/T 5231-2012 specification

for oxygen-free copper- - - - - - - - - - - - - ≤ 0.0030

ASTM specification for

oxygen-free copper- - - - - - - - - - - - - ≤ 0.0010

11

Table 2 shows some typical physical properties such as ρ, coefficient of linear expansion, c and λ of

HIPed alloy with 1 hour hydrogen reduction (viz., the oxygen content of 28 ppm) and the well-known

NARloy-Z alloy. It should be noted that the λ of HIPed alloy with 1 hour hydrogen reduction was

calculated based on the measured a value of 1×10-4 m2/s and the equation of “λ= a×c×ρ”. It can be

seen from this table that the ρ of obtained low oxygen HIPed alloy is slightly lower than that of

NARloy-Z, while the coefficient of linear expansion, c and λ of this HIPed alloy are a bit higher than

those of NARloy-Z. As a whole, the physical properties of HIPed alloy in this study are basically

consistent with these of NARloy-Z alloy.

Table 2 The physical properties of HIPed alloy with 1 hour hydrogen reduction and NARloy-Z alloy

Material HIPed alloy with 1 hour hydrogen reduction NARloy-Z alloy

ρ (g/cm3) 8.970 9.134

Coefficient of linear expansion (×10-6/6e 18.0 (24~500 )℃17.2

(24~260 )℃c (J/g·K) 0.40 0.37

λ (W/m·K) 362 295

Fig. 5 XRD patterns of HIPed Cu-Ag-Zr alloy with hydrogen reduction of 1 hour and prealloyed powder.

In order to check the structural changes during the oxygen reduction, the prealloyed powder and

HIPed material were examined by XRD, as shown in Fig. 5. From Fig. 5, the XRD pattern of HIPed

alloy shows four obvious crystalline peaks of α-Cu at the 2θ of ~43.5°, 50.5°, 74.0° and 89.5° and one

12

quite weak peak of unknown x-phase at the 2θ of ~36.8°, which probably corresponds to Zr, CuZr2 or

CuZr3 or Cu1.74Zr2.26 phase, as shown in the inset of Fig. 5. This XRD pattern of HIPed alloy is fully

agreed with that of prealloyed powder, as seen in Fig. 5, indicating there is almost no structural

change during the proposed oxygen treatment process.

To reveal the effects of oxygen content on the finally formed microstructure, Fig. 6 (a) and (b)

present the optical micrographs of HIPed Cu-3Ag-0.5Zr alloy with hydrogen reduction of 1 hour (viz.,

the oxygen content of 28 ppm) and without hydrogen reduction (viz., the oxygen content of 149 ppm),

respectively. In Fig. 6 (a) and (b), microstructural variations can still be found in these two materials

due to the different oxygen levels and associated effects on the prior particle boundaries (PPBs) and

grain size.

Fig. 6 Optical micrographs of HIPed Cu-Ag-Zr alloy. (a) Alloy with hydrogen reduction of 1 hour; (b) Alloy

without hydrogen reduction.

No PPB networks can be found in the microstructure of HIPed materials with 28 ppm oxygen

content (Fig. 6 (a)), while some shallow spherical PPB traces of raw particles is still visible in the

alloy with oxygen content of 149 ppm, as highlighted in the Fig. 6 (b). Note that obvious PPB trace is

not found in these micrographs due to the optimised powder preparation and HIP parameters which

broke up the PPBs. Although the mechanism of PPB formation in the copper alloy is not yet fully

understood, Cu-Ag-Zr alloy includes some easily oxidized elements, such as Zr and Cu, and will form

their oxides on the powder particle surfaces when exposed to the air. The low oxygen content is

important to control and decrease the PPBs content for copper alloy. The previous research in nickel-

based alloys found that the formation of PPB precipitation during HIP is mainly attributed to the

oxygen induced surface layers [7, 26-28] where these oxides further acted as the nuclei for the

13

(a) (b)

50 μm 50 μmSpherical PPB trace

Twins

Twins Spherical PPB trace

precipitation of carbides along the particle boundaries during HIP [29].In addition, as seen in Fig. 6

(a) and (b), the average grain size of the low oxygen content (28 ppm) material is slightly larger than

the higher oxygen content one. This slight increase in the mean grain size might be due to the less

volume fraction of the PPBs networks. Appa Rao, et al. [5] reported that a reduced duplexity grain

size in HIPed superalloy Inconel 718 could be attributed to the low level of oxygen content and less

PPBs, which allowed the powder particles to deform more homogeneously during HIPing and thereby

highly uniform recrystallization during slow cooling stage of HIP cycle. Thus the network of PPBs

can act a constraining factor for grain growth during the recrystallization process in the HIP.

Moreover, it is noted that both of these two HIPed alloys with different oxygen content have the

limited number of annealing twins, as marked in Fig. 6 (a) and (b) respectively. This is different from

phenomenon observed in HIPed superalloy Inconel 718 [7] or conventional copper alloys. In general,

as copper and nickel have relatively low stack fault energy compared to aluminium, and thus high

tendency to form large volume fracture of annealing twins. In addition, the twins formation is mainly

due to mechanical deformation or the annealing following plastic deformation, which are matched

with the plastic deformation of particles and then slow cooling (viz., annealing) during HIP [7]. Appa

Rao, et al. [7] suggested the formation of annealing twins in HIPed superalloys Inconel 718 was also

strongly influenced by the oxygen content, and the low oxygen particles show that the larger plastic

deformation seems beneficial to the recrystallization homogeneity and the increase in annealing twins

formation [7]. Based on these, HIPed Cu-Ag-Zr alloy with oxygen content of 28 ppm obtained in this

study should show more annealing twin bands than that with higher oxygen levels. However, only

limited annealing twins is observed in Fig. 6 (a), which may be as results of the different composition

between Cu-Ag-Zr alloys and Inconel 718 or the used HIP parameters, such as temperature, pressure

and time affecting the plastic deformation of particles and the slow cooling process. These HIP

parameters and the difference in annealing twins will be under further investigation.

Table 3 Room temperature and 500 °C tensile properties of HIPed alloys as a function of oxygen content.

HIPed alloyOxygen content/ppm

Temperature/

℃YS/MPa TS/MPa EL/%

14

hydrogen reduction of

1h28

RT

151 152 32732

545.4 44.3

no hydrogen reduction149 156 157 329

32

943.7 45.0

hydrogen reduction of

1h28

500

101 103 13313

217.6 18.4

no hydrogen reduction149 106 107 137

14

610.6 10.2

Tensile tests were carried out at room and elevated temperatures to evaluate the mechanical

performance of these two different oxygen content HIPed materials to confirm the effects of oxygen

content. The measured tensile properties for these two materials are summarised in Table 3 including

the yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) values at RT and 500 ℃

respectively. At RT, the YS, UTS and EL of low oxygen content HIPed materials are similar to those

values of the high oxygen content one. Nevertheless, at 500 °C, the EL of low oxygen content HIPed

material (~18%) increased ~70% compared to the high oxygen content one (~10%). Fig. 7 further

show the corresponding fracture surfaces of the failed specimens after tensile tests at 500 °C. Typical

brittle intergranular fracture morphologies are observed, which is correlated to their significantly

lower ductility at the elevated temperature compared to RT. A closer look of Fig. 7 (a) and (b) shows

a clear difference in these two fractographys, namely that low oxygen HIPed material mainly shows

regular grain boundary facets, while the high oxygen HIPed one shows spherical particle boundary

facets.

Compared to the high oxygen content materials, the considerably improved ductility of the low

oxygen content sample tested at 500 is found. This should be due to its lower oxygen content. The℃

deformation process can be described as follows. During the tensile deformation, the voids formed at

the interface of particle boundary/grain boundary due to deformation incongruity between PPB and

grain, which results in forming the crack source and providing a channel for crack propagation. In the

high temperature and atmospheric oxygen environments, due to the aggregation of impurities within

the grain boundaries, brittle oxidation is formed at grain boundaries. Thus, the cracks propagate along

the brittle PPBs and grain boundaries, leading to the particle boundary/grain boundary decohesion

15

then intergranular fracture. This rationalises the observed poor ductility at high temperature compared

to room temperature. More significantly, based on this analysis, it is clearly seen that the presence of

crack source and further grain boundary embrittlement leads to poor ductility at high temperature.

Hence, the higher the oxygen content and concentration of PPBs are, the more crack source shows,

and thus, then lower the ductility at high temperature is. A comparable result is also found in HIPed

Inconel 718 [7], suggesting that the decreasing in ductility of alloy at high temperature is due to the

persistence of PPBs decorated with highly stable and brittle oxides and carbides.

Fig. 7 Fractographs after tensile tests at 500 of HIPed Cu-3Ag-0.5Zr alloy. (a) Alloy with hydrogen℃

reduction of 1 hour; (b) Alloy without hydrogen reduction.

The observed differences in the fractographs in Fig. 7 is mainly due to the variation of the oxygen

content. The lower oxygen HIPed material shows less number of PPBs, and its crack propagation path

at high temperature is intragranular. In contrast, the high oxygen content alloy contains more PPB

networks, whose cracks generally propagate along these brittle PPB boundaries, which led to the

spherical particle boundary facets.

Fig. 8 further presents the fracture surfaces of the failed specimens after tensile tests at room

temperature of HIPed alloys with hydrogen reduction of 1 hour (a) and without hydrogen reduction

(b), respectively. From these fractographs, both HIPed alloys show the analogous fine dimples within

the grains and at particle boundaries, which is the predominant transgranular fracture morphology

leading to the good ductility. These similar fracture surfaces agree well with the high tensile ductility

at room temperature. HIPed Inconel 718 also showed a similar result of elongation at room

temperature [7].

16

(a) (b)

50 μm 50 μm

Fig. 8 Fractographs after tensile tests at room temperature of HIPed Cu-3Ag-0.5Zr alloy. (a) Alloy with

hydrogen reduction of 1 hour; (b) Alloy without hydrogen reduction.

The above results and analysis on the oxygen content, microstructure and mechanical properties

strongly confirm that the oxygen content of HIPed Cu-3Ag-0.5Zr alloy can be effectively decreased

by the proposed novel oxygen control method. As a result, the considerable improvement in

mechanical performance (ductility and fracture toughness) can also be achieved.

Subsequent heat treatment on PM/HIPed NARloy-Z alloy is typically performed to control

precipitation formation and distribution [20]. To obtain high performance powder materials through

optimal heat treatment process is however, beyond the scope of this study.

This low oxygen content control method is not limited to the powder preparation stage, but can

also be extended to the powder forming stage. One of the significant advantages for PM/HIP

technique is the near net shaped components with complex geometries. This low oxygen content

control method can be further applied to the HIP forming study of MCC liner with special-shaped

curved surface as seen in Fig. 9 (a), which schematically shows a container design of MCC liner with

multi-up and down gas pipes for a uniform low oxygen treatment. In addition, the proposed oxygen

control method together with appropriate container design (i.e., a container with multi-up and down

gas pipes for the certain larger and more complex component) may provide an effective way to obtain

low oxygen formed parts with complex structure, i.e., the commonly used multi-inner grids structure

(as a schematic diagram of container shown in Fig. 9 (b)). In addition, comparing with the traditional

PM/HIP process, this new method only inserts one simple step of hydrogen reduction before vacuum

degassing, utilizing the same container and heat treatment furnace with vacuum degassing. There is

no need for the introduction of more instrument in this new method, but quite few cost for the several

17

(a) (b)

10 μm 10 μm

stainless gas pipes and the use of hydrogen and heat treatment furnace is necessary. The cost of this

high-efficiency method is preliminarily speculated to be not beyond ~1% of the total cost of

conventional HIPed product.

Fig. 9 Schematic diagrams of container design. (a) container of MCC liner; (b) container of typical multi-inner

grids structure.

5. Conclusions

In this study, an effective oxygen removal technique for Cu-3Ag-0.5Zr alloy powder has been

developed. By adding two hydrogen flowing pipes to the sealed powder containers, this technique

establishes an effective hydrogen circulation prior to vacuum degassing at an elevated temperature.

Thus, hydrogen effectively reacts with oxygen and remarkably reduces the oxygen content of raw

powder. These results have shown that applying this newly developed technique, the oxygen content

within the raw powder of Cu-3Ag-0.5Zr alloy has been remarkably reduced from 140 ppm to 28 ppm,

which is near the level of oxygen-free copper and its alloys. As a result of low oxygen content,

commonly observed PPB networks in PM formed parts are absent. Moreover, compared to the

conventionally processed powder, ~70% of ductility improvement has been achieved at 500 ℃

without scarifying its strength. Thus, the developed novel effective oxygen removal technique is of

18

(a) (b)

great significance to provide a valuable idea for the low oxygen content control of powder materials

in PM industry.

Acknowledgements

The strong support from China Academy of Launch Vehicle Technology (CALT) and Aerospace

Research Institute of Materials & Processing Technology for this funded research is much

appreciated. The research was performed at the CALT-Imperial Advanced Aerospace Structure

Manufacturing Technology Laboratory at Imperial College London. Lei Zhao gratefully

acknowledges financial support from China Scholarship Council and helpful discussion with Prof

Jianguo Lin. Jun Jiang appreciates his fellowship funded by Imperial College London.

References

[1] D. Richter, G. Haour, D. Richon, Hot isostatic pressing (HIP), Mater. Des. 6 (1985) 303-305.

[2] D. Yang, Z. Hu, W. Chen, J. Lu, J. Chen, H. Wang, L. Wang, J. Jiang, A Ma, Fabrication of Mg-

Al alloy foam with close-cell structure by powder metallurgy approach and its mechanical properties,

Journal of Manufacturing Processes. 22 (2016) 290-296

[3] R. X. Zheng, H. Yang, T. Liu, K. Ameyama, C. L. Ma, Microstructure and mechanical properties

of aluminum alloy matrix composites reinforced with Fe-based metallic glass particles, Mater. Des.

53 (2014) 512-518.

[4]L.-h. Lang, Y. Xue, G.-l. Bu, H. Yang. Densification Behavior of Ti-6Al-4V Powder during Hot

Isostatic Pressing, STEEL RESEARCH INTERNATIONAL 81 (2010) 1328-1331.

[5]Y. Liu, L. Chen, H. Tang, C.T. Liu, B. Liu, B. Huang. Design of powder metallurgy titanium

alloys and composites, Materials Science and Engineering: A 418 (2006) 25-35.

[6] Seetharam, R., S. Kanmani Subbu, and M. J. Davidson. "Hot workability and densification

behavior of sintered powder metallurgy Al-B4C preforms during upsetting." Journal of Manufacturing

Processes 28 (2017): 309-318.

19

[7] G. Appa Rao, M. Srinivas, D.S. Sarma, Effect of oxygen content of powder on microstructure and

mechanical properties of hot isostatically pressed superalloy Inconel 718, Mater. Sci. Eng. A 435-436

(2006) 84-99.

[8] M. Yan, Y. Liu, Y.B. Liu, C. Kong, M. Qian, Simultaneous gettering of oxygen and chlorine and

homogenization of the β phase by rare earth hydride additions to a powder metallurgy Ti–2.25Mo–

1.5Fe alloy, Scripta Mater. 67 (2012) 491-494.

[9] R. Jiang, D.J. Bull, D. Proprentner, B. Shollock, P.A.S. Reed, Effects of oxygen-related damage

on dwell-fatigue crack propagation in a P/M Ni-based superalloy: From 2D to 3D assessment, Int. J.

Fatigue, 99 (2017) 175-186.

[10] H. C. deGroh III, D. L. Ellis, W. S. Loewenthal, Comparison of GRCop-84 to other Cu alloys

with high thermal conductivities, J. Mater. Eng. Perform. 17 (2008) 594-606.

[11]C. Gierl-Mayer, R. de Oro Calderon, H. Danninger. The role of oxygen transfer in sintering of

low alloy steel powder compacts: a review of the “internal getter” effect, Jom-Us 68 (2016) 920-927.

[12]C. Gierl-Mayer, H. Danninger, R.d.O. Calderon, E. Hryha. " Internal gettering '' metallothermic

reduction process ub tge early stage of sintering, international journal of powder metallurgy 51 (2015)

47-53.

[13]R. de Oro Calderon, C. Gierl-Mayer, H. Danninger. Application of thermal analysis techniques to

study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive

alloying elements, Journal of Thermal Analysis and Calorimetry 127 (2017) 91-105.

[14] J. Oh, W. Kim, B. Lee, C. Suh, H. Kim, J. Lim, New method for preparation of low oxygen Mo

powder by reduction of MoO3 using Ca, Powder Metall. 57 (2014) 291-294.

[15] Hyung-Seok Kim, Jung-min Oh, Chang-Youl Suh, Back-Kyu LEE, Jae-Won Lim, “Method of

producing low oxygen-content molybdenum powder by reducing molybdenum trioxide.” U. S. Patent

8979975, issued March 17, 2015.

[16] R. Berglund, H. Eriksson, J. Sundstrom, P. Arvidsson, “Method of controlling the oxygen

content of a powder.” U. S. Patent 20080268275, issued October 30, 2008.

[17] J. Oh, B. Lee, C. Suh, S. Cho, J. Lim, Preparation method of Ti powder with oxygen

concentration of < 1000 ppm using Ca, Powder Metall. 55 (2012) 402-404.20

[18] J.H. Sanders, P.S. Chen, S.J. Gentz, R.A. Parr, Microstructural investigation of the effects of

oxygen exposure on NARloy-Z, Mater. Sci. Eng. A 203 (1995) 246-255.

[19] J. Singh, G. Jerman, R. Poorman, B.N. Bhat, A.K. Kuruilla, Mechanical properties and

microstructural stability of wrought, laser, and electron beam glazed NARloy-Z alloy at elevated

temperatures, J. Mater. Sci. 32 (1997) 3891-3903.

[20] S. Chenna Krishna, K.Thomas Tharian, Bhanu Pant, R. Kottada, Microstructure and mechanical

properties of Cu-Ag-Zr alloy, J. Mater. Eng. Perform. 22 (2013) 3884-3889.

[21] National Standards of the People's Republic of China. Metallic materials-Tensile testing-Part 1:

Method of test at room temperature. GB/T 228.1-2010: December 23, 2010 (In Chinese).

[22] National Standards of the People's Republic of China. Metallic material-Tensile testing at

elevated temperature. GB/T 4338-1995: October 10, 1995 (In Chinese).

[23] National Standards of the People's Republic of China. Designation and chemical composition of

wrought copper and copper alloys. GB/T 5231-2012: December 31, 2012 (In Chinese).

[24] ASTM Standard Designation for Wrought and Cast Copper and Copper Alloys. Copper.org.

2010-08-25. Retrieved 2011-07-05.

[25] John J. Esposito, Ronald F. Zabora, Thrust chamber life prediction. Volume 1 Mechanical and

physical properties of high performance rocket nozzle materials. NASA CR-134806, 1975.

[26] R Baccino, F Moret, F Pellerin, D Guichard, G Raisson, High performance and high complexity

net shape parts for gas turbines: the ISOPREC® powder metallurgy process, Mater. Des. 21 (2000)

345-350.

[27] G. Raisson, J. Y. Guédou, D. Guichard, J. M. Rongvaux, Production of Net-Shape Static Parts by

Direct HIPing of Nickel Base Superalloy Prealloyed Powders, Adv. Mater. Res. 278 (2011) 277–282.

[28] Huei-Sen Wang, Yen-Ling Kuo, Chen-Ming Kuo, Chao-Nan Wei, Microstructural evolution and

mechanical properties of hot isostatic pressure bonded CM 247LC superalloy cast, Mater. Des. 91

(2016) 104-110.

[29] R. Menzies, R. Bricknell, A. Craven, STEM microanalysis of precipitates and their nuclei in a

nickel-base superalloy, Philos. Mag. A 41 (1980) 493-508.

21

22