Effect of Sample Size on the Properties of Powder Metallurgical Ta-10W Alloy After Vacuum Heat Treatment
Release time:
2025-11-11
0 Introduction
The Ta-10W alloy is one of the earliest developed tantalum-tungsten alloy series, having entered production in the 1950s for application as a high-temperature structural material in the aerospace industry [1-2]. Ta-10W alloy possesses characteristics such as high melting point, excellent high-temperature resistance, good chemical stability, and outstanding machinability [3-4]. As an ultra-high-temperature structural material, a material resistant to high-temperature strong acid corrosion, and a special functional material, it has become an indispensable key material in fields such as national defense, aerospace, nuclear energy, high-temperature technology, and chemical engineering [5-6]. Typically produced via vacuum electron beam melting followed by forging or extrusion, rolling, and annealing, Ta-10W alloy products primarily include plates, bars, tubes, and wires [7-8].
When fabricating large-sized shell components, common challenges include the high tungsten content of Ta-10W alloy, resulting in significant deformation resistance during high-temperature forging. Tools and dies are prone to deformation at elevated temperatures, while the coarse grains in ingots are difficult to break down. Intergranular oxidation cracking frequently occurs on forged surfaces, making it challenging to achieve the desired forging results. The powder metallurgy hot isostatic pressing (HIP) process can produce bulk materials with high density, fine grain structure, and superior comprehensive properties. Employing this process for large-scale shell components offers significant advantages: reduced raw material consumption, elimination of the need for large-scale high-temperature tooling design and manufacturing, and avoidance of heavy-tonnage forging equipment, leading to substantial cost savings. It is particularly suitable for producing large shell components with specific mechanical property requirements.
HIP can process various metallic materials. By incorporating multiple strengthening phases, the resulting materials exhibit high density, fine grain structure, and superior comprehensive properties [9-11]. Furthermore, the combination of powder metallurgy and HIP enables the fabrication of complex-shaped tantalum-tungsten alloy components. The oxygen content in tantalum-tungsten alloys directly influences material strength and ductility [12]. Following high-temperature vacuum heat treatment, the material exhibits elevated strength and ductility [13-16]. The Ta-W-Hf alloy prepared via this method features high-hardness grain boundaries with elevated oxygen content. Following high-temperature annealing, oxygen- and hafnium-rich nanoparticles precipitate within grains and at grain boundaries. The increased number of precipitates at grain boundaries reduces material strength by approximately one-third while enhancing toughness by 50%. Powder metallurgy Ta-W-Hf alloys can significantly modulate their comprehensive mechanical properties through heat treatment [17-19]. Powder metallurgy Ta-10W alloy lacks highly reactive elements like hafnium. During vacuum heat treatment, oxygen diffusion is the sole means of oxygen removal, thereby reducing the material's oxygen content. Elevated oxygen levels cause significant reduction in the elongation of Ta-10W alloy, potentially leading to brittle fracture. The oxygen content in powder metallurgical hot isostatic pressed Ta-10W alloy is primarily determined by the oxygen content in the powder. High-temperature vacuum heat treatment can reduce the alloy's oxygen content to some extent, thereby improving its mechanical properties.
The reduction of oxygen content in Ta-10W alloy through vacuum heat treatment occurs via diffusion. The larger the bulk size of the material, the greater the diffusion distance from the core to the surface, making oxygen reduction more challenging. This study investigated the oxygen content and mechanical properties of Ta-10W alloy powder, as well as block materials of two sizes (3mm×5mm×60mm and 10mm×10mm×60mm) subjected to vacuum heat treatment.
1 Experiments and Methods
1.1 Powder Preparation
Ta-10W alloy ingots produced by secondary electron beam melting were hydrogenated to form powder. The powder was mechanically crushed, sieved through a -0.125 mm mesh, and subjected to dehydrogenation in a vacuum furnace to obtain chemically uniform powder. The oxygen content of the powder was measured.
1.2 Hot Isostatic Pressing
The HIP equipment used in this study was manufactured by Sichuan Aviation Industry Group Chuanxi Machinery Co., Ltd., model HIP-350. The prepared raw powder was loaded into a pure titanium sleeve through the feed port and compacted. The feed port was then sealed with a pure titanium plug and welded under vacuum using electron beam welding. The post-welding photograph is shown in Figure 1(a).
After brazing, the components were pressed in a hot isostatic press at a temperature of 1500°C and a pressure of 190 MPa, with a holding time of 4 hours. Figure 1(b) shows a photograph of the sleeve after hot isostatic pressing and sintering. It can be observed that the sleeve has undergone deformation and shrinkage, exhibiting an orange peel-like surface texture. However, the sleeve maintains a good seal, with no signs of cracking or leakage detected. The sleeve was removed to extract the sample, whose density was measured using the displacement method, yielding a relative density exceeding 99.5%. Two specimen sizes were cut using electrical discharge wire cutting: 3 mm × 5 mm × 60 mm and 10 mm × 10 mm × 60 mm. The wire-cut contamination layer on the specimen surfaces was removed by grinding.
1.3 Vacuum Heat Treatment
1.3.1 Vacuum Heat Treatment of Tantalum Alloy Powder
The high-temperature vacuum heat treatment furnace used in the experiment was manufactured by Western Metal Materials Co., Ltd., model 2400-120. One hundred grams of Ta-10W alloy powder was placed in two tungsten crucibles. One crucible contained the sample covered with tantalum foil and embedded in clean vacuum electron beam melting ingot chips, while the other remained uncovered. The samples were heated at 1200°C under a vacuum of less than 5×10⁻³ Pa for 2 hours. After cooling to room temperature, the powder's oxygen content was measured.
1.3.2 Vacuum Heat Treatment of Small Specimens
Plate-shaped tensile specimens (3 mm × 5 mm × 60 mm) with surface contamination layers ground off were placed in tungsten crucibles for heat treatment. To prevent external oxygen contamination, each set of four specimens was embedded in clean electron-beam-melted Ta-10W machining chips within the tungsten crucible, with fresh chips used per furnace charge. Vacuum heat treatment was conducted at 1400°C, 1600°C, 1800°C, 2000°C, and 2200°C, each maintained for 2 hours under a vacuum pressure below 5×10⁻³ Pa. Specimens were removed after cooling to room temperature.
1.3.3 Vacuum Heat Treatment of Larger Specimens
Place 10 mm × 10 mm × 60 mm specimen blocks with surface contamination layers ground off into the furnace for heat treatment. Position the specimen blocks within a tungsten crucible, embed them in clean electron-beam-melted Ta-10W turning chips, and replace the chips for each furnace charge. Heat treatment was conducted at 1400°C/2h, 1600°C/2h, 1800℃/8h, and 2200℃/16h. The vacuum level was maintained below 5×10⁻³ Pa. After cooling to room temperature, the specimens were removed from the furnace. Material from the center of each specimen block was machined into 3mm×5mm×60mm plate-shaped tensile specimens.
1.3.4 Sample Analysis
Following vacuum heat treatment, oxygen content was measured using a LECO-TC600 oxygen-nitrogen analyzer (LECO, USA). Room-temperature tensile testing was conducted on an Instron 5982 universal testing machine (Instron, USA). Metallographic examination was performed using an Olympus PMG3 optical metallographic microscope (Olympus, Japan). The fracture microstructure of the tensile specimen was examined using a JSM-6460 scanning electron microscope (JEOL Ltd., Japan). The density of the samples was determined by the displacement method.
2 Results and Analysis
2.1 Vacuum Heat Treatment of Tantalum Alloy Powder
After hydrogenation and dehydrogenation followed by sieving, the oxygen content of the tantalum alloy powder was 1100×10⁻⁶. The oxygen content of covered and exposed powders after vacuum heat treatment is shown in Table 1.
After vacuum heat treatment at 1200°C for 2 hours, the oxygen content of the uncovered powder was 1350×10⁻⁶, an increase of 250×10⁻⁶ compared to the original powder, representing an approximate 23% rise. Powder covered with tantalum foil and embedded in clean electron beam-melted Ta-10W chips exhibited an oxygen content of 1050×10⁻⁶ after heat treatment, a decrease of 50×10⁻⁶ (approximately 4.5%), representing a smaller reduction. Comparing the two scenarios, under the experimental vacuum conditions, after 2 hours of heat treatment at 1200°C, the oxygen content of the powder increased significantly in the absence of shielding and chip adsorption. With shielding and chip adsorption, the oxygen content of the alloy powder decreased slightly. Considering the inherent measurement error in oxygen content detection, vacuum heat treatment achieved minimal reduction in oxygen content from the original powder under shielded and chip-adsorbed conditions.
2.2 Vacuum Heat Treatment of Small-Size Specimens
Comparing the oxygen content and tensile properties of unannealed 3mm×5mm×60mm specimens with those heat-treated at 1400℃, 1600℃, 1800℃, 2000℃, and 2200℃ (Figure 2), the oxygen content in the specimens gradually decreased with increasing heat treatment temperature. Compared to the untreated sample, the tensile strength of the specimens first increases and then decreases with rising heat treatment temperature; the yield strength exhibits the same trend. The elongation of specimens heat treated at 1400°C increased from 0% to approximately 3% post-treatment. Analysis suggests that the strength increase after 1400°C heat treatment results from the release of internal stresses accumulated during hot isostatic pressing within the material following vacuum heat treatment [20]. On the other hand, oxygen content in the material decreased from approximately 1000×10⁻⁶ to about 700×10⁻⁶ after 1400°C heat treatment. This reduction in oxygen content enhances grain boundary cohesion, imparting a degree of plasticity to the material.
As the heat treatment temperature increases, the oxygen content in the material continues to decrease, leading to a gradual decline in tensile strength and yield strength, while plasticity significantly improves. The reduction in oxygen dissolved in the solid solution causes a decrease in tensile strength and yield strength, accompanied by an increase in plasticity. The elongation of the specimens gradually increases with rising temperature, reaching approximately 23% at 2200°C.
The fracture morphologies of tensile specimens after heat treatment at different temperatures are shown in Figure 3. Figure 3(a) depicts the fracture surface of an unannealed specimen, featuring both intergranular fracture with relatively smooth fracture surfaces and transgranular fracture, corresponding to zero elongation at fracture. Figure 3(b) shows the fracture surface of a tensile specimen after vacuum heat treatment at 1400°C for 2 hours, primarily exhibiting intergranular fracture with a river-like pattern on the fracture surface. After vacuum heat treatment at 1600°C for 2 hours, the fracture surface of the tensile specimen is shown in Figure 3(c). The fracture is almost entirely intergranular, with a distinct river-like pattern on the fracture surface. Secondary cracks formed between some grains during fracture. After vacuum heat treatment at 1800°C for 2 hours, the fracture surface of the tensile specimen (Figure 3(d)) exhibited entirely intergranular fracture characteristics. Some fracture surfaces displayed a river-like pattern, while others showed preliminary features of ductile tearing with disordered and blurred lines. After vacuum heat treatment at 2000°C for 2 hours, the fracture surface of the tensile specimen (Figure 3(e)) exhibited blurred edges in the river-like patterns, with ductile tearing cracks along the fracture margins. Following vacuum heat treatment at 2200°C for 2 hours, the fracture surface of the tensile specimen (Figure 3(f)) displayed dense ductile dimples, characteristic of ductile fracture. It can be observed that as the vacuum heat treatment temperature increases, the fracture surface of the tensile specimen gradually transitions from intergranular brittle fracture to partial transgranular fracture, with the transgranular fracture region exhibiting ductile tearing. Ultimately, the fracture surface develops dense ductile dimples, and the elongation of the tensile specimen exceeds 20%.
Under identical vacuum conditions and holding times, the oxygen content in the specimens decreased more significantly with increasing heat treatment temperature. This indicates that elevating the vacuum heat treatment temperature substantially accelerates the outward diffusion rate of oxygen elements within the specimens.
2.3 Vacuum Heat Treatment of Larger Specimens
Specimens measuring 10 mm × 10 mm × 60 mm underwent heat treatment at 1400°C for 2 hours, 1600°C for 2 hours, 1800°C for 8 hours, and 2200°C for 16 hours. The oxygen content in the surface layer and core, tensile strength after core sampling, and relative density are shown in Table 2.
Compared to untreated specimens, vacuum heat treatment at 1400°C for 2 hours reduced surface oxygen content by approximately 30% while increasing core oxygen content by about 10%. The significant decrease in surface oxygen content indicates that oxygen diffusion outward has commenced under vacuum heat treatment conditions. The approximately 10% increase in core oxygen content may stem partly from detection errors and partly from phased reverse diffusion occurring within the larger specimen block, leading to elevated oxygen levels in the core. The tensile strength of the specimen decreased from 924 MPa to 540 MPa. Following machining, the tensile strength of the specimen is primarily determined by the oxygen content in the core material. The significant strength reduction stems from oxygen reverse diffusion, where oxygen diffusion within grains at the core increases oxygen content at grain boundaries. Elevated oxygen content diminishes intergranular bonding strength, manifesting as reduced mechanical properties [21].
Vacuum heat treatment at 1600°C for 2 hours yielded results similar to those obtained after 2 hours at 1400°C. The surface oxygen content of the material further decreased to 570×10⁻⁶. Similarly, the core oxygen content increased by approximately 10%, and the tensile strength of the specimen further decreased to 420 MPa. For larger-sized specimens heat-treated at 1400°C and 1600°C, the core oxygen content did not decrease but instead showed a slight increase. Compared to 3mm-thick specimens, the diffusion distance for oxygen from the center to the surface and escape increased from 1.5mm to 5mm in 10mm-thick specimens, representing a 300% increase relative to the smaller specimens. This extended diffusion distance necessitates a corresponding increase in diffusion time.
Research reports indicate [22] that the reaction constant for oxygen in metallic tantalum increases with rising temperature, and the diffusion coefficient of oxygen in metallic tantalum also shows an overall increasing trend with temperature. The diffusion coefficient D of oxygen in the tantalum matrix follows the pattern described by Equation (1).
Where: D₀ is the diffusion constant, D₀ = 3 × 10⁻⁶ m²/s; Q is the diffusion activation energy, Q = 121.42 × 10³ J/mol; R is the gas constant, 8.314 J/mol·K; T is the thermodynamic temperature, K.
The thermodynamic temperature T is positively correlated with the diffusion coefficient D. Increasing the heat treatment temperature facilitates an increase in the diffusion coefficient. To facilitate oxygen diffusion from the core to the surface and subsequent escape in larger specimens, both the vacuum heat treatment temperature and diffusion time must be extended. Therefore, the specimen heat treatment temperature was increased to 1800°C and 2200°C, with the holding time extended from 2 hours to 8 hours and 16 hours, respectively. Increasing both the diffusion time and diffusion coefficient for oxygen is expected to yield better oxygen reduction effects.
After 8 hours of heat treatment at 1800°C, the oxygen content in the surface layer and core of the material was 740×10⁻⁶ and 920×10⁻⁶, respectively, showing a significant decrease compared to the original oxygen content. However, compared to the 2-hour heat treatment at 1600°C, the oxygen content in both regions exhibited a slight, anomalous increase. During tensile testing, the specimens exhibited brittle fracture at very low strengths, yielding no measurable data. Further analysis revealed a decrease in material density to 98.6% relative density. After 16 hours of heat treatment at 2200°C, the surface oxygen content was measured at 830×10⁻⁶ and the core at 920×10⁻⁶. Tensile specimens fractured brittlely during lathe machining. Testing of specimens after 2200°C heat treatment revealed a further decrease in relative density to 96.2%. Metallographic micrographs (Fig. 4(a)) show the tantalum alloy matrix containing pores of varying sizes, with the largest pores measuring approximately 10 μm. Scanning micrographs of the fracture surface (Figure 4(b)) reveal numerous pores between grains. This indicates that the formation of pores led to a decrease in the macroscopic density of the material.
During prolonged high-temperature heat treatment, oxygen initially diffused out of the specimen and was absorbed by the machining chips due to their strong oxygen-absorbing and isolating capacity. The oxygen content in the specimen decreases slowly starting from the surface. Over time, oxygen elements in the core gradually diffuse outward, reducing the oxygen content in the core. The overall decrease in oxygen content in the small specimen occurs during this stage.
After a prolonged period, as the ability of the chips to absorb and block oxygen elements gradually diminishes and disappears, trace amounts of oxygen from the external atmosphere gradually diffuse into the interior of the specimen. Oxygen entering the tantalum alloy forms an α solid solution. At elevated temperatures, the solubility of oxygen in the alloy increases, but it may locally enrich at grain boundaries, forming small amounts of tantalum oxide. Due to its low melting point and high vapor pressure, this oxide volatilizes back into the gas phase. This cyclic diffusion oxidation process leads to localized porosity, reduced density, and severe degradation of material properties.
Considering multiple variables such as vacuum fluctuations and leakage rates in vacuum furnaces, the diminished oxygen absorption and isolation capabilities of tantalum alloy chips, and increased diffusion distances, the behavior of oxygen diffusion outward and inward in larger specimens remains complex. Whether localized oxidation and oxide sublimation occur after prolonged high-temperature heat treatment, leading to material porosity and density reduction, warrants further investigation and verification.
3 Conclusions
(1) Under a vacuum of 5×10⁻³ Pa, vacuum heat treatment of exposed tantalum-tungsten alloy powder at 1200°C for 2 hours resulted in an increase of over 20% in oxygen content within the powder. When using clean Ta-10W machining chips for embedding, the oxygen content of the tantalum-tungsten alloy powder showed a slight decrease.
(2) For 3 mm × 5 mm × 60 mm specimens encapsulated with clean Ta-10W turnings, oxygen content decreased with increasing vacuum heat treatment temperature above 1400°C. Room-temperature strength initially increased then decreased, while plasticity progressively improved.
(3) Similarly, with clean Ta-10W machining chips as the embedding medium, after heat treatment at 1400°C and 1600°C, the surface oxygen content of 10mm×10mm×60mm specimens decreased slightly, while the core oxygen content increased slightly, and the material strength decreased significantly. When the heat treatment temperature was increased to 1800°C and 2200°C, and the holding time was extended to 8h and 16h, respectively, resulting in the appearance of pores, reduced density, and material embrittlement.
Reference:
Chinese Library Classification: TF841 Document Identification Code: A DOI: 10.3969/j.issn.1009-0622.2022.04.009
Effect of Sample Size on the Properties of Powder Metallurgical Ta-10W Alloy After Vacuum Heat Treatment
Stardust Technology's spherical Ta10W alloy powder is produced using radiofrequency plasma spheroidization technology, representing a high-quality refractory metal powder. This powder exhibits high sphericity and a smooth surface, free from satellite particles and hollow grains. It combines high purity with low oxygen content and excellent flowability, demonstrating favorable bulk and tapped densities. Comprising 90% tantalum and 10% tungsten, it offers high-temperature resistance, corrosion resistance, and superior strength. With controllable particle size distribution, it is suitable for additive manufacturing, powder metallurgy, thermal spraying, and other processes. Widely applied in aerospace, nuclear industry, chemical engineering, and other high-end sectors. For further details, please contact our professional manager Cathie Zheng at +86 13318326187.
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