An investigation on the solid state sintering of mechanically alloyed nano-structured 90W–Ni–Fe tungsten heavy alloy

In this study, 90W–7Ni–3Fe heavy alloy was investigated for its microstructure development, mechanical properties and fracture behavior after solid state sintering. The nano-sized powders were synthesized by mechanical alloying (MA). The microstructure of solid state sintered heavy alloys consisted of tungsten matrix. The average tungsten grain size in the range of 1.7–3.0 μm was obtained. It was found that the grain size largely affected the mechanical properties. Tensile strength more than 1200 MPa was achieved at a sintering temperature of 1350 °C. Fracture mechanisms based on microscopical observations on the fracture surfaces were studied. Matrix failure, tungsten-intergranular cleavage and tungsten–matrix interfacial separation were found to be the possible failure mechanisms.     

Impact and dynamic deformation behavior of mechanically alloyed tungsten heavy alloy

93W-5.6Ni-l.4Fe tungsten heavy alloy was fabricated by mechanical alloying process using elemental powders of tungsten, nickel and iron, followed by sintering at temperatures of 1445~1485°C under hydrogen atmosphere. The tungsten heavy alloy sintered using mechanically alloyed powders showed finer tungsten particles about 5~18 μm with high density above 99% at shorter sintering time than that fabricated by conventional liquid-phase sintering process. Charpy impact energy of mechanically alloyed tungsten heavy alloy increased with increasing the matrix volume fraction and with decreasing the W/W contiguity. The high strain rate dynamic deformation behavior of tungsten heavy alloys using torsional Kolsky bar test exhibited different fracture modes dependent on microstructure. While the brittle intergranular fracture mode was dominant when the tungsten particles were contiguously interconnected in tungsten heavy alloys solid-state sintered below 1460°C, the ductile shear fracture mode was dominant when the tungsten particles were surrounded by ductile matrix phase in tungsten heavy alloys liquid-phase sintered above 1460°C.     

Effect of microstructural parameters on the properties of W-Ni-Fe alloys

The aim of this research was to examine the effect of microstructural parameters on the tensile properties of dif- ferent compositions of tungsten heavy alloys. The microstructural parameters (grain size, connectivity, contiguity, and solid volume fraction) were measured and were found to have a significant effect on the tensile properties of tungsten-based heavy alloys. The microstructural parameters of W-Ni-Fe alloys are sufficiently different to present a range of me- chanical properties. It is concluded that the mechanical properties of tungsten heavy alloys largely depend on the micro- structural parameters and their ductility is particularly harmed when grains are contiguous.     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

Variables on the precipitation of an intermetallic phase for liquid phase sintered W–Mo–Ni–Fe heavy alloys

The concentration of Mo in the liquid phase of W–Mo–Ni–Fe heavy alloys during isothermal hold dictates the precipitation of an intermetallic phase during cooling. If the concentration of Mo atoms in the matrix phase exceeds an equilibrium value (between 8 and 9 at.%), Mo atoms have a strong tendency to precipitate along with W, Ni and Fe from the matrix phase onto the solid matrix phase interface during cooling. The coprecipitation of W, Mo, Ni, and Fe results in the formation of an intermetallic phase in the interface between the solid grains and the matrix phase, (W₄Mo₆)(Ni₇Fe₃), which possibly has the same structure as MoNi. This intermetallic phase is difficult to eliminate by a fast water-quench practice.     

Milling media and alloying effects on synthesis and characteristics of mechanically alloyed ODS heavy tungsten alloys

Oxide dispersion strengthened (ODS) tungsten heavy alloys produced by mechanical alloying exhibit high creep strength at elevated temperatures and good penetration performance. The effect of process parameters during mechanical alloying is important in determining material properties. In this study, we have examined different grinding media and have varied the composition of alloying elements to investigate their effect on grinding performance and microstructure evolution. The composition of the milled powders can be changed due to the wear of the grinding media and can form different phases, which results in a significant effect on microstructural development and material properties. Our results show that alloys milled by a stainless steel grinding media encourage the formation of iron–tungsten carbides and iron–tungsten intermediate phases, which deteriorate the material densification and ductility. Conversely, the use of a tungsten carbide grinding media leads to an extreme refinement of the milled powders, whereby alloys form a uniform microstructure with a γ(Ni, Fe) phase configuration. This phase provides sufficient binding strength between the tungsten particles, such that the relative density and ductility of the materials were found to have been significantly enhanced.     

COMPUTER NUMERICAL SIMULATION OF MECHANICAL PROPERTIES OF TUNGSTEN HEAVY ALLOYS

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Macrosegregation induced sedimentation in liquid phase sintered Ni–W alloys

In this study, liquid phase sintering of Ni–W alloys containing a low W solid volume fraction was conducted. A solid-plus-liquid (mushy) zone forms at the compact bottom. A liquid head above this zone demonstrates macrosegregation. With increasing sintering time, macrosegregation is reduced concurrent with a decrease in the volume fraction of W particles and mushy-zone sedimentation.     

Microstructure Limitations of High Tungsten Content Heavy Alloys

Tungsten heavy alloys are two phase composites that are useful for several applications requiring high densities, such as radiation shields, counterbalance weights, and projectiles. This paper focuses on the density, strength and ductility variations at high tungsten concentrations (90 to 99.5 wt.% W). Fundamental relations are developed between properties, processing and micro-structure versus the tungsten content. At high tungsten levels, the strength and ductility are limited by the sintered microstructure. Furthermore, the high tungsten content heavy alloys are susceptible to impurity segregation to the tungsten-matrix interface. As a consequence, the mechanical behavior of high tungsten content alloys are sensitive to processing conditions.     

Fabrication, thermal stability and microhardness of sputtered Ni–P–W coating

Ternary Ni–P–W alloy coating was fabricated by the RF magnetron sputtering technique with dual targets of electroless nickel alloy and tungsten metal. The composition of both the alloy deposited and the sputtered targets were evaluated by electron probe microanalysis. The homogeneity of Ni–P targets fabricated by electroless nickel plating on copper plates was revealed from cross-sectional line profile analysis. Transitions in microstructure, in terms of the tungsten content in the as-deposited alloy deposit, were discussed using X-ray diffraction analysis. Results of microhardness tests showed that the surface hardness could be engineered by controlling the composition and microstructure in the Ni–P–W coating. A relatively high microhardness of approximately 1900 HK was observed for the ternary coating with high tungsten contents of 65 wt.%. The thermal stability could be enhanced by addition of tungsten into the deposit compared to the binary Ni–P sputtered coating.     

Effects of sintering conditions on mechanical properties of mechanically alloyed tungsten heavy alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix. This article is based on a presentation made in “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18, 2000 under the auspices of Far East and Ocean Fracture Society (FEOFS)et al.     

Microstructural control of and mechanical properties of mechanically alloyed tungsten heavy alloys

93W-5.6Ni-l.4Fe tungsten heavy alloys with controlled microstructures were fabricated by mechanically alloying of elemental powders of tungsten, nickel and iron by two different process routes. One was the full mechanical alloying of blended powders with a composition of 93W-5.6Ni-l.4Fe, and the other was the partial mechanical alloying of blended powders with a composition of 30W-56Ni-14Fe followed by blending with tungsten powders to form a final composition of 93W-5.6Ni-l.4Fe. The raw powders were consolidated by die compaction followed by solid state sintering at 1300°C for 1 hour in a hydrogen atmosphere. The solid state sintered tungsten heavy alloys were subsequently liquid phase sintered at 1445∼1485°C for 4-90 min. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed tungsten particles of about 6-15 μm much finer than those of 40 um in a conventional liquid phase sintered tungsten heavy alloy. An inhomogeneous distribution of the solid solution matrix phase was obtained in the two-step sintered tungsten heavy alloy using partially mechanically alloyed powders. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed larger elongation of 16% than that of 1% in the solid state sintered tungsten heavy alloy due to the increase in matrix volume fraction and decrease in W/W contiguity. Dynamic torsional tests of the two-step sintered tungsten heavy alloys showed reduced shear strain at maximum shear stress than did the sintered tungsten heavy alloys using the conventional liquid phase sintering.     

钨含量对钨合金动态剪切性能的影响

利用帽形试样在Hopkinson压杆上测试钨合金动态剪切应力应变关系,研究了钨含量在90%～97%（质量分数）范围内,应变率约为10^5s^-1时,钨含量对动态剪切性能的影响.结果表明,钨含量增加,动态剪切强度随之增加,断裂应变随之降低.动态剪切强度与钨的体积分数呈线性递增关系.断口分析表明,断口表现以粘结相撕裂和钨颗粒劈裂的混合断裂,钨颗粒劈裂比例近似等于钨颗粒体积分数.     

Fabrication of protective-coated SiC reinforced tungsten matrix composites with reduced reaction phases by spark plasma sintering

SiC reinforced tungsten matrix composites were fabricated via the spark plasma sintering process. In order to prevent an interfacial reaction between the SiC and tungsten during sintering, TiOx coated SiC particles were synthesized by a solution-based process. TiOx layer coated SiC particles were treated in high temperature nitriding conditions or annealed in a high temperature vacuum to form TiN or TiC coated SiC particles, respectively. The TiC layers coated on SiC particles successfully prevented tungsten from reacting with SiC; hence the proposed process resulted in successful fabrication of the SiC/W composites. The mechanical properties such as compressive strength and flexural strength of the composites were measured. Additionally, the effect of SiC on the high temperature oxidative ablation of tungsten was also investigated. The addition of SiC resulted in an improved oxidative ablation resistance of the tungsten-based composites.     

Formation of crystalline and amorphous solid solutions of W–Ni–Fe powder during mechanical alloying

The tungsten heavy alloy with the composition of 76.6W–17.3Ni–6.1Fe in atom percent was mechanically alloyed (MA) from the elemental powders of W, Ni and Fe. Nanocrystalline supersaturated solid solutions and amorphous phase were obtained during MA. Phase evolution, grain size and lattice distortion of these powders were determined and discussed. A thermodynamic model was developed based on semi-experimental theory of Miedema to calculate the driving force for phase evolution. The thermodynamic analysis showed that there is no chemical driving force to form the solid solution and the amorphous phase. The effect of the work of milling on the amorphization during MA was discussed and the model of multilayer amorphization during MA was applied to illustrate the feasibility of amorphization of powder with neither ΔH_mix0 nor D_BD_A. The driving force for amorphization is provided not by the negative heat of mixing or the stored energy in the grain boundaries but by the sharp concentration gradients in this system. Amorphization is mechanically driven and not by the negative heat of mixing. Crystallization is suppressed by sharp concentration gradients.     

Thermoelectric properties of ScCoSb, ScNi0.86Sb and MgNiSb compounds

Blends of 90 wt.% Ti and 10 wt.% W powders were consolidated by powder metallurgy, using an initial W powder size that was very fine (0.7 and 2 μm) or very coarse (<250 μm). Dissolution of W powders in the Ti matrix during consolidation was almost complete for the former blends (thus forming Ti–10W “alloys”) but very limited for the latter blend (thus forming a Ti–10W “composite”). The Ti–10W alloys exhibit much higher yield and tensile strengths than the Ti–10W composite, indicating that tungsten strengthens titanium more efficiently as a solute atom (solid-solution strengthening) than as a second phase (composite strengthening by load transfer). The Ti–10W alloys also exhibit much higher ductility than the Ti–10W composite, whose brittle W particles exhibit fracture or pull-out from the matrix.     

Influences of sub-micrometer Ta and Co dopants on microstructure and properties of tungsten heavy alloys

Tungsten heavy alloys are aggregates of particles of tungsten bonded with Ni/Fe or Ni/Cu via liquidphase sintering. The sub-micrometer Ta Co powder was added to this aggregate to strengthen the bonding phase. It is found that the main fracture pattern of the alloys is cleavage of tungsten grains and ductile rupture of bond phase,leading to improved tensile strength and elongation. Dopant Ta can act as grain size inhibitor in tungsten heavy alloys.     

High-temperature mechanical properties of aluminium alloys reinforced with titanium diboride （TiB2） particles

The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements.The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys (6061 and 7015) at high temperatures were studied.Titanium diboride (TiB2) particles were used as the reinforcement.All the composites were produced by hot extrusion.The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure.The fracture surface was analysed by scanning electron microscopy.TiB2 particles provide high stability of the aluminium alloys (6061 and 7015) in the fabrication process.An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys.Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure,and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.     

真空热处理对微波烧结93W-Ni-Fe合金显微组织及力学性能的影响

研究真空热处理对微波烧结挤压棒坯93W-Ni-Fe合金显微组织及力学性能的影响,采用高倍SEM和光学金相分别对合金断口和显微组织进行观察,采用能谱分析仪对合金真空热处理前后各元素含量进行定量分析,并对真空热处理样的相对密度、抗拉强度、延伸率和硬度进行测定和分析.结果表明:经真空热处理后,钨合金的各项力学性能都得到了提高,抗拉强度和延伸率提高显著,抗拉强度从920 MPa提高到了988 MPa,延伸率从9.7％提高到了18.6％;真空热处理后,显微组织中钨晶粒的连接度降低,合金断口中钨晶粒的穿晶解理断裂和粘结相的延性撕裂增多;真空热处理后合金粘结相中的钨含量明显降低.     

Investigation on the phase relationships in the Ir–Nb–Ni–Al system at the Ni-rich side

The phase relationship of the quaternary system Ir–Nb–Ni–Al for the Ni-corner was experimentally studied by investigating two newly produced alloys and testing the phase compositions of seven alloys previously investigated. The partial phase relationship around the Ni-rich side at 1300 °C was established.