Structural and Phase Transformations in Alloys during Spark Plasma Sintering of Ti + 23.5 at % Al + 21 at % Nb Powder Mixtures

We have studied the effect of sintering temperature on the structural and phase transformations of alloys produced by the spark plasma sintering of Ti + 23.5 at % Al + 21 at % Nb powder mixtures at temperatures in the range 1100–1550°C. The sintered alloys have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy (elemental X-ray mapping). The alloys sintered at temperatures of 1100 and 1200°C have been shown to have a nonuniform microstructure. According to electron microscopy results, the alloys consist of grains of the α₂ and Nb₂Al phases and small precipitates of the O-phase (intermetallic compound Ti₂AlNb). In addition, there are particles of unreacted niobium and titanium. The alloys sintered at a temperature of 1300°C have a uniform lamellar structure.     

The effect of structural state and temperature on mechanical properties and deformation mechanisms of WC-Co hard alloy

The publications reporting systematic investigations of the effect of structural state of WC-Co hard alloys (the cobalt binder content, WC grains size and contiguity) and temperature on mechanical properties and deformation mechanisms have been reviewed and generalized. The ductile-brittle transition, strain hardening, special features of WC-Co alloys deformations in various temperature ranges, and specificity of mechanical properties of the alloys with submicron WC grains have been discussed.     

High-strength state of two-phase composite materials part 2. Cermets

Numerical simulation of the cooling process was carried out to determine the residual stress-strain state of diamond-bearing and tungsten-cobalt hard alloys after sintering. The results show that the thermal stresses of these materials increase their tensile strength. The concentration dependence of the tensile strength of alloys of the WC-Co system is analyzed.Translated from Problemy Prochnosti, No. 3, pp. 17–24, March, 1991.The authors are grateful to M. G. Loshak, senior scientific worker of the Institute of Superhard Materials of the Academy of Sciences of the Ukrainian SSR, for fruitful cooperation and supplying the photograph of the microstructure of the hard alloy.     

Structure of the WC-Co group hard alloys after sintering in a gas atmosphere

The structure of the WC-Co group hard alloys after sintering in a vacuum and in a gas atmosphere without pressure and under gas pressure has been considered.     

Mechanism of Chromium Oxide Formation in Cobalt–Chromium–Molybdenum (F75) Alloys Prepared Using Spark Plasma Sintering

Metallic implants are known to have higher wear rates compared to ceramic. This causes more wear debris to be produced and results in the release of toxic metal ions to the surrounding areas. The strengthening mechanism in cobalt based cast orthopedic alloys depends upon carbides present in the microstructure, but these cause problems when dislodged between articulating surfaces, accelerating wear by abrasion and fretting. Thus, in order to improve the performance of these implants a novel method of processing the alloys, namely by spark plasma sintering (SPS) of fine powders, has been used as it generates hard oxides and not carbides in the microstructure. The oxide in the SPS processed alloy is identified as chromium oxide formed by a redox reaction between cobalt oxide found on the surface of cobalt particles and chromium. The oxygen associated with the cobalt powder is displaced and combines with the chromium during SPS. This oxide in the microstructure of the alloy can be more beneficial than carbides due to its higher hardness, resulting in lower wear rates and less wear particles. With the oxide in the microstructure, the hardness of the alloy becomes closer to that of ceramics. Also its lower density enables the alloy to be lighter. The chemical stability of the oxide ensures that it remains intact and due its insolubility in water, no carcinogenic or toxic reactions will occur.     

Synthesis and processing of nanostructured WC-Co materials

In this study a novel approach, termed the integrated mechanical and thermal activation (IMTA) process, was used to synthesize nanostructured WC-Co powder. As a result of the integration of mechanical and thermal activation, nanostructured WC-Co powder was synthesized below 1000°C, starting from WO₃, CoO and graphite powder mixtures. Furthermore, consolidation of the nanostructured WC-Co powder via high velocity oxy-fuel (HVOF) thermal spraying and solid state sintering was investigated. The results demonstrated the feasibility of converting the nanostructured WC-Co powder to coatings and bulk components, the properties of which are either comparable to or better than that of the conventional coarse-grained counterparts.     

放电等离子烧结制备AlCoCrFeNi高熵合金的组织演变与力学性能

采用放电等离子烧结方法(SPS)制备了AlCoCrFeNi高熵合金。通过差热分析、密度测试、X射线衍射、扫描电镜及力学性能测试,研究了SPS烧结温度对AlCoCrFeNi高熵合金的致密化行为、组织演变及力学性能影响。结果表明,随着SPS烧结温度的升高,材料的致密度与抗压缩强度明显提高。1200℃烧结后,AlCoCrFeNi高熵合金的致密度达到99.6%,抗压缩强度达到2195MPa,屈服强度达到1506MPa。在SPS烧结过程中,高熵合金从双相结构(BCC+B2)转变为三相结构(BCC+B2+FCC)。     

Hot isostatic pressing of ultrafine tungsten carbide-cobalt hardmetals

Hot Isostatic Pressing (HIP) has been successfully used to consolidate tungsten carbide-10 weight% cobalt (WC-10 wt% Co) powders mixtures with WC powder particle sizes in the range of 100 nanometers. Fully dense specimens of this composition have been obtained by HIP at 1000°C, a temperature well below those usually required for reaching the closed porosity stage in the WC-Co system. Conventional processing by vacuum sintering has also been carried out to study the individual effects of high isostatic pressure and vanadium carbide additions on densification and WC grain growth control of these hardmetals. The finest WC mean grain size after sintering has been obtained for the combined action of applied isostatic pressure and vanadium carbide (VC) additions. These results show that VC additions are effective in controlling WC grain growth even at temperatures as low as 1000°C.     

Synthesis of WC hard materials using coated powders

Nickel and cobalt were used as binder materials for tungsten carbide powders (WC) hard materials. Ni and Co binder were added individually to the WC powder by two different methods namely, mechanical mixing and chemical electroless coating. In this study WC powders of grain sizes 0.3–1.0 μm were electroless coated with either nickel or cobalt. The loading of either Ni or Co coating was 13 wt.%. The electroless-coating method conditions of both Ni and Co on WC powders are described. The coated powders were cold compacted and sintered in vacuum at different sintering temperatures. For comparison, identical materials compositions were prepared by mixing the powders constituents mechanically, compacted and sintered under the same conditions.The prepared powders and sintered materials were investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results revealed that coated WC materials have smaller values of porosity and more homogeneous microstructure while other properties, such as transverse rupture strength, and hardness exhibit greater values than those produced using mixing elemental powders. It is possible to outline the benefits of coated powder approach in the following: high homogeneity and better distribution of binder materials within WC hard materials, higher density and good interfacial bonding, capability of using fine powders, and possibility of using small alloying and/or reinforcement additions in a more uniform manner.     

Surface layers on cobalt base alloys by boron diffusion

Boronizing is used to improve the wear resistance of the surface of metals and alloys. Various powder mixtures containing boron compounds or amorphous boron were tested at different treatment temperatures. The boronizing time was varied also. The diffusion was observed by metallography and microhardness testing of sloping cuts of the alloys; sloping cuts were necessary because of the thin boronized surface layers with different boride compositions. Borides are formed by reactions with the diffusing boron. Their structures were determined by X-ray diffraction. Separate layers containing different borides were found and the composition of some of these borides was studied. Cast cobalt base alloys and cemented carbides contain different amounts of cobalt. Since boronizing leads to the formation of mainly cobalt borides, the boronized layers of these types of alloys are different.Some examples of applications and results will be given.     

纳米复合WC-Co粉末的烧结研究进展

纳米复合WC-Co粉末的烧结是纳米晶WC-Co硬质合金制末的烧结研究进展,包括纳米复合粉末的烧结特性、烧结机理和常用的烧结方法.     

High-temperature mechanical properties of WC-Co hard metals (Review)

Experimental results are discussed of the laboratory investigations into high-temperature mechanical properties of WC-Co hard metals as functions of the microstructure parameters (cobalt content, average sizes of carbide grains and cobalt interlayers), binder composition (carbon and cubic carbide contents) and the thermal and force action found in the literature. The effect of high temperatures on deformation characteristics in bending, tension and compression has been analyzed. The problems of short-and long-time strengths (high-temperature strength) and thermomechanical fatigue are discussed. The revealed mechanisms of the high-temperature deformation are considered.     

Effect of WC nanoparticle size on the sintering temperature,density, and microhardness of WC-8 wt % Co alloys

Using X-ray diffraction, scanning electron microscopy, and density measurements, we have studied the effect of WC particle size (20 to 150 nm) on the optimal sintering temperature of the WC-8 wt % Co alloy and the effect of sintering temperature (800–1600°C) on its phase composition, density, and microhardness. The results indicate that, during sintering of the starting powder mixture, the first to form is the ternary carbide phase Co₆W₆C. At sintering temperatures of 1100°C and above, this phase reacts with carbon to form Co₃W₃C. Sintering above 1000°C leads to the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co〈WC〉, along with the ternary carbide phases. The density and microhardness of the alloy have been measured as functions of sintering temperature. The use of WC nanopowder has been shown to reduce the optimal sintering temperature of the WC-Co alloy by about 100°C.     

On the development of nanostructured WC-Co hard alloys

A dimensional region of the existence of WC nanoparticles and nanostructured WC-Co hard alloys has been substantiated. It has been shown that the existing technologies do not allow to obtain pore- free WC-Co hard alloys with carbide particles of size 5–40 nm to be produced. A method of the formation of nanostructured hard alloys has been proposed.     

Porous FeAl alloys via powder sintering:Phase transformation,microstructure and aqueous corrosion behavior

This study investigates the phase transformation and microstructure of porous FeAl parts sintered from elemental powder mixtures using in-situ neutron diffraction and in-situ thermal dilatometry.A single B2 structured FeAl phase was determined in the sintered FeAl alloy.The combined effects of the Kirk-endall porosity,transient liquid phase,and phase transformations associated with powder sintering all contribute to the swelling phenomenon of the final sintered part.The aqueous corrosion test indicates that the corrosion products include iron oxides in the porous FeAl parts.The accumulation of corrosion products blocks the pore channel and decreases pore size and permeability over the soaking time.     

Study of the microporosity of WC-Co alloys

The existence of micropores in the structure of WC-Co alloys located at the grain and phase boundaries and commensurate with an interlayer of a cobalt phase has been established using scanning electron microscopy. It has been shown that the micropore size and shape change with a change in sizes of they cobalt interlayer and tungsten carbide grains.     

Microstructure and microhardness of high entropy alloys with Zn addition: AlCoFeNiZn and AlCoFeNiMoTiZn

High entropy alloys were designed from equiatomic multicomponent systems using powder metallurgy including mechanical alloying and sintering. The structure and morphology of the resulting alloys were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy techniques and their hardness values were also determined in the Vickers scale. The results indicate under the milling conditions used, the AlCoFeNiZn, AlCoFeNiMoTi and AlCoFeNiMoTiZn alloys crystallized forming BCC structures whereas the AlCoFeNi alloy presented two different phases, one with FCC structure and the other one with BCC. The synthesis method resulted in alloys with grain sizes in the nano scale having values between 4.1 and 9.4 nm on the powder form up to 40.1 nm after sintering phenomenon which lead to phase transformations which were more evident in the Mo-containing alloys. In addition, the AlCoFeNiZn and AlCoFeNiMoTiZn alloys did not show Zn traces after sintering as it was suggested by chemical analyses using energy dispersive spectroscopy, suggesting it is lost by evaporation during sintering process. Mo-containing systems exhibited the highest microhardness in both milled and sintered conditions.     

Effect of temperature-related factors on densification,microstructure and mechanical properties of powder metallurgy TiAl-based alloys

In the present work, the influence of temperature-related factors, including sintering temperature, heating step and temperature-control mode, on the densification, microstructure and mechanical properties of Ti-46.5Al-2.15Cr-1.90Nb-(B, Y, Mo) alloys prepared by SPS has been investigated and discussed in detail. The results obviously indicate that the sintering temperature plays a key role on densification and phase transition, when compared with the heating step and temperature-control mode. Based on the experimental results and theoretical analysis, the densification process and microstructural evolution of TiAl-based alloys during sintering are studied. Moreover, the mechanical properties of the sintered alloys are determined by the combined effects of the densification and microstructure. The obtained results will help to optimize the microstructure and properties for this kind of intermetallic alloys through controlling sintering parameters during powder metallurgy process.     

Dependence of the strength and plasticity of WC-6Co medium-grained hard alloys on deformation characteristics of the cobalt and carbide phases

The effect of deformation characteristics of the carbide phase and cobalt binder on the strength and plasticity of the WC-Co hard alloy has been studied using a calculation algorithm.     

Layer-by-layer laser synthesis of Cu–Al–Ni intermetallic compounds and shape memory effect

We have studied conditions for the synthesis of intermetallic phases in the Cu–Al–Ni system by selective laser sintering/melting, in particular by heating a powder mixture to 300°C. The effects of laser synthesis and heating on the microstructure of the intermetallic phases in the samples obtained have been studied using electron microscopy, optical metallography, and X-ray diffraction analysis. The results demonstrate high sinterability of stoichiometric mixtures. Resistivity measurements indicate that the samples exhibit a shape memory effect. We discuss the feasibility of producing biomicroelectromechanical systems using layerby- layer synthesis.