Search

The dense TiC-316 L stainless steel cermets with different TiC content were fabricated via conventional powder metallurgy (PM) process. The effects of TiC content on the microstructure and mechanical properties of TiC-316 L stainless steel cermets are studied. Scanning electron microscopy (SEM) is employed to observe the microstructure, fracture morphology, and crack propagation of cermets. The binder mean free path (MFP) and hard phase contiguity are analyzed based on the stereological principles. The relative density, hardness and transverse rupture strength (TRS) of the cermets are measured by the Archimedes' principle, Rockwell scale hardness tester and three-point flexural test, respectively. The average TiC grain size decreases first and then increases with the increase of hard phase content. At a content of 80 wt%, the maximum relative density (99.1%), hardness (88.9 HRA) and TRS (1373 MPa) are obtained. During crack propagation, both intergranular and transgranular fractures exist, of which the intergranular fracture prevail and coarse grains are more prone to transgranular fracture.

This work presents a study of tensile behavior for a Ti-28 at.%Ta alloy with low modulus and remarkable superelasticity. The Ti-28Ta alloy was firstly fabricated by melting the Ti/Ta powder mixture, followed by hot swaging. The microstructure and tensile behavior were examined by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy and digital image correlation. The results show that the as-fabricated Ti-28Ta alloy is consisted of β with minor nano-sized martensitic α″ and athermal ω. The yield strength and tensile strength of the Ti-28Ta alloy are 549.4 ± 17.0 and 673.8 ± 78.2 MPa, while its fracture elongation and Young's modulus reach to 28.55 ± 2.19% and 89.9 ± 6.1 GPa, respectively. The static and cyclic stress–strain tensile behaviors of the Ti-28Ta alloy were investigated. The results demonstrate that the Ti-28Ta alloy exhibits ductile and transgranular fracture behavior, excellent superelasticity and remarkable ductility.

In this study, we investigated the effect of titanium addition on microstructure and mechanical properties in Mo-Si-B alloys. The Mo-Ti-Si-B alloy (Mo-3.9Ti-3Si-1B, wt%), which has α-Mo, Mo3Si, Mo5SiB2 and TiO2 phases, was fabricated by a powder metallurgy (PM) method. The starting materials were pulverized by using a high-energy ball milling and the resultant powder was subjected to a reduction process followed by cold isostatic pressing (CIP) compaction and pressureless sintering. In the microstructure, intermetallic compound phases were uniformly distributed in the α-Mo matrix. Some titanium atoms solved into the α-Mo matrix and the others formed a TiO2 phase caused by reaction with oxygen at the grain boundary. Fracture toughness of the Mo-Ti-Si-B sintered body was recorded as 10.42 MPa·m1/2, which is lower than that of the Mo-Si-B sintered body without addition of titanium. In the Mo-Ti-Si-B sintered body, the fracture mode is similar to the Mo-Si-B sintered body where intergranular fracture through the Mo grain boundary and transgranular fracture cross the intermetallic compound phase. The decrease of fracture toughness is due to the relatively large TiO2 at the grain boundary, promoting intergranular fracture.

Due to the poor mechanical properties of solid lubricants, direct addition to the ceramic matrix will reduce the mechanical properties of the material. In this study, Al(OH)3 was coated on the surface of nano CaF2 by non-uniform nucleation method and added to Al2O3/Ti(C,N) ceramic matrix. Al2O3/Ti(C,N)/CaF2@Al2O3 was prepared by vacuum hot pressing sintering. Compared with the material directly added with nano-solid lubricant, the mechanical properties of ceramic tools added with coated nano-solid lubricant have been significantly improved. Among them, when the content of coated nano-solid lubricant is 10 vol%, the ceramic material has better comprehensive mechanical properties. SEM observation shows that the cross-sectional particle distribution of nano-coated powder ceramic material is relatively uniform and has good compactness. The fracture mode of ceramic materials is a mixed fracture mode of intergranular fracture and transgranular fracture.

Mo-12Si-8.5B alloys with different Zr contents (0 at.%, 1 at.%, 2 at.%, 3 at.%, 4 at.%) were manufactured via a mechanical alloying process followed by hot pressing sintering technology. The microstructure of Mo-12Si-8.5B alloy exhibited a continuous submicro- and micro-scale α-Mo matrix in which the sub-micron Mo 3 Si/Mo 5 SiB 2 particles were distributed. Addition of Zr to Mo-12Si-8.5B alloy promoted to form spherical nano-scale intermetallic Mo 2 Zr and ZrO 2 particles, which were mainly located at the grain boundaries (GBs) as well as partially within the grains. The microstructure of Mo-12Si-8.5B-xZr alloys was remarkably refined by these Mo 2 Zr/ZrO 2 nanoparticles. Additionally, results of mechanical properties indicated that the Zr addition improved the hardness, compression strength, yield strength and flexure strength of alloys. In particular, the Mo-12Si-8.5B-2Zr alloy exhibited extremely high compression strength (3.38 GPa), yield strength (3.17 GPa) and flexure strength (1.15 GPa). Quantitative analyses indicate that both fine-grained strengthening and Zr-rich particle strengthening mechanisms play a significant role in strengthening the Mo-Si-B-Zr alloys, the strengthening is dominantly governed by grain size reduction. Furthermore, Zr getters detrimental oxygen by synthesizing ZrO 2 distributed at grain/phase boundaries, which contributes to increasing the GBs cohesion. Fracture surfaces revealed that the fracture mode transformed from intergranular to transgranular fracture owing to Zr addition.

Oxide-dispersion-strengthened tungsten (ODS-W) is the most promising structural materials for military, aerospace and nuclear industries because of their excellent mechanical properties and stability at high temperature. However, conventional mechanical milling methods have reached certain limits in further improving the properties of W and its alloys. In this study, with the aim of synthesizing W based composite powder with homogeneously dispersed yttrium oxide (Y2O3) nanoparticles, a novel process methodology combining ultrasonic spray pyrolysis and subsequent hydrogen reduction has been described. Our results reveal that the W-Y2O3 composite powders consisted of multiple primary particles of approximately 30 nm with a three-dimensional network and are of high purity. TEM and XPS analysis results demonstrated that Y2O3 nanoparticles were homogeneously dispersed into the tungsten matrix. In comparison with pure W sintered body, the W-Y2O3 composite showed a refined grain structure, higher relative density and Vickers hardness, and the combination of an intergranular and a transgranular fracture mode and these were mainly attributed to the homogeneous dispersion of Y2O3 nanoparticles. Our work shows that ultrasonic spray pyrolysis and subsequent hydrogen reduction is a promising method for preparing high quality ODS-W with high density, fine grains and uniformly dispersed strengthening phase nanoparticles.

Refractory tungsten(W) materials are widely used in high-temperature field due to their high melting point. But there are some problems with W, such as low strength and poor toughness. The grain boundaries are very sensitive to impurities, which is one of the most important reasons for the degradation of tungsten properties. In this paper, we use silicon carbide (SiC) as a deoxidizer, which is designed to react with the oxygen atoms in tungsten at high temperature to purify and strengthen the grain boundaries. The effects of SiC addition on the sintering densification, microstructure and mechanical properties of W were investigated. Results show that SiO2 and W2C are produced by the interface reaction between SiC and W, which reduces the free energy of sintering system and promotes the densification process, the maximum relative density reaches 99.4%. When SiC content is 1 wt%, compared to pure tungsten, the tensile strength significantly increases from 220 MPa to 431 MPa, the hardness increases up to 697.13 HV. Furthermore, the fracture mode of this W is mainly transgranular fracture, which is due to interface reaction strengthening and purification grain boundary strengthening.

Nano-coated particles and whiskers were used as reinforcing additives for ceramic tool materials to take advantage of synergistic toughening and enhancement mechanisms. An Al2O3/TiC/CaF2@SiO2/SiCw ceramic tool material was fabricated by hot-pressing. Self-lubricating ceramic tool materials toughened by nano CaF2@SiO2 coated particles and SiC whiskers were studied. When the addition content of nano CaF2@SiO2 coated particles was 10 vol% and the content of SiC whiskers was 20 vol%, ceramic tool materials with the best comprehensive properties were obtained. The results showed that the hardness of the ceramic tool material was 16.52 GPa, the flexural strength was 698 MPa, and the fracture toughness was 6.89 MPa m1/2. Compared with the ceramic tool material with 10 vol% nano CaF2 particles and 20 vol% SiC whiskers, the above properties were improved by 7.06%, 31.74% and 19.06%, respectively. The SiC whiskers played a toughening role, while nano CaF2@SiO2 coated particles reduced the porosity, the compactness and also filled between the grain boundaries of the ceramic tool materials during sintering. This improved the interfacial bonding between ceramic grains, which increased the incidence of transgranular fracture in the ceramic tool materials.

Transition metal boride enable applications in fields of cutting tools as the hard phase, but their fabrication suffers from shortcomings in high temperature for binary and hard/soft phase balance. Here, we leveraged a WCoB-Co cermets prepared by using elemental powders of W, Co and B powders via in situ synthesis method and investigated the microstructure evolution of a composite structure of WCoB/Co. The phase evolution, microstructure evolution and mechanical properties were investigated using X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The complete sintering process consist of three solid-phase reaction stages and one liquid-phase sintering stage. The solid-phase reaction stage includes the following three reactions: B + 2Co → Co2B, 11 W +15 Co2B → 9WCoB + W2Co21B6, and 14 W + 2W2Co21B6 → 12WCoB + 23Co + Co7W6. The eutectic liquid phase, essential for the densification, forms at 1323 °C. The Rockwell hardness and density value reach 87.5HRA and 12.11 g/cm3, respectively. At 1380 °C, the traverse rupture strength (TRS) reaches the peak value of 1495 MPa. Moreover, the results show that transgranular fracture is the main fracture mode. The densification stage was completed at ~1400 °C. In particular, Co7W6 phase was discovered during the study of the evolution of WCoB-Co cermets. An increase of the B content was found to be beneficial to eliminate the Co7W6 phase that proved to be detrimental to mechanical properties.

The effects of ultrafine WC content on the microstructure, mechanical properties and wear resistance of regenerated coarse grained WC-10Co cemented carbides have been investigated by SEM, mechanical property tests and wear test. The results show that with the increase of ultrafine WC content, the average grain size of the regenerated cemented carbides increases first and then decreases. When the ultrafine WC content is 10%, the average grain size of the regenerated cemented carbides is the maximum (4.06 μm). Meanwhile, the irregular shape grain decreases and the growth of grain is improved. When the ultrafine WC content is 10%, the regenerated cemented carbides have the highest density (99.84%), which could alleviate the negative impact of coarse grains on hardness. The transverse rupture strength and fracture toughness of regenerated cemented carbides increases on account of crack deflection, crack branching, crack bridging, the increase of transgranular fracture of WC and dimple fracture of Co phase (TRS: 2520 MPa, KIC: 20.28 MPa·m1/2). Therefore, when the ultrafine WC content is 10%, the regenerated cemented carbides have more optimal mechanical properties. The addition of ultrafine WC particles balances the hardness and fracture toughness of the regenerated cemented carbides in the sliding wear experiment, which is found that the friction coefficient, wear rate, wear scar width and depth of regenerated cemented carbides are the smallest when the content of ultrafine WC is 15%.

Failure patterns of coated carbide tool were investigated by high-speed face milling of the hardened steel SKD11. Tool failure surface morphology, cutting force and machined surface roughness were also analyzed to reveal the failure mechanisms. The results indicated that the dominant failure pattern of coated carbide tool was breakage. The primary mechanism of tool breakage was fatigue fracture. Under different cutting speeds, the distinctive morphologies of fatigue fracture were presented on the failure surfaces. At low cutting speeds, many fatigue sources were observed on the rake face. The distance between fatigue sources and tool nose was approximately two times of the depth of cut. With the increase of cutting speed, the fatigue striations and riven patterns were observed at the fracture surface. In addition, the fatigue steps and crack deflection were found under high cutting speeds. The main fracture mode was intergranular fracture at lower cutting speeds. However, it was transgranular fracture at higher cutting speeds. Furthermore, the irregular fracture surfaces at low cutting speeds and at high cutting speeds contribute to a larger cutting force increment compared with the medium cutting speeds. The increment of surface roughness in the initial and severe wear stages was lower than that in the steady wear stage, while the deviation of surface roughness was relatively large.

Pure tungsten samples were prepared by the selective electron beam melting (SEBM) process. The effect of the SEBM process parameters on the density, microstructure and compression strength of pure tungsten was studied. In addition, the influence of substrate preheating temperature during SEBM was studied. A processing window for additive manufacturing of pure tungsten by SEBM was preliminarily determined. Pure tungsten samples with relative density of 99.5% and without obvious pores and microcracks were successfully fabricated. The as-built pure tungsten samples showed strong columnar grain structures. Compression strength along the columnar grains in the build direction was measured to be 1560 MPa. Fracture occurred predominantly along the columnar grain boundaries by decohesion, in addition to brittle transgranular fracture. Refinement and strengthening of the columnar grain boundaries are expected to improve the compression strength of the SEBM-fabricated pure tungsten.

The spherical indentation technique has been successfully implemented for inducing controlled damage, under both monotonic and cyclic loading conditions, in three microstructurally different polycrystalline diamond (PCD) grades. Damage evolution under increasing applied load and/or number of cycles has been assessed, and the attained results point out the important role of microstructure on nature, aspect and evolution of damage in these materials. At the microscopic level, transgranular fracture of diamond particles and crack propagation through the binder matrix are found to be the predominant crack paths for damage induced under contact loads in all the cases.

In this paper, the effect of the carbon contents on the microstructure, mechanical properties and fracture behavior of WC-40 vol.% Ni 3 Al composites prepared by conventional powder metallurgy was studied. Microstructure examinations of WC-40 vol.% Ni 3 Al specimens indicated that the WC grain is round and fine compared with faceted and large WC grain of WC-Co composites. With increasing carbon contents, the WC morphology of WC-40 vol.% Ni 3 Al composites is similar and the size of WC grain is approximate and the distribution of binder phase becomes relatively uniform. Furthermore, composition analysis of binder phase showed that, with the increase of carbon content, the solubility of W in Ni 3 Al reduced slightly. The ~ 9 wt.% solubility of W in Ni 3 Al is much lower compared with ~ 22.4 wt.% of WC-40 vol.% Co composites. The hardness and transverse rupture strength of WC-40 vol.% Ni 3 Al composites decrease with increasing carbon contents slightly. WC-40 vol.% Ni 3 Al composites present higher hardness and lower transverse rupture strength than that of WC-40 vol.% Co composites. With increasing carbon contents, the fracture behavior of the WC-40 vol.% Ni 3 Al composites is affected greatly and the fracture mode changes from transgranular fracture towards intergranular fracture.

An in-situ growth TaC whiskers toughening Al 2 O 3 ceramic matrix composite was prepared by two steps, which were in-situ synthesis of TaC whiskers in Al 2 O 3 matrix powder and hot pressing of the composite respectively. The preparation process, microstructure and toughening mechanisms of the composite were investigated in detail. TaC whiskers with a diameter of 0.2–1 μm and an aspect ratio of 10–30 were in-situ synthesized in the Al 2 O 3 matrix powder by a carbothermal reduction technique. The flexural strength, fracture toughness and Vickers hardness of the composite were 638 MPa, 6.5 MPa·m 1/2 and 17.4 GPa respectively. An improvement of the mechanical properties was ascribed to the TaC particles and whiskers which induced the toughening effects such as the crack deflection, interface debonding, whisker fracture and whisker pullout. The fracture mode of the composite was the combination of transgranular fracture and intergranular fracture which improved the mechanical properties of the composite.

This work compared phase composition, microstructures and mechanical properties of ZrB2—20 vol.% SiC—5 vol.%Yb2O3 prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength.

Several tungsten heavy alloys based on W–Ni–Cu and W–Ni–Fe were made recently at DMRL by liquid phase sintering route. These alloys were heat treated and characterized in terms of density, hardness and tensile properties. Although the densities of W–Ni–Cu based heavy alloys were found to be relatively higher than those of W–Ni–Fe based heavy alloys, the former exhibited inferior tensile properties and hardness values than the latter alloys. In both these heavy alloys, un-melted solid tungsten particles were bonded together by the respective low melting matrix alloy. The strength of the matrix or the bonding strength of the interface (between tungsten particles and matrix) was found to control the nature of final fracture. Poorer matrix strength or interfacial strength was found to initiate the fracture by separation of tungsten particles either by matrix failure or by interface failure. On the other hand, tensile fracture takes place predominantly by cleavage fracture of tungsten particles, if both the matrix and interface are stronger than the tungsten particles. The tensile fractured surfaces were examined under scanning electron microscope (SEM). Failure stress is observed to be lower in case of separation of tungsten particles (due to matrix and interface failure) than that in case of cleavage fracture (transgranular fracture) of tungsten particles. SEM image of fractured surfaces clearly indicated that failure in case of W–Ni–Cu based heavy alloys was due to matrix and/or interface (between tungsten grain and matrix) failure, where as, W–Ni–Fe based heavy alloys were failed predominantly by cleavage fracture of tungsten particles. This was evidenced by poorer tensile properties obtained for W–Ni–Cu based heavy alloys as compared to those of W–Ni–Fe based heavy alloys. The present sets of results indicate that heavy alloys based on W–Ni–Fe and W–Ni–Cu can be strengthened by minor additions of Co, Mo and Fe selectively. Higher strength (>850 MPa) of the heavy alloys are associated with the finer tungsten grain size and cleavage fracture of tungsten particles. Where as, the lower strength is associated with the larger tungsten grain size and interface and/or matrix failure. It is found in this study that as the tungsten grain size decreases, the tensile fracture mode changes gradually from interface failure to matrix failure to tungsten grain cleavage failure.

This study deals with the microstructure and mechanical properties of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides fabricated by conventional sintering. The conventional WC particles of 4 μm size and ultrafine particles of 0.2 μm were introduced in the system with varying ratio. The ratios of conventional WC particles to ultrafine WC particles were 2:1, 1:1, and 1:2. The microstructures of sintered WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides were sensitively dependent on the ratio of conventional WC particles to ultrafine WC particles. The rim phase increased with the increase in the amount of ultrafine particles. Hardness of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbide increased with increase in the amount of rim phase and decrease in the average grain size of WC particles. The bending strength showed the similar trend of the hardness. The fracture morphologies are reported. The fracture behavior changed from mixed mode to transgranular fracture mode, when the ratio of conventional WC particles to ultrafine WC particles was changed from 2:1 to 1:2.

The parameter critical stress represents the low-temperature fracture strength of a material, and corresponds to the intergranular fracture strength particularly for molybdenum having an equiaxial grain structure such as pure molybdenum. In this work the meaning of the critical stress for molybdenum having an elongated and large grain structure such as doped molybdenum is discussed. Results are summarized as follows. 1. For molybdenum having a different grain structure, the critical stress, σc is expressed by the following equations. (1) An equiaxial and relatively small grain structure (pure molybdenum) mσ ig ≪σ tg (m=1) and σ c =σ ig . (2) A slightly elongated grain structure mσ ig tg (m>1) and σ c =mσ ig . (3) An elongated and large grain structure (doped molybdenum) mσ ig ≧σ tg (m≫1) and σ c =σ tg . Here σig is the `true' intergranular fracture strength, σtg the transgranular fracture strength, m a constant depending on the grain shape such as aspect ratio. 2. Large scattering in the critical stress for doped molybdenum is attributed to the large scattering in the maximum strength at low temperatures. The maximum strength depends on the size of the crack initiation area.

Tensile tests at temperature 173–363 K were carried out for the powder-metallurgy molybdenum alloys containing 0·1 - 1·0 mass% Ti. Then critical stress and critical temperature representing low-temperature strength and ductility of the material, respectively, were evaluated. A small addition of Ti increased both intergranular and transgranular fracture strengths, and hence increased the critical stress. The ductility of molybdenum was improved and the ductile-to-brittle transition temperature was lowered by adding Ti, primarily due to the increase in critical stress. The fracture mode was predominantly intergranular with or without Ti additions.

The strength and ductility of a molybdenum alloy are influenced by carbon and its precipitates as well as the alloying element(s). In this investigation, the low-temperature tensile properties of MoRe and MoTi alloys with or without carbon addition were evaluated. Here the strength and ductility of the material were represented by two parameters, critical stress and critical temperature. Both Re and Ti additions increased the transgranular fracture strengths as well as the intergranular fracture strength, and consequently the critical stress increased and the critical temperature decreased. A small amount of carbon addition further increased the intergranular fracture strength and the critical temperature was substantially decreased. The critical temperature was approximately a function of critical stress for Mo-2% Re and 4%Re alloys and MoTi alloys. Very low critical temperatures for Mo-10%Re and 13%Re alloys are due to their characteristic temperature dependence of yield strength.