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The metal injection molding (MIM) method was applied to manufacture a shaped charge liner (SCL) used for petroleum perforating bombs. Its application could overcome the drawback of heterogeneous density distribution prepared using the traditional powder metallurgy and spinning process. The sintering results showed that because of the limitation in the sintering temperature, the relative density of the W–Cu alloy shaped charge liners prepared by metal injection molding (MIM W–Cu SCLs) ranged from 48.05% at 1050 °C to 52.52% at 1100 °C, which was far below those prepared by spinning. Further increase to higher temperature led to the W–Cu separation, cracks, and distortion of the SCL. The explosive test proved that the shaped charge liners prepared by metal injection molding (MIM SCLs) could achieve comparable or even better penetration performance than those prepared by spinning. The particle size of tungsten played a significant role in the penetration performance in which the sample prepared from a −250-mesh tungsten powder showed the highest penetration depth, which was 18.44% deeper than that of the spinning process. From the observation of ballistic holes, the jet of the MIM SCL was composed of dispersed W–Cu particles without a slug. The diameters of the holes bored by the MIM SCLs were larger than those SCLs produced by spinning, which proved that the MIM SCL jet is noncoherent. [ABSTRACT FROM AUTHOR]

Herein, powder metallurgy gas-solid nitriding was used to prepare a high-nitrogen Ni-free austenitic stainless steel with more than 1.0 wt.% N. Study of the microstructures and corrosion resistances of the as-prepared samples revealed that the corrosion weight loss, size, number of corrosion pits, and corrosion depth decreased as the N content increased from 1.19 to 1.37 wt.%. The self-corrosion current densities of the alloys were determined from their potentiodynamic polarization curves. The changes in the capacitive arc radius as demonstrated by the Nyquist curves were consistent with the conclusions of the immersion corrosion analysis; that is, a high N content leads to a high corrosion resistance. [ABSTRACT FROM AUTHOR]

In this study, gas-atomized Fe17Cr11Mn3Mo powders were nitrided to obtain different nitrogen contents for the purpose of studying the effects of nitrogen on predominant sintering mechanism during the initial stage of high nitrogen nickel-free stainless steel powder. Most of the nitrogen in both powders and sintered bodies was uniformly distributed in the form of solid solution, causing lattice distortion and thus hindering the diffusion of metal atoms. It was found that the higher the nitrogen content (0.19–0.84 wt%) in the sintered specimens, the lower the diffusion rate, and in turn the smaller the sintering-neck size. Theoretical analysis indicated that the predominant mechanism was surface diffusion. The additional activation energy caused by unit concentration of nitrogen hindering diffusion was 252.1 ± 40.4 kJ/mol, which was very close to the reported activation energy of nitrogen in FeCr steels. The results showed that the main obstacle to neck growth was solid solution nitrogen rather than surface nitrides. [Display omitted] • Most of the N in both powders and sintered bodies existed as solid solution. • The dominant sintering mechanism during the initial stage was surface diffusion. • The rate of diffusion decreased with increasing the sintered nitrogen content. • The main obstacle to neck growth was solid solution N rather than surface nitrides. • The activation energy that unit concentration of N hindering diffusion is 252 kJ/mol. [ABSTRACT FROM AUTHOR]

The effects of oxygen contents and sintering time on the sintering-neck growth behaviour of Fe 15Cr powder were studied. The oxygen content was increased from 0.029 to 0.059 wt% by heat-treating the powder under air at different temperatures. SEM observations revealed that the length of the neck decreased as the oxygen content increased. The sintering-neck growth rate decreased most significantly when the oxygen content was increased from 0.029 to 0.034 wt%. The results of electron probe microanalysis suggested that the increase in oxygen content led to encapsulation of the powder particles by oxide films, but the oxygen content inside the sintering neck remained similar to that of the matrix. Theoretical analysis indicated that the growth of sintering necks was hindered by surface oxides and interstitial oxygen in the matrix. When the oxygen content was 0.029 wt%, the sintering necks grew via surface diffusion; when the oxygen content exceeded 0.034 wt%, the dominant sintering mechanism changed to grain-boundary diffusion. The diffusion coefficient of surface diffusion was 2.41 × 10 −7  m 2 /s, which far exceeded that of grain-boundary diffusion; when the oxygen content was 0.034 wt%, the diffusion coefficient of grain-boundary diffusion was only 5.79 × 10 −9  m 2 /s. It was also found that grain-boundary diffusion required the breaking of Fe O bonds, so the diffusion coefficient gradually decreased to 1.12 × 10 −9  m 2 /s when the oxygen content was 0.059 wt%. The findings of this work will provide a theoretical basis for the optimisation of sintering processes of stainless steel with high oxygen level, such as the metal injection moulding process. [ABSTRACT FROM AUTHOR]

Porous NiTi-based shape-memory alloys (SMAs) were aged at different temperatures and holding times to study the effects of high O contents on the composition, microstructure, phase-transformation behaviour, and shape-recovery properties of SMAs. NiTi 2 and Ni 2 Ti 4 O x phases were precipitated during the aging process; the concentration was not significant until an aging temperature of 500 °C was applied for 2 h (O content = 1.87 wt.%). Aging the SMAs at 600 °C for 2 h produced a coarse NiTi 2 phase and an O content of 4.3 wt.%. Ti atoms were consumed during the oxidation process, which caused the excess Ni atoms to dissolve into the matrix or generate Ni-rich phases, such as Ni 4 Ti 3 or Ni 3 Ti. The presence of excess Ni atoms resulted in different phase-transformation behaviours. A high O content increased the elastic modulus of the porous NiTi-based SMAs, but decreased the shape-recovery rate. The samples aged at 600 °C exhibited inferior mechanical properties, and hence, it was concluded that high-temperature heat treatments should be avoided. [ABSTRACT FROM AUTHOR]

In TiC- and Ti(C,N)-based cermets, the wettability of the ceramic phase with the metallic binder is commonly increased through supplementation with Mo in the form of pure Mo powder or Mo 2 C. Herein, TiC–high Mn steel cermets were fabricated by conventional powder metallurgy techniques using Fe–Mo pre-alloyed powders as binders to guarantee uniform Mo distribution, and the cermet preparation process was optimized and investigated in detail. The microstructures of the thus obtained cermets were observed by scanning electron microscopy and compared to those of a Mo-free cermet and a cermet prepared using pure Mo powder. The grain size of Fe–Mo powder cermets exceeded that of the Mo-free cermet but was much smaller and more homogeneous than that of the Mo powder cermet. For Fe–Mo powder cermets, angular and tetragonal TiC particles were observed at Mo contents of <1.2 wt%, while round shapes became dominant at higher Mo contents. The hardness of Fe–Mo powder cermets increased with increasing Mo content, as did transverse rupture strength, which was maximal (2264 MPa) at a Mo content of 2.4 wt%, while impact toughness was maximal (11.2 J/cm2) at a Mo content of 1.2 wt%. The above values exceeded those reported for similar conventional cermets, and the use of Fe–Mo pre-alloyed powder as a metallic binder was therefore concluded to be an attractive strategy of increasing the strength and toughness of TiC–high Mn steel cermets. [ABSTRACT FROM AUTHOR]

This study investigates the sintering neck growth behavior of pure titanium powder with different oxygen contents. The growth rate of the sintering neck gradually decreased as the oxygen content increased from 0.06 wt% to 0.50 wt%. At the same time, it was observed that the oxygen in the sintering neck spread to the low oxygen region as the size of the neck increased. A theoretical analysis indicated that surface diffusion is the dominant diffusion mechanism. However, under the hindrance of oxygen in the matrix, both surface diffusion and grain boundary diffusion slow down. Calculating the increase in the self-diffusion activation energy of Ti atoms indicated that oxygen was more resistant to surface diffusion. Predictably, grain boundary diffusion will become the dominant diffusion mechanism in the initial stage of sintering when the oxygen content exceeds 2.628 wt%. Unlabelled Image • The sintering-neck length decreased with increasing O content in Ti powder. • The dominant sintering mechanism was always surface diffusion in the experiments. • O is more resistant to surface diffusion but have limited effects at lower levels. • The lower is solubility of O, the greater is inhibitory on surface diffusion. [ABSTRACT FROM AUTHOR]

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni 4 Ti 3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys. [ABSTRACT FROM AUTHOR]

Elemental powder (EP) sintering is attractive for use in fabricating the NiTi shape memory alloys due to its low cost and compositional flexibility. However, because of sintering challenges that are not adequately resolved, current EP NiTi alloys exhibit zero or trace tensile properties. In this study, a 10−4 Pa vacuum system was used to optimize the sintering of EP NiTi alloys. The unavoidable liquid phase and high-temperature sintering promoted elemental homogenization, which substantially improved the engineering properties of EP NiTi. Compared to previous studies, increasing the vacuum level caused deflagration reactions at a low temperature of 700 °C. It also rendered the materials more susceptible to macroscopic millimeter-scale defects at > 1050 °C due to the loss of the liquid phase. These problems could be avoided by optimizing the sintering process and completing high-temperature sintering at 1250 °C. High-temperature sintering promoted local elemental homogenization rather than extensive elemental migration. 3D–Computed Tomography reconstruction revealed that high-temperature sintering promoted shrinkage of the pore volumes and homogeneity of pore size distribution. This sintering process considerably improved the mechanical properties of the EP NiTi alloy. The tensile strength and related elongation were 701–763 MPa and 16.54–21.97%, respectively, and the alloy exhibited a shape memory of >90% in an 8% tensile evaluation. This optimization strategy, which integrates impurity control and the intermetallic phase sequence, promotes the practical applications of EP NiTi alloys. [ABSTRACT FROM AUTHOR]