Fine-grained93W-5.6Ni-1.4Fe Heavy Alloys Prepared By Spark Plasma Sintering

W-Ni-Fe heavy alloys are typical two phases composites used in the applicationsrequiring high density, such as counter weights, kinetic energy penetrators and radiationshields. Conventional liquid-phase sintered W-Ni-Fe alloys manifest a typical microstructurewhere coarsened spherical bcc tungsten grains are dispersed in a fcc Ni-Fe-W solid solutionmatrix. Further improvement of the mechanical properties is restricted. The performance isimproved to some extent after deformation. However，taken the material utilization, cost anddeformation degree, etc, into account, their expanded application in civilian and military fieldare confined. Tungsten grain refinment is proved to be the most effective approach to improvethe mechanical properties. The strength, hardness, toughness and other performances of thealloy will be improved with the decrease of tungsten grain size. For industrial applicationsand academic research, thus, it is of great interest to develop fine-grained W-Ni-Fe alloyswith high performance to meet the advanced requirements.The aims of this study are, first, to use the spark plasma sintering (SPS) method toprepare bulk93W-5.6Ni-1.4Fe heavy alloys from blended elemental powders and study theinfluence of various SPS parameters (sintering temperature, dwell duration and heating rate)on the densification and grain growth of the powders, and secondly to perform a formalsintering analysis, in order to help formulate a hypothesis concerning the mechanism(s) whichcontrols densification and grain growth. Regard the above as the basis, cyclic SPS andtransient liquid-phase SPS are developed and fine-grained93W-5.6Ni-1.4Fe heavy alloyswith enhanced performance are prepared successfully.Based on a detailed report on microstructure development and the local temperaturegradient in the vicinity of the pores, and by means of classical kinetics laws, the densificationand grain growth behavior are analyzed. The whole densification process can be roughlydivided into three stages. Particle rearrangement and neck formation and growth aredominating in the initial stage. The γ-(Ni, Fe) matrix phase has formed in this stage. Appliedpressure and particular “SPS effect” enhance the neck formation and growth. In theintermediate stage, dissolution-precipitation of W grains in the viscous matrix phase and Nienhanced grain boundary diffusion with viscous process (grain rotating and sliding) dominate the simultaneous densification and grain growth. During the final stage, grain growthpredominates and densification stagnates. The grain growth is controlled through both the gasphase diffusion and dissolution-precipitation mechanisms. Heating rate strongly influencedthe SPS densification and grain growth of93W-5.6Ni-1.4Fe heavy alloys. Higher heating rateincreases the viscous flow contribution to the densification, as well as the densification rate,resulting in an enhanced densification behavior at the SPS final stage. At the same time,coarsening induced by surface diffusion is minimized, and then grain growth is suppressed.Two master sintering curves (MSC) in different heating rate stage of93W-5.6Ni-1.4Feheavy alloys during SPS process are developed. The master sintering curve in low heating ratestage can accurately predict the densification behavior of93W-5.6Ni-1.4Fe heavy alloysduring SPS process, as well as the shrinkage and final density. In the high heating rate stage,the developed MSC is difficult in prediction of the final density of93W-5.6Ni-1.4Fe heavyalloys, resulting from the increase of macroscopic temperature difference. However, it can beused to predict the densification behavior of93W-5.6Ni-1.4Fe heavy alloys.Fine-grained93W-5.6Ni-1.4Fe heavy alloys are prepared by cyclic spark plasmasintering. Cyclic SPS at1400oC improves the tungsten grain morphology and the matrixdistribution, as well as the cohesion of tungsten-matrix interface, resulting in reduced W-Wcontiguities and enhanced performance of the alloys. The bending fracture is mainlycharacterized as tungsten-tungsten intergranular rupture, increased tungsten cleavage andmatrix rupture. Moreover, micro matrix ductile tearing and W-matrix separation at the W-Wseparated interfaces prevent the crack extension. Quasi-static compression yield strength atroom temperature of the93W-5.6Ni-1.4Fe heavy alloys is dependent on the microstructuralparameters such as tungsten grain size, matrix volume fraction and tungsten-tungstencontiguity. Dynamic mechanical behavior of the alloys is improved. Shear deformation zonealong the maximum shear stress plane is about30μm. However, the limited improvement ofthe tungsten grain morphology by Cyclic SPS weakens the deformation compatibility oftungsten grains within the shear deformation zone, and then the depth of the adiabatic shearband under high strain rate.Finer spherical tungsten grains (about6μm in diameter) were obtained by control of theSPS process (transient liquid-phase SPS). After SPS, the alloy shows a relative density of0.95 and tungsten-tungsten contiguity of0.53. Compared with conventional liquid-phase sinteredtungsten heavy alloys, transient liquid-phase SP-sintered93W-5.6Ni-1.4Fe heavy alloysexhibit high bending strength (about1580MPa), and high yield strength (about1050MPa atroom temperature and about640MPa at800oC), due to the fine-grained structure. SPS offersan additional strengthening effect on the yield strength of the93W-5.6Ni-1.4Fe heavy alloys.The dynamic performance of transient liquid-phase SP-sintered93W-5.6Ni-1.4Fe heavyalloys is improved. The width of local adiabatic shear band decreases with the tungsten grainsize, whereas the depth of local adiabatic shear band increases with the decrease of tungstengrain size. Therefore, spherical tungsten grain refinement is beneficial for the formation andextension of local adiabatic shear band, as well as the self sharpening behavior duringpenetration.