Ta-W合金高应变速率下亚结构转变研究进展

Ta-W alloy had been gradually developed as a penetrating weapon material due to its high density, high melting point and excellent high temperature strength. The high-speed penetration of the projectile was accompanied by the transformation of the internal substructure of the material. Compared with the static loading, the high strain rate loading process involved strain, strain rate, inertia, heating, and even secondary effects such as sound, light, and electricity. The penetration process was very short, so the analysis and processing were more complicated. In the process of dynamic loading, some substructure transformations in the material were often destructive. For example, the appearance of dynamic recrystallization (DRX) might become a potential factor of adiabatic shearing phenomenon, but at the same time, the generation of twins and stress-induced martensite in titanium alloys has been experimentally proved to effectively slow down the expansion of the shear band and caused the bifurcation of the shear band. Therefore, studying the substructure transformation and mechanism of Ta-W alloy at high strain rate was of great significance to improve the penetration performance of the projectile and to better apply the material to service at high strain rate. In the study of Ta-W alloy dynamic mechanics, the split Hopkinson pressure bar (SHPB) was widely used because of its higher strain rate and reliability compared with the thick-walled cylinder implosion test (TWC) and Taylor impact test. According to the geometry of the sample, it could be divided into dynamic compression experiment of cylindrical sample, dynamic compression experiment of hat-shaped sample and dynamic compression experiment of shear compression specimens(SCS). Different loading conditions could be achieved by controlling the geometric size of the sample, which had a very obvious influence on the damage of the sample and the generation and expansion of the shear band. In the characterization of the microstructure inside the Ta-W alloy after dynamic loading, the scanning electron microscope (SEM) could observe the microstructure inside the sample more macroscopically and realize the phase distribution and fracture analysis when the sample was damaged by adjusting the backscattered electron (BSE) mode and secondary electron (SE) mode. X-ray diffraction (XRD) could analyze the phase transition of the material during the high strain rate loading process and combine with the selected area electron diffraction (SAED) to realize the phase analysis of the small area. Focused ion beam (FIB) combined with transmission electron microscope (TEM) method could achieve fixed-point observation of small areas. Compared with ordinary backscattered electron diffraction (EBSD), the transmitted electron backscatter diffraction technology (t-EBSD) had higher spatial resolution and could characterize ultra-fine grains and deformed grains. Compared with TEM technology, the inside of Ta-W alloy could be analyzed more macroscopically without losing precision. This article summarized the substructure transformation of Ta-W alloys at high strain rates from three aspects: impact-induced phase transformation, dynamic recovery (DRV) and DRX in adiabatic shear band (ASB), and deformation twinning at high strain rates. Through the analysis of the microstructure of pure tantalum, Ta-5W, Ta-10W with different tungsten contents Ta-W alloys after deformation at high strain rates, it could be found that the impact-induced phase transformation was caused by the common dislocation configuration and dislocation density in the alloy. The results of the action, and experimental data and mathematical simulations proved that the occurrence of phase transition was closely related to the shear stress under impact load. Through the dynamic loading test of the Ta-W alloy hat-shaped sample, the SAED from the matrix to the adiabatic shear zone showed that the grain refined and misorientation increased in the adiabatic shear zone. However, the calculation of adiabatic temperature rose in the adiabatic shear zone showed that the temperature rise caused by the adiabatic effect was not enough to support the final step of rotating dynamic recrystallization (RDR). The grain boundary refinement was completed through the annihilation of dislocations, so only DRV occurred. However, limited to the accuracy of the instrument, the formation mechanism of the ASB and the internal substructure transformation needed to be further studied and proved. It could be obtained by compressing the Ta-W alloys with different initial dislocation densities at different strain rates. The formation of twinning at high strain rates was the result of the combined effect of the critical twin formation stress, dislocation configuration and dislocation density. It was proved by calculation that the twin contribution to the total response variable was low, which was different from the general perception. However, the mechanism of dislocation configuration and dislocation density in deformation twinning at high strain rates and the role of dislocations in twin nucleation still needed to be elucidated. Existing problems: The specific mechanism and role of shear stress in impact-induced phase transformation in Ta-W alloys were still unclear, and the DRX and DRV mechanisms within ASB, as well as the effect of dislocation density and structure on twinning and phase transformation, needed to be further studied. The research on the dynamic mechanical response of materials at high strain rates involved experiments and theories of many interdisciplinary subjects such as mechanics, materials, physics, and multi-level combined research and analysis from macro to micro was necessary.