Microstructure evolution and microhardness improvement of tantalum-modified tungsten heavy alloy processed by high-pressure torsion processing

Abstract

In this study, the 90(W-xTa)7Ni3Fe with tantalum content of 11 wt%, 18 wt% and 23 wt% were conducted at 400 °C with a pressure of 1.5 GPa by 10, 15 and 25 turns of high-pressure torsion (HPT) processing. Tungsten heavy alloy (WHA) with good consolidation and excellent mechanical properties was obtained. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) instruments were used to analyze the effects of tantalum content and HPT revolutions on the microstructure evolution and mechanical properties of tungsten heavy alloy. The results show that the fine grained microstructure is basically composed of hard W phase, soft Ta phase and γ-(Ni, Fe) bonded phase. Moderate Ta can promote the grain refinement by controlling the shear strain accumulation on W matrix, Ta phase and γ phase, and promote the formation of NiTa strengthening phase. Under the action of large shear strain, the W grains in 18 T samples occurred continuous dynamic recrystallization (cDRX) and dynamic recovery with slight grain growth from 2.20 μm after 15 turns to 2.62 μm after 25 turns, but the Ta grains experienced continuous shear refinement and cDRX refinement simultaneously with the average grain size of 2.07 μm - 2.63 μm. The ultrafine γ phase grains with 90.2 % proportion high angle boundaries were formed by dynamic recrystallization. Significant grain refinement, high density dislocations and powder consolidation give rise to the microhardness improvement, and 18 T samples showed the highest microhardness of 529.4 ± 4.1 HV_0.5. In the strengthening mechanism of WHA samples processed by HPT processing, dislocation strengthening plays a dominant role with over 40 % contribution to the total strength, whose contributing proportion gradually increased with the HPT revolutions, indicating that HPT process is a promising technique to develop ultrafine grained WHAs with superior mechanical properties.