Microstructure, distortion characteristics and mechanical behaviour of NiB modified 90W-6Ni-4Co heavy alloys

Abstract

In this work, the effect of NiB on densification, phase, microstructure, distortion and mechanical properties of conventionally sintered ternary 90W-6Ni-4Co heavy alloy has been investigated without altering (Ni +NiB)/Co ratio. No intermetallic phase was detected in the heavy alloys upon addition of NiB. Instead, the partial and complete exchange of Ni with NiB in 90W-6Ni-4Co has elicited relatively better densification (> 99% theoretical), higher bulk hardness (HV3 similar to 389-396 VHN), higher hardness of matrix (similar to 352 HV0.025) when the alloys were sintered at 1500 degrees C. In addition to W grains and matrix, the completely NiB-sub-stituted heavy alloy (90W-6NiB-4Co) sintered at 1500 degrees C exhibits the occurrence of W-rich third phase which contains similar to 75 wt% of W. Although the samples prevented slumping at 1475 degrees C, elephant foot formation was found inevitable in the base composition (90W-6Ni-4Co) when sintering was carried out at 1500 degrees C. However, the decreased distortion parameter (from 24.8 to 8.5) with optimal NiB addition affirms the enhanced shape retention capability of 90W-3Ni-3NiB-4Co and 90W-6NiB-4Co heavy alloys. Room temperature compression test of WHAs sintered at 1475 degrees C was performed at a fixed strain rate of 0.1 s(-1). No crack formation was observed in NiB containing samples up to 40% deformation. Moreover, the systematic addition of 3-6 wt% NiB resulted in lower coefficient of thermal expansion of NiB-containing heavy alloys in the temperature range of 400-600 degrees C. (C) 2021 Elsevier B.V. All rights reserved.