Systematic investigation of microstructure, distortion, mechanical and thermal properties of NiB and ZrB2-modified 90W-6Ni-4Co alloys

Abstract

The present work focuses on the role of NiB and ZrB2 addition on density, structural evolution, mechanical and thermal properties of 90W-6Ni-4Co (wt%) heavy alloys. The investigated tungsten heavy alloys were sintered at 1485 °C and 1500 °C with a holding time of 75 min in H2 atmosphere. Compared to the base alloy 90W-6Ni-4Co, the NiB-substituted alloys showed improved densification at both sintering temperatures. An in-depth study of the phase and microstructure of the 1500 °C sintered samples was carried out. X-ray diffraction analysis confirmed the presence of ZrO2 in ZrB2 containing WHAs. Microstructural analysis affirms that ZrB2 incorporation attributes for grain size refinement in sintered alloys. Furthermore, the elephant foot formation in the sintered samples was avoided by incorporating NiB individually or in combination with ZrB2 dispersoids. The distortion parameter (δ) of the samples sintered at 1500 °C was found to be decreased from 24.83 (0 wt% ZrB2) to 6.36 (2 wt% ZrB2). On the other hand, the combined addition of NiB and ZrB2 provides better shape retention with δ=2.82 for the tungsten heavy alloys with 3 wt% NiB and 2 wt% ZrB2. The maximum compressive stress of ∼2000 MPa was obtained in the 89W-6Ni-4Co-1ZrB2 samples. The maximum and minimum bulk hardness of 432 ± 5 HV3 and 365 ± 9 HV3 was obtained for 88W-3Ni-3NiB-4Co-2ZrB2 and 90W-6Ni-4Co heavy alloys. The coefficient of thermal expansion of the 90W-6Ni-4Co alloy was found to be the highest (7.65 × 10−6 /°C), which was reduced to 7.1 × 10−6 /°C and 6.8 × 10−6 /°C with the incorporation of 1 and 2 wt% ZrB2 respectively. W-rich channels were noticed in the microstructure of 1500 °C sintered samples. Overall, W-Ni/NiB-Co-ZrB2 alloys are very effective in enhancing the mechanical properties and distortion behavior compared to W-Ni-Co alloys.