PNNL

Abstract

Commercial 90W7Ni3Fe (90W, wt. %) tungsten heavy alloy (WHA) plates were hot-rolled (HR) at 900°C to the final consolidated thickness reductions of 62%, 74% and 87% (62R, 74R and 87R). An as-sintered (AS) 90W plate was acquired from the same vendor. In contrast to 62R and 74R, the 87R WHA was given a final processing heat treatment at 1400°C/3h (87RHT). HR deforms the WHA microstructure into an anisotropic brick (W) and mortar (NiWFe ductile phase) composite architecture. The microstructures and mechanical properties of all the WHA conditions were characterized both before and following annealing at 1300°C for 24h. Fracture toughness tests showed stable crack growth, with an average elastic-plastic fracture toughness, KJm ˜ 107±13 MPavm, in the AS and 87RHT conditions. In contrast, the 62R and 74R WHA elastic fracture toughness was significantly lower at KIm ˜ 35±2 MPavm. Lower toughness was attributed to higher WHA strength, since the W-particles experience warm, rather than HR at 900°C. The 1300oC/24h anneal results in similar average elastic-plastic toughness of values of KJm ˜ 125±12 MPavm in all the conditions, higher than the AS and 87RHT conditions, and much higher than for the 62R and 74R WHA. Scanning electron microscopy and electron backscatter diffraction showed that annealing recovers the hardening microstructures in the 62R and 74R WHA. Annealing also reduces the KJm anisotropy of the 87RHT plate. The implications of these results to the design of WHA, and the mechanisms mediating WHA fracture toughness are briefly discussed.