On the remarkable fracture toughness of 90 to 97W-NiFe alloys revealing powerful new ductile phase toughening mechanisms

Abstract

Tungsten is generally too brittle to serve a robust structural function. Here, we explore the fracture toughness of 90–97 wt%W Ni-Fe liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KJm ≈ 69–107 MPa√m) of the WHA, containing only 3–10 wt% of a Ni–Fe ductile phase (DP), is ≈ 9–13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). All the WHAs show extensive stable crack growth, and increasing blunting line toughness averaging ≈ 170 MPa√m, prior to significant crack extension. In contrast to classical ductile phase toughening, that is primarily due to macrocrack bridging, the WHA toughness increase mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatational shielding process zone microcracks in the macrocrack process zone. Tests down to –196 °C, to partially emulate irradiation hardening, show decreasing toughness and a transition to elastic fracture at a temperature of –150 °C for 90W to –25 °C for 97W. However, even at –196 °C, the leanest DP 97W WHA KIc is ≈ 3 times higher than that of monolithic W at RT. Possible effects of the small specimen size used in this study are briefly summarized.