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 to 97%W (by wt.) liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KIc or KJm ≈ 38 to 107 MPa√m) of WHA, containing only 3 to 10 wt.% of a NiFe-based ductile phase (DP), is ≈ 5 to 13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). RT tests for a range of precracked bend bar sizes generally show extensive stable ductile tearing (DT), except in the case of the 97 wt.%W alloy, where elastic fracture occurs in all but the smallest specimen tests. Nevertheless, even in this case the K_Ic = 38 ± 4 MPa√m, which is still almost 5 times higher than that for monolithic W. Tests with the smallest specimens 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 -25ºC for 97W. However, even at -196°C, the 97W K_Ic is ≈ 3 times that of monolithic W at RT. In contrast to classical ductile phase toughening by macrocrack bridging, WHA toughening mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatationally shielding process zone microcracks.

Keywords: Tungsten heavy alloy, liquid phase sintered WNiFe, tensile, fracture toughness, ductile phase toughening