Fracture and ductility in hot torsion of Nickel

Abstract

Fracture of Nickel 270 under conditions of warm working and hot working was studied using a hot torsion testing device. Under all conditions, fracture initiates as wedge-type cracks at the peak on the torque-twist curve. A hot fracture initiation criterion is proposed based on the initial deformation conditions. The higher the temperature and the slower the strain rate, the smaller the strain at which fracture initiates. Under hot working conditions, fracture propagates slowly and microstructural evidence suggests that deformation aided diffusion processes predominate. Under conditions of warm working, fracture propagates more rapidly and grain boundary sliding processes predominate. Metallographic and other experimental evidence show that the softening process under all test conditions is dynamic recrystallization. The influence of changes in the kinetics of dynamic recrystallization on ductility and microcracking is discussed.