High-Temperature Ductility of Ni–20% Cr-Base Alloys

Abstract

The ductility of a single-phase Ni–20% Cr alloy and two precipitation-hardened Ni–20% Cr-base alloys, hardened by Ni₃(Al, Ti) and Ni₃(Al,Nb), respectively, has been studied over a range of temperature at various strain rates. The results show that the alloy hardened by Ni₃(Al,Nb), while less ductile than the single-phase alloy, is superior to that hardened by Ni₃(Al,Ti) with respect to both tensile strength and elongation at fracture. In the single-phase alloy, there are two ductility troughs for a given strain rate when the strain rate is slow but in the two-phase alloys and at high strain rate in the single-phase alloy a single trough is obtained.

Metallographic examination has indicated that the ductility behaviour of the single-phase alloy can be correlated with structural changes, void formation leading to a low elongation at fracture but recovery and recrystallization increasing ductility, presumably by relieving internal stresses at the grain boundaries. It is suggested that an important contribution to the ductility of the two-phase alloys at slow strain rates is the formation of precipitate-free zones at the grain boundaries as a result of diffusion creep. The improvement in ductility as a consequence of replacing Ti by Nb as a hardening addition in these alloys can be attributed at least in part to the more rapid formation of these zones, possibly because of faster diffusion rates in the Nb-hardened alloy.