Characteristics of a new creep regime in polycrystalline NiAl

Abstract

Constant-load creep tests were conducted on fine-grained (≈23 µm) Ni-50.6 (at. pct) Al in the temperature range of 1000 to 1400 K. Power-law creep with a stress exponent,n ≈ 6.5, and an activation energy,Qc ≈ 290 kJ mol, was observed above 25 MPa, while a new mechanism withn~1 andQ c ≈ 100 kJ mol dominates when σ < 25 MPa, wherea is the applied stress. A comparison of the creep behavior of fine- and coarse-grained NiAl established that the mech-anism in then ≈ 2 region was dependent on grain size, and the magnitude of the grain-size exponent was estimated to be about 2. Transmission electron microscopy (TEM) observations of the deformed specimens revealed a mixture of dislocation tangles, dipoles, loops, and sub-boundary networks in the power-law creep regime. The deformation microstructures were in-homogeneous in then~ 2 creep regime, and many grains did not reveal any dislocation activity. However, bands of dislocation loops were observed in a few grains, where these loops appeared to have been emitted from the grain boundaries. The observed creep characteristics of the low-stress region suggest the dominance of an accommodated grain-boundary sliding (GBS) mech-anism, although the experimental creep rates were lower than those predicted by theoretical models by over seven orders of magnitude. The low value ofQ c in this region, which is ap-proximately one-third that for lattice self-diffusion, is attributed to the possible existence of interconnected vacancy flow channels, or “nanotubes,” at the grain boundaries.