Characteristics of a new creep regime in polycrystalline NiAl

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 26; no. 2; pp. 343 - 356
• Main Authors: RAJ, S. V, FARMER, S. C
• Format: Journal Article
• Language: English
• Published: New York, NY Springer 01-02-1995
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Creep; Exact sciences and technology; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Metals. Metallurgy; Physics; Deformation; Nickel alloys; Creep; Creep test; Polycrystals; Mechanical properties; Experimental study; Binary alloys; Aluminium alloys; Creep curve; Temperature effects; Fine grain structure; Microstructure; Creep deformation; Activation energy; TEM
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/bf02664671

Constant-load creep tests were conducted on fine-grained (approx. 23 microns) Ni-50.6 (at. pct) Al in the temperature range of 1000 to 1400 K. Power-law creep with a stress exponent, n approx. 6.5, and an activation energy, Q(c) approx. 290 kJ/mol, was observed above 25 MPa, while a new mechanism with n approx. 2 and Q(c) approx. 100 kJ/mol dominates when sigma is less than 25 MPa, where sigma is the applied stress. Comparing of the creep behavior of fine- and coarse-grained NiAl established that the mechanism in the n approx. 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 subboundary networks in the power-law creep regime. The deformation microstructures were inhomogeneous in the n approx. 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) mechanism, although the experimental creep rates were lower than those predicted by theoretical models by over seven orders of magnitude. The low value of Q(c) in this region, which is approximately one-third that for lattice self-diffusion, is attributed to the possible existence of interconnected vacancy flow channels, or 'nanotubes', at the grain boundaries. (Author)