Creep characterization of aluminum-magnesium solid-solution alloy through self-similar microindentation

Abstract

Indentation creep tests were performed on an Al-5.3 mol% Mg solid-solution alloy to examine whether mechanical properties can be extracted accurately from a testpiece, as small as a rice grain. A conical diamond indenter was pressed into a test surface with a constant load F at temperatures ranging from 0.60 to 0.65 Tm (Tm: the absolute liquidus temperature). When the representative flow stress σ̄m and the indentation strain rate ε̇in in the underlying material decreases to each critical value, σ̄c and ε̇c, as indentation creep proceeds, the stress exponent n for creep varies distinctively from 4.9 to 3.0. The measured σ̄c decreases with increasing temperature, while the corresponding indentation strain rate ε̇c increases under the same condition. This temperature dependence is in close agreement with the results derived from the dislocation theory. The activation energy Q for creep in range M(n ≅ 5, σm > σ̄c or ε̇in > ε̇c) is approximately equivalent to that for the lattice diffusion of pure aluminum, and the Q value in range A (n ≅ 3, σ̄m < σ̄c or ε̇ inn < ε̇c) is close to that for the mutual diffusion of this alloy. With load-jump tests, F was abruptly increased in the indentation creep test. In range M, instantaneous plastic deformation (IPD) takes place evidently even with a slight abrupt increase in load. On the other hand, the IPD does not occur in range A when load increment is within a certain value. However, the occurrence of IPD is observed under the condition of σ̄m > σ̄c or ε̇ in > ε̇c. The findings suggest that the creep rate-controlling process changes from the recovery control (n = 5) to the glide control (n = 3) below σ̄c. It is thus demonstrated that the indentation testing technique can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime. © 2006 The Japan Institute of Metals.