計装化押込み試験法によるAl-Mg固溶体のクリープ律速過程の遷移検出

抄録

  Indentation creep tests of an Al-5.3 mol% Mg solid-solution alloy were performed using a microindenter in order to examine whether creep properties can be extracted accurately from a testpiece which is as small as a rice grain. A conical indenter was pressed into a surface with a load of 0.39 N at temperatures ranging from 546 to 590 K. When the average equivalent stress σ_m in the region beneath the indenter decreases to a critical stress σ_c during the creep indentation, the creep stress exponent n changes from 4.9 to 3.0. The measured σ_c-value decreases from 122 to 52 MPa with increasing temperature, while the corresponding indentation strain rate ε_c increases from 1.22×10^-3 to 2.00×10^-3 s^-1. The temperature dependence of σ_c and ε_c is almost in agreement with the results derived from dislocation theory. The activation energy Q for creep in the stress range H (σ_m>σ_c) is approximately equivalent to that for the lattice diffusion of pure aluminum, 144 kJ mol^-1. The Q-value in the stress range M (σ_m<σ_c) is close to the activation energy for the mutual diffusion of this alloy, 130 kJ mol^-1. With load-jump tests at T=573 K, indentation load was abruptly changed from 0.39 N to various values ranging 0.19-0.59 N. In the stress range H, instantaneous plastic deformation (IPD) takes place evidently even when the load increment ΔF is very small. In the stress range M, the IPD does not occur when ΔF is within a certain value. However, the occurrence of IPD is observed when σ_m coincided with σ_c. The findings suggest that the creep rate-controlling process changes from recovery control (n=4.9) to glide control (n=3.0) below σ_c. These results agree well with those of conventional uniaxial creep tests. Consequently, the indentation testing technique can be used effectively to extract creep parameters from small-volume samples.