Detecting the transition of creep rate-controlling process in Al-Mg solid-solution alloy through instrumented indentation

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 am in the region beneath the indenter decreases to a critical stress sigma c during the creep indentation, the creep stress exponent 11 changes from 4.9 to 3.0. The measured sigma(c)-value decreases from 122 to 52 MPa with increasing temperature, while the corresponding indentation strain rate epsilon(c) increases from 1.22 x 10(-3) to 2.00 X 10(-3) s(-1). The temperature dependence of sigma(c) and epsilon(c) is almost in agreement with the results derived from dislocation theory. The activation energy Q for creep in the stress range H (sigma(m) > sigma(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 (sigma(m) < sigma(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 Delta F is very small. In the stress range M, the IPD does not occur when Delta F is within a certain value. However, the occurrence of IPD is observed when sigma(m) coincided with sigma(c). The findings suggest that the creep rate-controlling process changes from recovery control (n = 4.9) to glide control (n = 3.0) below sigma(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.