Experimental and computational creep characterization of Al–Mg solid-solution alloy through instrumented indentation

Abstract

Carefully designed indentation creep experiments and detailed finite-element computations were carried out in order to establish a robust and systematic method to extract creep properties accurately during indentation creep tests. Samples made from an Al–5.3 mol% Mg solid-solution alloy were tested at temperatures ranging from 573 to 773 K. Finite-element simulations confirmed that, for a power-law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent n, the activation energy Q _c for creep, the back or internal stress and so on can be evaluated robustly. The creep stress exponent n was found to change distinctively from 4.8 to 3.2 below a critical stress level, while this critical stress decreases rapidly with increasing temperature. The activation energy for creep in the stress range of n = 3.2 was evaluated to be 123 kJ mol⁻¹, close to the activation energy for mutual diffusion of this alloy, 130 kJ mol⁻¹. Experimental results suggest that, within the n = 3.2 regime, the creep is rate controlled by viscous glide of dislocations which drag solute atmosphere and the back or internal stress is proportional to the average applied stress. These results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime. It is thus confirmed that indentation creep tests of Al–5.3 mol% Mg solid-solution alloy at temperatures ranging from 573 to 773 K can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime.

Acknowledgements

The authors gratefully acknowledge insightful suggestions from Professor Subra Suresh during the course of this study. M.F. acknowledges the support by Overseas Researcher Fund of Nihon University. M.D. acknowledges the support by the Defense University Research Initiative on Nano Technology which is funded at the Massachusetts Institute of Technology by the Office of Naval Research under grant N00014-01-1-0808.

Notes

†Author for correspondence. Email: [email protected]

‡Author for correspondence. Email: [email protected].

†For convenience, the medium-stress range of n ≈ 3 is often denoted by the symbol M, and the low- and high-stress ranges of n ≈ 5 are represented by the symbols of L and H, respectively

Figure 6. (a) Experimentally measured indentation strain rate versus normalized average equivalent stress at 573 and 590 K, both on logarithmic scales. The n value changes distinctively from 4.8 to 3.2 at a critical stress level . (b) Corresponding computational results of indentation strain rate versus normalized average equivalent stress.

Additional information

Notes on contributors

Ming Dao†

†Author for correspondence. Email: [email protected]

Masami Fujiwara‡

‡Author for correspondence. Email: [email protected].