Hidenari Takagi

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

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.

PSEUDO-STEADY INDENTATION CREEP

Theoretical analysis and computational modeling have been performed to carry out constant-indentation strain rate tests. Results show that both the indentation pressure and indentation strain rate become constant after a lapse of loading time. Moreover, the finite element simulation reveals that the contour-line patterns of equivalent stress and equivalent plastic stain rate underneath a conical indenter evolve with geometrical self-similarity corresponding to indenter displacement. This finding confirms that pseudo-steady indentation creep occurs in the region beneath the indenter. We define representative points in the underlying material as those with equivalent stress equal to the indentation pressure divided by a constraint coefficient of 3. During the pseudo-steady indentation creep of a power-law material, the equivalent plastic strain rate at these points is proportional to the indentation strain rate for compatibility. These results point out that a constitutive equation for the tensile creep of a power-law material can be predicted by constant-indentation strain rate tests.

Creep characterization of power-law materials through pseudo-steady indentation tests and numerical simulations

A constant-indentation creep rate test (CICRT) has been carried out for an Al-Mg solid-solution alloy using a microindenter in the temperature range of 636–773 K. When a conical indenter is pressed into the specimen surface under a load condition of F=F0 exp(γt) (F the indentation load, F0 the initial load, γ the loading rate parameter, t the loading time), the indentation pressure and indentation creep rate approach constant values of ps and in(s), respectively. The representative points of the deformation in the underlying material are defined on a contour line of the equivalent stress σr = C1ps, where C1 is the so-called constraint coefficient of 1/3 reported by Tabor. The finite element simulation of a power-law material subjected to the CICRT shows that the relationship between the equivalent plastic strain rate r at these points and in(s) is r=C2in(s) and that C2 ≊ 1/3.6 in the case of a creep stress exponent of 3.0. The constitutive equation of r versus σr obtained from experimental data and the computed value of C2 is in good agreement with that evaluated from conventional uniaxial creep tests.