Ball indentation studies on the effect of aging on mechanical behavior of alloy 625

The effect of aging on mechanical behavior of Alloy 625 was investigated using the non-destructive Stress–Strain Microprobe (SSM) system. SSM is based on an automated ball indentation (ABI) technique, and involves multiple indentations by a small spherical indenter at a single penetration location under strain-controlled conditions. The technique permits evaluation of mechanical properties such as yield strength, ultimate tensile strength, strength coefficient and strain hardening exponent. Alloy 625 was aged at six different temperatures in the range of 873–1173 K for 500 h each. ABI tests were carried out at room temperature and at 473 K. The variation of yield and ultimate tensile strengths with aging temperature exhibited a peak in strength following aging at 973 K. The peak stress was 1.5 times the strength of the unaged material, and the strength after aging at 1173 K was nearly equal to that of the unaged material. The peak in strength is attributed to the precipitation of the γ” phase. The fall in strength due to aging above 973 K is attributed to the precipitation, growth and dissolution of δ-phase precipitates. These studies demonstrate that ABI can be used as a non-destructive technique to determine changes in mechanical properties of nickel base alloy components due to aging.