Precipitation-hardening of superalloy 693 and modeling of initial stages of hardening

Isothermal time-temperature-hardness behavior for Ni3Al type phase in austenitic superalloy 693 was investigated by micro hardness measurements of the aged specimens. Ageing of the alloy between 973 K and 1173 K indicated an initial increase in micro hardness with ageing time attaining a peak at 1.5 h followed by a drop of its value. The early stages of hardening for the alloy was modeled by the expression: ΔH = (Kt)n, where ΔH, t, n and K are increase in hardness, ageing time (h; in hour), time exponent and temperature-dependent rate constant (h−1) respectively. X-ray diffraction of solution-annealed and aged specimens indicated formation of Ni3Al type precipitation-hardening (γ′) phase, while scanning electron microscopy with energy dispersive X-ray spectroscopy could not clearly reveal the formation of γ′ phase. Transmission electron microscopy on peak-aged (1.5 h at 973 K, 1073 K, 1173 K) samples revealed formation of fine, face centered cubic, Ni3Al type (γ′) precipitates, while over-aged (3 h at 973 K, 1073 K, 1173 K) specimens indicated coarsening of precipitates. The initial stages of hardening for the alloy (at 973 K, 1073 K, 1173 K) are attributed to formation of fine Ni3Al type precipitates.