Age-hardening behavior and phase identification in solution-treated AEREX 350 superalloy

Abstract

This article presents results of an investigation on age-hardening behavior of superalloy AEREX 350. Microhardness testing was employed to evaluate the age-hardening response of the alloy while optical, scanning, and transmission electron microscopy techniques were used to characterize the major phases formed during the aging process. No significant hardening was found in solution-treated samples aged at temperatures up to about 680 °C. Aging at 700 °C up to 950 °C, however, caused a characteristic hardening response. This hardening was concurrent with the formation of γ', an ordered phase with L1₂ structure, as fine precipitate distributed throughout the fcc matrix. In the temperature range of 800 °C to 1055 °C, a new phase called η with D024 structure was formed. Two morphologies of η phase were found: the discrete blocky precipitates mainly at grain boundaries and elongated plates with a Widmanstätten appearance within the grains. The latter morphology was predominant at higher aging temperatures. This was attributed to accelerated diffusion of solute to the incoherent tips of Widmanstätten plates at high temperatures. No evidence of any precipitates was found in the microstructure of samples aged at 1060 °C, implying that this temperature was above the solvus temperatures of all precipitates in the AEREX 350 alloy. Based on the results presented in this investigation, it is suggested that considerable improvement in the properties of the alloy may be achieved through modification of the commercial heat-treatment practice.