Experimental full-field investigation of stress development in austenite during the thermally-activated transformation of a Cu-Al-Ni single crystal

The paper presents an experimental analysis of the stress field developing within austenite in a Cu-Al-Ni single crystal during its thermally-activated transformation to martensite. Starting from a virgin sample in austenitic state, fifty homogeneous cooling-heating cycles allowed to reach about 10 % of martensite at each cycle upon cooling. The in-plane strain fields were measured using Localized Spectrum Analysis (LSA) with a strain resolution of 2 × 10 -4 and a spatial resolution of 0.26 mm. The evolution of the stresses out of the martensite zones was then deduced from these strains using known elastic properties of the austenite crystal. Compression normal stresses of about -80 MPa on average were evidenced in the middle of the sample, remaining fully austenitic, while positive values were revealed in the periphery around martensite needles. In addition, shear stresses of opposite signs were observed on either side of the martensitic needles. Zero-shear paths were identified between parallel and closely spaced needles, as well as in the extension of their tips. The presence of stresses in the austenite, even far from the martensitic zones, can be explained by the elastic strains required to achieve the crystallographic compatibility at the interfaces, and the necessity to achieve global static equilibrium of the entire sample. Among other results, e.g. residual stresses at the end of each thermal cycle and a "strongly symmetrical" kinetics of transformation between cooling and heating, this study quantifies the heterogeneity of the stress fields which contribute to the sample's transformation although the macroscopic loading is purely thermal.