The thermally activated distortion with amplification effect and related variant selection in red gold alloys

Variant selection during the A1→L10 transformation in a polycrystalline red gold alloy close to equiatomic Au-Cu composition has been extensively studied by Electron Backscatter Diffraction (EBSD) in our previous work. The use of a mathematical description of the lattice distortion and the maximal work criterion allowed us to quantify the degree of selection. With the same approach, we investigate here an interesting shape distortion effect, discovered twenty years ago in equiatomic AuCu-Ga. The shape distortion of thin samples placed in bending condition and then heat-treated under stress is studied in details. The singular shape memory effect and the remarkable distortion amplification, which we call TADA effect, are explored by monitoring the sample radius of curvature and the advancement of the transformation. The underlying mechanisms of variant selection are revealed by EBSD analysis across the samples. The experimental crystallographic variant selection distribution is compared with the expected profile calculated with the Euler-Bernoulli beam theory. The good agreement demonstrates that variant selection during the transformation is at the origin of the macroscopic distortion of red gold alloys. The TADA effect was found to occur when external stresses are released, and strongly depends on the stress at the initial stage of the transformation. This unusual effect is assumed to result from the persistence of variant selection throughout the transformation.