Origin of tweed in Au–Cu–Al alloys

Abstract

The premartensitic tweed in Au–Cu–Al alloys, contrary to previous thought that resort to defects, is confirmed to be associated with the coherent embryos of an intermediate phase (I phase) embedded in parent phase. The parent → I phase transformation temperature was measured by differential scanning calorimeter and dynamic mechanical analysers, which shifts from 82.3 to 557.6 °C depending on the alloy composition. X-ray diffraction and transmission electron microscopes (TEM) results show that the parent → I phase transformation is a charge density wave transition that cannot be suppressed even by melt-spun method, which shows obvious compositional inhomogeneity between I phase and parent. The results imply that the parent → I phase transition is a fast displacive transformation coupled with diffusion. Moreover, accompanying the parent → I phase transformation, alloys demonstrate diversified microstructure revealed by TEM observation, from tweed to chessboard nanowires or twins. These findings provide the experimental evidence for that parent → I phase transformation in Au–Cu–Al alloys is originated from pseudospinodal decomposition as theoretically predicted.