Breaking the purity-stability dilemma in pure Cu with grain boundary relaxation

High purity metals are increasingly demanded in modern manufacturing industries, but their processing and applications are limited by a dilemma that purer metals are thermally and mechanically less stable. The reduced stability of pure metals originates from the weakened drag effect of impurity atoms on the mobility of grain boundaries (GBs) that are hard to stabilize without alloying. Following recent studies on stabilizing nanograined metals by tailoring structures of GBs, here we report that structural relaxation of GBs breaks the purity-stability dilemma in pure Cu. Contrary to the conventional impurity effect, thermal stability and hardness of nanograined Cu samples with relaxed GBs increase (rather than decrease) with higher purities. The discovered anomalous impurity effect, owing to suppression of GB relaxation process with impurity atoms, offers an alternative vector to stabilizing purer metals for advanced processing and applications.