Anomalous mechanical behavior of ultrafine-grained Al–Mg–Zr alloy at low temperature

The effect of deformation temperature on strength and ductility of ultrafine-grained (UFG) low-alloyed Al–Mg–Zr alloy before and after special deformation-heat treatment (DHT) consisting of low-temperature short-term annealing and a small additional deformation, was investigated in the temperature range 77–293 K. UFG structure was obtained by high-pressure torsion processing. It has been established that DHT leads to a substantially enhanced ductility (7–13%) while maintaining high strength (yield stress $\sim$300–435 MPa, ultimate tensile strength $\sim$370–490 MPa) over the entire temperature range studied. In the post-DHT state, an anomalous character of temperature dependences of the strength and ductility in the temperature range of 243–293 K is observed for the first time, which is unusual for coarse-grained and UFG Al-based alloys. A possible explanation for such anomalous temperature dependences of strength and ductility is proposed, based on the competition between various thermally activated processes at grain boundaries with inverse temperature dependences.