Creep behavior of ultra-fine grained Zn–4.5Al

Creep tests were performed at 295 and 373 K on a cryogenically ball-milled Zn–4.5Al alloy. Creep tests on the as-milled microstructure having an average grain size of 260 nm showed clear signs of a threshold stress. The same material after a targeted heat treatment showed no signs of a threshold stress for the same alloy with an average grain size of 510 nm. In both cases stress exponent (n) values close to 1 and activation energies close to that of grain boundary diffusion were noted. Potential causes of the threshold stress are proposed as being a nanocrystalline oxide dispersion or non-uniform solute segregation, each of which could potentially interfere with grain boundary vacancy transfer mechanisms.

Research highlights▶ The creep results exhibited an increasing stress exponent with decreasing stress suggesting the presence of a threshold stress for as-milled material. ▶ Upon annealing the mechanically alloyed material the threshold stress was no longer present. ▶ The stress exponent of 1 and the activation energy equal to grain boundary diffusion over the range of temperatures tested suggests Coble creep.