Effect of Ti addition on the thermal stability of nanocrystalline AZ61 Mg alloy

Abstract

Nanostructured alloys possess exceptional desirable properties but suffer from low thermal stability, as a consequence of a high content of grain boundaries where the atoms are in non-equilibrium lattice positions. Here nanocrystalline Ti/AZ61 magnesium matrix composite (AZ61Mg-Ti) was annealed isothermally between 573 K and 723 K. Subsequently, the thermal stability was analyzed, taking nanocrystalline AZ61 alloy as the comparative object. After annealing at 723 K for 600 min, the average grain size of AZ61Mg-Ti was approximately 95 nm, much less than that for the annealed AZ61 alloy (~450 nm). And Ti particles with size of ~410 nm and nano scale Ti3Al particles with size of ~ 8 nm were uniformly distributed in magnesium matrix. The kinetics of magnesium grain growth for nanocrystalline AZ61Mg-Ti, was well interpreted by generalized parabolic grain growth model, D8−D08=kt. The activation energy (Eg) for grain growth was calculated to be 134 kJ/mol, being 42 kJ/mol higher than that for pure Mg. For Eg, the contribution rates of drag effect and pinning effect to improve thermal stability were calculated to be 62% and 38%, respectively. Hardness evolution of annealed samples was studied and with raising annealing temperature the downtrend for AZ61Mg-Ti was more gradual than for AZ61 alloy due to the effect of the non-thermal sensitive Ti particles and the restricted Mg grain growth. After annealing treatment at 723 K for 600 min, the hardness was 1.27 GPa, higher than that for the annealed AZ61 (1.13 GPa).