AZ31 alloy produced by ECAP: metallurgical and mechanical investigation

The potentialities of pure Mg and Mg alloys as biocompatible materials for implants production have induced the research community to develop specific strategies for improving their mechanical and corrosion properties in simulated body fluid. Equal Channel Angular Pressing (ECAP) is considered a promising technique, but results are not well addressed yet due to the complex interaction between microstructure, mechanical properties and corrosion resistance. This work is focused on this interaction through advanced metallurgical and mechanical analysis of the AZ31 alloy subjected to four successive ECAP passes at 250 °C. Dynamic recrystallization takes place during ECAP, and the microstructure has been examined by light microscopy (LM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). It exhibits a progressive decrease of grain size to 6–7 µm after 4 passes and a significant texture change leading to the alignment of basal planes to ECAP shear plane. A slight decrease in dislocation density, the refinement of Al8Mn5 precipitates and the increase of the fraction of high-angle grain boundaries (HAGBs) are other typical aspects highlighted by the analyses. Such complex microstructural evolution affects the mechanical properties (yield stress and hardness) and compressive residual stresses giving rise to a performance peak after the first pass. The results, correlated with corrosion data from previous work, indicate that grain refinement, incremental fraction of HAGBs and compressive residual stresses have positive effects on corrosion resistance and stress corrosion cracking (SCC) behavior, whereas a negative contribution comes from texture evolution. In particular, corrosion resistance and SCC decrease with the fraction of grains with the [0002] orientation present on the surface of the samples. The best trade-off is achieved after the first ECAP pass that guarantees an optimal combination of mechanical and corrosion behavior. In this condition the AZ31 alloy has better performances than the ones obtained through other approaches and is very promising for biomedical applications.