Microstructure and mechanical properties of ECAP processed AZ91 and AZ80 magnesium alloys

Abstract

AZ91 magnesium alloy repreð€ˆ—ents about 90% of the cast alloys available today and offers a good combination of castability, mechanical strength and corrosion resistance. The alloy is ideally suited for applications where weight savings and fuel economy is exploited, e.g. in the automotive and aerospace industries although as-cast AZ91 has poor formability due to the presence of high fraction of coarse Mg17Al12 in its intermetallic phase. Studies have shown that equal channel angular pressing (ECAP), a form of severe plastic deformation, produces bulk samples of ultrafine-sized grains capable of improving strength and ductility. However, the micro structural evolution of the resultant magnesium alloys during the ECAP processing, its precipitate morphology and mechanical performance have not been fully understood. This project studies the microstructure development of AZ91 with different initial microstructures (viz. as-cast, forged, and solution treated) and as-extruded AZ80 magnesium alloys. The hardness and Young's modulus of a matrix and J3 (Mg17Al12) precipitate have been studied using the depth-sensing nanoindentation technique. The hardness of the J3 precipitate has been found to be about 3 times the matrix, whilst the elastic modulus of the precipitate is about twice the matrix. The strength of the alloy can be estimated using a crude-strengthening model at a given precipitate volume fraction. ECAP processing of up to 6 passes at 573 K of AZ91 and AZ80 alloys resulted in significant grain refinement. Analysis of the results shows that it is plausible that the yield strength increase upon ECAP processing is caused by such grain refinement, displaying a positive Hall-Petch relationship in the case of as-cast AZ91. On the contrary, both the results of forged AZ91 and extruded AZ80 alloys showed a decrease in the yield strength albeit resultant smaller grains possibly caused by texture modification introduced during the ECAP processing. It was also noted that the final grain size is affected by ECAP processing temperature, i.e. the lower the temperature, the smaller the resultant grain size. The FIBSEM observation shows that size, morphology and distribution of precipitates drive the level of ductility.