Achieving bimodal microstructure and enhanced tensile properties of Mg–9Al–1Zn alloy by tailoring deformation temperature during hard plate rolling (HPR)

Abstract

In the present work, we investigated the influence of deformation temperature on the formation of a bimodal microstructure and tensile properties of a Mg–9Al–1Zn (AZ91) alloy processed by a single pass hard plate rolling (HPR) with a thickness reduction of 55% followed by a short time annealing. Microstructural examinations by electron backscatter diffraction (EBSD) reveal that there exists a narrow deformation temperature window, i.e. it is only feasible to achieve a desired bimodal grain structure in the AZ91 alloy when HPRed at ∼350 °C. It is mainly because that partial dynamic recrystallization (DRX) occurs easily in the sample HPRed at ∼ 350 °C, facilitating the formation of a bimodal microstructure consisting of coarse un–recrystallized grains (>∼70 μm) and fine recrystallized grains (<∼5 μm). In contrast, reducing deformation temperature to ∼300 °C results in a coarse unrecrystallized microstructure containing substantial sub structure, while elevating the deformation temperature to ∼400 °C leads to an almost uniform recrystallized grain structure. Especially, the formation mechanism for the bimodal grain structure at the optimum deformation temperature is explored in terms of Schmidt factors analysis for basal slips (SFbasal) for grains of different size scales. Moreover, the 350 °C HPRed AZ91 sample exhibits a better combination of strength (ultimate tensile strength = ∼374 MPa) and ductility (elongation = ∼19.6%), as compared to the 300 and 400 °C samples. It is mainly attributed to the typical bimodal grain structure and the relatively high SFbasal featured by the fine recrystallized grains as well as the nanometer spherical particles distributing in fine grain interiors.