Twinning Behavior In AZ31Magnesium Alloy During Rolling And Its Effects On Annealed Microstructure And Properties

Rolled AZ31sheet exhibits poor formability due to its strong basal texture. Thislimits the use of magnesium alloy. In this work, AZ31sheets were rolled by differentrolling process (unidirectional rolling and cross rolling), and annealed at differenttemperature subsequently. Twinning behavior during rolling and its effects onmicrostructure and mechanical properties of annealed sheets are studied. Themechanism and feasibility to improve formability of rolled AZ31sheet bypre-cold-rolling and annealing are discussed.The results show that:①Pre-cold-rolling and static recrystallization annealing isan effective process to improve formability of rolled AZ31sheet. Compared with theoriginal sheet, the tensile test uniform elongation of rolled sheet annealed at300℃and400℃are improved by more than20%, and the Erichsen value are increased by morethan50%. The improvement of elongation is attributed to grain refinement andweakened basal texture. Because basal texture was weakened, Erichsen value wasincreased.②Types of twins in cross rolled and unidirectional rolled sheet are almostthe same. In both two types of as-rolled sheet, most of twins are the double twin{10-11}-{10-12} whose misorientation angle with matrix is38°. The angle between theprimary and secondary twin planes of this type of twin is the smallest among the all fourtype of double twin. There are few primary tension twins and contraction twins in theas-rolled sheet. The primary contraction twin is unstable, and will twin again to generatedouble twin.③Twins in unidirectional rolled sheet, primary contraction twins anddouble twins, have preferential orientation. Compared with unidirectional rolled sheet,orientation of twins in cross rolled sheet is more random. The orientation preference oftwins can be used to control microstructure and mechanical properties of rolled sheet.④Primarycontraction twins and double twins are the main nucleation site during staticrecrystallization. It is more difficult to nucleate in tension twins because of its lowdeformation store energy. After annealed at300℃or above, double twins can not bedetected in the sheet.⑤Texture of rolled sheets annealed at different temperature arebasal texture. Compared with the as-rolled sheet, the texture of rolled sheet annealed at150℃and200℃is not changed significantly, while the basal texture of rolled sheetannealed at300℃and400℃was weakened.After annealed at300℃and400℃, thetexture of unidirectional rolled sheet is slightly different from cross rolled sheet: the {0001} pole figure of unidirectional rolled sheet is elliptical, while the {0001} polefigure of cross rolled sheet is circular.