Nucleation And Growth Mechanism Of Twins In Magnesium Alloy During In-situ Tension

Due to the light weight and low density,magnesium and its alloys have been extensively used in aerospace,automobile,3C and other industry.Due to the lower critical resolved shear stress(CRSS),basal slip and {10(?)2} extension twinning become the two main deformation mode for magnesium alloys at room temperature Owing to the reorientation effect of the deformation grians,the formation of{10(?)2} extension twins is one of the important factors responsible for the formation of strong texture in magnesium alloys.The material used in the present thesis was commercially hot-extruded Mg-3Al-1Zn(AZ31)alloy bar,and the tensile deformation was carried out at room temperature with in-situ dynamic tensile test machine with a maximum load capacity of 2kN and along the extrusion radius direction(ERD)at room temperature.The in-situ tensile test combined with electron back-scattering diffraction(EBSD)method was used to observe the dynamic process of twinning of {10(?)2} extension twins in magnesium alloy during the tensile test for the first time.The nucleation and growth of twins can be clearly revealed by tracing the same position in the process of increasing of cumulative strain.When the cumulative strain reached 15%,283 twins were observed.Based on the statistical analysis of those 283 twins,incorporate with the theoritical analysis of Schmid Factor(SF)and strain accommodation,the nucleation and growth mechanism of {10(?)2}extension twins in AZ31 magnesium alloys during in-situ tension was studiedThe results show that Schmid Factor(SF)played an important role in the nucleation and growth of twins.The twin variants with high SF are easier to activate and have a relatively high growth rate,while for low SF twins,the nucleation and growth is difficult,and there is even the possibility of detwinning.However,not all the twins follow the SF rule.About 64%of the nucleated twins during the in-situ tensile test satisfy the SF rule,while for the twin growth this number is decreased to 50%.For the twins,which is not following the SF rule,the strain accommodation factors between the deformation modes in neighboring grains and extension twins are analyzed.From the calculation result,the shear from pyramidal slip and extension twinning in the neighboring grains is important for the nucleation and growth of extension twin.The shear accommodation imposed by extension twin in the neighboring grains both beneficial to the twin nucleation and twin growth.The shear accommodation imposed by pyramidal slip in the neighboring grains is beneficial to the twin nucleation,however,is unfavorable for the twin growth.