The role of deformation twinning on strain hardening and recrystallization in magnesium alloy AZ31

The quasi-static mechanical response of annealed, highly textured AZ31 rolled plate was measured in simple compression on two sample directions, each promoting a different twin family. A high plastic anisotropy with extreme strain hardening in both directions was observed. New insights on the influence of both twin types to this behavior were derived by correlating changes in the strain hardening rates with the microstructure evolution. Extension twins nucleated at low strains, grew to consume most of the structure, and saturated at strains below the peak hardening rates. Thin contraction twins were initially postulated to be responsible for the peak hardening rates by limiting the hard pyramidal slip length (producing a Hall-Petch type strengthening). However, subsequent experimental investigations raised questions on whether these are not present at the spacing required to account for the peak hardening rate. Thus, neither twin family was deemed responsible for the peak strain hardening rates.The effect of grain size on deformation twinning during strain hardening was also investigated by producing a similarly textured AZ31 plate with a larger grain size. A lower twinning stress was confirmed in this material. An additional change in strain hardening rate was associated with the nucleation of secondary extension twins forming perpendicular to the primary twin, limiting the growth potential of the pre-existing twins.The microstructure evolution during isothermal static annealing was investigated to critically evaluate the contributions of extension and contraction twins to the recrystallized microstructure after room temperature deformation, and establish the potential of these twins to alter the recrystallized texture. Contraction twins were potent sites for recrystallized nuclei producing many favorably oriented orientations for reduction of the strong c-axis fiber texture; while extension twins were not very effective in recrystallization. Recovery before the onset of recrystallization was extensive when the c-axis in most grains was compressed. Incomplete recrystallization within the hardest grains was deemed responsible for the retention of the c-axis fiber texture.These new insights were employed to identify the benefits of laser-roller hemming versus conventional hemming. Hemming is an automotive processing operation used to join inner and outer body panels involving severe forming and design requirements.Ph.D., Materials Science and Engineering -- Drexel University, 201