Study of β-precipitates and their effect on the directional yield asymmetry of friction stir processed and aged AZ91C alloy

Friction stir processing (FSP) has emerged as a solid state deformation process for achieving significant grain refinement in magnesium alloys. Cast AZ91C alloy is solution treated and friction stir processed to achieve an average grain size of ∼6.5 μm in the nugget zone. Ageing of the FSP alloy is conducted at different temperatures and times for continuous and discontinuous precipitation of the β-phase. In the as-FSP condition, significant yield asymmetry in the processing and transverse directions is observed due to strong basal texture produced in the processing direction. The yield asymmetry is found to get reduced upon ageing and minimized under optimized ageing conditions. Microscopic investigation of continuous and discontinuous β-precipitates is conducted using optical microscopy, SEM and TEM techniques and their effect on the directional yield asymmetry is discussed.