Grain refinement effects on the strain rate sensitivity and grain boundary sliding in partially solidified Al-5 wt%Mg alloy

This study investigated a mechanism of reducing solidification cracking susceptibility by grain refinement. Previously, the occurrence of grain boundary sliding (GBS) is implied as one mechanism, which is known for the characteristics of superplastic deformation.To verify GBS occurrence during solidification of an Al-5 wt%Mg alloy, the effects of refinement on the true stress-true strain curve and the effective power-law coefficient n_eff (the reciprocal of the strain rate sensitivity) were investigated in a partially solidified state. Furthermore, the contribution of GBS to deformation was examined using the index ξ_GBS defined in the research field of superplasticity. The results are following.(1) The grain refined structure elongation is greater than that in the coarse grain structure regardless of the solid fraction. Greater elongation is expected to result from both lower value of the coefficient n_eff and the higher contribution of the GBS, which provide more uniform deformation.(2) Maximum elongation of up to 20% was found at 480 °C (f_s: 0.939) in the grain refined structure. Based on the knowledge of superplastic behavior, the values of both n_eff≈2 and ξ_GBS≈70% at the temperature indicates superplastic-like deformation during the solidification, reducing solidification cracking susceptibility by grain refinement.