Microstructure and mechanical behavior of friction stir processed ultrafine grained Al–Mg–Sc alloy

Twin-roll cast (TRC) Al–Mg–Sc alloy was friction stir processed (FSP) to obtain ultrafine grained (UFG) microstructure. Average grain size of TRC alloy in as-received (AR) condition was 19.0 ± 27.2 μm. The grain size reduced to 0.73 ± 0.44 μm after FSP. About 80% of the grains were smaller than 1 μm in FSP condition. FSP resulted into 80% of the grain boundaries to have high angle grain boundary (HAGBs) character. Uniaxial tensile testing of UFG alloy showed an increase in yield strength (YS) and ultimate tensile strength (UTS) (by ∼100 MPa each) of the alloy with a very marginal decrease in total and uniform elongation (total – 27% in AR and 24% in UFG and uniform – 19% in AR and 14% in UFG). A theoretical model predicted that the grain refinement cannot take place via discontinuous dynamic recrystallization. Zener pinning model correctly predicted the grain size distribution for UFG alloy. From work hardening behaviors in both the conditions, it was concluded that grain boundary spacing is more important than the character of grain boundaries for influencing extent of uniform deformation of an alloy.

► An equiaxed ultrafine grained microstructure was obtained via FSP.
► Very high fraction of high angle grain boundaries after FSP was obtained.
► FSP UFG alloy exhibited considerably high uniform elongation (∼14%).
► Observed grain size range in UFG alloy matched predicted Zener model values.
► Grain boundary spacing is a critical parameter for the work hardening behavior.