Factors controlling ductility in ultrafine grain aluminum alloys under monotonic and cyclic loading

Ultrafine grained (UFG) Al processed by severe plastic deformation (SPD) methods are characterized by high strength and a limited uniform ductility. Friction stir processing (FSP), an offshoot of friction stir welding has emerged as a new technique to produce UFG alloys. FSPUFG Al alloys show a distinctly different behavior with higher uniform elongation and improved high cycle fatigue life as compared to other SPD processed variants. The objective of this work was to study the microstructural basis for such mechanical behavior in FSP-UFG Al alloys. To study the mechanical behavior of FSP Al alloys, a subsize bending fatigue and uniaxial tensile test beds were designed. These subsize test systems were validated using commercial alloys for which standard handbook test data are available. A commercial Al-Li alloy in different microstructural conditions was investigated using the subsize fatigue test bed. The results indicated that grain shape, texture, precipitate type and precipitate distribution controlled the high cycle fatigue life. To understand the mechanical behavior of UFG Al, an Al-Mg-Zn alloy was selected. The subsize high cycle fatigue tests conducted on this alloy show a significant difference in low angle boundary stability compared to their coarse grain counterpart. Key observations on the UFG AlMg-Zn alloy include: (a) Fatigue induced room temperature recrystallization. (b) Improved fatigue life at higher stresses (normalized) compared to corresponding coarse grained alloy in T6 condition. (c) Beneficial effect of distributed second phases on fatigue life through enhanced dislocation storage. Subsize tensile tests conducted on the UFG Al alloy suggests, that dislocation annihilation controls the deformation process. The experimental data indicated that such annihilation mechanisms were active even within the matrix. The role of low angle boundaries with consequent boundary diffusion was found to be significant in this regard. A diffusion based model proposed for UFG Al ductility fits the expected stress-strain trend.