Characterization of the mechanical behavior of ultrafine- grained metals using digital image correlation

Abstract. Nanocrystalline and ultrafine-grained materials show many very interesting properties compared to the conventional coarse-grained materials. For example, their strength values are usually much higher but, on the other hand, their ductility and deformability can be significantly lowered by the grain size refinement processes. One of the methods used to refine the grain size of metals is severe plastic deformation (SPD), which usually produces highly oriented structures that are strong but may deform non-homogeneously. In this paper, the deformation behavior of severe plastic deformation processed AZ31 magnesium alloy and commercially pure 1070 aluminum were characterized in compression and torsion using mechanical testing and high speed photography with digital image correlation. The results show that the AZ31 magnesium alloy deforms fairly uniformly in compression until fracture and the strains calculated from the movements of the ends of the anvils and the Lagrange strain on the surface of the specimen describe the overall deformation quite well. For the 1070 UFG aluminum, the shear deformation localizes very rapidly and the average strains measured from the surface or calculated from the displacements of the bars underestimate the true strain in the shear bands significantly. Also, the deformation in the torsion tests was not uniform shear but substantial strains were also measured in the axial direction. These strains were most likely due to small experimental errors, such as slight misalignment of the bars and the specimen in the test.