Mechanical characterization of surface modified Ti-based alloy before and after severe plastic deformation

Abstract

The intent of the study was to modify virgin biomedical Ti-13Nb-13Zr alloy in aim to realize whether applied modifications could improve mechanical properties. Two groups of modifications were applied to the prepared samples: structural modification achieved by severe plastic deformation (SPD) and modification of alloy surface obtained by electrochemical method. Structural modification was performed by using high pressure torsion (HPT) as one of the SPD methods. Pressure of 4.1 GPa was applied and up to 5 rotations were made at room temperature. After HPT treatment an ultrafine-grained (UFG) structure was obtained. The homogeneity of the UFG alloy was checked by using Vickers microhardness tester. Electrochemical anodization was done in the 1M H3PO4 + NaF electrolyte during 90 minutes, resulting in formation of an oxide layer on the alloy surface. Surface roughness was determined using an atomic force microscope. Tensile properties of the alloy before and after electrochemical anodization and HPT processing were determined. The scanning electron microscopy was used to characterize both morphology of anodized surfaces and the fracture surfaces after the tensile tests. The HPT treated alloy has higher values of tensile strength and modulus of elasticity and lower value of plasticity than the virgin alloy. Surface modification of virgin alloy leads to a decrease of these values compared to non-treated samples and practically has no influence on tensile properties on the UFG alloy. Mechanical properties of the surface – modulus of elasticity and nanohardness were determined using a nanoindentation test. The surfaces of anodized alloys (both virgin and HPT treated) have lower modulus of elasticity than surfaces of non-anodized ones.