Influence of plastic deformation and crystallographic texture on obtaining a stiffness gradient in Ti-30Nb-4Sn alloy by laser surface melting

This work is a study about the laser surface melting treatment in a Ti-30Nb-4Sn alloy as a method to create a stiffness gradient between surface and substrate. It was used cold rolled specimens with thickness reductions of 24, 36, 51, 68 and 85%. One sample with 85% of deformation undergone recrystallization heat treatment. It was carried out a study about the effects of cold deformation and recrystallization on the microstructure, texture evolution, elastic modulus and hardness. Microstructure of all samples showed to be composed of β and α" phases. Instrumented indentation (nanoindentation) was the technique used to measure hardness and elastic modulus. Hardness increased with deformation, while elastic modulus showed the opposite behavior. Recrystallization heat treatment caused elastic modulus and hardness to return to values of an undeformed specimen. The crystallographic texture was determined by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The 51% and 85% deformed samples showed similar texture components, in form of fibers: [302]α" || ND, [110]α" || ND and [001]α" || ND. The recrystallized sample showed two fiber texture components: [110] α" || ND and [203] α" || ND. The texture of β phase was only measured for the recrystallized sample, being found a [111]_β || ND texture component. Texture of recrystallized specimen was considerably stronger than as-deformed specimens. The decrease in elastic modulus after cold rolling was ascribed to dislocations and crystalline defects introduced by deformation. Nanoindentation measurements of deformed samples presented a significant difference in values between indentations performed on different cross sections (RD, TD and ND). This anisotropy was attributed to dislocation structures derived from cold deformation, rather than crystallographic texture. A stiffness gradient between surface and substrate was created in the work-hardened specimens after laser surface melting, showing higher elastic modulus in the treated surface. This effect was attributed to the reduction of dislocation density and other crystalline defects in the fusion and recrystallized zones. The recrystallized sample did not show any gradient of properties after laser surface melting. The EBSD mapping performed in the laser affected zone did not show any significant difference of texture between surface and substrate. (author)