Crystallographic orientation dependence of mechanical properties in the superelastic Ti-24Nb-4Zr-8Sn alloy

Abstract

Nanoindentation and electron backscattered diffraction techniques are used to study the dependence of crystallographic orientation on mechanical properties in individual grains of the superelastic polycrystalline Ti-24Nb-4Zr-8Sn alloy. Strain recovery, elastic modulus, and hardness are evaluated from the force-displacement curves using spherical and Berkovich indenters. Experimental data are reported in a standard stereographic triangle covering all possible crystallographic directions of loading in the bcc-\ensuremath{\beta} structure of the Ti-24Nb-4Zr-8Sn alloy. Our experiments show that both spherical and Berkovich indenters are suitable for probing anisotropy of elastic modulus whereas spherical indenter is more appropriate for probing anisotropy of superelastic response. The highest indentation strain recovery is measured along $\ensuremath{\langle}001{\ensuremath{\rangle}}_{\ensuremath{\beta}}$ and the lowest one is observed along $\ensuremath{\langle}111{\ensuremath{\rangle}}_{\ensuremath{\beta}}$. Our inverse pole figure distribution of indentation strain recoveries is in good agreement with inverse pole figure distribution of compressive lattice distorsions calculated from the crystallographic model of martensitic transformation in \ensuremath{\beta} titanium alloys. This established pattern in compression is very different from that seen during tensile deformation, and both our experiments and calculations confirm the strong tension-compression asymmetry of the strain response in superelastic \ensuremath{\beta} titanium alloys. The indentation modulus also shows significant crystallographic anisotropy: ${E}_{[001]\ensuremath{\beta}}<{E}_{[101]\ensuremath{\beta}}<{E}_{[111]\ensuremath{\beta}}$. In contrast, orientation dependence is lost when plasticity has set in, and hardness becomes independent of crystallographic orientation due to the fact that multiple directions are prone to generate slip systems in the bcc-\ensuremath{\beta} structure.