Orientation-Resolved Nanoindentation of Single Crystal Ti-15Mo: Inverted Elastic Anisotropy and Yield-Onset Behavior Beyond Simple CRSS Predictions
Ti-15Mo (wt%) is a well-established $β$-stabilized alloy whose mechanical response is strongly governed by phase stability and deformation mechanisms that operate at the single-grain scale. Because these intragranular mechanisms are intrinsically anisotropic, understanding the orientation dependence of the elastic and plastic response is critical to the texture-engineered processing of wrought components. Here, we quantify the nanomechanical anisotropy of single-crystal Ti-15Mo by combining EBSD with nanoindentation to correlate hardness, indentation modulus, strain rate sensitivity, and indentation yield response with the crystallographic surface normal near $\langle111\rangle,\langle124\rangle,\langle011\rangle,\langle023\rangle$, and $\langle013\rangle$ directions. While the modulus anisotropy is small (~10%), contrary to the prediction for a cubic crystal with A>1, orientations near $\langle111\rangle$ exhibit the lowest moduli. Yield onset is likewise softest near $\langle111\rangle$ and spans 41%, while hardness spans only 6.6% and follows neither the modulus nor the yield ordering. The alloy also exhibits yield behavior not captured by the simple CRSS/Schmid-factor models evaluated here. Strain-rate-jump tests yield V*=18-31b$^3$, consistent with kink-pair-nucleation-controlled plasticity; based on V*, the orientations separate into two groups suggestive of glide-dominated versus twin/transformation-participating response. We reveal an inversion of the elastic anisotropy expected for a homogeneous cubic $β$-phase and an orientation-dependent yield-onset response that is not reproduced by the simple Schmid-factor models examined here. Retained athermal $ω$ is proposed as the contributor because $β\rightarrowω$ collapse occurs on {111}$_β$ planes and may provide anelastic compliance through the reversible $ω\rightarrowβ$ transformation.