Transformation- and twinning-induced plasticity in phase-separated bcc Nb-Zr alloys: an atomistic study

Abstract

Several high-temperature body-centered cubic (bcc) structural materials such as Nb-, Zr- and Ti-based alloys undergo phase separation, which is a second-order phase transformation, whereby the host lattice decomposes into distinct bcc domains with different compositions. Using atomistic simulations, we studied the high-strain-rate response of bcc-forming Nb–xZr (x = 0, 25, 50 at.%) alloys. To induce phase separation in our starter alloy, we first employed hybrid Monte Carlo/Molecular Dynamics simulations in single crystals of Nb–xZr at 1000 K. Subsequently, these crystals were deformed along different crystallographic orientations ($$\langle 001\rangle$$ ⟨ 001 ⟩ , $$\langle 110\rangle$$ ⟨ 110 ⟩ and $$\langle 111\rangle$$ ⟨ 111 ⟩ ) at a strain rate of $$10^{+8} s^{-1}$$ 10 + 8 s - 1 , to investigate orientation dependent mechanical response. The phase-separated Nb–xZr microstructures exhibited distinct bcc domains enriched in either Zr or Nb. Notably, Nb-50 at.%Zr contained coarser Zr-domains compared to Nb-25 at.%Zr. The Zr-rich domains acted as “soft” inclusions, resulting in reduced peak strengths in the following order: pure Nb (Nb-0 at.%Zr) > Nb-25 at.%Zr > Nb-50 at.%Zr. This implies that phase separation causes softening in Nb–xZr. We also discovered two deformation pathways that depended on the crystallographic orientation: (i) For deformation along $$\langle 110\rangle$$ ⟨ 110 ⟩ and $$\langle 111\rangle$$ ⟨ 111 ⟩ directions: Elastic deformation was followed by dislocation plasticity on $$\{110\}\langle 111\rangle$$ { 110 } ⟨ 111 ⟩ slip systems; and (ii) For deformation along $$\langle 001\rangle$$ ⟨ 001 ⟩ direction: Elastic deformation was followed by the formation of a volumetric fcc structure, twinning on {112}$$\langle 111\rangle$$ ⟨ 111 ⟩ system, and the formation fcc-phase at the twin/matrix interfacial regions. This was ultimately accompanied by dislocation plasticity on $$\{110\}\langle 111\rangle$$ { 110 } ⟨ 111 ⟩ slip system. The bcc$$\rightarrow$$ → fcc displacive transformation facilitated {112}$$\langle 111\rangle$$ ⟨ 111 ⟩ twinning when Nb–xZr was deformed along $$\langle 001\rangle$$ ⟨ 001 ⟩ . Our investigation shows that softening of bcc alloys can result from a coupling of mechanisms involving local solute segregation, displacive phase transformation and twinning occurring across multiple slip planes.