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

MM Hasan and SG Srinivasan and D Choudhuri, JOURNAL OF MATERIALS SCIENCE, s10853-023-09078-y (2023).

DOI: 10.1007/s10853-023-09078-y

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 (< 001 >, < 110 > and < 111 >) at a strain rate of 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 < 110 > and < 111 > directions: Elastic deformation was followed by dislocation plasticity on 110 < 111 > slip systems; and (ii) For deformation along < 001 > direction: Elastic deformation was followed by the formation of a volumetric fcc structure, twinning on 112< 111 > system, and the formation fcc-phase at the twin/matrix interfacial regions. This was ultimately accompanied by dislocation plasticity on 110< 111 > slip system. The bcc ->.fcc displacive transformation facilitated 112< 111 > twinning when Nb-xZr was deformed along < 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.

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