First-principles study of the ternary effects on the plasticity of upgamma -TiAl crystals

Taegu Lee,Seong-Woong Kim,Seunghwa Ryu,Ji Young Kim,Won-Seok Ko

doi:10.1038/s41598-020-77891-5

Abstract

We studied the effects of important ternary elements, such as Cr, Nb, and V, on the plasticity of upgamma -TiAl crystals by calculating the point defect formation energy and the change in the generalized stacking fault energy (GSFE) surface from first-principles calculations. For all three elements, the point defect formation energies of the substitutional defects are lower in the Ti site than in the Al site, which implies that substitution on the Ti site is energetically more stable. We computed the GSFE surfaces with and without a substitutional solute and obtained the ideal critical resolved shear stress (ICRSS) of each partial slip. The change in the GSFE surface indicates that the substitution of Ti with Cr, Nb, or V results in an increase in the yield strength because the ICRSS of the superlattice intrinsic stacking fault (SISF) partial slip increases. Interestingly, we find that Cr substitution on an Al site could occur owing to the small difference between the substitutional defect formation energies of the Ti and Al sites. In that case, the reduction of ICRSSs of the SISF partial slip and twinning would lead to improved twinnability. We discuss the implications of the computational predictions by comparing them with experimental results in the literature.

Highlights

We studied the effects of important ternary elements, such as Cr, Nb, and V, on the plasticity of γ-TiAl crystals by calculating the point defect formation energy and the change in the generalized stacking fault energy (GSFE) surface from first-principles calculations
For substitutional defect formation in Al sites, the ideal critical resolved shear stress (ICRSS) of superlattice intrinsic stacking fault (SISF) slip decreased by 30–47%, and the ICRSS of twinning decreased by 32–60%, as presented in Table 3 and Fig. 7
From the point defect formation energy calculation, we found that all ternary substitutional atoms are more stable in the Ti site than in the Al site

Summary

Theoretical background and methodology

Slip system and deformation mechanism of the gamma TiAl crystals. The gamma phase has an. For substitutional defect formation in Al sites, the ICRSS of SISF slip decreased by 30–47%, and the ICRSS of twinning decreased by 32–60%, as presented in Table 3 and Fig. 7. The GSFE change pattern is depending on the Al content in TiAl and the kind of the ternary atom affects the magnitude of GSFE changes These results are consistent with previous experimental papers.[13,14] Our prediction on the yield strength due to a ternary atom occupying a Ti site is consistent with existing experimental s tudies[10,11,12,13,14] that report a strengthening effect induced by ternary alloying elements. To explain the ductility increase in the duplex structure by ternary atoms, it is necessary to consider the gamma phase and the interfacial effects between the gamma phase and the alpha phase as well as microstructural effects, such as grain size and lamellar size and spacing

Conclusions

Additional information