Abstract
We propose a theoretical framework to predict the deformation mechanism of the γ-TiAl single crystal without lattice defects by combining the generalized stacking fault energy and the Schmid factor. Our theory is validated against an excellent testbed, the single crystal nanowire, by correctly predicting four major deformation mechanisms, namely, ordinary slip, super slip, twinning, and mixed slip/fracture observed during the tensile and compressive tests along 10 different orientations using molecular dynamics simulations. Interestingly, although lattice defects are not taken into account, the theoretical predictions match well with existing experiments on bulk specimen with only a few exceptions; the exceptions are discussed based on the size-dependent deformation mechanism in the presence of preexisting dislocation sources. We expect that the method in this paper can be generalized to study various ductile intermetallic crystals where conventional Schmid law does not hold well.
Highlights
Over the past few decades, TiAl intermetallic compound has attracted much attention because of its use in applications such as LPT blades in aircraft engines and turbine wheel in automobile engines enabled by its outstanding high temperature mechanical properties including good thermal and oxidation resistance1–4
In order to overcome the limited applicability of the Schmid law in the γ-TiAl crystal, we suggest a theory to predict the deformation mode of the crystal without lattice defects by combining generalized stacking fault energy (GSFE) surface and the Schmid factor accounting for the partial slips
We reveal that four major deformation mechanisms of ordinary slip, super slip, twinning, and mixed slip/fracture occur depending on the loading condition, and that the theoretical prediction matches with the deformation mode observed in molecular dynamics (MD) simulations for all loading conditions tested in the study
Summary
We extended the methodology to the intermetallic system involving more complex GSFE surface with more deformation modes. We note that the lattice friction resistance of both L10 crystal and FCC crystal is not significant because the dislocation core structure of both crystals involves stacking faults and is planar. The lattice friction resistance is not taken into account, the deformation mode prediction based on the minimum crystal stress criterion works well for FCC crystals and for L10 crystal of the TiAl alloy in this study. Because we only consider three directions toward saddle points of the GSFE surface in the Step-1 deformation (Fig. 3a), negative Schmid factors appear in our analysis (footnote of Table 2), which is different from the conventional usage of Schmid law on ordinary dislocations in FCC crystals. Prior to the loading tests, we equilibrate the specimen at 300 K by using NPT ensemble simulation
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