Acta Metall Sin  2019, Vol. 55 Issue (5): 673-682    DOI: 10.11900/0412.1961.2018.00349
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First-Principles Study on the Impact of Antisite Defects on the Mechanical Properties of TiAl-Based Alloys
Zongwei JI1,2,3,Song LU3,Hui YU1,4,Qingmiao HU1(),Levente Vitos3,Rui YANG1
1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, SE-100 44, Sweden
4. School of Information Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
Abstract

Microalloying is an effective approach to improve the mechanical properties of TiAl-based alloys which have been applied as high-temperature structure materials. The antisite defects may be regarded as special alloying elements. However, the detailed information about the effect of antisite defects on mechanical behavior (full slip and twinning), which may be described theoretically by generalized stacking fault energy (GSFE), of TiAl-based alloys are scarce. In this work, the composition dependent GSFEs of off-stoichiometric γ-TiAl were calculated by using the first-principles exact muffin-tin orbitals method in combination with coherent potential approximation. With the calculated GSFE, the energy barriers for various deformation modes including twin (TW), ordinary dislocation (OD), and superlattice dislocation (SDI and SDII) were determined. The selection of the deformation mode under external shear stress with various directions was analyzed. The effects of the TiAl and AlTi antisite defects on the mechanical properties of γ-TiAl were then discussed. The results showed that the TiAl antisite defect decreases the energy barrier for the TW deformation leading by the superlattice intrinsic stacking fault (SISF) partial dislocation and increases the angle window of the applied shear stress within which TW deformation may be activated. Therefore, TiAl antisite defect is expected to improve the plasticity of γ-TiAl. The effect of AlTi antisite defect is opposite. The AlTi antisite defect decreases the energy barriers for the OD and SDII deformations leading by complex stacking fault (CSF) partial dislocation and increases their operating angle window, indicating that AlTi facilitates the slip of OD and SDII. Considering that the energy barrier for CSF is much higher than that for SISF, the plasticity induced by OD and SDII should be lower than that induced by TW. Calculations in this work explain the experimental finding that TiAl antisite defect improves the plasticity of γ-TiAl more significantly than AlTi antisite defect.

 ZTFLH: TG146.2
Fund: National Basic Research Program of China(2014CB644001);National Key Research and Development Program of China(2016YFB0701301)
Corresponding Authors:  Qingmiao HU     E-mail:  qmhu@imr.ac.cn
 Fig.1  Schematic for the slip/twinning systems on (111) plane of L10-TiAl, together with Burgers vectors for superlattice dislocation (SD, [10$\bar{1}$], [$\bar{1}$01] and $\frac{1}{2}$[11$\bar{2}$], named SDI, SDII and $\frac{1}{2}$[11$\bar{2}$] SD), ordinary dislocation (OD, $\frac{1}{2}$[1$\bar{1}$0]) and twinning (TW, $\frac{1}{6}$[11$\bar{2}$]) and the lowest energy full-dissociation paths. The different sizes of circles for atoms (red for Ti and blue for Al) stand for three consecutive (111) layers, marked by A, B and C. The red and blue backgrounds correspond to the superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF) leading paths, respectively. τ is the projection of external stress on the (111) plane (gray arrow). θ is the angle measured from [11$\bar{2}$] direction to τ. APB stands for the antiphase boundary Fig.2  Generalized stacking fault energies (γGSFE) for the SISF partial dislocation leading deformation models (TW and SDI) of TiAl, Ti(Al0.9Ti0.1) and (Ti0.9Al0.1)Al alloys (γSISF and γUSISF, γCSF and γUCSF, γTW and γUTW, γAPB and γUAPB stand for the stable and unstable stacking fault energy for SISF, CSF, TW, and APB, respectively)Color online Fig.3  γGSFE for the CSF partial dislocation leading deformation models (OD and SDII) of TiAl, Ti(Al0.9Ti0.1) and (Ti0.9Al0.1)Al alloys Table 1  Calculated stacking fault energies of stoichiometric γ-TiAl, together with previous ab initio and experiment results Fig.4  Calculated γSISF (a), γCSF (b), γAPB (c) and γTW (d) of Ti(Al1-mTim) and (Ti1-mAlm)Al against concentration of antisite defects (m) (The Ti-rich and Al-rich cases are separated with the vertical dashed line at zero) Fig.5  Calculated γUSISF, γUCSF, γUAPB and γUTW of Ti(Al1-mTim) and (Ti1-mAlm)Al against m (The Ti-rich and Al-rich cases are separated with the vertical dashed line at zero) Fig.6  Slip energy barrier (γEB) of various deformation modes in γ-TiAl alloys as a function of m (The Ti-rich and Al-rich cases are separated with the vertical dashed line at zero) Fig.7  Effective slip energy barriers ($γˉ$) of the leading partial dislocations (SISF and CSF) as functions of the direction of the shear stress (θ ) (The critical angles for the transition from SISF to CSF are represented by the vertical lines) Fig.8  $γˉ$ for the SISF partial dislocation leading deformation models TW and SDI as functions of θ (The critical angles for the transitions from TW to SDI and from SISF to CSF are represented respectively by the vertical dashed and solid lines) Fig.9  $γˉ$ for the CSF partial dislocation leading deformation models OD and SDII as functions of θ (The critical angles for the transitions from OD to SDII and from SISF to CSF are represented respectively by the vertical dashed and solid lines)