| 其他摘要 | Research on Ti-Al alloys had been conducted for many years because of their wide application as materials of major importance in the aerospace and automotive industries. However, development of TiAl alloys for industrial application has been greatly retarded by their brittleness at room temperature. Therefore, the phase transformation of TiAl↔Ti3Al is very important and attract wide interests of investigation in recent years because the two-phase alloys (γ-TiAl + α2-Ti3Al) were found possessing the improved ductility (toughness) than the single phase. It is obvious that the composition of substrates will affect this phase transition process, but up to now, the detailed information of the component discrepancy in the TiAl↔Ti3Al transition is not clear yet. The effect of the dislocation and the interface should also be identified. Besides, the effect of Nb additions on the TiAl mechanical properties is a great controversy. To reveal these problems, both the first-principles and the molecular dynamics simulations were combined to investigate the effects of the composition on the phase transformation of TiAl↔Ti3Al and its corresponding dynamic deformation process.
Firstly, the energy-strain curves of the different dislocation initiation and gliding behaviors in TiAl and Ti3Al were studied to reveal the easy-going deformation modes. It was found that the energy barriers decrease in the sequence of 1/2[ ], 1/2[ ], 1/6[ ], 1/6[ ]. The 1/2[ ] dislocation is very stable and hard to be induced, while 1/2[ ] is tend be decomposed into two 1/6< > partial dislocations. The 1/6[ ](111) in TiAl and 1/3 [10 0](0001) in Ti3Al are easier to induce than other dislocations, but they are metastable and tend to continue further transformation. It’s the directional p-d bonding lead to the higher shearing barrier in TiAl. The improved mechanical properties of the duplex phases may partly due to the reduction of Al-Ti directional covalence in sheared TiAl and the strengthened Al-Ti interactions in sheared Ti3Al,
Secondly, aimed to reveal the role of composition during the phase transition in TiAl alloys, the shear deformation simulation were systematically conducted on the fct and hcp models with different Ti:Al ratios using the first-principles method. The formation energy calculation shows that the stabilities of fct and hcp phases are similar when the Ti:Al approximately close to 1.5. The energy-strain curves show that there is a common composition soften region (Ti:Al=1.25～2). Within the region, both the shearing energy barriers of FCC→HCP and FCC←HCP are low. But the transformation of HCP→FCC is preferential out of the region. Further, a crucial stress induced phase transition mechanism was proposed based on the previous result.
Then, to investigate the dislocation and the interface behaviors in L10-TiAl, the dynamic and static deformation process along [ ] direction were studied combining the MD and ab initio methods. The MD result presented the detailed structure transition process, such as the initiation of the SISF, the subsequent formation of TWIN and HCP faults, and finally the partly transformation of HCP to TWIN. The static study of the energy variation at the different fault transition stages revealed that, it is the cooperation of the energy barrier and the stacking fault energy governs the deformation modes and the dislocation behaviors.
Lastly, the effect of different Nb additions (0～20.85 at.%) on the TiAl mechanical properties were systematically studied using the first-principles method. All the Nb additive configurations were found possessing the less stability and bigger c/a than those of the pure TiAl. The deformation related energy-strain and energy barrier versus Nb addition curves reveals that Nb has a staggered strengthening effect on TiAl mechanical properties. The analysis of the charge density difference and the partial density of states revealed that, when the two adjacent Nb is about two (111) layers away or more than that, there is a weak strengthening effect coming from the enhanced covalent Nb-Ti and Nb-Al bondings in the (111) plane. However, when the Nb addition increased so that adjacent Nb occurred in the nearest (111) layers, there is a strong strengthening effect coming from both the (111) inter-plane and inter-layer.
Our research presents a compelling answer to the above motioned problems in the stress induced phase transition at room temperature. The study of the dynamical structural evolution and large scale alloying effect overcome the experimental limits. The results provide some predictions for further studies, and are beneficial to understand the related experimental phenomenon. |
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