Generalized Stacking Fault Energy And The Properties Of Dislocations In NbCr₂ And HfCr₂

The Laves phase chromium-based alloys are considered as potential high temperature structural materials because of their excellent physical and chemical peoper ties. Their close-packed structure make them have high hardness, high melting points, good oxidation resistance, creep resistance and corrosion resistance. These excellent high temperature performance make them have a very promising development prospects. Nevertheless, their significant brittleness properties at the ambient temperature limits their widely applications. So it is necessary to improve their ductility. Alloying is considered to be an important way to improve their ductility.By adding the transition metal elements to change the electron concentration, crystal structure, and produce vacancies, so as to cause phase trasition, change dislocation and chemical bonding characteristics to improve their ductility.Generally, the ambient temperature brittleness is closely related to their plastic deformation characteristics which are controlled by their micro-defects structures, such as stacking faults and dislocations. Therefore, study the micro-defects structures of the intermetallic compounds to understand the plastic deformation mechanism and mechanical properties for improving the ductility.In this theories, we calculated the elastic constants and generalized stacking fault eneygies by using the first principles method. Discussed the mechanical properties of C14 and C15 Laves phases NbCr₂ and HfCr₂. In addition, we investigate the generalized stacking fault eneygy and dislocation properties of C14 and C15 NbCr2 and HfCr₂ alloyed V, Zr, Mo, Ta and W. The main contents are presented as follows :?1? We have calculated the generalized stacking fault eneygy curves along 1/2<110>and1/6<112>direction on {111} plane, considered the two kinds of transformation paths in combination with the obtained generalized stacking fault eneygies, we discussed their deformation mechanism. The synchroshear mechanism could be regarded as energetically more favourable?along the1/6<112> direction?. Based on the Griffith cleavage facture theory to calaulate the cleavage energy and combined with the unstable stacking fault energy to introduced on the Rice criterion to study influence of dopping alloy concentration of 2.8 at.% on brittle-plastic behaviours of NbCr₂ and HfCr₂. The results show that NbMo and TaHf, WHf can improve the ductility of NbCr₂ and HfCr₂. The core structure and Peierls stress of 1/6<112>{111} edge dislocation, screw dislocation and mixed dislocation in NbCr2 and HfC r2 with and without the alloying elements have been investigated within the improved Peierls-Nabarro?P-N? dislocation equation using the Foreman's method. The results show that the edge dislocation width is bigger than the screw width, but the Peierls stress of edge dislocation is smaller than the screw dislocation, which indictate that to move an edge dislocation is easier tha n screw dislocation, lead to the edge dislocation width increase. And that NbMo andHfTa,WHf can increase the core width and reduce the Peierls stress of NbCr2 and HfCr₂.?2? Based on synchroshear mechanism, the formation of intrinsic stacking fault and the shear slide in C14 Laves phase have been modeled. Through the first-principle theory calculated the generalized unstable stacking fault energy curves of C14 Laves phase along the1/3<1010>{0001}NbCr₂ and HfCr₂. The ideal C14 Laves phase through the synchroshear process, a complete C14 Laves phases structure turn into a part of C15 Laves phases structure. The obtained intrinsic stacking fault energy values were all negative which indicate that the C15 structure is more stable than C14 structure. By using the Rice criterion which based on the generalized unstable stacking fault energy and the surface energy, to study influence of dopping alloy concentration of 3.3 at.% on brittle-plastic behaviours of NbCr2 and HfCr₂. The results show that NbMo and TaHf, WHf can improve the plasticity of NbCr2 and HfCr₂. The core structure and the Peierls stress of 1/3<1010>{0001}edge dislocations in NbCr2 and HfC r2 with and without the alloying elements by adopting the improved P-N dislocation equation and using the Foreman's method to solved the edge dislocation. The results show that NbMo andHfTa,WHf can increase the core width and reduce the Peierls stress of NbCr2 and HfCr₂.