Development of MoSi_2 based silicides as refractory materials operating at ultra-high temperatures and severe enviroment.

Cll_b、C40和D8_8结构的过渡金属硅化物是超高温材料的候选材料之一，可以在1500℃以上的高温下工作。选取了MoSi_2、WSI_2、CrSi_2、TaSi_2和Ti_5Si_3。研究了复合材料的塑性变形行为和强化机理，提出了提高塑性的途径。MoSi_2。晶体在900°C以上的温度下可以变形，变形模式主要为<331>(110)和<331>(103)滑移。在高温下，MOSi_2的变形是通过激活<100>和<110>的普通位错来完成的，这些位错倾向于形成节点。当温度高于1200℃时，普通位错的激活显著提高了塑性。研究了这些硅氧体的蠕变行为。这些硅氧体具有较高的蠕变活化能和良好的抗蠕变性能。从相稳定性的角度看，当1/4<111>位错运动在（110）平面上产生纯层错时，Cll_b结构晶体（110）平面上的原子排列与C40结构晶体（0001）平面上的原子排列完全相同。因此，层错的形成与cll_b结构相对于c40结构的稳定性密切相关。Cr对Mo的部分替代导致被层错分隔的1/4[111]超偏晶的活度大幅增加，这有望改善塑性。

As one of the candidates for ultra-high temperature materials operating at high temperatures more than 1500ﾟC, transition metal silicides with the Cll_b, C40 and D8_8 structures have been chosen. MoSi_2, WSI_2, CrSi_2, TaSi_2 and Ti_5Si_3 were selected among these silicides. In this work plastic deformation behavior and strengthening mechanism have been investigated and then some approach for ductility improvement have been carried out. MoSi_2. crystals can be deformed at temperatures more than 900ﾟC and the major mode of deformation is<331> (110) and<331> (103) slips. At higher temperatures deformation of MOSi_2 is accomplished by the activation of<100>and<110>ordinary dislocations which tend to form nodes. The ductility is remarkably improved with the activation of ordinary dislocations at temperatures above 1200ﾟC. The creep behaviour of these silisides has also been investigated. These silisides have high activation energy for creep and exhibit good creep resistance. From the view point of phase stability when pure stacking faults is created on (110) plane by the motion of 1/4<111>dislocation, the atomic arrangement on (110) planes in crystals with the Cll_b structure become to be identical to those on (0001) plane in crystals with the C40 structure. Therefore, formation of the stacking fault is closely related to the stability of the Cll_b-structure relative to the C40-structure. The partial replacement of Mo by Cr results in a substantial increase in the activity of the 1/4[111]superpartials separated by the stacking fault, which is expected to improve the ductility.