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

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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.