Anomalous Plasticity in MoSi_2 Single Crystals and its utilization

在1995年，我们首次报道了即使在室温这样低的温度下，偏离[001]取向的MoSi_2-x-h单晶也可能发生塑性流动，而接近[001]取向的单晶只有在1300 ℃以上的温度下才能在宏观尺度上发生塑性变形。我们首先研究了在MoSi_2单晶中所观察到的这种偏离[001]取向的反常塑性是否在其它具有C11_b结构的过渡金属双硅化物如WSi_2和（Mo，W）Si_2中也能观察到，具有C_（40）结构的化合物如Mo（Al，Si）_2、VSi_2、NbSi_2、TaSi_2和具有C_（54）结构的CrSi_2、TiSi_2。其次，我们研究了利用MoSi_2的反常塑性进行结构变形的可能途径。四种不同C_（40）二硅化物的变形实验结果表明：（1）VSi_2、NbSi_2和TaSi_2只有在（0001）上的滑移沿着进行时才可能发生塑性流动<1210>，并且在整个温度范围内没有观察到其它滑移系;（2）VSi_2，NbSi_2和TaSi_2中的基滑移是以常规方式发生的，而CrSi_2和Mo（Si，Al）_2中的基滑移是通过同步剪切机制发生的。（3）NbSi_2中的塑性流动起始温度为500 ℃，在1400 ℃预应变后再低温变形时，NbSi_2中的基滑移临界切应力降低了300 ℃。（4）NbSi_2中基滑移临界切应力随温度升高而降低，利用MoSi_2的异常塑性的一种方法是形成MoSi_2基复合材料，NbSi_2和TaSi_2形成垂直于MoSi_2中解理面的界面。

IN 1995 we first reported that plastic flow is possible even at temperatures as low as room temperature for single crystals of MoSi_2 wirh orientations away from[001], while single crystals with orientations near [001] can be plastically deformed on a macroscopic scale only at temperatures above 1300ﾟC.In this study, we first examined whether such anomalous plasticity oberved in MoSi_2 single crystals with orientations away from [001] is observed in other transition metal disilisides with the C11_b structure such as WSi_2 and (Mo, W) Si_2, those with the C40 structure such as Mo(Al, Si)_2, VSi_2, NbSi_2, TaSi_2 and CrSi_2, TiSi_2 with the C54 structure. Secondly, we investigated possible ways to utilize the anomalous plasticity for structural spplications of MoSi_2.Deformation experiments of four different C40 disilicides result in the following findings : (1) plastic flow of VSi_2, NbSi_2 and TaSi_2 is possible only when slip along <1210> on (0001) is operative and no other slip systems are observed in the whole temperature range investigates, (2) basal slip in VSi_2, NbSi_2 and TaSi_2 occurs in a conventional manner unlike in CrSi_2 and Mo (Si, Al)_2 in which the corresponding slip occurs through a synchroshear mechanism, (3) while the onset temperature for plastic flow in NbSi_2 is 500ﾟC for as-grown crystals, it is lowered by 300ﾟC when the crystal is prestrained at 1400ﾟC and then re-deformed at low temperatures, (4) the critical resolved shear stress (CRSS) for basal slip in NbSi_2 decreases with increasing temperature, but exhibits a moderate peak around 1300ﾟC.One way to utilize the anomalous plasticity of MoSi_2 is to form MoSi_2-base composites with NbSi_2 and TaSi_2 forming interfaces perpendicular to cleavage planes in MoSi_2.