Plastic deformation of single crystals of Mo(Si, Al)2with the C40 structure

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Abstract
The deformation behaviour of single crystals of Mo(Si, Al)2 with the hexagonal C40 structure, which are formed through alloying MoSi2 with Al, has been studied as a function of crystal orientation and Al content in the temperature range from room temperature to 1500°C in compression. Plastic flow is possible only above 1100°C for orientations where slip along 〈1120〉 on (0001) is operative and no other slip systems are observed in the whole temperature range investigated. These are in contrast with the observation in single crystals of MoSi2 with the tetragonal C11b structure that plastic flow is observed from temperatures as low as room temperature where two different slip systems are operative. The critical resolved shear stress for basal slip in Mo(Si,Al)2 decreases rapidly with increasing temperature and the Schmid law is valid for this slip. Basal slip appears to occur through a synchroshear mechanism. A dislocation model is proposed for the synchroshear mechanism, in which a dislocations (b = ⅓〈1120〉) dissociate into two synchro-partial dislocations with the same Burgers vector (b = ⅙〈1120〉) and each synchropartial dislocation further dissociates into two partials on two adjacent planes. The difficulty for the operation of slip systems other than basal slip in Mo(Si,Al)2 as well as mechanisms of C40—C11b transformations are discussed in terms of the dislocation model proposed.

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