Plastic deformation of bulk and micropillar single crystals of Mo5Si3 with the tetragonal D8m structure

Abstract

The plastic deformation behavior of single crystals of Mo5Si3 with the tetragonal D8m structure has been investigated in compression as a function of crystal orientation and temperature (1200–1500°C) in the bulk form and as a function of crystal orientation and specimen size at room temperature in the micropillar form. The slip system of {112¯}<111> is identified to be the only one that operates at high temperatures above 1200°C in bulk crystals, while any plastic flow is not detected at room temperature in micropillar crystals. The critical resolved shear stress (CRSS) for {112¯}<111> slip at room temperature estimated from the extrapolation of the temperature dependence of CRSS obtained for bulk crystals is considerably higher than fracture stresses obtained for micropillar crystals, indicating that the room-temperature brittleness is due in principle to the difficulty in dislocation motion arising from the very high CRSS value. The value of fracture toughness evaluated with a chevron-notched micro-beam specimen with a notch plane parallel to (001) is 1.54 MPa•m1/2, which is comparable to those reported for other transition-metal (TM) silicides of the TM5Si3-type. The selection of {112¯} slip plane and the dissociation of the 1/2<111> dislocation on the slip plane are discussed based on generalized stacking fault energy (GSFE) curves theoretically calculated by first-principles calculations.