Tensile deformation and fracture behaviour in NbSi2 and MoSi2 single crystals

Tensile deformation behaviour in NbSi2 and MoSi2 single crystals with the C40 and C11(b) structure, respectively, was examined in a temperature range up to 1500 degreesC and compared with that under compression, focusing on the temperature dependence of the critical resolved shear stress (CRSS), elongation/fracture strain and crack initiation process. Two different types (Type A and Type B) of geometry of NbSi2 tensile specimens were used to evaluate elongation and fracture behaviour. No difference in the CRSS for the {0001}[2 (1) over bar(1) over bar0] slip of NbSi2 and that for the {011)<100] slip in MoSi2 was found in both modes of tension and compression. In contrast, the ductile-brittle transition temperature (DBTT) depended strongly on the loading mode and the type of geometry. Elongation and process for crack initiation in NbSi2 changed remarkably depending on the deformation temperature. Four new slip systems of {10 (1) over bar0}1/3[1 (2) over bar 10], {10 (1) over bar0}1/3[1 (2) over bar 10], {10 (1) over bar0}[0001] and {11 (2) over barY}[112X](XxY= -6) were found near the crack path in NbSi2 deformed at 1000degreesC. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.