Plastic behaviour of 4H-SiC single crystals deformed at temperatures between 800 and 1300 °C

We studied the mechanical behaviour of 4H-SiC single crystals tested under compression at temperatures between 800 degrees C and 1300 degrees C. The tests were conducted at a constant strain rate ((epsilon) over dot = 1.5 x 10(-5) s(-1) and (epsilon) over dot = 3.0 x 10(-5) s(-1)) and constant load (creep) in a stress range between 28 and 110 MPa. Transmission electron microscopy (TEM) observations allowed us to correlate the results of the mechanical tests with the dislocation microstructure of deformed specimens. Both the mechanical results and the TEM observations revealed the existence of a change in deformation mechanisms at a critical temperature T-c similar to 1000 degrees C. The parameters characterizing the deformation mechanisms - the stress exponent and the activation energy - were evaluated from the mechanical tests at temperatures above and below T-c. The value obtained for the stress exponent was n = 2.8 +/- 0.8 in the tests at constant strain rate and T > T-c, and n = 2.9 +/- 0.3 in the creep tests over the entire temperature range. The constant strain rate tests gave activation energies of Q = 0.7 +/- 0.3 eV for T < T-c and Q = 3.3 +/- 0.6 eV for T > T-c, and the corresponding values given by the creep tests were between Q = 1.4 +/- 0.1 eV and Q = 2.3 +/- 0.6 eV for T < T-c, and Q = 3.5 +/- 0.7 eV for T > T-c. The TEM observations showed that at T > T-c most basal dislocations were dissociated into two partial dislocations that glide simultaneously, leaving only a small stacking fault gamma between them. In contrast to this behaviour, at T < T-c the deformation takes place by the glide of just one of the partials-that of lower activation energy (leading partial) with a core of Si atoms, without the twin partial of higher activation energy (trailing partial) and C atom core having been nucleated. The glide of the leading partial leaves behind it a large stacking fault zone of the glide plane. In both temperature ranges, the slip traces are long and straight along simple crystallographic directions, suggesting that the movement of the dislocations is controlled by the Peierls mechanism over the entire temperature range studied. The results of the mechanical tests, together with optical microscopy of the lateral faces of the deformed specimens and TEM observations are used as a basis for an analysis and discussion of the most significant features of the plasticity of the material and of the deformation mechanisms operating in the two temperature ranges. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.