The flow stress of the wide band-gap semiconductor, 4H-SiC, has been measured by uniaxial compression tests over the temperature range 500–1400 °C at different strain rates. At low strain rates, 4H-SiC shows a transition in the yield stress at a temperature Tc.
In addition, the brittle-to-ductile transition (BDT) temperature TBDT
of the same material has been determined on precracked samples at different values of strain rate
by 4-point bend tests. Intriguingly, the transition temperature Tc
in yielding is very close to the brittle-to-ductile transition temperature TBDT
in the fracture behavior. In previous transmission electron microscopy (TEM) investigations, significant microstructural differences were found between low-temperature (T<Tc)
and high-temperature (T>Tc)
deformed crystals. There, the results showed that in the samples deformed below the transition temperature Tc,
deformation proceeds by the generation and motion of single leading partial
dislocations on different (0001) planes. Moreover, all the partials appeared to have the same core, silicon. On the other hand, at temperatures above Tc,
the samples deformed by the generation and motion of perfect
dislocations dissociated in the form of leading/trailing partial pairs separated by a ribbon of stacking fault. Based on the present mechanical tests and previous TEM results—together with experimental evidence from other semiconductors—a model is presented in which Tc
are identified and correspond to the temperature at which crystal shear takes place by different dislocation types. Below Tc,
single leading partials are responsible for crystal shear, whereas above Tc,
perfect dislocations (i.e., leading/trailing pairs) accomplish the slip. Since generation of a leading partial from crack tip sources basically shuts the sources off, the crystal remains brittle below this transition temperature. At Tc=TBDT,
trailing partials are nucleated from the same sources to clean up the stacking fault and allow multiplicative generation of dislocation avalanches resulting in transition to the ductile mode. © 2003 American Institute of Physics.