The brittle-ductile transition in silicon

Modelling the upper yield point and the brittle–ductile transition of silicon wafers in three-point bend tests

“…Each intercept in the Arrhenius plots corresponds to a characteristic distance the dislocations have to move to nucleate the F o r P e e r R e v i e w O n l y 3 sources. These models are supported by experiment and computer simulation [8,9]. The remarkable sharpness of the transition in crystals with few initial dislocations is attributed to the sources, once nucleated, being highly stressed for dislocation emission.…”

“…In the precracked specimen the crack has to be shielded before the applied stress reaches the value needed to cause brittle fracture. In both cases the plasticity is brought about by dislocation avalanches produced by new sources nucleated by the motion of dislocations either to the surface (present experiments) or to or along the precrack [8,9]. As in the case of precracked specimens the nucleation of the sources is controlled by dislocation velocity, and in both cases the dislocations move a distance characteristic of the experimental and specimen conditions, resulting in "structure sensitivity" of the BDT.…”

“…This dynamic simulation model is similar to that used previously in treating emission from loaded crack tips (e.g. [8]), except that the stress fields here are different. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 results show that the primary dislocations seem to reach the tensile surface before the "cross-slip" dislocations, while this is not the case in the computer simulation.…”

“…The observed transitions are sharp and the activation energy controlling the strain-rate dependence of the transition temperature (T c ) was found to be close to that controlling dislocation velocity (for reviews see [8,9]). The intercepts of the Arrhenius plots however vary greatly depending on the test geometry, crystal growth technique, procedure of introducing the crack, and composition.…”

“…This suggests that although the temperature dependence in all cases is controlled by dislocation velocity, T c at any given strain-rate is structure sensitive. Two different models were suggested, applicable to different geometries, in which the critical event in a constant strain-rate test is the nucleation of crack tip sources, which, in the high stress field near the crack tip, rapidly emit avalanches of dislocations sufficient to shield the crack completely before the applied stress intensity factor reaches K IC for brittle fracture [8,9]. In these models it is the nucleation of crack tip sources by gliding dislocations which controls the strain-rate dependence of T c .…”