Shapes and velocity relaxation of dislocation avalanches in Au and Nb single crystals

We capture the spatiotemporal velocity dynamics of dislocation avalanches in face-centered cubic (FCC) gold and body-centered cubic (BCC) niobium crystals by compression testing of cylindrical microcrystals. In niobium, avalanche peak-velocities are over one order of magnitude lower, have distinctly rougher avalanche shapes, and relax more slowly to zero velocity than in gold. The avalanche dynamics (including averaged avalanche shapes) can be described reasonably well by mean-field predictions for avalanches near the depinning transition in the case of gold, but not for niobium. A detailed analysis shows consistent deviations (non-trivial exponents) from the predicted functional forms for both gold and niobium if the exponent that describes the velocity decay of the shape function is treated as a free parameter. While the avalanche size statistics and related scaling exponents are similar, these noticeable differences in the dislocation-avalanche dynamics demonstrate material-specific variations not predicted by "universal" behavior. The marked difference in avalanche shapes is discussed in terms of edge- and screw-dislocation mobility of an operating spiral arm source.