  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.
