  Several experiments show that crystalline solids deform in a bursty and
intermittent fashion. Power-law distributed strain bursts in compression
experiments of micron-sized samples, and acoustic emission energies from
larger-scale specimens, are the key signatures of the underlying critical-like
collective dislocation dynamics - a phenomenon that has also been seen in
discrete dislocation dynamics (DDD) simulations. Here we show, by performing
large-scale two-dimensional DDD simulations, that the character of the
dislocation avalanche dynamics changes upon addition of sufficiently strong
randomly distributed quenched pinning centres, present e.g. in many alloys as
immobile solute atoms. For intermediate pinning strength, our results adhere to
the scaling picture of depinning transitions, in contrast to pure systems where
dislocation jamming dominates the avalanche dynamics. Still stronger disorder
quenches the critical behaviour entirely.
