  The plastic deformation of metal alloys localizes in the Portevin--Le
Chatelier effect in bands of different types, including propagating, or type
'A' bands, usually characterised by their width and a typical propagation
velocity. This plastic instability arises from collective dynamics of
dislocations interacting with mobile solute atoms but the resulting sensitivity
to the strain-rate lacks fundamental understanding. Here we show, by employing
high-resolution imaging in tensile deformation experiments of an aluminum
alloy, that the band velocities exhibit large fluctuations. Each band produces
a velocity signal reminiscent of crackling noise bursts observed in numerous
driven avalanching systems from propagating cracks in fracture to the
Barkhausen effect in ferromagnets. The statistical features of these velocity
bursts including their average shapes and size distributions obey predictions
of a simple mean-field model of critical avalanche dynamics. Our results thus
reveal a novel paradigm of criticality in the localization of deformation.
