  We demonstrate that plastic deformation in solids is associated with a
dynamic transition that is reminiscent to the transition from a superconducting
to a mixed phase in Type II superconductors. We report analytic calculations,
extensive molecular dynamics and sequential umbrella sampling Monte Carlo
simulations of a two dimensional triangular crystalline solid undergoing
plastic deformation under strain. The solid consists of particles connected by
harmonic springs. Non-affine displacement fluctuations of the solid are
enhanced using an external field, causing a rich deformation behaviour. The
external field leads to a mixed phase, where defect and stress-free
crystallites are surrounded by a network of amorphous regions with large local
internal stress --- a "stress Meissner" effect. The transition shows slow
ageing dynamics caused by the presence of many competing, non-crystalline
free-energy minima. Under uniform uniaxial strain, this amorphization
transition gives rise to irreversible plastic deformation with the amorphous
regions appearing as band-like structures. Our results may be checked by
careful experiments on colloidal crystals using holographic optical tweezers.
