  We present a phenomenological time-dependent Ginzburg-Landau theory of
nonlinear plastic deformations in solids. Because the problem is very complex,
we first give models in one and two dimensions without vacancies and
interstitials, where large strains produce densely distributed slips but the
mass density deviations remain small except near the tips of slips. Next we set
up a two-dimensional model including a vacancy field (or local free-volume
fraction), where relevant is the sensitive dependence of the elastic shear
modulus on the vacancy density. In our simulation, if strains are applied to
nearly defectless solids but in the presence of such elastic inhomogeneity, the
vacancy density and the mass density can become considerably heterogeneous for
large strains on spatial scales much longer than the atomic size. These
strain-induced disordered states are metastable or long-lived once they are
created.
