  We present a dynamical theory of a two-dimensional martensitic transition in
an elastic solid, connecting a high-temperature phase which is nondegenerate
and has triangular symmetry, and a low-temperature phase which is triply
degenerate and has oblique symmetry. A global mode-based Galerkin method is
employed to integrate the deterministic equation of motion, the latter of which
is derived by the variational principle from a nonlinear, nonlocal
Ginzburg-Landau theory which includes the sound-wave viscosity. Our results
display (i) the phenomenon of surface nucleation, and (ii) the dynamical
selection of a length scale of the resultant patterns.
