  We consider a singularly-perturbed two-well problem in the context of planar
geometrically linear elasticity to model a rectangular martensitic nucleus in
an austenitic matrix. We derive the scaling regimes for the minimal energy in
terms of the problem parameters, which represent the {shape} of the nucleus,
the quotient of the elastic moduli of the two phases, the surface energy
constant, and the volume fraction of the two martensitic variants. We identify
several different scaling regimes, which are distinguished either by the
exponents in the parameters, or by logarithmic corrections, for which we have
matching upper and lower bounds.
