  Needle-like twins are observed experimentally within the transition layer at
the martensite-twinned martensite interface. We utilize a phase-field approach
to investigate this microstructure. Our goal is to simulate the morphology of
the transition layer and to perform a detailed analysis to characterize its
interfacial and elastic micro-strain energy. To illustrate the micromechanical
framework developed for that purpose, sample computations are carried out for a
CuAlNi shape memory alloy undergoing the cubic-to-orthorhombic martensitic
transformation. A particular focus of the study is on size-dependent morphology
through examining the impact of twin spacing. Additionally, our results reveal
that certain twin volume fractions lead to the emergence of twin branching, as
a way to minimize the total free energy stored in the microstructure.
