Nanoscale Undulation of Elastic Fields During Deformation Twinning in FCC Metals

Finely twinned microstructures are widely observed in metals and alloys but the underlying formation mechanisms remain debatable. In particular, the role of internal stresses in promoting these inhomogeneous patterns is still not clear. By incorporating a geometrically nonlinear microelasticity theory into phase-field framework, we study the evolution of elastic fields resulting from the growing deformation twins (DT) at grain boundaries in fcc metals. Simulations in two model systems, i.e., Ni and CoCrFeMnNi (a high-entropy alloy), show that as the external applied stress increases, the internal elastic fields begin to develop undulations with stripelike patterns owing to the significant geometrical nonlinearity associated with DT. This elastic undulation, absent in linear modeling, is initially nonuniform inside the grain and becomes global and coarsened, exhibiting a characteristic wavelength of ~1-2 nm. The predicted elastic inhomogeneity leads to a stack of alternating crystal orientations favored by the undulating local stress fields. The resemblance of our predicted stress undulation and the stripelike patterns in experiments may suggest a universal mechanistic origin of the nanotwinned microstructures widely observed in deformation twinning and displacive transitions.