Surface-Induced Phase Transformations: Multiple Scale and Mechanics Effects and Morphological Transitions

Strong, surprising, and multifaceted effects of the width of the external surface layer Delta(xi) and internal stresses on surface-induced pretransformation and phase transformations (PTs) are revealed. Using our further developed phase-field approach, we found that above some critical Delta*(xi), a morphological transition from fully transformed layer to lack of surface pretransformation occurs for any transformation strain epsilon(t). It corresponds to a sharp transition to the universal (independent of epsilon(t)), strongly increasing the master relationship of the critical thermodynamic driving force for PT X-c on Delta(xi). For large epsilon(t), with increasing Delta(xi), X-c unexpectedly decreases, oscillates, and then becomes independent of epsilon(t). Oscillations are caused by morphological transitions of fully transformed surface nanostructure. A similar approach can be developed for internal surfaces (grain boundaries) and for various types of PTs and chemical reactions.