Displacive Phase Transformation and Surface Effects Associated with Confocal Laser Scanning Microscopy

The influence of the free surface on martensitic transformation was examined by comparing the highest temperature at which martensite forms (M_S), as measured using dilatometry, with surface observations using confocal laser scanning microscopy. It is found that the proximity of the surface during confocal microscopy permits martensitic transformation to occur at a higher temperature with a reduced free energy change. This is because the strain energy from the shape deformation accompanying the growth of martensite is reduced at a free surface. The second observation is that plates of martensite tend to coalesce as they approach the free surface where there is reduced constraint. The general observations are backed by calculating the strain energy caused by a subsurface edge dislocation as a function of the orientation of its Burgers vector relative to the free surface.