Effect of strain on NiTi surface-optical reflectivity

Abstract
The mechanical interaction of living cells upon a NiTi surface was simulated with external loads. The surface structural response was studied. Optical reflectivity, which is a sensitive indicator of structural changes at the surface, was used. The behaviour of reflectivity under tension and compression within the range of strains up to 5% was analysed. The probability of reversible martensitic transformations (MT) in the surface oxide layer was demonstrated. The peculiarities of these MT in comparison with those in the bulk of the alloy were considered. Irreversible structural changes occurred in surface oxide under external stresses, despite complete strain recovery after unloading. A correlation emerged between the strains caused by external stresses and the residual augmentation of reflectivity. The probable mechanisms of irreversible structural changes initiated at the NiTi surface by external load action are discussed.