STRUCTURE FRAGMENTATION IN A SUBSURFACE NICKEL TITANIUM LAYER CAUSED BY ITS IRRADIATION WITH PULSED SILICON ION FLUXES

摘要

Using the methods of electron backscatter diffraction, an investigation of variations in microstructure of the subsurface nickel titanium layer after its irradiation with pulsed, medium-energy silicon ion fluxes is performed. It is shown that following this ion-beam irradiation of the specimen surfaces, the subsurface-layer structure changes and undergoes fragmentation down to as deep as 5-15 μm, which is smaller than the average grain size of the initial alloy. It is found out that the fragmented-structure layer is characterized by the presence of a martensitic В19￠ phase and a high concentration of interfaces and grain boundaries; the linear dimensions of the fragments exceed 1μm, the structure refinement in the layer below the irradiated surface is nonuniform and depends on crystallographic orientation of the initial grain. A reason for intensive fragmentation of individual grains of the initial B2 phase after ion-beam treatment is assumed to be the proximity of the orientation axes of the principal slip planes to that of the incident ion beam flux. This might have resulted in an earlier, compared to other grains, onset of plastic deformation in these grains and, as a result, partial fragmentation of their structure.