Tribologically induced amorphization in the subsurface of aged Ni-rich TiNi alloy during dry sliding

Abstract

The microstructure modification due to friction energy transformation in near-surface zone is crucial to the tribological performance of materials. To study the surface and subsurface response of materials exhibiting stress-induced martensitic transformation to repeated sliding, we have investigated the quite novel microstructure changes induced during the dry sliding of an aged Ti-51.5 at.% Ni alloy against the GCr15 spheres. In the process of reciprocating sliding, the B2 austenite primarily transforms into amorphous phase in the worn subsurface area. Remarkably, B2 clusters of nanocrystals below 10 nm are randomly embedded in the amorphous ribbon in the topmost region whereas the medium-range-ordered debris in the order of 3–5 nm are retained in the deeper layer. More importantly, it can be found that large number of nanocrystalline phase and amorphous structure coexist in the strip at a depth about 2.5 μm. Additionally, XRD patterns of the worn tracks show the appearance of B19’, being different from the B2 in the original microstructure. The martensitic transformation therefore plays a crucial role in accommodating associated plastic deformation for the formation of an amorphous layer. The distinctive elementary mechanisms identified here will be essential for the future modeling of the tribologically deformed layer of the alloys.