Study of hardness and wear behaviour of NiTi shape memory alloys

In order to improve the understanding of the hardness and wear behaviour of TiNi shape memory alloys and their relation with Ms temperature and microstructure, five TiNi alloys were prepared with compositions varying from 49 to 51 at.%. Ni, austenized at 800 degrees C for 10 min, quenched in water and annealed at 500 degrees C for I h before air-cooling. Also, one alloy for each microstructure, which presented the most promising results, was selected for studying and relating the precipitates size, quantity, type, distribution and coherency with the properties above mentioned. The results showed two wear behaviours according to the phases present and the Ms temperature of the studied material: On beta-phase alloys the wear behaviour was influenced by the presence of semi-coherent precipitates as well as its high capacity of transformation, which increased the absorbed energy for restoring the structure and then, decreasing the damage mechanisms, while in martensite-phase alloys it was conditioned by the reorientation and/or re-transformation of the existent plates. Also, it was observed that the presence of precipitates affected strongly the hardness and wear behaviour, as they restricted the nucleation, free-growth, mobility and re-accommodation of the martensite plates, which conditioned the capacity for absorbing energy, reducing thus the wear damage. (C) 2007 Elsevier B.V. All rights reserved.