Microstructure and Mechanical Properties of a Superelastic NiTi-1Hf (at. ) Bearing Alloy

摘要

Over the past decade NASA has been developing high strength superelastic alloys based on 55Ni-45Ti (at. %) formulations that meet the requirement of advanced bearings needed for challenging aeronautics and space applications [1]. These bearings are superior to the traditionally used bearings that are typically made of materials such as austenitic stainless steels, superalloys, ceramics or high carbide tool steels, in terms of having both the corrosion resistance and tolerance to severe static loads. 55Ni-45Ti bearings also have high hardness, low density, good machinability, electrical conductivity, nonmagnetic behavior, and lubricant compatibility needed for tribological applications. In spite of 55Ni-45Ti bearings being so attractive, a processing challenge associated with quench-cracking and residual stresses of bearing components during heat-treatment has been observed that poses a concern, In order to resolve this issue, low Hf amount of 1 at % was added to replace INi in 55Ni-45Ti, and a new alloy, 54Ni-45Ti-lHf (at. %) was produced and processed with a modified heat treatment which reduced residual stresses and cracking during quench [2]. In order to assess the effect of this heat treatment on mechanical properties, detailed mechanical testing and microstructural characterization were conducted at each stage of heat treatment, the results of which are described in this work.