Mechanical and Tribological Characterization of Nitinol 60 Parts Manufactured by Different Powder Metallurgy Processes
NiTinol 60 (60NiTi) is a Ni-rich NiTi alloy comprising approximately ~60 wt.% (55 at.%) Ni and ~40 wt.% (45 at.%) Ti. 60NiTi, when employed in a hardened state, displays outstanding properties. It exhibits excellent corrosion resistance and a high level of hardness, has a low elastic modulus and an extensive elastic range that imparts high mechanical contact load resilience. It is dimensionally stable, has good bio-compatibility, can be machined into precision components and is non-magnetic. This unique set of characteristics make the intermetallic 60NiTi suitable for a wide variety of applications in the marine, medical, aerospace and food processing industries. As an example, this alloy has recently been employed as a rolling element bearing material that is highly corrosion resistant and endures high levels of shock loads before experiencing any permanent deformation.
This work investigates the possibility of using hardened 60NiTi in two other distinct areas of interest. For the first area of interest, the sliding wear performance of 60NiTi is compared to 440C steel. This type of steel has high tensile strength and Rockwell C hardness similar to 60NiTi and is conventionally considered as a wear resistant alloy for applications such as gears where contacting materials slide over each other. This research, apart from defining the conditions under which the responses of these alloys differ and answers questions which arise from testing, helps to understand the conditions under which 60NiTi is a suitable alloy as a wear resistant material in sliding conditions. Results obtained in this research, in addition to introducing 60NiTi as a material for load bearing applications other than rolling, will help the engineering community to properly apply this intermetallic in situations where materials slide over each other. Furthermore, findings obtained in this research give an insight into main wear mechanisms and mechanical properties and clarify the mechanisms that at a microscopic level cause damage in 60NiTi and generally NiTi alloys.
In addition, recent studies showed that the addition of hafnium, even in low concentrations, provides benefits in terms of hardening treatment and phase stability in 60NiTi. These findings have led to consider NiTi-Hf alloy (~58 wt.% Ni-~39 wt.% Ti-~3 wt.% Hf) as a substitute or alternative for 60NiTi alloy. The exact composition by weight percent is 57.6 % Ni-39.2 % Ti-3.2 % Hf (54 at.% Ni- 45 at.%Ti-1 at.% Hf) and is designated as 58Ni39Ti-3Hf. Despite the positive interest in this alloy, direct comparisons of the sliding behavior of 58Ni-39Ti-3Hf and the forerunner 60NiTi alloy have not been made. This research evaluates and compares the sliding performance of 58Ni39Ti-3Hf with baseline 60NiTi to investigate the effect of Hf-addition and define the conditions under which 58Ni-39Ti-3Hf can be considered as an alternative to 60NiTi for sliding applications. Moreover, reasons behind the observed differences are investigated and discussed through different mechanical and microstructural experimentations. Results obtained from these tests help to understand the effects that Hf-addition has on the mechanical properties of baseline 60NiTi.
The second area of interest focuses on processing 60NiTi to a porous structure suitable for general bone replacements. Implants that are intended for use in general bone replacements such as spinal or cranial inserts are expected to have a porous structure. In such cases, other than good biocompatibility and dimensional integrity and fulfilling the mechanical prerequisite for biomedical applications, final parts should contain interconnected and open pores of 30-80 Vol.% and pore sizes in the range of 100-600 μm. In this research, elemental Ni and Ti powders are used to process and manufacture 60NiTi parts by a conventional press-and-sinter method. This study, while working toward evaluating the possibility of obtaining the mentioned 60NiTi structure through a conventional press-and-sinter method, investigates the effects of different processing factors such as sintering temperature, heating rate and sintering holding time on the microstructure and mechanical properties of these parts. In addition, a series of heat-treatment methods are developed to homogenize and harden porous 60NiTi in a cost-effective way. Outcomes of these heat-treatment methods with regards to obtained microstructures and mechanical properties are discussed and analyzed.