Loading...
Development of nickel-titanium-hafnium alloys for impact resistant tribology performances
Mills, Sean H.
Mills, Sean H.
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2019
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
Abstract
On the international space station, low-stress high cycle rotary bearings housed in the water recycling system require high wear resistance during fatigue cycling and good corrosion resistance. Moreover, these components experience high stress levels during transportation to space. To mitigate accumulation of plastic strain, loads need to be distributed to a larger contact area, while maintaining a high level of resistance to rolling/sliding fatigue damage. High hardness, high compressive elastic strength, and good corrosion resistance, cavitation erosion resistance, nonmagnetic behavior make Ni-rich NiTiHf alloys optimum candidates for tooling, wear, and specialized bearing applications. Moreover, low effective modulus of these alloys allows them to elastically accommodate large amounts of deformation by distributing loads to a larger contact area, thereby mitigating plastic strain accumulation, while maintaining a high level of resistance to rolling/sliding fatigue damage. Conventional superelastic binary Ni-Ti alloys are known to experience high hardness and high residual stresses upon rapid quenching, resulting in cracking and machining distortion, whereas secondary precipitates can over-coarsen if cooled slowly, thereby reducing the material hardness. This dissertation is designed to elucidate the effects of hafnium additions in Ni-rich NiTiHf alloys, by varying the nickel contents by 50.3 – 56.0 at. % and hafnium contents by 1.0 – 8.0 at. %, and optimizing heat treatments in order to tune structure-property relationships. The structure of a newly discovered cubic precipitate phase in Ni56Ti36Hf8 alloy was characterized using transmission electron microscopy (TEM). The tribological performance and underlying deformation mechanisms in Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys were studied using rolling contact fatigue (RCF) testing and TEM. The Ni56Ti36Hf8 alloy, which is strengthened by the unique precipitate microstructure, exhibited a notable increase in RCF performance and less sub-surface damage compared to Ni55Ti45 and Ni54Ti45Hf1 alloys. Chapter 1 is a comprehensive review on Ni-rich NiTi-alloys that discusses fundamentals on mechanical behavior, transformations, precipitation and strengthening mechanisms specifically for NiTi and NiTiHf alloy compositions that are suited to tribological applications. Subsequent sections provide in-depth information on ternary alloying and design considerations for rolling contact fatigue performance and space-age applicability to elucidate the optimum NiTi alloy for functional bearing use. Chapter 2 provides information on common materials processing practices in NiTi and NiTiHf alloy development. Moreover, several electron microscopy techniques that are used throughout this study and details regarding the preparation of TEM samples are discussed. Chapter 3 explores the effects of heat treatments on the microstructures and hardness of both Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys. Their suitability for tribological applications was evaluated using a combination of transformation electron microscopy techniques, ab initio density functional theory calculations, and Vicker’s hardness testing. Chapter 4 utilizes transmission electron microscopy and ab intio density functional theory techniques to characterize a new cubic phase in the Ni56Ti36Hf8 alloy which is more precisely identified to be New phase with symmetry of the Pm3 ̅m space group and a lattice parameter a = 8.816 Å. In this structure, nickel atoms preferentially segregate to the perimeter lattice sites of the 54-atom unit cell. Chapter 5 investigates the rolling contact fatigue performance and subsurface deformation of Ni56Ti36Hf8 alloy compared with 1st generation Ni54Ti45Hf1 alloy and baseline Ni55Ti45 alloy. FIB samples were extracted from the surface of RCF rod specimens after varied levels of deformation during 3 ball-on-rod RCF tests. Chapter 6 screens tribological performance of several 2nd generation NiTiHf alloys in comparison to 1st generation Ni54Ti45Hf1 alloy and baseline Ni55Ti45 alloy. Screening tests include Vickers micro-hardness, rolling contact fatigue, monotonic compression and cyclic compression. Moreover, the peak-hardened microstructures of the RCF tested alloys were captured, tying underlying microstructural mechanisms to properties and performance. The dissertation is concluded in Chapter 7 where the evaluation of performance and connections to microstructure are summarized. Chapter 7 also addresses open areas of research as well as a general outlook on the class of NiTiHf alloys and future applications that can benefit from the fundamental work on this material system.
Associated Publications
Rights
Copyright of the original work is retained by the author.