Author ORCID Identifier

https://orcid.org/0000-0003-1731-6637

Year of Publication

2020

Degree Name

Master of Science in Mechanical Engineering (MSME)

Document Type

Master's Thesis

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. Haluk E. Karaca

Abstract

Shape memory alloys such as NiTiHf and NiTi have the ability to generate large recovery stresses when they are constrained after pre-straining and then heated above their Austenite Finish Temperature (Af). In this work Ni49.9Ti50.1 (at.%), the most well-known SMA with impressive shape memory properties but limited temperature range and Ni50.3Ti29.7Hf20, a promising high temperature shape memory alloy, were characterized to reveal their stress generation capabilities. The effects of pre-straining on stress generation were investigated via martensite reorientation method of NiTi and NiTiHf alloys by loading the samples till preselected values, unloading and then heating when the initial phase was martensite.

Moreover, an alternative stress generation testing, superelastic pre-straining, by loading the samples till preselected values in superelastic testing and then heating was employed to reveal stress generation capabilities of NiTiHf single crystals. It was revealed that stress generation via superelastic response can extend the temperature range for envisioned applications and can result in higher maximum working stress levels. Additionally, the orientation dependence of the recovery stress generation capabilities of Ni50.3Ti29.7Hf20 (at.%) single crystals were assessed along the orientations of [111], [110] and [100] and compared to NiTi and NiTiHf polycrystalline alloys.

It is revealed that the amount of stress generation depends greatly on the orientation, pre-straining amount and the testing temperature (or pre-stress amount) of the material. Maximum working stress of higher than 2 GPa was generated after superelastic loading in the [111]-oriented NiTiHf single crystals. Moreover, in the same material, recovery stress of 1.24 GPa was generated after pre-straining at room temperature via martensite reorientation.

It was revealed that superelastic testing results in high maximum working stress but lower stress generation and can operate at higher temperatures compared to martensite reorientation method. Combined use of martensite reorientation and superelastic prestraining methods could result in broad operation temperature and stress range for constrained recovery applications of shape memory alloys.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.180

Available for download on Monday, May 17, 2021

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