Year of Publication

2017

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. Haluk E. Karaca

Second Advisor

Dr. Yang-Tse Cheng

Abstract

Owing the capability of recovering large deformations through reversible phase transformation, shape memory alloys (SMAs) are well-known for their unique behaviors such as shape memory effect (SME) and superelasticity (SE), which can also be characterized by instrumented indentation techniques. Nickel titanium (NiTi) SMAs have been extensively used for nano/micro-indentation studies and widely applied to biomedical and other elaborate medical devices.

In this study, indentation responses of NiTi, NiTiHf, NiTiHfPd and NiTiHfCu alloys were investigated using spherical and Berkovich indenters at room temperature. Spherical and Berkovich indentation hardness, modulus, and work/depth recoverable ratio of these NiTi-based alloys were revealed as a function of maximum loading level at nano and macro scales. It has been revealed that indentation responses are highly composition, aging and load level dependent. Perfect work/depth recovery was observed in superelastic NiTiHfPd alloys using the spherical indenter.

Temperature-dependent shape memory properties of equiatomic NiTi, Nickel rich NiTi, and as-received and aged NiTiHf alloys were investigated using a spherical indenter between 30-340 ºC under selected load levels. Ti-6Al-4V was also tested for comparison. Spherical indentation response of aged high temperature NiTiHf alloys showed a clear relationship between the work recoverable ratio and transformation temperatures, superelastic and plastic behavior. It was concluded that indentation response can be used to measure local superelasticity response, determine phase transformation temperatures and reveal the temperature intervals of the deformation mechanisms of shape memory alloys. Spherical indentation hardness and modulus as a function of temperature can be used to exam the phase transformation, but cannot provide sufficient information regarding the superelastic and plastic behavior.

Orientation dependence of the shape memory properties in aged Nickel rich Ni50.3Ti29.7Hf20 single crystals were investigated along the [100], [110] and [111] orientations under room and high temperatures through indentation techniques. Indentation hardness, modulus and work /depth recoverable ratio were investigated as a function of temperature and indentation depth/load. It was found that indentation response of work recovery ratio is orientation independent, however, shape memory properties (e.g. transformation temperatures) determined from the indentation responses are almost orientation independent.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2017.344

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