SHAPE MEMORY BEHAVIOR OF SINGLE AND POLYCRYSTALLINE NICKEL RICH NICKEL TITANIUM ALLOYS

Author

Irfan Kaya, University of KentuckyFollow

Date Available

4-29-2015

Year of Publication

2014

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Mechanical Engineering

First Advisor

Dr. Haluk E. Karaca

Abstract

NiTi is the most commonly used shape memory alloy (SMA) and has been widely used for bio-medical, electrical and mechanical applications. Nickel rich NiTi shape memory alloys are coming into prominence due to their distinct superelasticity and shape memory properties as compared to near equi-atomic NiTi shape memory alloys. Besides, their lower density and higher work output than steels makes these alloys an excellent candidate for aerospace and automotive industry. Shape memory properties and phase transformation behavior of high Ni-rich Ni₅₄Ti₄₆ (at.%) polycrystals and Ni-rich Ni₅₁Ti₄₉ (at.%) single-crystals are determined. Their properties are sensitive to heat treatments that affect the phase transformation behavior of these alloys.

Phase transformation properties and microstructure were investigated in aged Ni₅₄Ti₄₆ alloys with differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) to reveal the precipitation characteristics and R-phase formation. It was found that Ni₅₄Ti₄₆ has the ability to exhibit perfect superelasticity under high stress levels (~2 GPa) with 4% total strain after 550°C-3h aging. Stress independent R-phase transformation was found to be responsible for the change in shape memory behavior with stress.

The shape memory responses of [001], [011] and [111] oriented Ni₅₁Ti₄₉ single-crystals alloy were reported under compression to reveal the orientation dependence of their shape memory behavior. It has been found that transformation strain, temperatures and hysteresis, Classius-Clapeyron slopes, critical stress for plastic deformation are highly orientation dependent.

The effects of precipitation formation and compressive loading at selected temperatures on the two-way shape memory effect (TWSME) properties of a [111]-oriented Ni₅₁Ti₄₉ shape memory alloy were revealed. Additionally, aligned Ni₄Ti₃ precipitates were formed in a single crystal of Ni₅₁Ti₄₉ alloy by aging under applied compression stress along the [111] direction. Formation of a single family of Ni₄Ti₃ precipitates were exhibited significant TWSME without any training or deformation. When the homogenized and aged specimens were loaded in martensite, positive TWSME was observed. After loading at high temperature in austenite, the homogenized specimen did not show TWSME while the aged specimen revealed negative TWSME.

Recommended Citation

Kaya, Irfan, "SHAPE MEMORY BEHAVIOR OF SINGLE AND POLYCRYSTALLINE NICKEL RICH NICKEL TITANIUM ALLOYS" (2014). Theses and Dissertations--Mechanical Engineering. 37.
https://uknowledge.uky.edu/me_etds/37