PRECIPITATION, ORIENTATION AND COMPOSITION EFFECTS ON THE SHAPE MEMORY PROPERTIES OF HIGH STRENGTH NiTiHfPd ALLOYS

Author

Emre Acar, University of KentuckyFollow

Date Available

4-28-2015

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

NiTiHf high temperature shape memory alloys are attractive due to their high operating temperatures (>100 ^oC) and acceptable transformation strain compared to NiTi. However, NiTiHf has limitations due to their lack of ductility and low strength, resulting in poor shape memory properties. In this study, Pd has been added to NiTiHf alloys in an attempt to improve their shape memory behavior. A combined approach of quaternary alloying and precipitation strengthening was used.

The characterization of a Ni_45.3Ti_29.7Hf₂₀Pd₅ (at. %) polycrystalline alloy was performed in compression after selected aging treatments. Transmission electron microscopy was used to reveal the precipitation characteristics. Differential scanning calorimetry, load-biased (constant stress) thermal cycling experiments and isothermal stress cycling (superelasticity) tests were utilized to investigate the effects of aging temperature and time. The crystal structure and lattice parameters were determined from X-ray diffraction analysis. Significant improvement in the shape memory properties of Ni_45.3Ti_29.7Hf₂₀Pd₅ was obtained through precipitation strengthening. The effects of chemical composition (effects of Hf content replacing with Ti) on the shape memory properties of NiTiHfPd alloys were also revealed.

Orientation dependence of the shape memory properties in aged Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals were investigated along the [111], [011] and [-117] orientations. The shape memory properties were determined to be strong functions of orientation and aging condition. A perfect superelastic behavior (with no irrecoverable strain) with 4.2 % recoverable compressive strain was obtained in the solutionized condition at stress levels as high as 2.5 GPa while 2 % shape memory strain under a bias stress of 1500 MPa was possible in an aged [111] oriented single crystal. A mechanical hysteresis of 1270 MPa at -30 ^oC, which is the largest mechanical hysteresis that the authors are aware of in the SMA literature, was observed along the [111] orientation.

Finally, thermodynamic analyses were conducted to reveal the relationships between microstructure (e.g. precipitate size and interparticle distances) and martensitic transformations in Ni_45.3Ti_29.7Hf₂₀Pd₅ SMAs. Precipitate characteristics were found to be effective on the elastic energies for nucleation, propagation with dissipation energy and these energies influenced the TTs and the constant stress shape memory properties in Ni_45.3Ti_29.7Hf₂₀Pd₅ alloys.

Recommended Citation

Acar, Emre, "PRECIPITATION, ORIENTATION AND COMPOSITION EFFECTS ON THE SHAPE MEMORY PROPERTIES OF HIGH STRENGTH NiTiHfPd ALLOYS" (2014). Theses and Dissertations--Mechanical Engineering. 40.
https://uknowledge.uky.edu/me_etds/40