Authors: Tejas Umale
Source: OakTrust (Texas A&M University Libraries) (article, 2020-07-24)
Field: Materials Science — Materials Chemistry

Effect of Composition and Phase Equilibria on Transformation and Precipitation Characteristics in Ni–Ti–Hf High Temperature Shape Memory Alloys Shape memory alloys (SMAs) are a class of material that can “memorize” their original shape and recover it when stimulated with temperature or stress or both. For the increasing need of high temperature SMAs for advanced aerospace, transportation and energy applications, the NiTiHf alloys standout from its other NiTi-based competitors because of its economical and stable precipitation strengthening. The Ni-lean NiTiHf alloys possess high transformation temperatures (TTs) but suffer from high thermal and dimensional stability because of the presence of brittle (Ti,Hf)₂Ni phases and low matrix strength which makes them vulnerable during practical applications. Whereas, the Ni-rich NiTiHf alloys despite possessing slightly lower TTs than Ni-lean counterparts, they depict high strength, high thermal and dimensional stability. The TTs and strength of the Ni-rich NiTiHf alloys can be enhanced further with H-phase precipitation. The Ni-rich NiTiHf alloys despite possessing such impressive properties the compositional space is not explored systematically. Also, despite notable enhancement of shape memory properties upon H-phase precipitation, the systematic thermodynamic and kinetic studies on the H-phase is still missing. The aforementioned studies once conducted, can aid in developing novel NiTiHf alloys with required transformation characteristics. \nIn the present study, efforts have been made to understand the compositional dependence of the NiTiHf alloys on the TTs. More than 42 NiTiHf alloys of different compositions in the Ni-lean, equiatomic and Ni-rich regimes were prepared by vacuum arc melting with highest accuracy and were tested for TTs. Also, the thermodynamics of H-phase precipitation is explored by using the high throughput diffusion multiple experiments and the phase diagram for the Ni-Ti-Hf system was established along with H-phase equilibrium with other phases. The kinetic study of the H-phase precipitates was performed using the Small Angle X-ray Scattering, which enabled to understand the evolution of precipitate size and volume fraction with time at different heat treatment temperature. Also, a new machine learning based approach was implemented to explore the NiTiHf alloys with required TTs.\n \nTherefore, by combining the chemistry, thermodynamics and kinetics-based studies of the NiTiHf system, novel NiTiHf HTSMAs can be developed for required aerospace, energy, oil-gas industries and various other applications. Also, the machine learning based approach can be used to fill in the knowledge gaps and further assist in developing NiTiHf alloys for specific engineering applications. Precipitation ; Materials science ; Transformation (genetics) ; Phase (matter) ; Metallurgy ; Shape-memory alloy ; Composition (language) ; Thermodynamics ; Chemistry ; Physics ; Meteorology Shape Memory Alloy Transformations ; Titanium Alloys Microstructure and Properties ; Intermetallics and Advanced Alloy Properties