Novel Shape Memory Alloys: Session 5
Program Organizers: Othmane Benafan, NASA Glenn Research Center

Friday 8:00 AM
July 14, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Marcus Young, University of North Texas

8:00 AM  
The Development of Nickel-Titanium-Hafnium Superelastic Alloys for 3D Printing Biomedical Implants: Behnam Amin-Ahmadi1; Joseph Pauza1; Tom Duerig2; Ronald Noebe3; Aaron Stebner1; 1Colorado School of Mines; 2Confluent Medical Technologies; 3NASA Glenn Research Center
    NiTiHf-based shape memory alloys have been receiving considerable attention in aerospace, defense and biomedical applications due to their ability to recover large deformations at high temperatures and/or stress levels, high damping capacity and wear resistance. Precipitation strengthening (formation of coherent H phase precipitate structures) provides exceptional strength during transformation under isothermal (superelastic) and isobaric (actuation) uses, even without cold work. This behavior of NiTiHf alloys makes them a promising candidate for biomedical implants produced by additive manufacturing. However, they still need to be engineered in terms of strength, ductility and superelastic cyclic-behavior for secure employment. This study investigates superelastic behavior of low Hf content (<10 at.%) NiTiHf-based alloys by changing composition/aging treatments. It is suggested that variations in precipitate size, chemical composition of the matrix and morphology of precipitates through aging may affect critical stresses for martensitic transformation, hysteresis, ductility and dislocation slip.

8:20 AM  
Tensile Fatigue Characterization of NiTiHf20 High Temperature Shape Memory Alloys: Omer Karakoc1; Joel Sam1; Ceylan Hayrettin1; Demircan Canadinc1; Ibrahim Karaman1; Michael A. Bass2; James H. Mabe2; 1Texas A&M University; 2Boeing Company
    Main goal of this study is to develop a fundamental understanding of the effect of upper cycle temperatures (UCT), microstructure and stress levels on the thermo-mechanical cyclic response and fatigue behavior of NiTiHf20 high temperature shape memory alloys (HTSMAs). Fatigue testing primarily focused on tensile specimens cycled until fracture. Tensile specimens with 300C UCT exhibited fatigue lives 2 times of that of the specimens with the UCT of 350C. Moreover, the specimens containing nanometer size precipitates displayed higher actuation work output than the specimens having larger precipitates. It is also noted that the stress level plays an important role in determining fatigue life. Increase in stress level caused serious drop in fatigue life of the HTSMA samples. Tests results indicate that UCT of the thermo-mechanical cycles and microstructure play a significant role on the evolution of the actuation strain and plasticity. The mechanisms responsible for these differences will be discussed.