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.

8:40 AM  
Shape Memory Response of NiTiHf(Pd) Alloys Fabricated by Conventional and Selective Laser Melting Methods: Haluk Karaca1; Soheil Saedi1; Guher Toker1; Osman Ozbulut2; Narges Moghaddam3; Mohammad Elahinia3; Othmane Benafan4; 1University of Kentucky; 2University of Virginia; 3University of Toledo; 4NASA Glenn Research Center
    NiTiHf(Pd) alloys can be utilized as high temperature/strength SMAs and their properties are highly composition and heat treatment dependent. In this study, shape memory properties of Ni-rich NiTiHf, Ni-lean NiTiHf and NiTiHfPd alloys are investigated systematically to reveal the effects of composition alteration and heat treatments on their transformation temperatures, microstructure and mechanical response. Moreover, a Ni-rich NiTiHf alloy was fabricated by selective laser melting method and its shape memory properties are determined and compared with NiTiHf alloys fabricated by conventional methods. It will be shown that shape memory properties of these alloys can easily be tailored by composition alteration, processing parameters and heat treatments. These alloys can show superelasticity at temperatures above 100C, shape memory effect under high stress levels (>1 GPa), large damping capacity (>40 and a wide operating window (> 200C). These alloys are found to be very promising for aerospace, automotive and civil engineering applications.

9:00 AM  
Development of Advanced Nickel-Titanium-Hafnium Alloys for Tribology Applications: Sean Mills1; Aaron Stebner1; Ronald Noebe2; Christpher DellaCorte2; 1Colorado School of Mines; 2NASA Glenn Research Center
    High hardness, compressive and torsional elastic strength and corrosion resistance of binary Ni-Ti alloys makes it an optimum candidate for specialized bearings. However, when compared with high performance steel bearings, intermetallic materials are more prone to rolling contact fatigue degradation. Under rapid quenching processes, binary Ni-Ti alloy as been known to experience high residual stress. Failure due to cracking and machining distortion is a possibility. Not quenching leads to low hardness. This dissertation is designed to elucidate the effect of hafnium ternary alloying on and bearing raceway performances. Multimodal studies on Ni54Ti45Hf1 will include rolling contact fatigue testing as a basis for mechanical testing. Electron microscopy and time/temperature/transformation study of Ni54Ti45Hf1 alloy will provide a better understanding for the ternary system. Future work includes characterizing failure mechanisms, performances (hardness, strength, life) versus residual stresses, and map of alloy design space to allow optimization of the NiTiHf system for tribology applications.

9:20 AM  
Thermal Stability of Ni-rich Ni-Ti-Hf and Ni-Ti-Zr High-Temperature Shape Memory Alloys Near Their Working Temperatures: Aquilina Mara Prez-Sierra1; Ruben Santamarta Martinez1; Jaume Pons1; Ibrahim Karaman2; Ronald Noebe3; 1University of the Balearic Islands; 2Texas A&M University; 3NASA Glenn Research Center
    Slightly Ni-rich NiTiHf and NiTiZr alloys have recently become serious candidates as high temperature shape memory alloys (HTSMA) since they can exhibit good mechanical and functional properties. However, the microstructures and functionalities of any HTSMA might be modified by long exposure of the material to high working temperatures. In the present study, the thermal stability of three alloys (Ni50.3Ti29.7Hf20, Ni50.1Ti24.9Hf25 and Ni50.3Ti29.7Zr20) near their respective transformation temperatures has been studied by DSC and TEM for three distinct microstructures: precipitate free and those containing two different sizes and densities of H-phase nanoprecipitates. The results demonstrate that NiTiHf alloys show better stability under austenite aging, martensite aging and thermal cycling than the NiTiZr, with Ni50.3Ti29.7Hf20 showing the most stable transformation upon aging in austenite. The presence of H-phase precipitates enhances the thermal stability of the three compositions, especially for those microstructures containing large precipitates.

9:40 AM  
Superelastic Behavior of Ni-rich Ni-Ti-Hf and Ni-Ti-Zr High-Temperature Shape Memory Alloys in Compression: Aquilina Perez-Sierra1; Jaume Pons1; Ruben Santamarta1; Ibrahim Karaman2; Ronald Noebe3; 1University of the Balearic Islands; 2Texas A&M University; 3NASA Glenn Research Center
    Recently, Ni-rich Ni-Ti-Hf/Zr alloys have attracted interest in the scientific community as high temperature shape memory alloys due to their good functional properties and thermal stability, especially when H-phase nanoprecipitates have been previously in-duced. In this work, compression experiments at different temperatures and TEM ob-servations have been carried out for three polycrystalline materials (Ni50.3Ti29.7Hf20, Ni50.1Ti24.9Hf25 and Ni50.3Ti29.7Zr20) after three different initial thermal treatments: solu-tion treated, aged 550C for 3h, and furnace cooling. The results show that the best di-mensional stability and superelastic behaviour is obtained after 550C/3h for the three compositions, although the superelastic window is lower than 40C. Furnace cooled al-loys do not show superelastic behavior but do exhibit high toughness, as they can with-stand stresses up to 2 GPa and reach ductility levels above 12% without failure. Finally, solution heat treated Ni50.3Ti29.7Zr20 shows superelasticity even though no martensitic transformation peak can be observed by DSC.

10:00 AM Break