Novel Shape Memory Alloys: Session 2
Program Organizers: Othmane Benafan, NASA Glenn Research Center

Tuesday 2:00 PM
July 11, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Yuji Sutou, Tohoku University

2:00 PM  Invited
Large Strain Deformation And Recovery Of Shape Memory Uranium Niobium Alloys: Donald Brown1; Bjorn Clausen1; Amy Clarke2; Kester Clarke2; Robert Field2; 1Los Alamos National Lab; 2Colorado School of Mines
    U-6wt.% Nb (14at.%) has been shown to display the shape memory effect (SME) to ~7% strain, where deformation proceeds by twin rearrangement via boundary migration and is recoverable by a thermal cycle into the high temperature cubic phase. In-situ neutron diffraction during deformation has shown by monitoring the evolving texture that further deformation beyond the SME regime proceeds via a distinct mechanical twinning mechanism. During unload in this regime, ~2% strain is recovered by significant recovery of these twins. Moreover, if cycled to the high temperature phase after large strains, the material no longer recovers its initial shape, but does recover its initial crystallographic texture and mechanical response, i.e. subsequent deformation will result in recovered SME. This process can be repeated for a finite number of cycles before the secondary twin mechanism ceases to operate and the material fails.

2:40 PM  
Diffusive and Diffusionless Transformations of Martensitic Ti-Nb Alloys: Matthias Boenisch1; Ajit Panigrahi2; Mariana Calin1; Mihai Stoica1; Thomas Waitz2; Michael Zehetbauer2; Werner Skrotzki3; Juergen Eckert4; 1IFW-Dresden; 2University of Vienna; 3Technische Universität Dresden; 4Erich Schmid Institute of Materials Science
    In this work we study the thermal stability and the temperature-induced β ↔ α'' martensitic transformation (MT) of fully and partially martensitic Ti-Nb alloys. We examine the phase transformation sequences and precipitation reactions occurring during isochronal cycling in dependence on composition using calorimetry and dilatometry in combination with in-situ X-ray diffraction. Heating causes the orthorhombic unit cell of α''-martensite to become structurally closer to bcc austenite β. Depending on composition, α'' decomposes into hcp α and β, or reverts martensitically back to β. During decomposition of α'', volumes of Nb-depleted α''lean and Nb-enriched α''rich form. An α''-like phase is observed during early stages of α precipitation from β. The elastic and irreversible contributions of the thermoelastic energy balance of β ↔ α'' are quantified. Beyond a critical Nb concentration, the latent heat becomes very small (< 5 J/g) and the β → α'' MT remains incomplete.

3:00 PM  
Slip Deformation of Single Crystalline α″ Martensite in Ti-Nb Shape Memory Alloy: Masaki Tahara1; Nao Okano1; Tomonari Inamura1; Hideki Hosoda1; 1Tokyo Institute of Technology
    Ti-Nb base alloys have been developed as new Ni-free biomedical shape memory alloys. To improve the shape memory effect and superelasticity, many efforts have gone into increasing the critical stress for slip deformation. However, until now there are only a few reports of the slip deformation mode in the α″ martensite phase (C-centered orthorhombic structure) of β-Ti alloys. In this study, the slip deformation modes of the stress-induced α″ martensite was investigated in the single crystalline Ti-27mol%Nb alloy. The specimens were parent β phase at room temperature, and the single variant of α″ martensite (i.e. single crystalline martensite) was induced by compression stress. The slip deformation was occurred by further compression test in the stress-induced single crystalline martensite, and the surface relief of specimens were observed by optical microscope and SEM. Two types of the dislocation slip with a Burgers vector parallel to [110]α″ and [101]α″ directions were observed.

3:15 PM  
Functional Fatigue Behaviour of Single- and Oligocrystalline Fe-Mn-Al-Ni: Malte Vollmer1; Mario Kriegel2; Philipp Krooß1; Anja Weidner2; Horst Biermann2; Ibrahim Karaman3; Yuri Chumlyakov4; Thomas Niendorf5; 1University of Kassel; 2TU Bergakademie Freiberg; 3Texas A&M University; 4Tomsk State University; 5Universität Kassel
     New iron-based shape memory alloys (SMAs) with a disordered matrix and ordered nano-sized precipitates like Fe-Ni-Co-Al-X and Fe-Mn-Al-Ni-X attracted much attention in recent years due to their excellent pseudoelastic reversibility featuring strains of more than 10% in single- and oligocrystalline state. Furthermore, Fe-Mn-Al-Ni is characterized by its low Clausius-Clapeyron-relationship of 0.53 MPa/K. However, one of the main criteria for a widespread industrial application of these alloys particularly in seismic engineering, is the cyclic pseudoelastic behaviour, also referred to as functional fatigue and studies focusing on this aspect are still rare in literature. This work aims at analysing the functional fatigue behaviour of single- as well as oligocrystalline Fe-Mn-Al-Ni samples. Cyclic pseudoelastic tests were accompanied by in situ measurements using several techniques like optical microscopy, scanning electron microscopy, electron-backscatter diffraction and acoustic emission. Moreover, post mortem transmission electron microscopy was conducted in order to identify elementary mechanisms responsible for cyclic degradation.

3:30 PM Break