Novel Functional Behaviors: Beyond Shape Memory Effect & Superelasticity: Session 3
Program Organizers: Ibrahim Karaman, Texas A&M University

Wednesday 10:20 AM
July 12, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: James Monroe, Thermal Expansion Solutions/Texas A&M

10:20 AM  
Calorimetric Measurements of the Elastocaloric Entropy Change in Shape Memory Alloys: Lluis Manosa1; Enric Stern-Taulats1; Adria Gracia-Condal1; Antoni Planes1; 1University of Barcelona
     The elastocaloric effect refers to the reversible thermal response of a solid subjected to uniaxial stress. Among the materials exhibiting elastocaloric effects, shape memory alloys are the best candidates for solid-state cooling devices based on this effect [1]. Typically, the elastocaloric effect is quantified by the temperature or entropy changes that take place when the stress is applied either adiabatically or isothermally. While direct measurements of adiabatic temperature changes are readily feasible and have been reported for a variety of alloys, determination of entropy changes has only been possible by means of indirect methods. We have developed a differential scanning calorimeter which operates under the application of uniaxial stress which enables a direct determination of entropy changes. We have used this purpose-built set-up to obtain the elastocaloric isothermal entropy changes in shape memory alloys.1.- L. Mañosa and A. Planes, Adv. Mater. In press.

11:00 AM  
Shape Memory Alloys for Potential Novel Multicaloric Refrigeration Applications: Daoyong Cong1; 1University of Science and Technology Beijing
    Shape memory alloys exhibit shape memory effect and superelasticity as a result of a temperature- or stress-induced martensitic transformation. Magnetic shape memory alloys could even display a magnetic-field-induced martensitic transformation due to the different magnetic states of martensite and austenite. Besides shape memory effect and superelasticity, the external-field-induced martensitic transformation in shape memory alloys could also lead to a variety of other functional properties, such as magnetocaloric effect and magnetoresistance (in case of magnetic shape memory alloys), elastocaloric effect, barocaloric effect, and anomalous thermal expansion behavior due to the change of crystal lattice during transformation. In this presentation, we will introduce our recent work on the multicaloric effects in shape memory alloys. The multicaloric effects include magnetocaloric effect, elastocaloric effect, and their coupled effects. The alloy systems involve both conventional shape memory alloys and magnetic shape memory alloys.

11:20 AM  
Elastocaloric Cooling Capacity of Shape Memory Alloys – Role of Deformation Temperatures and Mechanical Cycling: Yan Wu1; Huseyin Sehitoglu1; Elif Ertekin1; 1University of Illinois at Urbana Champaign
    The elastocaloric cooling (EC) capability of shape memory alloys (SMAs), originating from the entropy change upon endothermic reverse phase transformation, has the potential to reduce the dependence on conventional scheme for refrigeration. In this study, the EC properties were explored for SMAs starting with CuZnAl, NiTi, NiTiCu that are well known and then Ni2FeGa and NiTiHf13.3 that are relatively new. Their correlations with entropy changes, functional fatigue resistance, superelastic temperature windows were also investigated. Remarkably, we found a temperature decrease of 14.2˚C for CuZnAl, 18.2˚C for NiTi, 15.2˚C for NiTiCu, 13.5˚C for Ni2FeGa, and 6.95˚C for NiTiHf13.3 upon adiabatic stress relieving, which is also proportional to the entropy difference between phases except for NiTiHf where strain localization engendered early failure. NiTi and Ni2FeGa showed better stability in superelastic response, hence EC effect. The latter possesses an operational temperature window near 200˚C, which is the highest among the chosen SMAs.

11:35 AM  
Thermal Instability of Nb Nanowires Embedded in NiTi Shape Memory Alloy: Reza Bakhtiari1; Daqiang Jiang2; Hong Yang1; Yinong Liu1; 1University of Western Australia; 2China University of Petroleum, Beijing
    A profound understanding of the heat response of nanowires embedded in martensitic phase transformation matrix is needed for optimizing processing conditions to assure utilization of their exceptional mechanical properties of the nanowires inside the nanocomposites. In this study we investigated the effect of isothermal annealing on morphological changes of in-situ Nb nanowires embedded in a NiTi shape memory alloy matrix by means of scanning electron microscopy. The nanowires are found to become thermally unstable at above 550C and start to fragment and decay into chains of short rods and spheres at higher temperatures due to Rayleigh instability. The fragmented nano inclusions continue to coarsen by the mechanism of Ostwald ripening through NiTi matrix as the medium for diffusion.

11:50 AM  
Nano-compression Superelastic Behaviour in Cu-Al-Be Shape Memory Alloy Micro/nano Pillars: Valeria Fuster1; José Fernando Gómez-Cortés1; Iñaki López-Ferreño1; María Luisa Nó1; José María San Juan1; 1University of the Basque Country
     Shape memory alloys are firm candidates to be incorporated as sensors and actuators into MEMS. A good shape memory behaviour and superelasticity have been recently reported in Cu-Al-Ni SMA [1, 2]. However, up to now this is the only SMA system explored, with good properties at nano-scale. Our goal was to carry out a quantitative characterization and analysis of the superelastic behaviour in the Cu-Al-Be SMA system, because of the good properties at macroscopic scale. A series of pillars covering a broad-range of size diameters have been milled by FIB from single crystal slides. These pillars have been studied by nano-compression tests. A good superelastic behaviour and shape memory effect have been observed under different experimental conditions, which open the door for new small-scale applications. [1] J. San Juan, M.L. Nó, C.A. Schuh, Nature Nanotechnology 4, 415 (2009).[2] J. San Juan et al., Applied Physics Letters 104, 011901 (2014).

12:10 PM Break