Cyclic Stability of Two-Way Shape Memory Effect in Aged Ni50.3Ti32.2Hf17.5 Polycrystals after Various Thermomechanical Treatments.

In the present paper, the cyclic stability of the high-temperature two-way shape memory effect was studied in high-strength Ni50.3Ti32.2Hf17.5 polycrystals after various thermomechanical treatments-training (thermocycling under stress) and stress-induced martensite aging. The effect of training and stress-induced martensite aging on the microstructure, the two-way shape memory effect, and its cyclic stability was determined. It was found out that both thermomechanical treatments induce the high-temperature two-way shape memory effect at T > 373 K, with a strain of 1.5% in tension. The influence of cyclic tests (up to 100 stress-free cycles of cooling/heating) on the two-way shape memory effect strain, the transformation temperatures, and the microstructure was established. Different degradation mechanisms of the two-way shape memory effect were established after thermocycling and stress-induced martensite aging.

Measuring stress-induced martensite microstructures using far-field high-energy diffraction microscopy.

Modern X-ray diffraction techniques are now allowing researchers to collect long-desired experimental verification data sets that are in situ, three-dimensional, on the same length scales as critical microstructures, and using bulk samples. These techniques need to be adapted for advanced material systems that undergo combinations of phase transformation, twinning and plasticity. One particular challenge addressed in this article is direct analysis of martensite phases in far-field high-energy diffraction microscopy experiments. Specifically, an algorithmic forward model approach is presented to analyze phase transformation and twinning data sets of shape memory alloys. In the present implementation of the algorithm, the crystallographic theory of martensite (CTM) is used to predict possible martensite microstructures (i.e. martensite orientations, twin mode, habit plane, twin plane and twin phase fractions) that could form from the parent austenite structure. This approach is successfully demonstrated on three single- and near-single-crystal NiTi samples where the fundamental assumptions of the CTM are not upheld. That is, the samples have elastically strained lattices, inclusions, precipitates, subgrains, R-phase transformation and/or are not an infinite plate. The results indicate that the CTM still provides structural solutions that match the experiments. However, the widely accepted maximum work criterion for predicting which solution of the CTM should be preferred by the material does not work in these cases. Hence, a more accurate model that can simulate these additional structural complexities can be used within the algorithm in the future to improve its performance for non-ideal materials.

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys.

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of -100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here.

Defect-driven synthesis of self-assembled single crystal titanium nanowires via electrochemistry.

One-dimensional single crystal nanostructures have garnered much attention, from their low-dimensional physics to their technological uses, due to their unique properties and potential applications, from sensors to interconnects. There is an increasing interest in metallic titanium nanowires, yet their single crystal form has not been actualized. Vapor-liquid-solid (VLS) and template-assisted top-down methods are common means for nanowire synthesis; however, each has limitations with respect to nanowire composition and crystallinity. Here we show a simple electrochemical method to generate single crystal titanium nanowires on monocrystalline NiTi substrates. This work is a significant advance in addressing the challenge of growing single crystal titanium nanowires, which had been precluded by titanium's reactivity. Nanowires grew non-parallel to the surface and in a periodic arrangement along specific substrate directions; this behavior is attributed to a defect-driven mechanism. This synthesis technique ushers in new and rapid routes for single crystal metallic nanostructures, which have considerable implications for nanoscale electronics.

In vitro cytotoxicity evaluation of a 50.8% NiTi single crystal.

To our knowledge, the biocompatibility of nickel-titanium (NiTi) single crystals has not been reported. Yet certain orientations of single crystals present several advantages over the polycrystalline form in terms of maximal strain, fatigue resistance, and temperature range of superelasticity. Therefore we tested the in vitro biocompatibility of 50.8% NiTi single crystals in the orientation <001> after four different heat treatments in a helium atmosphere followed by mechanical polishing. The study was performed on the material extracts after immersion of the specimens in cell culture medium (DMEM) for 7 days at 37 degrees C. Cytotoxicity studies were performed on L-929 mouse fibroblasts using the MTT assay. J-774 macrophages were used to assess the potential inflammatory effect of the extracts by IL1-beta and TNF-alpha dosages (sandwich ELISA method). Exposure of L-929 to material extracts did not affect cell viability. In addition, IL1-beta and TNF-alpha secretion was not stimulated after incubation with NiTi extracts compared to the negative controls. These results were predictable since atomic absorption spectroscopy did not detect nickel ions in the extracts with a resolution of 1 ppm. Within the limits of in vitro testing, our results demonstrate that the TiNi(50.8%) single crystals do not trigger a cytotoxic reaction.