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Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P 2 1 / m symmetry instead of experimentally identified P 2 / m. The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost. [Display omitted] [ABSTRACT FROM AUTHOR]

Laser diffusion has previously been studied to incorporate dopants in semiconductors and to carburize steel for surface hardening without melting the substrate, among others. The optical and electromagnetic properties of materials can also be modified by this diffusion method to tailor the material response at different frequencies of the electromagnetic spectrum. Platinum atoms have been diffused into titanium and tantalum sheets by a laser chemical vapor diffusion method using a metallorganic compound of platinum and laser beams of different polarizations. Thermal decomposition of the precursor at the laser-heated spot on the surface of the substrate generates platinum atoms that diffuse into the substrate, producing laser-platinized samples. The transmittances of the samples are determined by measuring the strength of the transmitted magnetic field oscillating at 63.86 MHz. The laser-platinized samples produced by linearly polarized lasers exhibit higher transmittances than the samples obtained by using azimuthally polarized lasers. [ABSTRACT FROM AUTHOR]

• Precipitates obstruct the motion of austenite-martensite interfaces in niti. • Thermoelastic effects quantitatively explain the barrier strength. • Cyclic transformations induce inelastic deformation at precipitate-matrix interfaces. • Deformation originates in a thin amorphous interfacial layer. Precipitation of secondary phases is a common strategy used to control both the structural and functional properties of shape memory alloys. It can be used to promote nucleation of the martensitic transformation as well as improve cyclic stability. Less is understood about how precipitates affect the progression of an ongoing transformation, i.e., motion of austenite-martensite interfaces. In this study, we performed molecular dynamics simulations of the interaction of austenite-martensite interfaces moving in the NiTi alloy with Ni 4 Ti 3 precipitates. It was found that the nanoscale precipitates obstruct interface motion until a sufficient undercooling is reached. The simulation results can be quantitatively explained with thermoelastic effects – elastic deformation of the precipitates acts to oppose the thermodynamic driving force favoring the transformation. A simple model is proposed to predict a more difficult transformation in shape memory alloys with higher concentrations of and/or harder precipitates. Additionally, simulations of cyclic transformations implicate inelastic deformation at the precipitate-matrix interface as one mechanism responsible for the cyclic drift in transformation characteristics. Deformation originated in a thin, amorphous interfacial layer and expanded with increasing cycles. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] The objective of this study was to investigate the interactions between phase transformation and viscoplasticity during uniaxial constant force thermal cycling (UCFTC) of a Ti-rich Ni-Ti-20Hf (at.%) high-temperature shape memory alloy (HTSMA). These tests were conducted (up till failure) at 1, 10 and 50 ∘ C / min , to vary the duration of exposure to high temperatures, viz. the amount of viscoplasticity, and to examine the rate-dependency of actuation. The macroscopic results from the tests were used to investigate the evolution of transformation temperatures, hysteresis, transformation and irrecoverable strains for the cycles in which the effect of potential damage mechanisms could be assumed to be negligible. The phenomena that affected the behavior were: viscoplasticity at 1 ∘ C / min , transformation-induced plasticity (TRIP) at 10 and 50 ∘ C / min , and accumulation of retained martensite at all the three rates. More interestingly, the response at 1 ∘ C / min indicated a unique interplay between the effect of viscoplasticity over phase transformation and static recovery. The retained martensite was identified through a series of DSC and XRD analyses, and its contribution to TRIP strain was quantified through a UCFTC test. Furthermore, a test involving alternating isothermal creep and UCFTC at 10 ∘ C / min was conducted to investigate an effect of viscoplasticity produced by creep on the behavior, while reducing the viscoplasticity during thermal cycling. The alternating test revealed an effect of phase transformation over the viscoplastic strain rate. The experimental investigations demonstrated a rate-dependent phase transformation behavior, and a two-way coupling between phase transformation and viscoplasticity, bringing out the importance of understanding viscoplastic deformations in phase-transforming materials. [ABSTRACT FROM AUTHOR]

Martensitic transition temperatures (MTTs) can be tuned by alloying binaries with other elements to create multi-component shape memory alloys (SMAs). However, it is inefficient to use the trial-and-error approach to find compositions with desirable operating temperatures because of the large number of combinations of metals possible to form ternaries, quaternaries, etc. Thus it is crucial to develop the theoretical capability of accurately predicting MTTs as a function of composition, in order to provide experimentalists with necessary and reliable guidance. Previous work has focused on developing and applying first-principles methods to compute phase transitions and MTTs in binary SMAs such as NiTi (nitinol), but certain technical problems associated with the multi-component SMAs remain unsolved. In this work, we employed ab initio molecular dynamics (MD) and thermodynamics integration to study the NiTiHf-based high-temperature ternary SMAs. We overcome the technical challenges to accurately obtain the Gibbs free energy in cubic ternaries where the reference structures are unknown. Specifically, we examined the cubic, monoclinic and orthorhombic structures of Ni 0.5 Ti 0. 5 − x Hf x for x ∈ [ 0 , 0. 5 ] , and our results suggest that the cubic-to-monoclinic martensitic transition occurs when x < 0. 08 , for x > 0. 17 the martensitic transition is between the cubic and orthorhombic phases, whereas in between our calculations cannot distinguish these two martensite structures near the MTT. The computed MTTs vs Hf content x are in good agreement with measured data. Thus our current work paves the way for computational design of multi-component SMAs with desired properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: A recent microstructure-based FEM model that couples crystal-based plasticity, the B2↔B19′ phase transformation and anisotropic elasticity at the grain scale is calibrated to recent data for polycrystalline NiTi (49.9 at.% Ni). Inputs include anisotropic elastic properties, texture and differential scanning calorimetry data, as well as a subset of recent isothermal deformation and load-biased thermal cycling data. The model is assessed against additional experimental data. Several experimental trends are captured – in particular, the transformation strain during thermal cycling monotonically increases and reaches a peak with increasing bias stress. This is achieved, in part, by modifying the martensite hardening matrix proposed by Patoor et al. [Patoor E, Eberhardt A, Berveiller M. J Phys IV 1996;6:277]. Some experimental trends are underestimated – in particular, the ratcheting of macrostrain during thermal cycling. This may reflect a model limitation that transformation–plasticity coupling is captured on a coarse (grain) scale but not on a fine (martensitic plate) scale. [Copyright &y& Elsevier]

This study investigates the high-temperature shape memory behavior of NiTiHf alloys fabricated via selective laser melting process. Specifically, the effects of laser power (100 W and 250 W) on their transformation temperatures, strain, and microstructure were investigated and compared to the ingot. The transformation temperatures of SLM fabricated alloys increased from 150 °C to 350 °C with elevated laser power due to Ni evaporation. The sample fabricated with 100 W showed sharp transformation peaks, good shape memory behavior with recoverable strain of 1.67% and superelasticity. The sample fabricated with 250 W had broad transformation peaks with low recoverable strain of 0.7% during thermal cycling. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A NiTi-20Hf high temperature shape memory alloy (HTSMA) was additively manufactured by selective laser melting (SLM) technique using NiTiHf powder. The thermomechanical and shape memory response were compared to the conventional vacuum induction skull melted counterpart. Transformation temperatures of the SLM material were found to be above 200 °C and slightly lower due to the additional oxygen pick up from the gas atomization and melting process. The shape memory response in compression was measured for stresses up to 500 MPa, and transformation strains were found to be very comparable (up to 1.26% for as-extruded; up to 1.52% for SLM). [ABSTRACT FROM AUTHOR]

Substituting Ni with Au in NiTi leads to dramatic increases in transformation temperatures, meeting one of the requirements for a viable high temperature actuator material. Consequently, four alloys containing between 49 and 51 at.% Ti, a fixed 40 at.% Au, and balance Ni, were prepared and investigated in detail using load-biased thermal cycling (LBTC), scanning electron microscopy (SEM), aberration corrected scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS). LBTC experiments demonstrated work output well above 400 °C, with full recovery up to 100 MPa. The alloys exhibit minimal variation in shape memory properties despite the relatively large composition range from Ti-lean to Ti-rich, in stark contrast to most other NiTi-based systems, which demonstrate extreme compositional sensitivity. Electron beam analysis revealed the presence of two types of secondary phases present in all compositions, which are subsequently characterized. Differences in secondary phase content as a function of alloy composition is shown to have a moderating effect on the transforming matrix composition - an important asset for this alloy system - potentially easing processing requirements and opening up shape memory alloys to new fabrication techniques. Unrecovered strain during cycling at higher loads is analyzed from a theoretical perspective to gain insight into the mechanisms of defect formation responsible for functional fatigue. [ABSTRACT FROM AUTHOR]

Polyimide aerogels show low thermal conductivity and excellent mechanical properties despite their low density. Here, we examine the effect of fabric incorporation into a polyimide aerogel matrix during the gelation process. For this purpose, plain weave fabrics based on high-modulus carbon fibers as well as ultra-high-molecular-weight polyethylene fibers (known as Spectra®) were considered. For a minimal density penalty, only one or two fabrics were incorporated into the aerogel materials. The carbon fabric-based composites outperformed the UHMW polyethylene fabric-based counterparts. The out-of-plane thermal conductivity of the aerogel composites was reduced by approximately half at room temperature with the addition of only one carbon fabric. The incorporation of only two carbon fabrics doubled the flexural strength of the composite materials, but one or two layers of fabric were not enough to statistically alter the ballistic impact performance of these materials in terms of energy absorption per areal density. Both types of fabric-reinforced composite samples showed an almost 4-fold increase in the sound absorption coefficient in comparison with the pristine polyimide aerogels for 2–2.5 kHz. This study shows that for a small change in density and almost no change in volume, one or two carbon fabrics can significantly improve multiple material properties of polyimide aerogels simultaneously. [ABSTRACT FROM AUTHOR]

• Torsional behavior of selective laser melted NiTi tubes was investigated. • Ti-rich precipitates formation tailored the transformation temperatures of NiTi tubes. • Local shear-strain distributions were observed via in-situ strain monitoring. • 2.3% of stable recovery strain was achieved under torsional loading. The large reversible strain upon heating (shape memory effect) or unloading (superelasticity), high power-to-weight ratio, good functional stability, compact size, and lightweight make the rotary NiTi shape memory alloys actuators an interesting candidate for various engineering applications. Additive manufacturing (AM) provides a single step freeform manufacturing process that not only fabricates the complex geometries but also tailors the properties of the printed parts profoundly. In the selective laser melting (SLM) technique, the process parameters (PPs) and scanning strategy show the remarkable effect on the microstructure, the properties, and the size accuracy of as-built parts. Impurity pick-up during the AM process is an unintended incident altering the microstructure and thermomechanical properties of fabricated parts significantly. In this paper, slightly Ni-rich NiTi powder is utilized to fabricate the NiTi tubes with three different thicknesses via the SLM method. It is shown that the bidirectional scanning strategy results in the size deviation of thin-wall tubes. Transformation temperatures (TTs) of the as-fabricated samples are assessed and compared with those of the starting powder. A large shift in TTs is found between the powder and the SLM tubes. The x-ray diffraction pattern shows the martensite phase at room temperature for the starting powder, while the as-built tubes are in the austenite phase coexisting with a secondary phase of Ti-rich oxide. Scanning electron microscopy (SEM) confirms Ti-rich Ti 4 Ni 2 O x precipitates form along the grain boundaries. The characterization of tubes under pure torsional loading shows the localized shear strain on the tube surface. The thermomechanical behavior of the as-fabricated tubes is investigated and shown to exhibit superelastic response with a stable transformation strain of 2.3% after 10 cycles. [ABSTRACT FROM AUTHOR]