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Experiments have confirmed the feasibility of an innovative, new class of very simple, reliable, low mass, low packaging volume, and low-cost self-deployable structures for space and commercial applications.
Experiments and analyses have confirmed the feasibility of an innovative, new class of very simple, reliable, low mass, low packaging volume, and low-cost self-deployable structures for space and commercial applications.
Thermoelastic deformation mechanisms in polycrystalline biomedical-grade superelastic NiTi are spatially mapped using in situ neutron diffraction during multiaxial loading and heating. The trigonal R-phase is formed from the cubic phase during cooling to room temperature and subsequently deforms in compression, tension, and torsion. The resulting R-phase variant microstructure from the variant reorientation and detwinning processes are equivalent for the corresponding strain in tension and compression, and the variant microstructure is reversible by isothermal loading. The R-phase variant microstructure is consistent between uniaxial and torsional loading when the principal stress directions of the stress state are considered (for the crystallographic directions observed here). The variant microstructure evolution is tracked and the similarity in general behavior between uniaxial and torsional loading, in spite of the implicit heterogeneous stress state associated with torsional loading, pointed to the ability of the reversible thermoelastic transformation in NiTi to accommodate stress and strain mismatch with deformation. This ability of the R-phase, despite its limited variants, to accommodate stress and strain and satisfy strain incompatibility in addition to the existing internal stresses has significance for reducing irrecoverable deformation mechanisms during loading and cycling through the phase transformation.
Polymeric materials are usually tailored for specific functionality. A single polymer exhibiting multiple simultaneous functionalities often requires intricate molecular architecture, which is difficult to manufacture at scale because of its complex synthesis routes. In this work, a facile, partly renewable composition—prepared via reactive melt processing—that induces tunable functionalities such as 3D printability, shape recovery, and self-healing while exhibiting satisfactory mechanical properties is reported. The system with a hydrogen-bonded 3D network consists of thermally reversible nano-scale agglomerates of sustainable, rigid phenolic oligomers and crystallizable flexible polymer. Local molecular mobility and temperature-dependent relaxation and recovery of the non-equilibrium networked states enable exploiting these simultaneous functionalities. Transitions involving solidification and structure stabilization at ambient temperature spanning several hours after preheating only at 70 °C directly contrast typical thermoplastic or thermoplastic elastomer behaviors. Results from this study can inform the design of future rheology modifiers and materials for soft robotics.
Herein, we demonstrate how to predict and experimentally validate phase diagrams for multi-component systems from a high-dimensional virtual space of all possible phase diagrams involving several elements based on small existing experimental data. The experimental data for bulk phases for known systems represents a sampling from this space, and screening the space allows multi-component phase diagrams with given design criteria to be built. This approach uses machine learning methods to predict phase diagrams and Bayesian experimental design to minimize experiments for refinement and validation, all within an active learning loop. The approach is proven by predicting and synthesizing the ferroelectric ceramic system (1-ω)(Ba 0.61 Ca 0.28 Sr 0.11 TiO 3 )-ω(BaTi 0.888 Zr 0.0616 Sn 0.0028 Hf 0.0476 O 3 ) with a relatively high transition temperature and triple point, as well as the NiTi-based pseudo-binary phase diagram (1-ω)(Ti 0.309 Ni 0.485 Hf 0.20 Zr 0.006 )-ω(Ti 0.309 Ni 0.485 Hf 0.07 Zr 0.068 Nb 0.068 ) designed for high transition temperature (ω ≤ 1). Each phase diagram is validated and optimized through only three new experiments. The complexity of these compounds is beyond the reach of today’s computational methods.
The structural, magnetic, and transport properties of the Mn{sub 2}Ni{sub 1.8}In{sub 0.2} alloy have been presented. It was observed to form in tetragonal structure. Multiple magnetic phase transitions have been detected in magnetization data over the temperature range from 2 to 350 K. From the resistivity, ac susceptibility, and heat capacity data, the superconducting transition are observed at T = 5.6 K. The alloy is considered to be one of the potential candidates from the Mn-rich alloys, which has shown both magnetic and superconducting properties, which can be useful from application point of view.
Cu-Zn-Al foams of varying porosity fractions using mechanical alloyed powder have been made through powder metallurgy route. Here, NH{sub 4} (HCO{sub 3}) was used as a space holder. Mechanically alloyed Cu-Zn-Al is made using a planetary ball mill taking the ratio of Cu/Zn/Al = 70:25:5 (by weight ratio). The ball/powder ratios were varied in the four ranges 10:1, 15:1, 20:1, and 25:1. Green compacts of milled powder and space holder samples were sintered at three stages at three different temperatures 350, 550, and 850 °C for 1 h at each stage. The crystalline size and particle size as a function of ball/powder ratios were examined. The compressive deformation responses of foams are varied with relative density and the ball/powder ratio. The plateau stress and energy absorption of these foams increase with an increase in relative density but decreases with increase in ball/powder ratio, even though crystalline size decreases. This has further been explained on the basis of particle morphology as a function of ball/powder ratio.
The link between microstructure and mechanical properties is investigated for a superelastic Ni–Mn–Ga microwire with 226 μm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse γ precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load–unload cycles, reproducible stress fluctuations of ~3 MPa are measured on the loading superelastic stress plateau of ~50 MPa. During cycles at various temperatures spanning -70 to 55 °C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of ~13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic γ precipitates and the twinning martensite matrix. Lastly, the temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.
Current approaches to tailoring the thermal expansion coefficient of materials or finding materials with negative thermal expansion rely on careful manipulation of either the material's composition and/or the complex fabrication of composites. By contrast, we report a new principle that enables the precise control of macroscopic thermal expansion response of bulk materials via crystallographic texture manipulation and by taking advantage of anisotropic Coefficients of Thermal Expansion (CTE) in a large class of martensitically transforming materials. Through simple thermo-mechanical processing, it is possible to tailor the thermal expansion response of a single material––without manipulating its composition––over a wide range of positive and negative values. We demonstrate this principle by gradually tuning the macroscopic CTE in a model NiTiPd alloy between a positive (+14.90 × 10 -6 K -1 ) and a negative (-3.06 × 10 -6 K -1 ) value, simply by incrementally increasing tensile plastic deformation in the martensite phase. This surprising response is linked to the large positive, +51.33 × 10 -6 K -1 , and negative, -34.51 × 10 -6 K -1 , CTE anisotropy, at the lattice level, along the different crystal directions in martensite. Similar CTE anisotropy is also shown experimentally in CoNiGa and TiNb alloys. In a model TiNb alloy, giant macroscopic CTEs of +181 × 10 -6 K -1 and -142 × 10 -6 K -1 are measured. A connection between the CTE anisotropy and the martensitic transformation in these and other materials systems such as NiTi, pure uranium, and PbTiO 3 is later made. It is shown that negative or positive thermal expansion crystallographic directions are connected to the crystallographic relationship between the austenite and martensite lattices, and can easily be predicted using the lattice parameters of austenite and martensite phases. Our current observations and analyses suggest that the tunability of the macroscopic CTE through thermo-mechanical processing is universal in materials––both ceramic and metals––that undergo martensitic transformations
Compression-compression cyclic deformation of nanocrystalline NiTi tubes intended for medical stents and with an outer diameter of 1 mm and wall thickness of 70 μm was studied using micropillars produced by FIB with the loading axis orthogonal to the tube axis. These micropillars were cycled in a displacement-controlled mode using a nanoindenter equipped with a flat punch to strain levels of 4, 6 and 8% in each cycle and specimens were subjected to several hundred cycles. Furthermore, the cyclic response of two NiTi tubes, one with Af of 17 °C and the other with an Af of -5 °C is compared. The texture of the tube with the Af of -5 °C was measured at the microscopic level using transmission electron microscopy and at the macroscopic level by X-ray diffraction and good agreement was noted. Characteristics such as i) a reduction in the forward transformation stress, ii) increase in maximum stress for a given displacement amplitude, and iii) a reduction in the hysteresis loop area, all with increasing number of cycles, observed typically during cyclic deformation of conventional macroscopic specimens, were captured in the micropillar cyclic tests. Our observations lead to the conclusion that micropillar compression testing in a cyclic mode can enable characterizing the orientation-dependent response in such small dimension components that see complex loading in service, and additionally provide an opportunity for calibrating constitutive equations in micromechanical models.
Magnetocaloric refrigeration is a solid-state cooling approach that assures high energy efficiency and low environmental impact. It remains uncompetitive with conventional vapor-compression technologies due to lack of high-performing materials that exhibit large magnetocaloric effects in low magnetic fields. In this work we report a game-changing enhancement of the magnetocaloric response in a transition-metal-based Ni–Co–Mn–Ti. Mechanically and chemically stable rapidly solidified ribbons exhibit magnetic entropy changes as high as ~27J·kg –1 K –1 for a moderate field change of 2T, comparable to or larger than the best known materials for near-room temperature applications. The ribbons can be easily manufactured in large quantities and the transition temperature can be adjusted by varying Co concentration.
Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V 45 Ti 30 Ni 25 (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. Furthermore, the bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed.
Taking full advantage of intrinsic high strength of nano-reinforcements has been proven difficult in a composite until the concept of strain matching was experimental verified in a nanowire Nb/TiNi in-situ composite, which was obtained via solidifying eutectic Nb/TiNi and subsequent severe wire drawing. However, the volume fraction of the nano-reinforcement was dictated by eutectic reaction, which severely limited the development of high-performance composites. Here, the martensitic transformation nanocomposites were successfully acquired with increased volume fraction and changeable morphology of the nano-reinforcement through hot packaged ARB (PARB) and subsequent wire drawing. The nano-reinforcement with lamellar configuration in the composite could exhibit a large lattice strain of similar to 3.8%, which demonstrated the applicability and effectiveness of the strain matching principle in nano-lamellar composite. The obtained composites show a yield strength over 2.0 GPa and a quasi-linear elastic strain as larger as similar to 6.5% with adjustable apparent elastic modulus from 30 to 50 GPa.
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Nanocrystalline (NC) NiTi were previously reported to obtain non-hysteretic superelasticity and nearly zero temperature-dependence of superelastic stresses. However, the influence of the non-transforming regions in the specimens was not ruled out. In this study, we fabricate NC NiTi specimens with grain size down to 15 nm by wire drawing and annealing. Our specimens have no detectable non-transforming regions and thus are of complete transformation capability. We demonstrate that the hysteretic superelasticity is observed in all specimens of different grain sizes and that the minimum temperature-dependence of superelastic stresses is 4.7 MPa/K. Possible reasons for the discrepancy have been discussed.
Nanowires (or nanoribbons) were identified as the most potential structural reinforcements for composites due to their ultrahigh strength, but it is difficult to transcribe their ultrahigh strength into the composite. Here, an in-situ W nanoribbbons-NiTi matrix composite was fabricated by mechanical reduction of a W-NiTi ingot, and the micromechanical deformation behavior was studied by in-situ high energy X-ray diffraction during tensile loading-unloading. It is revealed that the large elastic deformation of the W nanoribbons in a NiTi matrix is dictated by and synchronized with the martensitic transformation of the matrix. The bearing strength of the W nanoribbons, up to 11.6 GPa, was achieved in this composite. A significant load transfer took place from the matrix to the nanoribbons, exhibiting the characters of locality and rapidity. In conclusion, the stress fraction carried by the W nanoribbons with the volume fraction of 3% is up to 45% for the strength of the composite.
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We report the influence of the microstructure on the martensitic transformation in polycrystalline Cu−Zn−Al thin films with 18R structure. The films are grown in two steps. First, Cu−Al thin films are obtained by DC sputtering. Second, the Zn is introduced in the Cu−Al thin films by the annealing them together with a bulk Cu−Zn−Al reference. The crystalline structure of the films was analyzed by X-ray diffraction and transmission electron microscopy. The martensitic transformation temperature was measured by electrical transport using conventional four probe geometry. It was observed that temperatures above 973 K are necessary for zincification of the samples to occur. The resulting martensitic transformation and its hysteresis (barrier for the transformation) depend on the grain size, topology and films thickness. - Highlights: • Polycrystalline Cu−Al−Zn thin films with nanometric grain size are sintered. • Influence of thermal annealing process on the microstructure is analyzed. • Martensitic transformation of Cu−Al−Zn thin films is strongly affected by the microstructure.