Shift Of Transformation Temperatures Research Articles

NiTiCu shape memory alloys with ultra-low phase transformation range as solid-state phase change materials

Shape memory alloys (SMAs) have been demonstrated as effective phase change materials (PCMs) for thermal energy storage (TES) applications. NiTi and NiTiHf SMAs have shown high TES performance, as quantified by PCM figure of merit (FOM) but their use in applications requiring narrow operation temperature windows is limited by large overall phase transformation ranges (OTR). This work investigates NiTiCu SMAs as PCMs with high FOM and low OTR. A full-factorial design of experiments is used to examine 24 NiTiCu compositions. The compositions were fabricated using vacuum arc melting and their phase transformation and thermophysical properties were characterized using calorimetry, thermal diffusivity, and density measurements. The NiTiCu compositions spanned martensitic transformation temperatures between -22 and 84 °C and exhibited greater FOM (250–1050 106J2K−1s−1m−4), compared to traditional PCMs (typically <100 106J2K−1s−1m−4), with major benefits associated with higher density and higher thermal conductivity values. In addition, the NiTiCu compositions in this study show ultra-low OTR (12–20 °C) compared to NiTi and NiTiHf SMAs (>50 °C), enabling utility in narrow operating temperature windows. Thermal cycling was also performed revealing extreme stability of martensitic transformation with only 0.04 °C shift in transformation temperatures after 80 thermal cycles, which is the lowest reported to date in SMA literature.

Enhanced elastocaloric effect and mechanical properties of Gd-doped Ni-Co-Mn-Ti-Gd metamagnetic shape memory alloys

Improving elastocaloric effect (eCE) and mechanical properties is the key to developing high-performance eCE cooling materials. In this paper, the metamagnetic shape memory alloys Ni35.5Co13.5Mn35Ti15-xGdx (x = 0–0.9) were successfully prepared by arc melting method, and the effects of Gd substitution for Ti on the martensitic transformation (MT) temperature, phase composition, microstructure, eCE and mechanical properties of the alloys were systematically investigated. With the introduction of Gd elements, the transformation temperature shifts to a low temperature, and the mechanical properties of the alloy are also enhanced. The fracture strength increased from 980 MPa (x = 0) to 1142 MPa (x = 0.9). At room temperature (RT), a large adiabatic temperature change of + 13.5 K/− 12.7 K is obtained after loading/unloading, and a constant temperature change of + 5.6 K/− 5.5 K is obtained without apparent degradation under the compressive stress of 400 MPa during 61 cycles for all-d metal Heusler Ni36.5Co13.5Mn35Ti14.9Gd0.1 alloy. The adiabatic temperature change of 13.5 K exceeds the elastocaloric temperature change of most NiMn-based alloys available at present, and the mechanical properties of Ni-Co-Mn-Ti-Gd alloy are at a high level among the NiMn-based alloys. The precipitation of the Gd-rich second phase along the grain boundaries is intrinsically responsible for the improved mechanical properties.

AUSTENITE – FERRITE TRANSFORMATION TEMPERATURES OF C MN AL HSLA STEEL

The article is aimed to investigate a shift of transformation temperatures of C-Mn-Al HSLA steel with different cooling rates. The transformation temperatures from austenite to ferrite have been determined by dilatometry using thermal-mechanical simulator Gleeble 1500D. To define the start and finishing temperatures of the austenite-ferrite transformation intersectional method was used. Effect of cooling rate on transformation temperature has been evaluated for 0.17, 1, 5, 10, 15, 20, 25°C.s-1. There was found out that rising the cooling rate results in moving transformation temperature range to lower temperatures. The transformation temperatures have been also compared with temperatures calculated using equations of several authors. Some of them have considered cooling rates only. Cooling rates have effect on final microstructure. The effect has been evaluated by measuring hardness (HV10) relating the cooling rates from 0.17 to 25°C.s-1. Increasing cooling rates resulted in increase of hardness. Moreover, Thermo-Calc software was used to determine the Ae3 and Ae1 equilibrium temperatures. Equilibrium transformation temperatures Ae3-Ae1 were higher than experimentally measured by dilatometric method using Gleeble 1500D.

Torsional behavior and microstructure characterization of additively manufactured NiTi shape memory alloy tubes

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 Ti4Ni2Ox 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.

DSC and microstructure analysis of high temperature Ni-Ti-Hf, low hysteresis Ni-Ti-Cu and conventional super-elastic and shape memory Ni-Ti alloy ingots and wires

The progression of martensitic transformations in calorimetric experiments on NiTi-based shape memory alloys (SMA) depends significantly on the material composition, processing history and the thermo-mechanical treatment which is performed to prepare the material for use in application. The phase transformations in Ni-Ti alloys are first order transitions, i.e. the microstructure symmetry changes occur spontaneously and superheating and supercooling effects emerge. Thus, the thermal scanning rate in differential scanning calorimetry (DSC) affects the observation of important material parameters for the characterization of shape memory alloys like onset temperatures, hysteresis width and change in enthalpy of phase transformations. This work depicts how the transformation characteristics as observed by DSC measurements vary significantly by altering the alloy composition and processing conditions of the material. An in-depth calorimetric study on a wide range of NiTi-based alloys which were prepared as heat treated ingot and processed wire was performed. In order to evaluate also the rate effect on the transformation of distinct alloys, DSC rates from 1 to 30 °C/min were tested, highlighting the interplay between thermal scanning velocity and material parameters. Manifold results are presented on how chemistry, microstructure, DSC sample geometry and processing stresses in the materials contribute to award the alloys diverging sensitivity to the thermal scanning rate. Deviations in binary Ni-Ti composition alter the sensitivity to heating and cooling rate, e.g. martensitic transformation in annealed super-elastic (SE) wire is stronger affected by the rate than shape memory wire due to differences in the precipitation behavior. Ni-Ti-Hf high temperature shape memory alloys show a rather small enlargement of hysteresis, but a huge shift in transformation temperatures, while Cu addition to Ni-Ti causes very sensitive hysteresis changes up on modifying thermal rates which is assigned to B19 orthorhombic transformation. Transformation parameters of wires are less affected by DSC rates than ingots due to their high surface to volume ratio, while sample weight and surface finish is observed to have a minor impact. Small grains and residual strains owing to processing history of drawn wire tend to stabilize the transformation. In materials which exhibit R-phase transformation, the low heat capacity of this phase could be responsible for its minor sensitivity to variations in DSC rate.

Enhanced elastocaloric effect and mechanical properties of Fe-doped Ni–Mn–Al ferromagnetic shape memory alloys

The influence of Fe substitution for Ni on mechanical properties and elastocaloric effect (eCE) in Ni50-xFexMn32Al18 (x = 2, 3 and 4) ferromagnetic shape memory alloys (FSMAs) has been investigated. These alloys undergo a first-order martensitic transformation from B2 (austenite) to L10 (martensite) on the cooling process. As the Fe content increases, the martensitic transformation temperature shifts toward lower temperature region, and the amount of γ phase increases gradually in Ni50-xFexMn32Al18 (x = 2, 3 and 4) alloys. Moreover, the maximum compressive strength (1147 MPa) is obtained in Ni46Fe4Mn32Al18 alloy. The application and removal stress of 300 MPa can produce a large adiabatic temperature change of about 5.1 K and no significant degradation of eCE after 30 stress cycles in Ni46Fe4Mn32Al18 alloy. These results show that Ni50-xFexMn32Al18 alloys would be the promising candidates for new multifunctional materials.

Tuning the Ground State and Its Relationship to Zero-Field-Cooled Exchange Bias in NiMnSnAl Alloys

The crystal structure, phase transitions, and exchange bias (EB) in Al-doped Ni50Mn36Sn14-xAlx Heusler alloys were investigated. With the increase of Al content, the cell volume decreases gradually, and the martensitic transformation temperature shifts to a higher temperature. Due to the positive “chemical pressure” by Al substitution, the antiferromagnetic interaction strengthens, leading to the decrease of magnetization and the ground state evolution from “reentrant” spin glass to spin glass. Zero-field-cooled EB is triggered in samples exhibiting spin-glass ground state. These results suggest a viable way to design Ni-Mn-Z-based alloys showing zero-field-cooled EB by tuning the exchange interactions and ground state via lattice modulation.

Magnetic phase diagram, magnetocaloric effect, and exchange bias in Ni43Mn46Sn11−xGax Heusler alloys

Abstract In this work, the effect of Ga substitution on the crystal structure, phase transitions, magnetocaloric effect, and exchange bias properties, was systematically investigated in Ni43Mn46Sn11−xGax alloys. With increasing Ga content, the Curie temperature of austenite slightly decreases, while the martensitic transformation temperature shifts to a higher temperature and finally disappears, due to the emergence of γ phase. Large low-field magnetic entropy changes around the martensitic transformation were observed as the transition from a paramagnetic martensitic phase to a ferromagnetic austenitic phase took place. Ni43Mn46Sn11 has a reentrant-spin-glass ground state, and Ga doping can enhance the antiferromagnetic interaction arising from lattice contraction, leading to close of intermediate ferromagnetic windows and establishment of canonical spin-glass for x > 2. Prominently, the zero-field-cooled exchange was achieved in the spin-glass region with 5 ⩽ x ⩽ 10 .

Large Magnetocaloric Effect and Magnetoresistance in Fe and Co Co‐Doped Ni‐Mn‐Al Heusler Alloys

The Ni40Co10−xFexMn33Al17 (x = 4, 6, 8) alloys are prepared by arc‐melting method. The crystallographic structure, magnetic and magnetoresistance properties are systematically studied. The large magnetic change is obtained across the martensitic transformation from the ferromagnetic austenite phase to weak‐magnetic martensite phase. With the increase in Fe/Co ratio, the martensite transformation temperature shifts to the lower temperature. The large values of magnetoresistance of 33.3, 25.7, and 13.1% are achieved at the field of 3 T for x = 4, x = 6, and x = 8 alloys around the martensitic transformation, respectively. These results indicate that Ni‐Fe‐Co‐Mn‐Al alloy provides the motive power for further research in practical applications.

NiTi-Polyimide Composites Prepared Using Thermal Imidization Process

We manufactured NiTi plate-polyimide composite samples and analyzed their thermomechanical behavior. The residual stresses formed in the composite result from the shift of transformation temperatures and shape changes during thermal cycling. We demonstrate the use of finite element analysis for modeling the shape changes. The shape changes result from the difference in coefficients of thermal expansion and from the changes of Young’s modulus and of the coefficient of thermal expansion in the NiTi shape memory alloy.

Large reversible magnetocaloric effect in a Ni-Co-Mn-In magnetic shape memory alloy

Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications. Here, we report a large reversible magnetocaloric effect in a Ni49.8Co1.2Mn33.5In15.5 magnetic shape memory alloy. A large reversible magnetic entropy change of 14.6 J/(kg K) and a broad operating temperature window of 18 K under 5 T were simultaneously achieved, correlated with the low thermal hysteresis (∼8 K) and large magnetic-field-induced shift of transformation temperatures (4.9 K/T) that lead to a narrow magnetic hysteresis (1.1 T) and small average magnetic hysteresis loss (48.4 J/kg under 5 T) as well. Furthermore, a large reversible effective refrigeration capacity (76.6 J/kg under 5 T) was obtained, as a result of the large reversible magnetic entropy change, broad operating temperature window, and small magnetic hysteresis loss. The large reversible magnetic entropy change and large reversible effective refrigeration capacity are important for improving the magnetocaloric performance, and the small magnetic hysteresis loss is beneficial to reducing energy dissipation during magnetic field cycle in potential applications.

Phase transformation and mechanical properties of Ti50Ni50 alloy after high-velocity impact

To simulate the impact of space debris, Ti50Ni50 alloy is impacted at 1.2 km s−1 using a powder gun. Phase transformation, microstructure and mechanical properties in Ti50Ni50 alloy induced by impact are investigated. The results show that the phase transformation temperature shifts to a lower temperature in impacted Ti50Ni50 alloy. Variants lose their self-accommodated morphology, martensite plates widen, the variants interfaces migrate and bend under force, and many dislocations exist in the martensite plates. The microhardness reduces and the shape recovery ratio enhances gradually with the increase in distance from the crate. This is attributed to plastic deformation and working hardening caused by high-velocity impact.

Relations Between the Phase Transformation Yields, the Hystereses and the Extents of Transformation in Magnetic Shape Memory Alloys

The Clausius-Clapeyron relation, which is introduced in the case of multi-axial thermo-magneto-mechanical loadings, shows an additive effect between Magnetism and Mechanics on the shift of transformation temperatures. For linear dependence of these temperatures, it permits to link, firstly, the phase transformation Yields on stress and on magnetic field (or the critical stress and the threshold magnetic field), secondly, the hystereses and, thirdly, the extents of temperature during transformation in magnetic shape memory alloys, such as, NiCoMnIn.

Flexible Bamboo-Structured NiCoMnIn Microfibers with Magnetic-Field-Induced Reverse Martensite Transformation

The bamboo-structured Ni45Co5Mn36.8In13.2 fibers with martensitic transition temperature at room temperature were fabricated by melt spinning followed by annealing. A magnetic-field-induced reverse martensitic transformation from martensite to austenite with a shift of transformation temperature of 2.6 K/T was obtained in the microfibers. Different from the intrinsic brittleness evidenced in bulk alloy, the microfibers stored at 320 K (47 °C) were easily flexible, which is due to the fact that the formation of bamboo structures reduces the mismatch stress/strain near grain boundaries.

Viscoelastic sigmoid anomalies in BaZrO3–BaTiO3 near phase transformations due to negative stiffness heterogeneity

Abstract

Influence of Magnetic Field on Magnetostructural Transition in Ni&lt;sub&gt;46.4&lt;/sub&gt;Mn&lt;sub&gt;32.8&lt;/sub&gt;Sn&lt;sub&gt;20.8&lt;/sub&gt; Heusler Alloy

A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Phase transformation of constrained BaTiO 3 particles in a Sn matrix

The phase transformation properties of barium titanate (BaTiO 3 ) particles encapsulated in a Sn matrix have been investigated by high-energy X-ray diffraction both during and after the sintering process. The tetragonal to cubic phase transformation temperature of BaTiO 3 in the sintered state is 11 °C lower than that in the pre-sintered state. The origin of this shift in transformation temperature is attributed to a component of hydrostatic stress acting on the BaTiO 3 particles in the sintered state.

Synthesis, sintering and characterization of PNZST ceramics from high-energy ball milling process

Pb 0.99Nb 0.02[(Zr 0.60Sn 0.40) 0.94Ti 0.06] 0.98O 3 powders were synthesized from oxide mixtures of Pb 3O 4, Nb 2O 5, ZrO 2, SnO 2, and TiO 2 using a Fritsch P4™ vario-planetary ball milling system. The perovskite structure of PNZST powder can be obtained well after 14 h milling and crystallite size of the powders greatly reduced to 20–30 nm. The resultant powders have better sinterability characteristics to obtain dense ceramic bodies at low temperature. The dielectric constant of samples shows a maximum at 1150 °C and the PE MC to PE SC transformation temperature shifts to higher temperature with an increase in grain size. The field-induced strain reaches maximum longitudinal strain of 0.32% and the P– E hysteresis loop shows much higher squareness at 1150 and 1200 °C. The measured dielectric and antiferroelectric properties of the PNZST ceramics were found to be comparable to those from other processing techniques.

Magnetization, shape memory and hysteresis behavior of single and polycrystalline FeNiCoTi

We report on the shape memory characteristics and magnetic behavior of polycrystalline and single crystalline FeNiCoTi. Predeforming the samples in the martensitic state and biasing of the martensite variants produced anisotropy in the magnetization behavior allowing the ‘easy axis’ to be identified as the ‘ a-axis’ in the martensitic state. Based on these results, we provide an estimate of the magnetic anisotropy energy as 8.34×10 5 ergs/cm 3. The results confirm the different magnetization behavior in the martensitic and austenitic states, and the shift in transformation temperatures upon application of a magnetic field. Shape memory strains near 2.5% are demonstrated under constant stress temperature cycling and upon heating at zero stress after deformation. We present a thermodynamics based theory that explains the origin of the hysteresis in this class of alloys emanating from the dissipation of energy due to plastic deformation. We predict the thermal hysteresis (135 K), and the shift in transformation temperature (14 K) with applied magnetic fields in agreement with the experimental results. The possibility of utilizing these classes of alloys as magnetic shape memory alloys is discussed.

Electrical resistivity of NiTi with high transformation temperature

For a better understanding of the transformation behaviour on NiTi, the electrical resistivity of Ni 49.5Ti 50.5 was precisely measured by a four-probe potentiometric method. The temperature dependence of the electrical resistivity is linear in the regions of the high- and low-temperature phases, and a drastic change in electrical resistivity is observed in the temperature range of the transformation. The transformation remains of the one-stage between the high- and low-temperature phase even with thermal cycling, although with increasing the number of thermal cycles the electrical resistivity versus temperature curve shifts to the low temperature side and the electrical resistivity increases in the single phase temperature region which is attributable to the generation of transformation-induced defects. The one-stage transformation changes to a two-stage transformation due to the appearance of an intermediate phase after an incomplete thermal cycle, and with subsequent thermal cycling the peak in the electrical resistivity is enhanced to stabilise the intermediate phase. The as-cut Ni 49.5Ti 50.5, of which the surface is damaged, shows a two-stage transformation, even during the first cooling after cutting, and a resistivity-peak due to such a two-stage transformation is also enhanced with thermal cycling. It is thought that the amount of defects plays important roles in the shifts of transformation temperatures and in the transfer from one-stage to the two-stage transformation, and contributes to the stability of each phase.