Changes In Transformation Temperatures Research Articles

Meso-scopically homogeneous superelastic transformation and related elastocaloric effect in a textured Ti-based shape memory alloy

Homogeneous superelastic behavior in shape memory alloys (SMAs) has been confirmed to be crucial for functional and structural fatigue properties, and more importantly, for stable elastocaloric effect (eCE). Here, the Ti–22Nb–4Zr–2Ta plate was prepared by successive cold rolling at 98.5%, followed by annealing at 1023 K for 30 min, forming a strong recrystallized texture, denoted as {112}β < 110>β. Such favorable texture contributed to a completely recoverable superelastic strain of ∼2.4% with narrow hysteresis of 35/45 MPa under isothermal/quasi-adiabatic conditions, which resulted in homogeneous martensitic transformation (MT) and adiabatic temperature changes (ΔTadi) of 4.8 K upon loading and 4.4 K upon unloading. The observation on the evolution of space-resolved strain and temperature throughout the sample revealed that the intrinsic feature of diffuse transformation along with favorable texture contributed to meso-scopically homogeneous transformation and related eCE. A standard deviation of strain was determined to be only 0.05%, corresponding to a small temperature deviation of 0.2–0.3 K, which quantitatively demonstrated the homogeneity, potentially high solid-state cooling performance efficiency, and long structural and functional fatigue life in Ti-based SMAs.

Enhancing the thermal stability and recoverability of ZrCu-based shape memory alloys via interstitial doping

ZrCu-based shape memory alloys (SMAs) are regarded as emerging smart materials with high phase transformation temperatures. However, unfavorable thermal stability and poor strain recovery rate hinder commercial applications. Herein, these issues were addressed by doping interstitial B atoms, and the impacts of B content on microstructure, martensitic transformation behavior and properties of ZrCu-based SMAs were systematically investigated. It was shown that (Zr50Cu25Ni7.5Co17.5)99.98B0.02 SMAs possessed excellent thermal stability and shape memory effect. The change of martensitic transformation temperature for this alloy was 15.7 °C after several cycles. The shape recovery rate reached 92.5 % at 8 % compressive pre-strain, which was attributed to the de-twinning of (0 0 1) compound twins and the appearance of (1 1 1) type I twins. Furthermore, it was elucidated for the first time by first-principles calculations that B atoms were covalently bonded with Zr and Cu atoms occupying the octahedral interstitial positions in the B19′ and Cm phases. This study offers an available pathway to exploit efficient and advanced products.

Combined shape memory alloy phenomena: A novel approach to extend applications of shape memory alloys

The present study introduces a novel approach enabling new applications of shape memory alloys by combining different shape memory alloy phenomena during use. It is shown that the shape memory effect can be exploited and combined with superelasticity. This is realized by a change of transformation temperatures due to the formation of nano-sized precipitates. In contrast to the conventional design of components only aiming at one phenomena, i.e., tailoring the precipitate size first to set the predefined functional properties, in the present approach the precipitates are formed during activation of the shape memory effect.

INVESTIGATION OF THE EFFECT OF SINTERING TEMPERATURES ON THE PRODUCTION OF POROUS NITI ALLOY BY LOOSEN SINTERING METHOD

This article aims to produce NiTi shape memory alloys, which show superelasticity and shape memory effect, as well as good biocompatibility and corrosion properties, in open and porous sizes ranging from 100-500 μm, which are required for use as implants. This structure of pores is necessary to allow tissue growth and fluid flow inside the implants. Many powder metallurgy methods have been used in producing porous NiTi shape memory alloys. However, the packaging pressure used in these methods has not successfully created the desired pore distribution, shape, and size. The methods by which it can be produced are costly in terms of cost. In the study, production was carried out by sintering the powder mixture poured into molds without pressure with the help of binder polymers. This sintering process was carried out in an argon atmosphere for 1 hour at temperatures 1050, 1125, and 1200 °C. The study shows that pressureless loosen sintering can produce porous NiTi alloys, which is the more straightforward method. The pore distribution and proportions were examined. Homogeneous distribution and pores in desired sizes are created. It has also been determined that the binder polymer has a space-retaining effect. It was determined at which temperature the alloy sintered at different temperatures contained the desired B2 austenite phase for superelasticity. Austenite start and finish temperatures were determined for the alloy produced at each sintering temperature. As a result of this research, it was determined which phase was denser at which temperature, and the phase transformation temperatures were found. The exact temperature measurements can be calculated by changing the sintering time parameter. In addition, the change in phase transformation temperatures can be measured by heat treatment after sintering. Porous NiTi alloys can be used as dental and medical implants thanks to their excellent biocompatibility and corrosion resistance. This method will decrease production costs, and more people will have access to this material. In general, the mechanism of sintering methods is joining the points in contact with the packaging pressure by necking. In this study, the combination of the grains with the polymer without packaging pressure with the thermal expansion mechanism reveals the originality of the study.

Degradation of recovery properties after heat treatment in hot-forged NiTiFe alloy

NiTiFe is a kind of low-temperature shape memory alloy (SMA) with great application potential in aerospace, such as pipe joints. Heat treatment can effectively regulate the microstructure and recovery properties (recovery strain and recovery stress) of SMAs. In this study, the microstructure, recovery properties, and transformation temperatures of hot-forged Ni47Ti50Fe3 shape memory alloy (SMA) before and after heat treatment (HT) with different HT temperatures (550–950 ℃) were analyzed. The results show that when the HT temperature is above 650 ℃, the density of high- and low-angle grain boundaries was reduced. The recovery properties of the hot-forged NiTiFe alloy deteriorate with increasing HT temperature. The initial hot-forged samples showed the best recovery strain of 8.32% and recovery stress of 449 MPa, which were reduced to 5.99% and 299 MPa, respectively, after 950 ℃ HT. The R↔B2 phase transformation temperature changes little with the increasing HT temperature. But the reverse martensitic phase transformation finish temperature increased from − 75 ℃ to − 50 ℃ after tensile deformation at − 180 ℃. The EBSD and HRTEM analyses of the microstructure after the free and constrained recovery tests were carried out. The {114}B2 and {112}B2 twins remained after the free recovery process, and more of those twins remained after the constrained recovery process. Although the recrystallization and grain growth can reduce the defects density which are the obstacles to the form of detwinning martensite, it also makes the deformation twins prone to form during the tensile and constrained recovery process, thus leading to poor recovery properties.

Study on interfacial characteristics and properties of NiTi/Al–Mg joint by vaporizing foil actuator welding

As a new type of impact welding technology, Vaporizing Foil Actuator Welding (VFAW) has the advantages of high welding efficiency and small heat affected zone, which could avoid the decline of shape memory properties of the NiTi alloy in the NiTi/metals joint of fusion welding. Therefore, NiTi/Al–Mg VFAW experiments under different discharge voltages and standoffs were carried out to analyze the effects of process parameters on interface morphology, mechanical and functional properties. The results showed that high strength welding between NiTi and Al–Mg alloys were realized. With the increase of discharge voltage and standoff, the interface morphology changed to flat, a small amount of melting layer, wave, continuous melting layer, and the joint performance increased first and then decreased. The difference in properties between NiTi and Al–Mg alloys led to the change of microstructure and nano-hardness at the interface. Equiaxed recrystallized grains and small slender grains appeared on Al–Mg and NiTi alloy respectively. The nano-hardness of both sides increased, but the hardening range of the Al–Mg alloy side was smaller. The changes of phase transformation temperatures of the NiTi/Al–Mg joints were small compared to NiTi base, indicating the VFAWed NiTi alloy still maintained the original shape memory function. This work deepens the understanding about the properties of NiTi/Al–Mg VFAW, and is helpful for expanding the application of NiTi shape memory alloy.

Evolution of transformation behavior and tensile functional properties with process parameters for electron beam wire-feed additive manufactured NiTi shape memory alloys

Electron beam wire-feed additive manufacturing (EBAM) has been shown to be an ideal method to additively manufacture NiTi alloys with good tensile functional properties. However, the influences of process parameters on the microstructure and thermomechanical response of EBAM processed NiTi alloys have not been revealed adequately up to now. In this study, the evolution of formation quality, microstructure, phase transformation behavior, and functional properties with the variation of process parameters were comprehensively studied for the NiTi alloys fabricated by EBAM. The results show that the process parameters may significantly affect the surface formation quality of the as-built NiTi alloy parts. All samples exhibit continuous and coarse columnar grains in the building direction in spite of the large variation of EBAM process parameters. One-stage A↔M transformation behavior was found for all as-built samples, while the phase transformation temperatures vary for different process parameters. The change in phase transformation temperature mainly results from different amount of Ni-evaporation, on which the beam power has the most significant effect. Most of the samples, which have low transformation temperatures, exhibit good tensile superelasticity after post heat treatment, while the sample with high transformation temperature shows good shape memory effect at as-built state. This study shows that by regulating process parameters, the EBAM can be used to fabricate NiTi alloys with different thermomechanical responses according to the demands of practical applications, while no need to change the raw material.

Adjustment of chemical composition with dissimilar filler wire in 1.4301 austenitic stainless steel to influence residual stress in laser beam welds

Residual stress is a considerable challenge in welding. Due to the local heat input high temperature gradients occur between weld seam and base material, which lead to thermal and transformation induced stress. With targeted alloying in the weld seam the martensitic phase transformation can be shifted to lower temperatures, called Low Transformation Temperature (LTT) effect. This effect uses the volume expansion during the martensitic phase transformation, which counteracts the stress accumulation while cooling of the specimen. In most cases, a flux cored filler wire with a typical alloy composition of 10%-Cr and 10%-Ni is used to exploit the LTT effect. In this paper, however, the aim is to alloy in-situ while combining conventional materials of different types. Combinations of high-alloyed base material (1.4301) with low-alloyed solid filler wire (G3Si1) are analysed. For comparison, similar welds with high-alloyed solid filler wire (G19 9) are also carried out. The chemical compositions are generated within a laser beam welding process and examined by EDS-Analysis. In order to measure the influence of chemical compositions on residual stress, hole drilling measurements are performed. Furthermore, dilatometry tests help to determine the change of phase transformation temperature as well as elongation. EBSD analysis show the changes in phase transformation due to the variation of chemical compositions. It is proven, that a reduction of residual stress by the means of targeted alloying with conventional materials in an austenitic stainless steel is possible.

Acceleration of B2/L21 order-disorder transformation in Ni2MnAl Heusler alloys by in-magnetic-field annealing

Ferromagnetic L21 phase of Ni2MnAl with high order degree is difficult to synthesize because of low L21/B2 order-disorder transformation temperature. It is found that magnetic field accelerated the transformation from the antiferromagnetic disordered B2 to ordered L21 phase. Magnetization increased by in-magnetic-field annealing at 673 K, which indicated the acceleration of the ordering to L21 phase. Magnetic field effects on the B2/L21 order-disorder transformation were discussed based on magnetic-field-induced change of transformation temperature, enhancement of degree of order at annealing temperature, and reduction of critical radius of L21 nuclei. Growth kinetics of L21 phase were evaluated by Johnson-Mehl-Avrami equation, which indicated that 15-T annealing accelerated the growth of L21 phase 5.7 times faster than zero-field annealing. Our results showed that magnetic field can contribute to shortening the annealing time for synthesis of ferromagnetic L21-Ni2MnAl alloy.

The effect of solution treatment temperature on SIEM and shape recovery of Fe–Mn–Si shape memory alloy

Solution treatment process is stepwise process in manufacturing of iron based shape memory alloy. It is well known that the grain size of austenite in the solution treatment condition depends on the austenitizing/solutionizing temperature. Moreover, the solution treatment temperature influences the transformation temperatures of SMAs. It has been found by researchers that solution treatment temperature brings about modification in the grain size, the grain boundary angles, and the annealing twins. The major objective of the present work is therefore to assess the influence of modifications in the microstructural features by using different solution treatment temperatures. Optical microscopic studies reveal the microstructural changes brought about by changing the solution treatment temperature. DSC shows that somewhat change of transformation temperatures in regarding to the conjugated solution treatment temperature, but the martensite transformation temperatures are still in the range of ambient temperature. X-ray diffraction results refer to the enhancement of SIEM in the alloy that was solution treated at 1000 °C. TEM observations prove that uniform stacking faults have been generated at the austenite of the alloy that has been solution treated at 1000 °C. In addition, SADP shows that ε-martensite is the domain structure generated after 3.1% pre-strain of the sample that has been solution treated at 1000 °C. The results indicate that solution treatment at an optimal temperature of 1000 °C leads to increase of shape memory effect of iron base shape memory alloy.

Martensitic transformation, magnetic entropy, and adiabatic temperature changes in bulk and ribbon Ni48Mn39.5Sn12.5\u2212xInx (x\u2009=\u20092, 4, 6) metamagnetic shape memory alloys

Martensitic transformation, magnetic entropy, and direct adiabatic temperature changes in Ni48Mn39.5Sn12.5−xInx (x = 2, 4, 6) metamagnetic Heusler bulk and grain-constrained ribbon alloys were studied. All alloys showed a typical L21 structure in austenite and the 4O structure in martensite. Their relative volume contributions changed depending on In content. With increasing In concentration, the martensitic transformation temperature increased, whereas the Curie temperature of austenite decreased. The magnetic entropy change under magnetic field of 5 T attained maximum of 20 J/kgK in the bulk and 14.4 J/kgK in the ribbon alloys with the Ni48Mn39.5Sn8.5In4 nominal composition. The corresponding adiabatic temperature change under 1.7 T yielded 1.3 K for the Ni48Mn39.5Sn8.5In4 bulk alloy. Despite grain confinement, melt spinning was found to stabilize martensite phase. Changes observed were discussed with relation to strengthened covalency imposed by In substitution.Graphic abstract

Effects of grain refinement on the microstructures and damping behaviors of a Cu–Al–Ni–Mn–Ti shape memory alloy

Grains of a Cu–Al–Ni–Mn–Ti shape memory alloy (SMA) were remarkably refined by the Al–La–B inoculant. Evolution of microstructures and changes of both martensitic transformation (MT) temperatures and damping property of the Cu–Al–Ni–Mn–Ti SMA were also systematically investigated. It was found that with increasing the adding amount of inoculant, the grain size decreased first and then increased, whereas the damping increased first and then decreased. When the adding amount of inoculant reached 2.5 wt%, the smallest grains and the highest damping could be obtained. The improvement of damping could be attributed to the increased density of interfaces between matensite lamellae and between twins. Moreover, with increasing the adding amount of inoculant, all characteristic temperatures of both reverse and forward MT shift to high temperature side. This could be ascribed to the decreased Al content in the Cu–Al–Ni–Mn–Ti SMA due to the formation of Al11La3 phase. In addition, this is also related to the changes of the density of grain boundaries and the order degree of the Cu–Al–Ni–Mn–Ti SMA.

Effect of Ti/Ni and Hf/Zr ratio on the martensitic transformation behavior and shape memory effect of TiNiHfZr alloys

Abstract The functional properties of shape memory alloys are strongly sensitive to the composition of alloys. In this study, the effect of Ti/Ni ratio and Hf/Zr ratio on the martensitic transformation behavior, shape memory effect and microscopic structures of two different quaternary TiNiHfZr alloys (TixHf15Zr5Ni80-x and Ti31.5HfyZr20-yNi48.5) have been investigated systematically. It is found that increasing Ti/Ni ratio of TixHf15Zr5Ni80-x alloys dramatically increases the martensitic transformation temperature and a maximum value (5%) of the recovered strain during shape recovery on heating is obtained for the alloy with x = 30. On the contrary, changing Hf/Zr ratio of Ti31.5HfyZr20-yNi48.5 alloys does not result in big change of martensitic transformation temperature or recovered strain. The evolution of macroscopic properties of TixHf15Zr5Ni80-x alloys with x can be understood by considering the balanced effect between the Ni content change of the matrix, and the volume fraction of Ti2Ni-like precipitates, which are often brittle and not desired for SMAs. Our results suggest that the TixHf15Zr5Ni80-x (x > 30.5 at. %) alloys are promising shape memory alloys for high temperature applications above 250 °C.

On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

The giant magnetocaloric effect was quantified in CoMn1-xFexGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P63/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple (thermo) magnetization measurements. It was found that increasing Fe content led to the decrease of both the martensitic transformation temperature and entropy change (Delta S) at the point of the phase transition. Moreover, first-time magnetocaloric measurements resulted in irreproducible entropy change versus temperature diagrams, which was attributed to the release of internal pressure in bulk samples that disintegrated into powder upon transformation. CoMn1-xFexGe demonstrated larger magnetic field-induced entropy changes and giant magnetocaloric effect (MCE) compared to other CoMnGe alloys doped with Si, Sn, Ti, and Ga. However, the observed brittleness and apparent change in volume at the magnetic transition was posited to influence the material’s potential for regenerative applications.

Influence of Heating Rate on the Transformation Temperature Change in Selected Steel Types

The paper is focused on the polymorphic transformations of selected steel types. Generally, when creating CCT diagrams, the transformation temperature values Ac1 and Ac3 are determined under at very low heating rates (max. 0.1 °C?s-1). The only exceptions are so-called in-situ CCT diagrams, which are determined based upon the real temperature cycles. However, in the case of dynamic processes such as welding technology or heat treatment with magnetic induction heating, cannot be used information from conventional CCT diagrams. Incorrectly selected temperatures lead either to incomplete austenitizing during heat treatment or to a different development of phase transformations in HAZ of welds. Especially, when these processes are carried out without soaking temperature. That is why as the main aim of this paper was to determine the heating rate influence for steels S460MC, S690QL and X23CrMoV12-1 on the shift of their transformation temperatures Ac1 and Ac3. The in-tensity of the heating rate effect on the transformation temperature depends on the steel structure and amount of alloying elements. Consequently, steels with different structures, different type of processing and mechanical properties were deliberately selected to make the assessment as complex as possible.

Effect of process parameters on the phase transformation behavior and tensile properties of NiTi shape memory alloys fabricated by selective laser melting

Abstract In this work, we show that selective laser melting (SLM), apart from producing complex structures, can also act as a metallurgical method to modify the Ni/Ti ratio of NiTi shape memory alloys, and thus providing a feasible approach to tailor the transformation temperatures and to modify the mechanical performance of NiTi alloys. NiTi samples were fabricated by SLM with a large variation of process parameters, e.g. scanning speed (v) from 400 to 1200 mm s−1, hatch spacing (h) from 40 to 110 μm, and laser power (P) from 60 to 200 W. It is found that the martensite transformation temperature changes monotonously with the respective change of v, h or P. The composition analysis suggests that the different amount of Ni-loss under various SLM process conditions is the main reason for the evolution of transformation temperatures. Most importantly, good mechanical (total elongation >10 %) and functional properties under tensile mode have been obtained despite of the large variation of SLM process parameters and the presence of extensive defects. The good tensile properties and tailorable phase transformation temperatures will provide great potential to make novel NiTi smart structures.

Mechanical and elastocaloric effect of aged Ni-rich TiNi shape memory alloy under load-controlled deformation

This study investigated the elastocaloric effect of Ni-rich Ti49.6Ni50.4 shape memory rods under compression. As-received rods, rods aged at 300 °C for 1 h, and rods aged at 275 °C for 125 h were investigated to reveal the effects of aging on the elastocaloric properties. These samples were first trained under constant stress of 1200 MPa or 1500 MPa, which decreased the B19′ transformation temperature and stabilized the R-phase transformation. Associated with the changes in transformation temperatures, the pseudoelasticity performance and elastocaloric effect of the samples decayed with increasing training times and then became stable when the training time exceeded a certain value. The sample aged at 275 °C for 125 h, which was most hardened by the precipitation hardening effect, exhibited better stability during the training process due to less plastic deformation and thus retained an obvious elastocaloric cooling effect. Because the precipitation treatment broadened the transformation range of the sample aged at 275 °C for 125 h by introducing a two-stage B19’→R→B2 reverse transformation, the elastocaloric cooling effect of 7 °C could be utilized at a relatively large temperature range of −20 °C to 100 °C.

Influence of hydrostatic pressure on martensitic transformation and strain behavior for Co52V29+xGa19−x Heusler alloys

In the present work, polycrystalline Co52V29+xGa19-x(0 ≤ x ≤ 2) Heusler alloys have been prepared. The related experimental results confirm that each sample undergoes a first-order martensitic transformation (MT) from the L21 cubic austenitic state to the D022 tetragonal martensitic state. With increase of V concentration, both transformation temperature and strain are heightened simultaneously, thus giving rise to an increment of transition entropy change $$\left( {\Delta S_{{{\text{tr}}}} } \right)$$ . In addition, the influence of hydrostatic pressure on MT and transition strain have been also carefully studied. With increase of pressure, the transformation temperatures of these samples move to high values apparently, but their reversible strain only exhibit a slight enhancement and quickly approach the saturated values. For different compositions, however, there is no obvious difference in the rate of transformation temperature change with pressure. Such a behavior opens up the potential applications of the functional properties associated with the hydrostatic pressure-induced MT for this system.

Effects of aging temperature and aging time on the mechanism of martensitic transformation in nickel-rich NiTi shape memory alloys

The effects of aging temperature and time on the shape memory behavior of the high nickel-rich Ni54Ti46 (at.%) alloys were investigated. The changes in transformation temperatures, microstructure, lattice structure and mechanical behavior with aging were revealed. Multiple-stage phase transformation was observed for aged specimens due to the formation of Ti3Ni4, Ti2Ni3, and TiNi3 precipitates. It was revealed that the transformation temperatures can be tailored by aging where Ms was −3 °C after 550 °C–24 h aging and 29 °C after 400 °C–3 h aging. Aged samples showed a shape memory effect with a recoverable strain of <1.6% where 550 °C–3 h aged Ni54Ti46 alloy showed a maximum recoverable strain of 1.55% under 500 MPa. The maximum work output was determined as 7.75 J/cm3 for 550 °C–3 h aged Ni54Ti46 alloys. Partial shape recovery was also observed in aged samples.

Comparison of cold-hardening bending and direct electric resistance heat treatment on the mechanical properties and transformation temperature of NiTi archwire: An in vitro study

The aim of this investigation was (1) to compare the effects of cold-hardening bending and direct electric resistance heat treatment (DERHT) methods; and (2) to compare the effects of offsets and angulations on the mechanical properties and thermal analysis of NiTi alloy archwire. Nickel-titanium (NiTi) archwires (Ormco, Glendora, CA), 0.016×0.022-inch, were bent by cold-hardening bending and DERHT methods into 1-mm, 2-mm, or 3-mm offset for a 3-point bending test, and at angles of 30°, 45°, or 60° to the horizontal plane for testing the change in transformation temperature (Af) measured by differential scanning calorimetry (DSC). The data were analysed using ANOVA followed by the Scheffe post-hoc test. The 3-point bending test results of the cold-hardening bending and DERHT methods were not significantly different between the 1-mm, 2-mm and 3-mm offset groups (95% CI: -0.05 to 0.97; P=0.082, 95% CI: -0.65 to 0.74; P=0.983 and 95% CI: -0.61 to 0.98; P=0.813, respectively). Increasing the offset resulted in a significantly decreased force in the 3-point bending test (P<0.001). The Af temperatures of the cold-hardening bending and DERHT methods were not significantly different for the 30°, 45°, and 60° bending angulations (95% CI: -1.93 to 1.39; P=0.876, 95% CI: -1.2 to 0.87; P=0.878, 95% CI: -2.24 to 1.18; P=0.636, respectively). Af temperatures were not influenced by different bending angulations. NiTi archwire shape can be modified by using both cold-hardening and DERHT bending methods, because the mechanical properties and Af temperature are not affected. However, the bending distance has an effect on the mechanical properties.