Transformation Temperature Hysteresis Research Articles

Atomistic simulations of AuTi high-temperature shape memory alloys

Herein, the atomic-scale phase transformation and deformation processes of AuTi shape memory alloys for high-temperature applications were investigated using molecular dynamics simulations based on a newly developed interatomic potential for the Au-Ti binary system. The developed second nearest-neighbor modified embedded-atom method potential exhibited good accuracy and transferability to reproduce various physical properties of the target alloy system, particularly in properties related to the reversible phase transformation of AuTi high-temperature shape memory alloys. As a conceivable application of the interatomic potential, the temperature-induced phase transformation of AuTi single-crystal shape memory alloys and the effect of off-stoichiometry on the phase transformation behavior were examined. Molecular dynamics simulations revealed that the transformation temperature and thermal hysteresis decreased with increasing Ti content, consistent with the reported experimental trend, suggesting that the reduced volume difference between the austenite and martensite phases was closely related to the reduced thermal hysteresis. We further examined the mechanical response and resultant superelasticity of nanocrystalline AuTi alloys to demonstrate that the cyclic deformation of nanocrystalline alloys exhibited a transformation ratcheting behavior caused by the plastic deformation of amorphous-like grain boundary region.

The influence of particle size and heat treatments on the transformation energies of a gas atomized Cu-Al-Ni-Mn shape memory alloy

Forward and reverse transformation temperatures of shape memory alloys are significantly affected by their thermal and processing history. Controlling these temperatures is of great importance when designing materials displaying shape memory effect. In this direction, the present work shows the influence of particle size and heat treatments on thermodynamic properties of a gas-atomized Cu-Al-Ni-Mn shape memory alloy. Commercially pure elements were used in the gas atomization process and differential scanning calorimetry was used to measure As, Af (austenitic start and final transformation temperatures), Ms, Mf (martensitic start and final transformation temperatures), transformation temperature hysteresis and chemical/non-chemical energies involved in the martensitic-austenitic transformation. The influence of quenched-in defects and annealing was also studied. Results show a correlation between particle size and transformation energies, which can be attributed to atomic ordering in the austenitic phase upon cooling, after the solidification process. This is further confirmed by the increase in the reverse and forward transformation temperatures after thermal cycling and annealing.

Valence Electron Ratio for Design of Shape Memory Alloys with Desired Phase Transformation Temperatures

Dependence of phase transformation temperatures of TiPd- and TiPt-based shape memory alloys on valence electron ratio (VER), number (ev/a), and average atomic number of the alloys (Z) are investigated. The alloys have medium numbers of valence electrons (6.6 ≤ ev/a ≤ 7.3) that are near 7 and a wide range of the average atomic number (Z = 25–54). The forward and reverse phase transformation temperatures, Ms and (As), increase with average atomic number of the alloys, respectively. Clear correlations between transformation temperatures and VER are found. Ms and (As) both decrease from around 1184 °C (1175 °C) to as low as 20 °C (32 °C), respectively, with increasing VER from 0.134 to 0.270. Temperature hysteresis of the thermoelastic transformation in these alloys tends to decrease with increasing VER. Dependence of transformation temperatures and temperature hysteresis of these shape memory alloys on VER is discussed based on the variations of elastic moduli and atomic bonding due to composition change.

Evolution of transformation characteristics of shape memory alloys during cyclic loading: transformation temperature hysteresis and residual martensite

This paper investigates the evolutions of transformation temperatures and residual martensite of shape memory alloys during cyclic loading. Pseudoelastic NiTi specimens were firstly subjected to stress-controlled cyclic loading; then differential scanning calorimetry and x-ray diffraction characterizations were conducted on samples cut from specimens after different cycles as well as virgin one. Results show that (i) transformation temperatures rise while the transformation temperature hysteresis increases at initial cycles and then remarkably decreases when the SMA is highly distorted; (ii) due to the accumulation of residual martensite, the more fatigue cycles applied to the SMAs, the less latent heat absorbed/released during the transformation. Furthermore, residual martensite volume fraction was calculated with respect to plasticity and a linear character was observed.

The effect of ternary alloying with vanadium on Ti-Pt shape memory alloys

Ti-Pt alloys have attracted much research due to high transformation temperatures (1050°C) which makes them suitable for high temperature applications in automotive and aerospace industries. The binary alloy exhibits negligible shape memory effect and the amount of platinum (Pt) required to produce this alloy makes it very expensive which limits its practical application and commercialization. Ternary alloying of Ti-Pt could lead to solid solution strengthening and improved shape memory properties. Furthermore, ternary alloying by replacing Pt could reduce the cost of the alloy. However, it could lead to changes in the microstructure, crystal structure, transformation temperatures and transformation temperature hysteresis that would significantly affect the shape memory behaviour of the alloy. This paper investigates the effect of ternary alloying of Ti-Pt with varying vanadium contents (0-18.25 at.%) on the as-cast condition. The effect of ternary alloying on the microstructure, transformation temperature and transformation temperature hysteresis were studied. The Ti50-Pt50-x-Vx (x = 6.25 to 18.25 at.%) alloys were produced by the button arc melting method using elemental powders of titanium, platinum and vanadium. Results showed the formation of martensitic a-Ti50(Pt,V)50 with Ti3Pt, (Ti,V)3Pt, Ti4Pt3-like and oxide phases in Ti-Pt-(V) alloys. The phase transformation temperature of Ti-Pt alloys decreased with increasing vanadium content up to 10.2 at.% V.

Investigation of phase evolution and control over phase transformation temperature and thermal hysteresis using stoichiometry and co-doping in VO2 thin films

Structural phase transition temperature and its associated hysteresis in VO2 have been controlled by high valent dopant induced local structural modification which acts as phase nucleation site during phase transformation. Monoclinic phase intensity loops plotted against temperature has been observed to exhibit thermal hysteresis. It is shown that the size, shape and central position of hysteresis loop depend on stoichiometry and doping concentration. Highest reduction in phase transformation temperature and thermal hysteresis width has been observed in case of W+6-Mo+6 co-doping and W+6 doping, respectively. Hence energy barrier associated with the structural phase transition has been successfully manipulated to vary Tc and hysteresis width. These findings have implications for designing the phase switching devices and smart window applications.

On the prediction of martensite formation in metals

A new approach to predict athermal martensite formation in metals is presented. It is based on computing the driving force of the transformation including a strain energy term induced by atomic shear displacements and energy terms due to substitutional and interstitial lattice distortions. The model is applied to prescribe the martensite and austenite start temperatures in Fe-, Ti- and Co-based alloys with no adjustable parameters. Expressions for Ms variations with composition are derived for multicomponent systems. The transformation temperature hysteresis is predicted in Co alloys showing that this approximation can be used to design alloys with the shape memory effect.

Development of Rolling Technology for an Iron-Based Shape-Memory-Alloy

Low cost Fe-Mn-Si based shape memory alloys (SMA) have drawn much attention during the last two decades as a cost-effective alternative to the expensive Ni-Ti based SMA. In particular, the alloy Fe-17Mn-5Si-10Cr-4Ni-1(V,C) (mass%), which has been developed at Empa shows very promising properties with regard to potential commercial applications in civil and mechanical engineering. This alloy has a higher reverse transformation temperature and larger thermal hysteresis in comparison to the Ni-Ti based alloys, which is adequate for producing stable recovery stresses at room temperature. Furthermore, recovery stresses of up to 300 MPa after heating to only 160°C can be achieved without so-called ‘training’ treatment. Furthermore, the alloy can be easily and cost effectively produced under standard air melting and casting conditions. For availability of these heavily microstructure dependent skills for civil and mechanical engineering, e.g. as prestressing elements in concrete structures or coupling/clamping devices, a process chain for manufacturing is necessary. Therefore, a hot and cold rolling technology for strip production with thermal heat treatment processes was developed at TU Bergakademie on base of experimental simulation results. The last one helps to understand the dependencies of deformation parameters, the deformation behavior and their influence to the microstructure evolution in correlation to the recovery.This paper discusses the basic material properties, recovery stress formation behavior and finally the feasibility of the alloy as reinforcing elements in civil engineering applications by using a rolling technology for flat products.

Effects of Grain Size and Co Addition on the Transformation Temperatures of Ti–Ni–Zr Thin Films

The transformation temperatures and temperature hysteresis of Ti–Ni–Zr films were found to be very sensitive to grain size. However, films annealed at 1223 K exhibited transformation temperatures and temperature hysteresis nearly the same as those of the corresponding bulk alloys. Annealing of Ti–Ni–Zr–Co films with 10.5–26.7 at.% Zr and 0–12.7 at.% Co at 1223 K for 10 h showed that Co substitution for Ni in the Ti–Ni–Zr films was beneficial to reducing the temperature hysteresis. Correspondingly, a Ti29.3Ni42.7Zr20.9Co7.1 bulk alloy annealed at 1223 K for 10 h exhibited a high martensitic transformation temperature (432 K) and a small temperature hysteresis compared to Ti–Ni–Zr bulk alloys (21 K). The present study verified the usefulness of sputtering for the exploration of new shape-memory alloys.

Martensitic transformation and superelasticity in off-stoichiometric Co2Cr(AlSi) Heusler alloys

Martensitic transformation behaviors and magnetic properties in off-stoichiometric Co2Cr(AlSi) Heusler alloys were investigated mainly by TEM, DSC, SQUID and mechanical test. In Co57Cr21Al11Si11 alloy, martensitic transformation from Heusler-type L21 to D022 phase was confirmed, where Ms and Af temperatures were 429 and 452K, respectively, suggesting thermoelastic martensitic transformation because of the small transformation temperature hysteresis. In polycrystalline Co57Cr25Al10Si10 alloy with large grains, superelasticity with a recovery strain of over 2% was detected by compression test at 298K. Thus, a novel Co-based shape memory alloy was discovered in the Co–Cr–Al–Si system.

Effects of Co and Al addition on martensitic transformation and microstructure in ZrCu-based shape memory alloys

The effect of ternary alloying element Al and quaternary alloying element Co on the martensitic transformation of ZrCu-based shape memory alloy was investigated. The results show that the addition of Al and Co in ZrCu alloy decreases both the martensitic transformation temperature and the martensitic transformation temperature hysteresis. Transmission electron microscope (TEM) observations reveal that the Cm martensite structure is the preferential formation phase. The intervariant structures in ZrCuAlCo alloy are (021) type I twins, while the dominant substructures inside the martensite variant are the (001) compound twins. With the increase of Co content, tensile fracture strength and strain are improved obviously.

Effect of Nb Addition on Martensitic Transformation Behavior of AuTi-15Co Based Biomedical Shape Memory Alloys

The effect of quaternary addition of Nb being from 0 to 2mol% on the phase constitution, phase transformation behavior and shape memory behavior of Au-50mol%Ti-15mol%Co (AuTiCo) alloys is studied. X-ray diffraction analysis shows that, with increasing Nb concentration, the primary phase appeared at room temperature changes from B19 orthorhombic martensite phase to B2 cubic parent phase, and that all the lattice parameters are not largely changed by Nb addition. The maximum transformation strain evaluated in 2mol%Nb-added AuTiCo is 7.4%. Differential scanning calorimetry revealed that the reverse martensitic transformation finish temperature (Af ) decreases from 414K in Nb-free AuTiCo to 303K by 2mol%Nb addition. The ratio of Af change by Nb addition is evaluated to be 155K/mol%Nb, that is comparably larger than 131K/mol%Nb in NiTi. These results indicate that Nb addition is effective to stabilize B2 parent phase. AuTiCo alloy containing less than 2mol%Nb exhibits shape memory effect with 90% shape recovery ratio. Besides, AuTiCo containing 2mol%Nb exhibits 2.8% superelastic shape recovery at room temperature as well as 7.4% maximum transformation strain. Therefore, Nb addition to AuTiCo is effective to decrease the martensitic transformation temperature and to improve superelasticity, although the transformation temperature hysteresis increases by Nb addition, similar to Nb-added NiTi. [doi:10.2320/matertrans.M2014418]

Martensitic Transformation and Mechanical Properties of Fe-added Au-Cu-Al Shape Memory Alloy with Various Heat Treatment Conditions

ABSTRACTIn order to improve shape memory properties of Au-Cu-Al based shape memory alloys, the possibility to utilize thermo-mechanical treatment was investigated in this study, and effects of heat-treatment temperature on microstructure, martensitic transformation and mechanical properties of cold-rolled Au-30Cu-18Al-2Fe (AuCuAlFe) alloy were clarified by X-ray diffraction analysis (XRD, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and tensile tests at room temperature (RT). Here, Fe addition to AuCuAl improves ductility. Cold rolling with the thickness reduction of 30% was successfully carried out in AuCuAlFe at RT. An exothermic heat was observed in DSC at temperature from 402K, suggesting that recovery started at 402K. Besides, the transformation temperature hysteresis increased by the cold-rolling. The alloy was completely recrystallized after the heat treatment at 573K for 3.6ks. Tensile tests revealed that the yield stress was raised by cold rolling and largely by the subsequent heat treatment at 433K, which corresponded to the recovery start temperature by DSC. The yield stress decreased with increasing heat treatment temperature over 453K, probably due to recrystallization. AuCuAlFe cold-rolled and subsequent heat-treated at 573K exhibited the lowest yield stress as well as stress-plateau region, indicating that the thermo-mechanical treatment is effective to improve shape memory properties of Au-Cu-Al based alloys.

Effects of nanoprecipitation on the shape memory and material properties of an Ni-rich NiTiHf high temperature shape memory alloy

Shape memory properties of a Ni50.3Ti29.7Hf20 (at.%) polycrystalline alloy were characterized after selected heat treatments. The effects of heat treatment temperature and time on the transformation temperatures (TTs) and temperature hysteresis were determined by differential scanning calorimetry. Thermal cycling under constant compressive stress was carried out to reveal the changes in transformation strain, temperature hysteresis, and TT as a function of stress. Isothermal stress cycling experiments were conducted to reveal the critical stresses, transformation strain, and stress hysteresis as a function of temperature. The crystal structure and lattice parameters of the transforming phases were determined by X-ray diffraction at selected temperatures. Precipitate characteristics and martensite morphology were revealed by transmission electron microscopy. Precipitation was found to alter the martensite morphology and significantly improve the shape memory properties of the Ni-rich NiTiHf alloy. For the peak aged condition shape memory strains of up to 3.6%, the lowest hysteresis, and a fully reversible superelastic response were observed at temperatures up to 240°C. In general, the nickel-rich NiTiHf polycrystalline alloy exhibited a higher work output (≈16.5Jcm−3) than other NiTi-based high temperature alloys.

High-Throughput Characterization of Thin Film Shape Memory Alloys

Thin film shape memory alloys (SMAs) are promising materials for micro-machines because of their high-output power, large strain, and actuation without high voltage. To search for compositions of suitable thin film SMAs for each application, combinatorial technique is useful tool, however the technique requires high-throughput characterization method for thermal property which is an important property of thin film SMAs such as two way martensitic transformation temperature and thermal hysteresis. In this paper, novel high-throughput characterization method for such properties using thermography is proposed and demonstrated. Compositionally integrated thin film SMA samples (TiNiPd) with only 1mm2 of each area are characterized at once. The difference of martensitic and reverse martensitic transformation temperature against those temperatures measured by a conventional method, differential scanning calorimetry, is about 5K.

Effect of heat treatment on transformation behavior of Ti–Ni–V shape memory alloy

Abstract Effects of annealing and aging processes on the transformation behaviors of Ti–50.8Ni–0.5V (atomic fraction, %) shape memory alloy were investigated by means of differential scanning calorimetry (DSC). The A → R/R → A (A – parent phase, R – R phase) type one-stage reversible transformation occurs in 350–400 °C annealed alloy, the A → R → M/M → R → A (M – martensite) type two-stage transformation occurs in 450–500 °C annealed alloy, the A → R → M/M → A type transformation occurs in 550 °C annealed alloy, and A → M/M → A type transformation occurs in the alloy annealed at above 600 °C upon cooling/heating. The transformation type of 300 °C aged alloy is A → R/R → A, and that of 500 °C aged alloy is A → R → M/M → A, while that of 400 °C aged alloy changes from A → R/R → A to A → R → M/M → R → A with increasing aging time. The effects of annealing and aging processes on R and M transformation temperatures and temperature hysteresis are given. The influence of annealing and aging temperature on transformation behaviors is stronger than that of annealing and aging time.

Effect of heat treatment temperature on the microstructure and actuation behavior of a Ti–Ni–Cu thin film microactuator

Ti–Ni–Cu/SiO 2 two layer diaphragm-type microactuators were fabricated by sputter deposition and micromachining. The influence of heat treatment temperature on the actuation behavior was investigated under quasi-static conditions. The interfacial structure of Ti–Ni–Cu/SiO 2 and internal structure of the Ti–Ni–Cu layer were also investigated using transmission electron microscopy. The reaction layer formed between the Ti–Ni–Cu and SiO 2 layers, and preferentially grew into the SiO 2 side. The reaction layer formed at 1023 K mainly consisted of Ti 4(Ni,Cu) 2O. The maximum height of the diaphragm decreased with increasing heat treatment temperature. The growth of the reaction layer also affected the microstructure of the Ti–Ni–Cu layer. The density of fine platelets and Ti 2Ni precipitates decreased with increasing heat treatment temperature from 873 to 923 K, and they disappeared at 973 K due to the fact that the reaction layer mainly consisted of a Ti-rich phase. The microactuator heat treated at 973 K showed the highest transformation temperature with the lowest transformation temperature hysteresis, which is attractive for high speed actuation.

Experimental and numerical analysis of recovery stress in [formula omitted] shape memory alloys: application to tightening

Ni47Ti44Nb9 Shape Memory Alloys (SMA) are widely considered for tightening applications. The most common one are shape memory rings whose diameter decreases during heating. The alloy is composed of a NiTi matrix containing niobium precipitates. A specific thermomechanical treatment increases by more than one hundred degrees the transformation temperature hysteresis, what increases the range of use and improves the tightening efficiency. Tightening pressures exhibited by Ni47Ti44Nb9 rings with two various thicknesses are experimentally investigated. The test bed is composed of an Inconel 718 pipe instrumented by strain gauges sensors. Measured strains lead to the tightening pressure thanks to the large elastic resistance of Inconel. Evolutions with temperature are recorded. A thermo-mechanical constitutive law, specific for Ni47Ti44Nb9, has been developed. It is based on the Mori-Tanaka scale transition technique by considering the precipitates as elastic-plastic inclusions embedded in the SMA matrix. Its behavior is described by the Chemisky, Duval et al. constitutive model. The elastic-plastic constitutive law for inclusion is a classical one proposed by Simo and Hughes. The resulting effective law is implemented, and validated in ABAQUS via UMAT subroutine. The developed approach is adopted for the simulation of the experimental tests on Ni47Ti44Nb9 rings, and comparisons are performed.

Martensitic transformation temperature hysteresis narrowing and magnetocaloric effect in ferromagnetic shape memory alloys Ni–Mn–Ga

The hysteresis narrowing and magnetocaloric effect (MCE) were studied in the Ni–Mn–Ga alloys. Ni 54Mn 24Ga 22 and Ni 51.9Mn 27Ga 21.1 were selected in order to obtain specific characteristic temperatures of alloys and to study an influence of several factors on the hysteresis width ( δ) of basic austenite–martensite (AM) transformation. The effect of martensite crystal lattice softening due to intermartensitic transition (IT) taking place immediately after AM transformation on δ was examined in alloy 1. MCE was measured in alloy 2, in which Curie temperature ( T C ) and austenitic start temperature ( A S) coincides. It was demonstrated that the hysteresis width δ reaches about 2 K, if IT is enough close to AM transition. MCE was found to have a maximum at T C≈ A S.

Effect of disperse Ti3N4 particles on the martensitic transformations in titanium nickelide single crystals

The effect of the size and volume fraction of Ti 3 N 4 particles in Ti-(50.3-51.5) at % Ni single crys- tals on their martensitic transformation temperatures and temperature hysteresis is studied. Aging at T = 673- 823 K leads to a nonmonotonic change in the martensitic transformation temperatures and temperature hyster- esis, which is related to a change in the Ni concentration in the matrix, the hardening of the high-temperature phase, a change in the elastic and surface energies generated upon the martensitic transformations, and the inter- nal stresses that appear because of the difference in the lattice parameters of the particles and the matrix. As a result of the high strength of the B 2 phase and the high elastic and surface energies that are generated upon the martensitic transformations due to the precipitation of particles of size d < 40 nm at an interparticle distance λ < 50 nm, the martensitic transformation temperatures decrease down to the suppression of the R - B 19' transi- tions upon cooling to 77 K. A thermodynamic description for the martensitic transformations in heterophase crystals is proposed, and an analogy between the martensitic transformations in heterophase Ti-Ni single crys- tals with nanoparticles of size d = 20-100 nm and those in single-phase Ti-Ni polycrystals with a grain size of 50-200 nm is found.