Reverse Martensitic Transformation Temperature Research Articles

Phase field study on phase stability at high temperatures in nanograined Fe–Ni alloy prepared by plastic deformation

The phase stability at high temperatures in nanograined Fe–Ni alloy prepared by surface mechanical attrition treatment (SMAT) and ball milling (BM) modes are drastically different. For example, the onset temperature (As) of reverse martensitic transformation (RMT) of nanocrystal prepared by SMAT is higher than that of coarse crystal, while BM exhibits the opposite result. The elastoplastic phase field model for RMT was first proposed to investigate the correlation between RMT and its forward martensitic transformation (FMT), in turn, the above experimental phenomena were revealed in this paper. The prepared nanocrystals with the same diameter (15 nm) by SMAT and BM mode were taken as representative examples. The simulation results indicated that the strain energy is relaxed by the applied strain during FMT in nanocrystalline prepared by smaller plastic deformation mode (e.g. SMAT), but it is enhanced by larger plastic deformation mode (e.g. BM). The stored strain energy in BM mode is much larger than that in SMAT mode, and hence the driving force provided for subsequent RMT is also larger in BM mode. Therefore, the phase stability of the martensitic nanocrystals prepared by BM mode at high temperatures is much lower than the counterpart by SMAT mode, which agrees well with the experimental measurements.

Effect of Al doping on the kinetics of reverse martensitic transformation in Ni-Mn-In Heusler alloys

In this work, the effects of Al on the microstructure and kinetics of reverse martensitic transformation in Ni50Mn34In16-xAlx (x = 0, 0.5, 1, and 1.5 (% at)) Heusler alloy was studied. The structural and phase transformation behavior of the samples was evaluated by XRD, FE-SEM, and DSC techniques. In order to evaluate the transformation kinetics parameters in the non-isothermal state upon the heating rates of 5, 10, and 15 K/min, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isoconversional methods were utilized. The results indicated that the reverse martensitic transformation temperatures and activation energy shifted to higher values due to doping the Al content up to 1 at.%. Conversely, the substitution of In by 1.5 at.% Al leads to a decrease in the transformation temperatures and activation energy. Furthermore, due to the lack of dependence on the activation energy and Avrami exponent on the degree of transformation, the reverse martensitic transformation is single-stage for all samples.

Effect of annealing temperature and time on martensitic transformation temperatures and mechanical properties of the Ti–50.7at.%Ni shape memory alloy

The study covers the effect of recrystallization annealing temperature and time on the characteristic temperatures of martensitic transformations and mechanical properties of the Ti–50.7at.%Ni shape memory alloy in the form of wire after cold drawing at room temperature. Six modes of post-deformation annealing with different temperatures and holding times were studied for the alloy to obtain structures with different sizes of recrystallized grains. The recrystallized grain size was determined by electron backscatter diffraction (EBSD). It was shown that the size of recrystallized grains increases from 2.5 to 9 μm, with both an increase in the annealing temperature (600– 700 °С) and an increase in the holding time (0.5–5.0 h). The characteristic temperatures of direct and reverse martensitic transformations were determined using differential scanning calorimetry. It was shown that the threefold growth of the recrystallized grain size reduces the starting temperature of the direct martensitic transformation, and extends the temperature range of the reverse martensitic transformation. The results of mechanical tests (stretching tests) at room temperature showed that an increase in the grain size leads to a decrease in the dislocation yield strength and an increase in the phase yield strength. It was established that the dislocation yield strength obeys the Hall–Petch law, and the phase yield strength is determined by the test temperature position relative to the starting (or peak) temperature of the direct martensitic transformation. Heat treatment modes for specific products should be recommended taking into account these two competing factors, as well as reverse martensitic transformation temperatures determining the alloy strain recovery temperatures.

Effects of Cr and Sn additives on the martensitic transformation and deformation behavior of Ti-Cr-Sn biomedical shape memory alloys

The effects of Cr and Sn additives on the phase constitution, lattice deformation strain, transformation temperature, and deformation behavior of Ti-Cr-Sn alloys were systematically investigated to inform the development of a superelastic alloy with high lattice deformation strain. The addition of Cr and Sn changes the reverse martensitic transformation temperature by −190 K/mol% Cr and −141 K/mol% Sn, respectively. The lattice deformation strains (η1, η2, and η3) were evaluated in terms of the lattice parameters of the parent β phase to the martensite α″ phases. In addition, the composition range of the alloys that are expected to exhibit superelasticity and high deformation strain near the human body temperature was established based on the composition dependence of lattice deformation strain and transformation temperature. The deformation behavior of Ti-Cr-Sn alloys at room temperature changes from the shape memory effect to superelasticity with increasing Cr and Sn contents. In particular, Ti-2.5Cr-8.5Sn, Ti-3.0Cr-7.5Sn, Ti-3.0Cr-8.0Sn, Ti-3.5Cr-7.0Sn, and Ti-4.0Cr-6.5Sn alloys exhibit superelasticity at room temperature and a high lattice deformation strain (η2) that exceeds 6.32%. It is concluded that Cr and Sn are more effective than other β-stabilizing elements at lowering the martensitic transformation temperature of β-Ti shape memory alloys without reducing the lattice deformation strain.

Temperature Dependent Stress–Strain Behavior and Martensite Stabilization in Magnetic Shape Memory Ni51.1Fe16.4Ga26.3Co6.2 Single Crystal

The superelastic properties and stress-induced martensite (SIM) stabilization have been studied in a shape memory Ni51.1Fe16.4Ga26.3Co6.2 single crystal. The single crystal, characterized by a thermally induced forward martensitic transformation temperature around 56 °C in the initial state, has been submitted to compression mechanical testing at different temperatures well above, near and below the martensitic transformation (MT). After each mechanical test, the characteristic MT temperatures and the transformation enthalpy have been monitored by means of differential scanning calorimetry. At temperatures below MT, the stress–strain (σ–ε) curves show a large strain, around 6.0%, resulting from the detwinning process in the martensitic microstructure, which remains accumulated after unloading in the detwinned state of the sample as a typical behavior of the shape memory alloys (SMAs). After just two “σ–ε + heating” cycles the accumulation of strain was not observed any more indicating the formation of a two-way shape memory effect which consists in a spontaneous recovery of the aforementioned detwinned state of the sample during its cooling across the forward MT. Whereas the thermally induced shape recovery in conventional SMAs occurs at the fixed value of the reverse MT temperature, the heating DSC curves of the mechanically deformed martensite in the present work show a burst-like calorimetric peak at the reverse MT arising at temperatures essentially higher than the thermally activated one. This behavior is the result of the SIM stabilization effect. After a short thermal aging in the stress-free state, this effect almost disappears, showing a slight impact on the MT characteristic temperatures and the enthalpy. At temperatures higher than the transformation one, the SIM is not stabilized, as the mechanically induced martensite fully retransforms into austenite after the unloading. From the σ–ε curves, the critical stress, σc, as well as the values of Young’s moduli of martensite and austenite are determined showing linear dependences on the temperature with a slope of 3.6 MPa/°C.

Deformation induced martensite stabilization in Ni45Mn36.7In13.3Co5 microparticles

A Ni45Mn36.7In13.3Co5 alloy has been deformed by hand crushing and ball milling, both in martensite and in austenite, to analyze the effect of the induced plastic deformation on the relative stability between structural phases. A huge martensite stabilization, with shifts on the temperature of the first reverse martensitic transformation of up to 100 K, is observed not only in the deformed martensite but also in the grinding-induced martensite. It is also found that, regardless of the degree of plastic deformation, the stabilized martensite displays a non-modulated tetragonal structure, which accommodates the generated internal stresses better than the thermal induced modulated monoclinic martensite. In all cases, the destabilization process is shown to finish around the Curie temperature of the austenite, which actually represents a high temperature limit for the mechanically-stabilized martensite. This phenomenon is analyzed and explained in terms of the effect of magnetic entropy contribution to the driving force for reverse MT.

Correlation between particle size and magnetic properties in soft-milled Ni45Co5Mn34In16 powders

The effect of microstructural defects induced by mechanical milling has been studied in a Ni–Mn–In–Co metamagnetic shape memory alloy. The martensitic transformation and Curie temperatures do not change with grinding, thus pointing out to a null variation of long range atomic order as a consequence of the deformation. Nevertheless, the enthalpy change of the martensitic transformation highly decreases. This, and the large thermal stabilization of the martensite (with shifts on the temperature of the first reverse martensitic transformation up to 60 K), indicate the presence of a huge amount of internal stresses and microstructural defects in the obtained micro-particles. The presence of such defects considerably affects the saturation magnetization in austenite whereas almost no effect is observed in martensite. The magnetocaloric effect has been evaluated in samples with three different particle sizes. In spite of the MCE value is lower than in the bulk, the broader temperature range for the martensitic transformation in the powders makes the relative cooling power be comparable to that in the bulk. The as-milled micro-particles can be then considered as good preliminary candidates for magnetic refrigeration applications at the microscale.

The Influence of Thermomechanical Treatment on the Strain Behavior of the Fe–Ni–Co–Ti Ferromagnetic Alloy Nanocomposite with Shape Memory Effect

The paper describes mechanical tests of a nanocomposite produced from the ferromagnetic shape memory Fe−Ni−Co−Ti alloy under uniaxial tension over a wide temperature range. The production of the nanocomposite was preceded by preliminary thermomechanical treatment (TMT), involving drawing, quenching, and ageing, for precipitation hardening. The TMT imparted high superelastic strain and shape memory effect to the nanocomposite. The preliminary TMT with strain ψ = = 7.4−22.5% aged at T = 650°C for 5−10 min was experimentally found to correspond to the optimal combination of the maximum superelastic strain and shape memory effect. This contributed to the phase and twinning plastic deformation of the nanocomposite over the test temperature range Ms < Ttest < Af (where Ms is the start temperature of forward martensitic transformation on cooling and Af is the finish temperature of reverse martensitic transformation on heating). A plateau with constant stress was found in the two-phase Mf < Ttest < Ms region on the tensile curve at drawing strain ψ = 22.5%. A significant increase in the preliminary strain (more than 40%) substantially stabilized the austenitic matrix, thus inhibiting the martensitic transformation and reducing reversible effects because the austenite grain size refines when the lattice defect density increases. The austenite grain size distribution was assessed versus the chosen TMT conditions. When the austenite grain size increased, the superelastic strain recovery became higher. The factors leading to greater superelasticity were analyzed within different phenomenological models. The TMT has a crucial role in the variation of structure and mechanical properties, in turn promoting inelastic effects at different temperatures.

The microstructural characteristics and high temperature mechanical properties of quaternary Ti–V–Al–Co shape memory alloys

Abstract In the present study, the effect of Co addition on the microstructural features, martensitic transformation beahviors and mechanical properties of Ti–V–Al alloys was investigated systematically. The results revealed that phase constituents varied with Co content increasing, which can be summarized as follows: sole α martensite phase → coexistence of α martensite phase and β austenite phase → fully β austenite phase. Correspondingly, the reverse martensitic transformation temperature continuously decreased with Co content increasing. Moreover, the moderate Co addition can improve yield strength and elongation of Ti–V–Al alloys as a consequence of the solid solution strengthening effect. The maximum yield stress of 811 MPa and superior elongation of 23.8% can be obtained in Ti–V–Al–Co2.0 and Ti–V–Al–Co1.0 alloy, respectively. Besides, Co addition led to the significant improvement of the higher temperature yield strength of quaternary Ti–V–Al–Co alloys, compared with the Ti–V–Al alloy without Co addition.

Microstructure and mechanical properties of Ti–V–Al–Cu shape memory alloy by tailoring Cu content

The microstructure, martensitic transformation behavior, mechanical and shape memory properties of the Ti–V–Al–Cu light weight shape memory alloys were investigated systematically. The results showed that the phase constitution gradually evolved from single α″ martensite phase to the complete β phase with the Cu content increasing in the solution treated Ti–13V–3Al-xCu alloys at room temperature. The spear-like martensite variant constituted the typical self-accommodation configuration, and {111} type I twins and <211> type II twins coexisted in the Ti–V–Al–Cu alloys. Moreover, Ti2Cu precipitate can be observed in the Ti–V–Al–Cu alloys with the higher Cu contents. The martensitic transformation temperature of Ti–V–Al–Cu alloys decreased continuously with the increased Cu content. The Ti–V–Al–Cu alloys with the higher transformation temperature and higher strength as well as the superior shape recovery characteristics can be obtained by tailoring Cu content. It was the perfect combination of solid solution strengthening, grain refinement and precipitation strengthening to promote the enhancement of matrix strength. The maximum recoverable strain was 4.4%, for 6% pre-strain and the reverse martensite transformation temperature was 334 °C in the optimal Ti–13V–3Al-0.5Cu alloy.

Functional properties of superelastic orthodontic nickel-titanium archwires with protective titanium nitride coatings

Today there are many manufacturers of orthodontic archwires composed of a nickel-titanium (TiNi) alloy with different elastic-force characteristics. A limited number of orthodontic archwires is available for initial tooth alignment, since reactive stresses do not always satisfy the condition 160 ≤ σc ≤ 200 MPa. The use of orthodontic archwires with polymer coatings having better aesthetics is increasing. However, they show excessive wear and color change during a long-term orthodontic treatment. The aim of this paper is to study and optimize the functional characteristics of superelastic archwires composed of Ti-50.8 аР % Ni alloy with TiN coatings deposited at varying deposition times. A three-point bending test was carried out to evaluate the functional properties. The distance between the supporters was 10 mm. The archwires were subjected to bending at a temperature of 23 ± 3 °C. Each test was continued until deformation of 1.5; 3; 4.5 and 6 % was reached. It has been found that titanium nitride coatings deposited on the Ti-50.8 at. % Ni surface alloy by the vacuum-plasma method act as the barrier layer to prevent the release of nickel ions into biological environment. Heat treatment (~400 °C) during deposition allows the required elastic-force characteristics and functional properties of the material to form. The optimal reactive stress (160-200 MPa) and the reverse martensitic transformation temperature occurred near room temperature can be obtained due to an appropriate selection of the deposition parameters. In the martensitic phase at room temperature, the archwire can be deformed. When the archwire sample is placed in oral cavity and heated to temperatures above 30 °C the material is in the superelastic state. Further research is needed in terms of coating stability during deformation when the material in superelastic state, as well as conducting corrosion testing, studying biocompatibility of archwire samples with titanium nitride coatings in order to successfully implement the proposed technology in dental practice. The prototypes of orthodontic TiNi archwires with protective and decorative TiN coatings will be obtained for medical application.

Martensitic transformation, shape memory effect and superelasticity of Ti–xZr–(30–x)Nb–4Ta alloys

Martensitic transformations, mechanical properties, shape memory effect and superelasticity of Ti–xZr–(30–x)Nb–4Ta (x = 15, 16, 17 and 18; at%) alloys were investigated. X-ray diffraction (XRD), optical microscopy (OM) and transmission electron microscopy (TEM) results indicated that the Ti–16Zr–14Nb–4Ta, Ti–17Zr–13Nb–4Ta and Ti–18Zr–12Nb–4Ta alloys were mainly composed of α″-martensite, while the Ti–15Zr–15Nb–4Ta alloy was characterized by predominant β phase. The reverse martensitic transformation temperatures increased when Nb was replaced by Zr, indicating stronger β-stabilizing effect for the former. The Ti–15Zr–15Nb–4Ta alloy displayed superelasticity during tensile deformation with a recovery strain of 3.51%. For the other three alloys with higher Zr content, the martensitic reorientation occurred during tensile deformation, resulting in shape memory recovery upon subsequent heating. The maximum shape memory effect was 3.46% in the Ti–18Zr–12Nb–4Ta alloy.

The Influence of Sintering Parameters in the Microstructure and Mechanical Properties of a Cu–Al–Ni–Mn–Zr Shape Memory Alloy

Shape memory alloys from the Cu–Al–Ni system arouse interest for industrial applications due to their high thermal stability and low cost. In this study, the Cu–11.8Al–3.2Ni–3Mn–0.5Zr alloy is prepared through a powder metallurgy processing technique. Specimens are produced by a conventional uniaxial press followed by sintering procedure under argon atmosphere. The sintering temperature is varied in the range from 950 to 1060 °C, whereas sintering time is varied from 0.5 to 2 h. The results denote that a temperature close to the alloy's solidus temperature is necessary to achieve higher densities. Moreover, martensite‐type microstructures are observed, despite the slow cooling rate applied after sintering. The compression strengths are in the order of 500 MPa with the exception of the specimens sintered for 1.5 h, which reaches an average of 700 MPa. This is explained by the lower porosity resulting from liquid phase sintering. The forward and reverse martensitic transformation temperatures, however, are quite similar for all examined conditions.

Magnetic and Structural Transitions Tuned through Valence Electron Concentration in Magnetocaloric Mn(Co1–xNix)Ge

The structural and magnetic properties of magnetocaloric Mn(Co1–xNix)Ge compounds have been studied. Two responses to the increase of valence electron concentration on substitution of Ni (3d84s2) for Co (3d74s2) in the orthorhombic phase (Pnma) are proposed: expansion of unit-cell volume and redistribution of valence electrons. We present experimental evidence for electronic redistribution associated with the competition between magnetism and bonding. This competition in turn leads to complex dependences of the reverse martensitic transformation temperature TM (orthorhombic to hexagonal (P63/mmc)) and the magnetic structures on the Ni concentration. Magnetic transitions from ferromagnetic structures below x = 0.50 to noncollinear spiral antiferromagnetic structures above x = 0.55 at low temperature (e.g., 5 K) are induced by modification of the density of states at the Fermi surface due to the redistribution of valence electrons. TM is found to decrease initially with increasing Ni content and then increa...

A Study of NiTi Alloy Mechanical and Corrosion Behavior Subjected to Mechanical and Cyclic Thermal Loading in Corrosive Environment

The corrosion behavior of a nickel titanium alloy (NiTi) have been investigated. The NiTi alloy is subjected to cyclic thermal loading in corrosive environments (3.5 wt% NaCl and Ringer solution). Thermogravimetric measurement versus temperature is perfumed to obtain forward and reverse martensitic transformation temperatures. Electrochemical Avesta cell is used to study corrosion behavior of NiTi alloy under thermal cycling in absence of mechanical loading. Electrochemical Impedance Spectroscopy and cyclic voltammetry measurements are used to study corrosion behavior at different temperatures and phases. The change in corrosion rate during phase transformations are being investigated. An equivalent circuit model is used to calculate the electrochemical parameters from EIS measurements for forward and reverse martensitic transformations. The results shows the breakdown of passive oxide film on the surface with martensitic transformation. This breakdown would be an origin for fatigue crack initiation and an explanation for fatigue lifetime reduction of NiTi in corrosive environments.

Microstructure and Shape Memory Behavior of Ti–Nb Shape Memory Alloy Thin Film

Ti–Nb shape memory alloy (SMA) thin film is a promising candidate applied as microactuator in biomedical field. In this study, the microstructure and shape memory behavior of Ti–Nb SMA thin films in different heat treatment conditions have been investigated. Fine ω phases embedded in the β phase matrix suppress the martensitic transformation of the films. As a result, the as-deposited and most of the annealed films consist of the β and α″ dual phases. The annealed Ti–Nb thin film shows excellent superelasticity effect when deformed above the reverse martensitic transformation temperature, that is 3.5% total recovery strain can be obtained when 4% pre-strain is loaded.

High temperature Ni45Co5Mn25 − xFexGa20Cu5 ferromagnetic shape memory alloys

We anticipated that replacing Mn by Fe in the Ni50−yCoyMn25−xFexGa25−zCuz magnetic shape memory alloys would increase their Curie temperature and spontaneous magnetization. In particular, in the set of alloys Ni45Co5Mn25−xFexGa20Cu5 (x=0, …8), a record-breaking combination of high martensitic transformation temperature (400K) and Curie temperature (458K) has been achieved for Ni45Co5Mn21Fe4Ga20Cu5. This alloy shows a non-modulated tetragonal martensite with the lowest lattice distortion ratio, c/a=1.15, near the reverse martensitic transformation temperature. Because of the high Curie temperature and low c/a ratio, the Ni45Co5Mn25−xFexGa20Cu5 and Ni50−yCoyMn25−xFexGa25−zCuz alloys are promising candidates for high temperature magnetic shape memory applications.

Phase Constitution and Martensitic Transformation Behavior of Au-51Ti-18Co Biomedical Shape Memory Alloy Heat-Treated at 1173K to 1373K

Phase constitution and martensitic transformation behavior were investigated for a Au–51Ti–18Co alloy heat-treated at 1173 K to 1373 K for 3.6 ks. The Au–51Ti–18Co alloy was fabricated by Ar arc-melting technique and subsequently by hot-forging at 1423 K for 10.8 ks. X-ray diffraction analysis revealed that B2 parent phase, B19 martensite phase and AuTi3 simultaneously appeared regardless of the heat-treatment temperatures. By increasing the heat-treatment temperature, the volume fraction of AuTi3 was slightly reduced. Besides, the lattice transformation strain which was calculated from the precisely-determined lattice parameters was evaluated to be 7 % in the Au–51Ti–18Co alloy in all the heat-treated conditions. This value is comparable to that of NiTi practical alloys. From differential scanning calorimetry (DSC) analysis, reverse martensitic transformation temperature was slightly increased with the heat-treatment temperature. From the lattice transformation strain point of views, the Au–51Ti–18Co has a large potential for novel biomedical shape memory alloy.

Martensitic transformation and shape memory effect at high temperatures in off-stoichiometric Co2VSi Heusler alloys

Martensitic transformation in off-stoichiometric Co2VSi Heusler alloy was firstly confirmed in this work. The martensite phase presents an ordered 2M non-modulate D022 structure transformed from L21 Heusler structure. In the Co63.5V17.0Si19.5 alloy, the forward and reverse martensitic transformation temperatures are extremely high which reach up to 683°C and 693°C, respectively, higher than most known Heusler-type high-temperature shape memory alloys. And results also indicate that the single martensite phase alloy exhibits a good ductility and strength with the compressive strain of over 15% and strength of about 2300MPa, respectively. What's more, the recoverable strain being maximal to around 3.1% can be obtained at high temperature (750–800°C) due to its reverse martensitic transformation.

Phase Formation, Thermal Stability and Mechanical Properties of a Cu-Al-Ni-Mn Shape Memory Alloy Prepared by Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing process used to produce parts with complex geometries layer by layer. This rapid solidification method allows fabricating samples in a non-equilibrium state and with refined microstructure. In this work, this method is used to fabricate 3 mm diameter rods of a Cu-based shape memory alloy. The phase formation, thermal stability and mechanical properties were investigated and correlated. Samples with a relative density higher than 92% and without cracks were obtained. A single monoclinic martensitic phase was formed with average grain size ranging between 28 to 36 μm. The samples exhibit a reverse martensitic transformation temperature around 106 ± 2 °C and a large plasticity in compression (around 15±1%) with a typical “double-yielding” behaviour.