Thermoelastic Martensitic Transformation Research Articles

A Microstructural Study of Cu-10Al-7Ag Shape Memory Alloy in As-Cast and Quenched Conditions

Shape memory alloys (SMAs) represent an exceptional class of smart materials as they are able to recover their shape after mechanical deformation, making them suitable for use in actuators, sensors and smart devices. These unique properties are due to the thermoelastic martensitic transformation that can occur during both thermal and mechanical deformation. Cu-based SMAs, especially those incorporating Al and Ag, are attracting much attention due to their facile production and cost-effectiveness. Among them, Cu-Al-Ag SMAs stand out due to their notably high temperature range for martensitic transformation. In this study, a Cu-based SMA with a new ternary composition of Cu-10Al-7Ag wt.% was prepared by arc melting and the samples cut from this casting alloy were quenched in water. Subsequently, the phase composition and the development of the microstructure were investigated. In addition, the morphology of the martensite was studied using advanced techniques such as electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The analyzes confirmed the presence of martensitic structures in both samples; mainly 18R (β1′) martensite was present but a small volume fraction of (γ1′) martensite also was noticed in the as-quenched sample. The observation of fine, twinned martensite plates in the SMA alloy with symmetrically occurring basal plane traces between the twin variants underlines the inherent correlation between microstructural symmetry and the properties of the material and provides valuable insights into its behavior. The hardness of the quenched sample was found to be lower than the as-cast counterpart, which can be linked to the solutioning of Ag particles during the heat treatment.

Impact of thermal activation on the growth dynamics of thermoelastic martensitic transformation

The dynamics of thermoelastic martensitic transformations (MTs) become not straightforward with lowering temperatures due to the manifestation of the thermal activation process. The author recently proposed a new concept of “viscosuperelasticity” of which mathematical expression for the thermal activation of habit plane motion enables to quantitatively reproduce the MT dynamics under both the isothermal and anisothermal (dynamic cooling/heating) conditions. In this study, the time-dependent evolution of MTs is evaluated with the concept of viscosuperelasticity for some representative cases of MT-hosting alloy systems. This concept enables seemingly strange low-temperature behaviors to be explained as manifestations of thermal activation. Furthermore, undiscovered exceptional dynamics are also predicted to occur. This study guides the way to manifest the impact of time from experimentally accessible MT behaviors and provides a systematic understanding of low-temperature MT dynamics.

Active operative heat storage application using thermoelastic martensitic transformation in TiNi-based alloy

We propose a new concept of heat-storage material that utilizes thermoelastic martensitic transformation in a TiNi-based alloy. In the material, the stored thermal energy is maintained over a wide temperature range between the thermal absorption and desorption temperatures, and the stored energy is released on demand by external stress on the ground of a latent heat of the stress-induced martensitic transformation. The heat-storage temperature range was effectively increased through a thermomechanical treatment without changing the amount of latent heat. The tensile force applied to the treated sample released to the environment more heat than the unbiased desorption temperature. The amount of heat released was in agreement with the values expected from the Clausius–Clapeyron equation and differential scanning calorimetry profiles. The amount of heat released was confirmed to remain unchanged up to the 9th cycle when the release process was repeated at 286 K by applying tension after heat absorption.

Effects of laser welding on the martensitic transformation of a shape-memory Ti-Ni alloy

The objective of this study is to investigate the martensitic transformation behavior of a Ti-Ni alloy subjected to a laser welding process. Materials characterization techniques were used to analyze the influences of this welding procedure on the phase transformation behavior. This study can provide valuable insights for the improvement of Ti-Ni alloy welding processes used for technological applications, such as in the aerospace industry. In this study, samples were prepared under three conditions: thermal treatment at 400°C, thermal treatment at 500°C, and as-received. The characterization techniques used were: Optical Microscopy (OM), Differential Scanning Calorimetry (DSC), Electron Dispersion Spectroscopy (EDS), X-Ray Diffraction (XRD), and Vickers Microhardness (HV). The results allow for analysis of thermoelastic martensitic transformation and variations in the compositions of the alloy elements. The research, in general, aims to repurpose welding waste in Ti-Ni alloys for the production of mini sensors. The enthalpy was higher for the alloy treated at 500°C and welded. The research findings led to the observation of martensitic phase transformation in the laser-welded Ti-Ni alloy, which suggests that it can be used as a sensor/actuator.

Insensitivity of nucleation rate to order-disorder interfaces in reversible thermoelastic martensitic transformations

Insensitivity of nucleation rate to order-disorder interfaces in reversible thermoelastic martensitic transformations

Regularities in the Evolution of Thermoelastic Martensitic Transformations during Cooling/Heating in the Free State and under Load of Titanium Nickelide Alloyed with Niobium.

This article presents the results of studies of the features of the development of thermoelastic martensitic transformations during cooling/heating in the free state and under load of Ti50Ni49.7-XNbXMo0.3 alloys (X = 0.5, 1.0 and 1.5 at% Nb) with shape memory effects. Using X-ray diffraction analysis, it was found that all the alloys studied at room temperature contained a multiphase mixture consisting of intermetallic compounds with the TiNi (B2, B19'), Ni56Ti29Nb15, and Ti2Ni compositions. Scanning electron microscopy was used to study the microstructure of TiNi (Nb,Mo) alloys and it was found that the distribution of fine Ni56Ti29Nb15 particles in the matrix depends significantly on the concentration of the alloying element. A correlation was established between changes in the structural-phase state in TiNi (Nb,Mo) alloys and the occurrence of the B2↔B19' martensitic transition in the free state and under load. Based on physical and mechanical studies, the temperature ranges of the martensitic transformations (MT) in the free state and under load were established. Based on the thermodynamic description of the MT and the analysis of the characteristic temperatures of the MT, it was found that the MT mechanism is strongly dependent on the concentration of the alloying element.

Shape memory and elastocaloric properties of melt-spun NiMn-based Heusler alloys

In the present work, towards further development of NiMn-based Heusler-type elastocaloric alloys a series of melt-spun ribbons have been prepared from three multicomponent Ni-Mn-Sn-based metamagnetic shape memory alloys (MetaMSMAs) and two multicomponent Ni-Mn-Ga-based ferromagnetic shape memory alloys (FSMAs). Their thermoelastic martensitic transformation (TMT), crystal structure and thermomechanical properties have been studied in detail. Whereas FSMAs ribbons showed a conventional stress-strain behavior at TMT and conventional elastocaloric effect (eCE), MetaMSMAs ribbons, in turn, exhibited highly pronounced inverse stress-strain and inverse eCE characteristics at TMT. The abnormal thermomechanical behavior of the MetaMSMA ribbons is produced by the presence of significant internal stresses due to appropriate amount of quenched-in defects resulting from the rapid solidification of melt-spun alloys. The results of the present study can serve as a guide for the design of new eCE-efficient materials in the form of ribbon.

The Effect of High-Temperature Thermomechanical Treatment on the Microstructure and Mechanical Properties of Cu–Al–Ni–(B) Alloys with a Thermoelastic Martensitic Transformation

The Effect of High-Temperature Thermomechanical Treatment on the Microstructure and Mechanical Properties of Cu–Al–Ni–(B) Alloys with a Thermoelastic Martensitic Transformation

The Effect of High-Temperature Thermomechanical Treatment on the Microstructure
 and Mechanical Properties of Cu–Al–Ni–(B) Alloys with a Thermoelastic Martensitic Transformation

Polycrystalline boron-alloyed α + β Cu–Al–Ni compositions subjected to high-temperature thermomechanical treatment (HTMT) via forging and rolling are studied for the first time. Optical, scanning, and transmission electron microscopy and X-ray diffraction analysis were used in combination with measurementsof tensile mechanical properties to study the peculiarities of the microstructure, phase composition, and mechanical properties of these alloys. The peculiarities of the microstructure and mechanical behavior of alloys differing in their aluminum and boron contents, which were subjected to HTMT, have been determined.The alloys were prepared in the fine-grained state, which determines the increase in the functional strength and plastic characteristics. A schedule of HTMT of bulk Cu–Al–Ni–(B) alloys is proposed.

Microstructure and Mechanical Behavior of Cu-Al-Ni-B Alloys with Thermoelastic Martensitic Transformation

For the first time, using optical, scanning, and transmission electron microscopy and X-ray phase analysis in combination with measurements of tensile mechanical properties, we obtained data on the structural features of the polycrystalline shape-memory eutectoid Cu-Al-Ni-(B) alloys doped by aluminum (of 10 and 14 wt% Al in total amount), nickel (of 3, 4, and 4.5 wt% Ni), and boron (0.02–0.3 wt% B) in various compositions. The effect of boron on the grain sizes, structure, phase composition, and mechanical properties of shape memory (SM) alloys has been studied. The localization of aluminum borides in the structure was investigated and an effect of grain growth inhibition in the (α + β) and β Cu-Al-Ni-B alloys was established, both in the cast state of the alloys considered and after their heat treatment.

Effects of V addition on microstructure and pseudoelastic response in Fe–Mn–Al–Ni alloys

Avoiding quench cracks and controlling the formation process of NiAl precipitates are essential to regulate the microstructure and pseudoelastic properties of Fe–Mn–Al–Ni shape memory alloys (SMAs). In this study, we investigated the effect of 1.5 at.%V addition on the quench sensitivity of Fe–Mn–Al–Ni SMAs and the precipitation behavior of NiAl precipitates in air-cooled Fe–Mn–Al–Ni–V SMAs. It is found that the V addition significantly inhibits the precipitation process of non-transforming γ-phase. Polycrystalline sample of bamboo structure was prepared by abnormal grain growth and tested by incremental strain cycling tensile test at room temperature. The polycrystalline Fe–Mn–Al–Ni–V SMAs obtain fully reversible thermoelastic martensitic transformation without quenching, which also prevents the formation of quenching cracks. Single crystal samples were further prepared by directional recrystallization. High number-density and relatively large-size coherent β-NiAl precipitates were obtained to strengthen the matrix in the single crystal without aging, resulting in large stress hysteresis and irrecoverable strain. After aging Fe–Mn–Al–Ni–V single crystal for 3 h at 200 °C, both the stress hysteresis and irrecoverable strain decreased, thereby achieving good pseudoelasticity in Fe–Mn–Al–Ni–V SMAs.

The Effect of Boron Addition on the Structure
 and Mechanical Properties of Cu–Al–Ni–B Alloys
 with a Thermoelastic Martensitic Transformation

For the first time, data on the peculiarities of the structure of the Cu–Al–Ni–(B) alloys differing in the contents of alloying elements, namely, containing 10–14 wt % aluminum, 3, 4, 4.5 wt % nickel, and 0.02–0.3 wt % boron were obtained using optical, scanning, and transmission microscopy and X-ray diffraction analysis along with measurements of tensile mechanical properties. The effect of boron on the grain size, structure, phase composition, and mechanical properties of the shape-memory alloys has been studied. The localization of aluminum boride precipitates in the structure has been studied, and the effect of grain growthretardation in (α + β) and β Cu–Al–Ni–B alloys in both cast and heat-treated states has been found.

The Effect of Boron Addition on the Structure and Mechanical Properties of Cu–Al–Ni–B Alloys with a Thermoelastic Martensitic Transformation

The Effect of Boron Addition on the Structure and Mechanical Properties of Cu–Al–Ni–B Alloys with a Thermoelastic Martensitic Transformation

Influence of ceramic reinforcement characteristics on the microstructure and wear behavior of NiTi matrix composites

Because the important properties of the NiTi alloy, such as the shape memory effect, pseudoelasticity, and wear resistance, originate from a thermoelastic reversible martensitic transformation, these properties can be controlled using internal and external stresses. In this research, to improve the wear resistance and to minimize the mismatch stress in the matrix, YSZ microparticles with a thermal expansion coefficient close to that of an NiTi alloy were used as the reinforcement. In samples consolidated using spark plasma sintering, the role of YSZ reinforcement on microstructure and wear behavior was investigated and compared with NiTi-ZrO2 samples. The microstructural studies showed that using YSZ instead of monoclinic zirconia as the reinforcement can produce a lower mismatch stress and, therefore, a higher fraction of austenitic phase remains in the matrix. The austenite phase possesses strain recovery, which is beneficial for tribological applications. Therefore, NiTi-YSZ samples with a higher percentage of austenite and better mechanical properties demonstrated superior wear resistance. For example, the addition of 5 wt% YSZ to NiTi reduced the wear rate from 5.74 × 10−3 to 1.03 × 10−3 mm3/N.m. Investigation of wear mechanisms showed that abrasive, delamination and adhesive mechanisms mainly govern the wear process.

Thermoelastic Martensitic Transformation and Shape Memory Effect in [001]-Oriented Single Crystals of High-Entropy Alloy

Thermoelastic Martensitic Transformation and Shape Memory Effect in [001]-Oriented Single Crystals of High-Entropy Alloy

Development of an alternating heat engine, actuated by shape memory alloys

The paper introduces a new type of heat engine actuated by shape memory alloy (SMA) commercial wires. The originality of the design consists in using two SMA active elements that are martensitic at room temperature (RT) and compensate each other in such a way that each element becomes, on a turn, austenitic while the other is martensitic and vice-versa. The SMA wires were manufactured from Ti50Ni45Cu5 SMAs undergoing thermoelastic martensitic transformation characterized, among other things, by superior stiffness of austenitic state as compared to martensitic state. This feature has enabled thermoelastic SMAs to develop work generating shape memory effect (SME), while undergoing reverse martensitic transformation, during heating. Based on this principle, solid state heat engines have been built even since the discovery of SMAs. The paper reviews the most prominent types of SMA driven solid state engines, before introducing a new type of engine. In the present heat engine constructive solution, solar energy is used to directly heat up the active element, which has a straight austenitic “hot shape” and was bent into a curved shape at RT, while being in martensitic state. During heating, the element becomes fully austenitic, partially recovers its straight shape, by decreasing its curvature radius, and bents the bias martensitic element which is softer. The latter will be brought with its most deformed area into the focus of an optical lens, thus being instantly heated up. It develops, in its turn, work generating SME. Based on this principle, a linear-alternative heat engine has been patented. Its functioning principle was tested for different numbers of heating–cooling cycles. By means of an original testing setup, using hot air heating and sprayed water cooling, the variations of displacement with temperature were recorded and the critical temperature for then start of reverse martensitic transformation (As) was determined.

Near-room-temperature martensitic actuation profited from one-dimensional hybrid perovskite structure

Martensitic transformation, usually accompanied by ferroelastic and thermoelastic behaviors, is an interesting and useful mechanical-related property upon external stimuli. For molecular crystals, however, martensitic systems to show reversible stimuli-actuation behaviors are still limited because of a lack of designability and frequent crystal collapse due to large stress releases during the transformations. Here, a one-dimensional hybrid perovskite semiconductor (NMEA)PbI3 (NMEA = N-methylethylammonium) was prepared by following a dimensionality reduction design principle. The crystal undergoes reversible ferroelastic and thermoelastic martensitic transformations, which are attributed to weak intermolecular interactions among the chains that easily trigger the interchain shearing movement. The actuation behavior occurring during the phase transition is very close to room temperature and demonstrated to behave as a mechanical actuator for switching. This work provides an effective approach to designing molecular actuators with promising applications in next-generation intelligence devices.

Investigation of some thermomechanical processing effects on the structure and properties of a TiNiCu shape memory alloy

A Ti50Ni45Cu5 shape memory alloy (SMA) was cast, homogenized and hot rolled into 0.5 mm-thick lamellas. After homogenization, the intercrystalline Ti2(Ni, Cu) based solid solution, gradually decomposed with the increase of hold temperature and time, as pointed out by optical, scanning electron and atomic force microscopy. After hot rolling, by wire spark erosion, rectangular specimens with the dimensions 0.5 mm × 2 mm × 6 mm were cut each one with a mass of 0.44 g. Since the specimens experienced a thermoelastic martensitic transformation, while being martensitic at room temperature (RT) they were soft and could be easily bent against cylindrical calibers, in order to induce their “cold” (martensitic) shape. When heated, up to 100 °C with a hot air gun, they readily recovered, by shape memory effect (SME) their initial straight form, also known as “hot shape”. Thermomechanical training cycles were subsequently applied, consisting in: (i) RT bending; (ii) hot air gun heating and (iii) air cooling. These cycles were repeated up to 50 times. The effects of the thermomechanical training cycles were subsequently analyzed, by differential scanning calorimetry, on fragments cut from the active sections of the specimens. It was noticed that training basically increased thermal hysteresis. Finally, experiments were performed which demonstrated the capacity of Ti50Ni45Cu5 SMA elements to develop.

Elastocaloric Properties of Polycrystalline Samples of NiMnGaCu Ferromagnetic Shape Memory Alloy under Compression: Effect of Improvement of Thermoelastic Martensitic Transformation.

Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change (ΔS) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both ΔT and ΔS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs. T were considered. Maximum values of ΔS = 55.9 J/KgK and ΔT = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test.

In Situ Observation of Thermoelastic Martensitic Transformation of Cu-Al-Mn Cryogenic Shape Memory Alloy with Compressive Stress.

The thermoelastic martensitic transformation and its reverse transformation of the Cu-Al-Mn cryogenic shape memory alloy, both with and without compressive stress, has been dynamically in situ observed. During the process of thermoelastic martensitic transformation, martensite nucleates and gradually grow up as they cool, and shrink to disappearance as they heat. The order of martensite disappearance is just opposite to that of their formation. Observations of the self-accommodation of martensite variants, which were carried out by using a low temperature metallographic in situ observation apparatus, showed that the variants could interact with each other. The results of in situ synchrotron radiation X-ray and metallographic observation also suggested there were some residual austenites, even if the temperature was below Mf, which means the martensitic transformation could not be 100% accomplished. The external compressive stress would promote the preferential formation of martensite with some orientation, and also hinder the formation of martensite with other nonequivalent directions. The possible mechanism of the martensitic reverse transformation is discussed.