Phase Transformation Hysteresis Research Articles

Microstructure and Phase Transformation Behavior of NiTiCu Shape Memory Alloys Produced Using Twin-Wire Arc Additive Manufacturing

NiTiCu thin walls were produced by twin-wire arc additive manufacturing (T-WAAM) using commercial NiTi and Cu wires as the feedstock materials. This approach aims to solve the problems typically associated with large phase transformation hysteresis in NiTi shape memory alloys. The microstructure, mechanical properties, and phase transformation behavior of the as-deposited NiTiCu alloy were comprehensively examined. The results revealed that the as-deposited NiTiCu alloy was well-formed, with its microstructure showed columnar, equiaxed, and needle-like grains, depending on the location within the deposited walls. The microhardness gradually increased from the first to the third layer. The Cu content was 20.80 at%, and Cu-based precipitates were formed in the as-deposited NiTiCu. The volume fractions and lattice parameters of the matrix and precipitates in the as-deposited NiTiCu material were analyzed using high-energy synchrotron X-ray diffraction. The martensitic phase was identified as a B19 crystal structure, and the as-deposited NiTiCu underwent a one-step B2-B19 phase transformation. The tensile strength and fracture strain were approximately 232 MPa and 3.72%, respectively. In particular, the addition of Cu narrowed the phase transformation hysteresis of the as-deposited NiTiCu alloy from 24.4 to 7.1 ℃ compared with conventional binary NiTi alloys. This study expands the potential of T-WAAM in modifying the phase transformation behavior of NiTi-based ternary alloys.

Wire-based electron beam additive manufacturing of NiTiNb shape memory alloy: Microstructure, phase-transformation behavior and mechanical properties

NiTiNb shape memory alloy (SMA) is considered to have great potential in aerospace due to its wide phase-transformation hysteresis. Here, we innovatively used electron-beam additive manufacturing (EBAM) to fabricate a NiTiNb SMA thin-walled structure. The phase-transformation temperature, microstructure evolution, and mechanical properties were investigated in detail. The as-built NiTiNb SMA comprised a NiTi matrix and different morphologies of β-Nb phase at room temperature. The microstructure homogeneity and mechanical properties of NiTiNb prepared by EBAM significantly improved compared with those of NiTiNb powder-based AM technology. Our work provided a promising method for the preparation of defect-free and microstructure homogeneous NiTiNb SMA by AM. This method further stimulated the potential of additively manufactured SMA components for industrial applications.

High yield stress and narrow phase transformation hysteresis of thermomechanical-processing NiTiCu shape memory alloy

NiTiCu shape memory alloy is subjected to thermomechanical processing which deals with local canning compression followed by annealing at 300, 450 and 600 °C, respectively. Transformation temperatures of annealed NiTiCu specimens increase with elevating annealing temperature. Following annealing at 300 °C, NiTiCu specimen exhibits high yield stress of about 2000 MPa due to the existence of nanocrystalline grains. NiTiCu sample annealed at 600 °C possesses an extremely narrow phase transformation hysteresis width of about 1 °C. Nanotwins are found in NiTiCu sample annealed at 300 °C, while a large number of twin variants appear in NiTiCu samples annealed at 450 and 600 °C. Nanocrystalline grains contribute to suppressing the occurrence of twin variants. Furthermore, nanocrystalline grains are not beneficial to the formation of martensite variants. In the case of compressive loading, martensite reorientation and detwinning take place more readily in NiTiCu samples annealed at 450 and 600 °C.

The Mechanical and Functional Behaviors of Mn–15 wt%Cu Alloy Produced by Selective Laser Melting

Mn–Cu‐based alloys possess excellent mechanical and functional properties (damping capacity and shape memory effect). This work utilizes a typical additive manufacturing technique, selective laser melting (SLM), to fabricate Mn–15 wt%Cu alloys to explore the mechanical and functional behaviors. The results indicate that the γ‐(Mn, Cu) and a small amount of γ′‐(Mn, Cu) phase are the main phase compositions in the as‐SLMed Mn–15 wt%Cu alloy, as well as twins and second‐phase precipitation particles. The microstructure shows a fine cellular γ‐(Mn, Cu) dendrite with primary dendrite spacing of ≈0.9 μm in the columnar grains with a size of ≈8 μm. The martensitic transformation start temperature (MS) and phase transformation hysteresis are ≈132.6 and ≈5.1 °C, respectively. The existing nanoscale chemical segregations of Mn and Cu are attributed to the spinodal decomposition of γ‐(Mn, Cu). A compositional modulation is observed with a wavelength of ≈20 nm and an amplitude fluctuation of Mn (55–85 wt%) and Cu concentrations (15–45 wt%). Finally, the as‐SLMed Mn–15 wt%Cu alloy boasts good tensile properties (359, 268 MPa, and 3.6%), damping (internal friction of 0.032 when the strain amplitude is 9 × 10−4), and shape memory performances (one‐way ηow = 37–48%, and two‐way ηtw = 16–20% under the pre‐deformation strain of 2–3%).

Coexistence of five domains at single propagating interface in single-crystal Ni[sbnd]Mn[sbnd]Ga shape memory alloy

Coexistence of both austenite and martensite during phase transformation is a common feature of all Shape Memory Alloys (SMAs). The martensite has different variants featuring characteristic deformations rotationally linked to each other due to the symmetries of the austenite parent phase, and the martensite variants can form twins with different mean characteristic deformations. Multiple-domain microstructures (consisting of austenite, martensite twins and individual martensite variants) evolve collectively within an SMA sample during the phase transformation, contributing thus to the material's macroscopic response (e.g., its global deformation). Particularly, the multiple domains can exist at the diffuse austenite-martensite interface nucleating and propagating in a single crystal in certain conditions. This implies an energy barrier for this interfacial structure, influencing the interface kinetics and the driving force (energy dissipation) of the phase transformation. In this paper, we report an experimentally observed interface consisting of five domains (austenite, one martensite variant and three twins) in a Ni–Mn–Ga single-crystal initially consisting of one martensite variant and subjected to a uniaxial thermal gradient. The compatibility analysis (performed from the characteristic strains of the three martensite variants having approximately a tetragonal symmetry) reveals that the five-domain interface is not a perfectly compatible pattern like the basic habit plane (consisting of only one twin compatible with austenite). However, its level of non-compatibility is similar to that of the quite common X-interface (four-domain coexistence) which is observed in many SMAs. Further, the significant effects of the thermal loading path and the material initial state (the initial martensite variant) on the domain pattern formation are demonstrated and analyzed. The experimental observation and the theoretical analysis of the domain patterns can provide hints to better understand diffuse interface kinetics and phase transformation hysteresis.

Preparation of VO2 films via microspacing in-air sublimation method

The microspacing in-air sublimation (MAS) is a promising method for single crystals preparation due to its high efficiency and universality. However, this strategy has seldom been reported in other domains, such as film preparation. Vanadium dioxide (VO2) has been widely used in electronics and thermochromic-based applications, which demand simple, efficient, and cheap procedures. This paper reports a new cost-effective and high-efficiency fabrication method of VO2(M) thin films. High-quality VO2 films were prepared on sapphire and mica substrates by combining the MAS method with post-annealing from a precursor of vanadium acetylacetone oxide (VO(acac)2). The VO2/Al2O3 film, even though the thickness is less than 60 nm, shows a relative switching efficiency of 71.31% at 3500 cm−1. The film also exhibits more than three orders of magnitude change in electrical resistance, while the hysteresis of phase transformation was less than 5 °C. This strategy opens a new gate for preparing dense VO2 films with high flatness and high phase change performance without expensive and complex instruments.

Effect of the Initial Columnar‐Grained Inhomogeneity of Electrical Steels on the Transformation Temperature

The effects of various initial structures and heating and cooling rates on the columnar‐grained transformation temperature are studied by a thermal dilatometer and electron backscatter diffraction. The results show that deformation can reduce the thermal hysteresis more effectively than decrease the heating rate, and the phase transformation hysteresis of the deformed sample is the smallest during heating, followed by the columnar‐grained sample with a slow heating rate, whereas the columnar‐grained sample with a fast heating rate has the largest phase transformation hysteresis. Moreover, the dilatational amount changes greatly with cooling rate. The coarse columnar grains can severely restrain dilatation. The smaller dilatational amount is related to the incomplete phase transformation of the columnar grains and also related to the suppression of the dilatational amount of the small grains by the surrounding columnar grains. In contrast, the largest dilatational amount is caused by the relatively sufficient phase transformation. In addition, the changes of the transformation temperature and the dilatational amount are mainly induced by the grain size effect and the inhomogeneity of the structures, but {100} texture can affect the uniformity of grain size.

Investigating the various properties of cold sprayed CuAlNi shape memory alloys developed by post annealing process

CuAlNi shape memory alloy (SMA) has wide scope in vibration damping applications for machine tools. A bespoke cold spray based additive manufacturing system was used to fabricate CuAlNi SMA precursor structures of two different compositions with optimized deposition parameters, which were subsequently alloyed via annealing. The two compositions of CuAlNi chosen for the study were Cu rich in wt% (Cu-82%, Al-14%, Ni-4%) and the Ni rich in wt% (Cu-12%, Al-2%, Ni-80%) shape memory alloy material. The various properties of developed samples were compared with commercially available CuAlNi structure. The properties compared include surface morphology, crystal structure, phase transformation temperature, and mechanical properties were studied with scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) compression testing respectively. The shape memory characteristics were also analyzed using dynamic mechanical analyzer (DMA). Among the two samples, Cu rich sample exhibited better SMA qualities than Ni rich sample as reported in the literature. The micro-hardness of Cu rich sample was to be 356 ± 3 VHN comparatively lower than Ni rich sample. The DSC results revealed the phase transformation hysteresis for Cu rich sample to be very low at 5 ± 3°C. The Cu rich CuAlNi SMA showed very good shape memory characteristics in the DMA results in the range of 50°C to 100°C.

Phase Transformation and Recovery Stress of Ni47Ti44Nb9 Alloy During Constrained Heating and Cooling

The phase transformation and recovery stress of pre-deformed Ni47Ti44Nb9 alloy are systematically investigated during constrained heating, cooling and thermal cycling. Recovery stress is determined by a phase transformation, thermal expansion, and contraction. Reverse martensitic transformation occurs during constrained heating and then results in a sharp increase in the recovery stress. Meanwhile, the wide phase transformation hysteresis shrinks during thermal cycling. A few martensitic transformations gradually occur across a wide temperature range during constrained cooling, resulting in a negative decreasing stage or a wide plateau of recovery stress. When cooled further to a critical value of 20 °C, the recovery stress and temperature follow the Clausius–Clapeyron equation, and large amounts of martensitic transformations appear, resulting in the sharp decrease of the recovery stress.

In-situ TEM analysis of the phase transformation mechanism of a Cu–Al–Ni shape memory alloy

Minimizing phase transformation (PT) hysteresis is of crucial importance for reliability of shape memory alloy (SMA)-based devices, where the lattice strain/stress caused by thermal hysteresis leads to functional degradation. As a result, understanding structural factors that control PT pathway is critical for development of materials with high reversibility. In this study, two distinct PT mechanisms (from γ1' martensite to β1 austenite phase) in Cu–Al–Ni SMAs were revealed by in-situ TEM observation. A growth-dominant conventional PT mechanism shows in the fast quench sample, whereas a nucleation-dominant PT mechanism that suppresses interface propagation and induces high thermal hysteresis displays in the slow quench sample. By characterizing the atomic scale composition and microstructure we discover that slow quenching induces nanoprecipitation that changes chemistry of the matrix alloy, which in turn causes the higher PT hysteresis and temperature, while fast quenching avoids the formation of these nanoprecipitates. Our finding provides valuable insights into the fabrication of SMAs with better reliability.

<i>In-Situ</i> TEM Analysis of the Phase Transformation Mechanism of a Cu-Al-Ni Shape Memory Alloy Fabricated with Different Quenching Speeds

A shape memory alloy (SMA) recovers its original shape after deformation through martensitic phase transformation (PT), which is diffusionless and involves shear deformation. Minimizing PT hysteresis is of crucial importance for reliability of SMA-based devices, where the stress caused by thermal hysteresis leads to functional degradation. As a result, understanding structural factors that control PT, including phase nucleation and growth, is critical for development of materials with high reversibility. In this study, the PT mechanism (from γ1' martensite to β1 austenite phase) in Cu-Al-Ni SMAs was investigated by preparing alloy with different quenching rates, resulting in very different transition temperatures. By characterizing the atomic scale composition and microstructure we show that slow quenching induces nanoprecipitation that change chemistry and strain field of the matrix alloy, while fast quenching avoids the formation of these nanoprecipitates. In-situ monitoring of the PT process in TEM demonstrate a growth-dominant conventional PT mechanism with low hysteresis in the fast quench sample. On the other hand, the alloy with precipitates shows a nucleation-dominant PT mechanism with suppression of phase growth that induces high thermal hysteresis. Our finding provides valuable insights into the fabrication of SMAs with better reliability.

Mechanisms of nanocrystallization and amorphization of NiTiNb shape memory alloy subjected to severe plastic deformation

Abstract NiTiNb shape memory alloy (SMA) has been widely used for pipe coupling because it possesses the wider phase transformation hysteresis. In the present study, Ni 47 Ti 44 Nb 9 (at.%) SMA, which is composed of B2 austenite phase and β-Nb phase, was subjected to local canning compression at the deformation degrees of 25%, 50% and 75%, respectively. Local canning compression causes Ni 47 Ti 44 Nb 9 SMA to be in a three-dimensional compressive stress state, which contributes to severe plastic deformation of the involved alloy. Severe plastic deformation leads to nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA. Dislocation slip plays a dominant role in plastic deformation of Ni 47 Ti 44 Nb 9 SMA. Physical mechanisms for nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA were revealed on the basis of statistically stored dislocation (SSD) and geometrically necessary dislocation (GND). As a soft phase, β-Nb phase is subjected to plastic yield more easily than B2 austenite phase. However, nanocrystalline phase and amorphous phase are more easily induced in B2 austenite phase than in β-Nb phase. The dislocations emitted from β-Nb phase move toward the interface between β-Nb phase and B2 austenite phase and consequently pile up at the corresponding interface, which plays a significant role in nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA subjected to severe plastic deformation. The formation of nanocrystalline phase deals with the transformation from low angle grain boundary to high angle grain boundary. Nanocrystalline phase can be viewed as a transition from coarse grain to amorphous phase. The occurrence of amorphous phase is attributed to the fact that a high density of dislocations are induced during severe plastic deformation of Ni 47 Ti 44 Nb 9 SMA.

Local Evolution of the Elastocaloric Effect in TiNi-Based Films

Strain and temperature profiles of magnetron-sputtered ferroelastic TiNi-based films of 20 μm thickness are investigated during tensile load cycling with respect to strain, strain rate, and cycle number in order to assess their potential for elastocaloric cooling. Two different ferroelastic film specimens are considered, binary TiNi and quaternary TiNiCuCo films, which strongly differ regarding their phase transformation hysteresis and fatigue behavior. In situ digital image correlation and infrared thermography measurements reveal a correlated response of strain and temperature bands that is determined by mesoscale stress and temperature fields on the kinetics of phase transformation. In the case of binary TiNi films, this response is also strongly affected by cycling-induced fatigue causing vanishing band formation and decreasing elastocaloric effect size. In contrast, TiNiCuCo films show negligible fatigue and retain the local characteristics of the elastocaloric effect. Compared to TiNi films, they exhibit not only a reduced temperature change, but also a reduced work input for pseudoelastic cycling resulting in an improved material’s coefficient of performance of 15.

Elucidating the origins of phase transformation hysteresis during electrochemical cycling of Li–Sb electrodes

In this combined computational and experimental study, we identify the origins of phase transformation hysteresis during cycling of Sb electrodes, which undergoes alloying reactions upon lithiation.

Multiple effects of Ni-rich precipitates in Ni–Ti–Al thin films

Abstract Phase transformation and mechanical properties of two distinct Ni-rich Ni–Ti–Al thin films deposited at various temperatures were investigated in this paper. It is found that phase transformation temperatures and hardness of the films obviously increase with growth of Ni-rich precipitates, and their phase transformation hysteresis decreases. Especially, martensitic transformation start temperature of the Ni 49.7 Ti 45.3 Al 5 films exceeds room temperature and increases by 20 °C, and the Ni 44 Ti 32 Al 24 Heusler alloy films show near-zero transformation hysteresis. Besides, hardness of the two films increases by 12.8% and 22%, respectively. The evolutions are caused by the growth of Ni-rich precipitates which strengthens the matrix. The precipitation would sharply consume Ni content of the matrix that changes phase transformation behavior from the Ni-rich to Ti-rich films.

Influence of cooling rate on phase transformation and microstructure of Ti-50.9%Ni shape memory alloy

Heat treatment of Ti-50.9%Ni (mole fraction) alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature (Ms) and martensitic finish transformation temperature (Mf) decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis (Af-Mf) increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.

Vibration reduction of the impact system by an SMA restraint: numerical studies

The dynamic behaviour of an impact oscillator with a shape memory alloy (SMA) restraint is modelled and analyzed. This impact oscillator has the secondary support made from an SMA and the thermo-mechanical description of the SMA element follows the formulation proposed by Bernardini et al. [1,2]. The thermo-mechanical coupling terms included in the energy balance equation allow to undertake the non-isothermal analysis. Due to the mechanical characteristics of the SMA element and the non-smooth nature of the impacts, five different modes of operation can be distinguished. The undertaken numerical investigations suggest that the system can exhibit complex dynamic responses, which if appropriately controlled can be used for vibration reduction. A comparison with an equivalent elastic oscillator is made. It is found out that the low amplitude regimes are not affected by the SMA element. On contrary, for the large amplitude responses, a significant vibration reduction may be achieved due to the phase transformation hysteresis loop. Various bifurcation scenarios are constructed and the influence of the SMA element is discussed. In particular, the analysis of the frequency and amplitude variations of the external excitation is given and the parameter ranges where the vibration reduction is possible are identified.

Interface propagation and microstructure evolution in phase field models of stress-induced martensitic phase transformations

Analytical solutions for diffuse interface propagation are found for two recently developed Landau potentials that account for the phenomenology of stress-induced martensitic phase transformations. The solutions include the interface profile and velocity as a function of temperature and stress tensor. An instability in the interface propagation near lattice instability conditions is studied numerically. The effect of material inertia is approximately included. Two methods for introducing an athermal interface friction in phase field models are discussed. In the first method an analytic expression defines the location of the diffuse interface, and the rate of change of the order parameters is required to vanish if the driving force is below a threshold. As an alternative and more physical approach, we demonstrate that the introduction of spatially oscillatory stress fields due to crystal defects and the Peierls barrier, or to a jump in chemical energy, reproduces the effect of an athermal threshold. Finite element simulations of microstructure evolution with and without an athermal threshold are performed. In the presence of spatially oscillatory fields the evolution self-arrests in realistic stationary microstructures, thus the system does not converge to an unphysical single-phase final state, and rate-independent temperature- and stress-induced phase transformation hysteresis are exhibited.

Numerical algorithms for cyclic phase transformation hysteresis in a shape memory plate subject to axisymmetric plane stress

AbstractWe present numerical algorithms for calculating stress fields in an annulus composed of a shape memory material under conditions of quasi‐static edge loading at constant temperature. The algorithms track the material microstructure in terms of the volume fraction of austenite (A) and martensite (M), the latter of which provides a transformation strain. The dependence on load path imparts significant hysteresis in the stress induced transformation between A and M. A previous study that was restricted to proportional loading in the direction of forward transformation (J. Appl. Mech. 2005; 72:44–53) is here generalized to consider arbitrary loadings. The shooting algorithm that was robust for the previously considered proportional loadings is found to be subject to numerical instability for the most general transformation possibilities considered here. This motivates the development of an alternative iterated mapping algorithm that is found to generate a robust semi‐analytical finite difference procedure. The algorithm efficiently determines the operative transformation type, as is illustrated in cases where forward and reverse loading are occurring simultaneously at different plate locations. At those locations where phase transformation is inactive, the algorithm continues to account for martensite reorientation that alters the local transformation strain. Copyright © 2006 John Wiley & Sons, Ltd.

Phase Transformation Hysteresis in a Plutonium Alloy System: Modeling the Resistivity during the Transformation

AbstractWe have induced, measured, and modeled the β - α' martensitic transformation in a Pu-Ga alloy by a resistivity technique on a 2.8-mm diameter disk sample. Our measurements of the resistance by a 4-probe technique were consistent with the expected resistance obtained from a finite element analysis of the 4-point measurement of resistivity in our round disk configuration. Analysis by finite element methods of the postulated configuration of α' particles within model σ grains suggests that a considerable anisotropy in the resistivity may be obtained depending on the arrangement of the α' lens shaped particles within the grains. The resistivity of these grains departs from the series resistance model and can lead to significant errors in the predicted amount of the α' phase present in the microstructure. An underestimation of the amount of α' in the sample by 15%, or more, appears to be possible.