Microstructure and high-temperature shape-memory effect in Ni54Mn25Ga21 alloy

1 Introduction Shape memory alloys (SMAs) have developed rapidly in the past few decades as new functional materials, with commercial applications in pipe couplings, medical implants, electrical connectors and various actuators etc[1?3]. But the highest …     

Modeling and simulation of irradiation effects on martensitic transformations in shape memory alloys

A model is proposed for the calculation of the irradiation induced changes of the martensitic transformation temperatures of shape memory alloys. It considers the transition temperatures being determined by a chemical and a non-chemical term in the Gibbs free energy that can be affected by the irradiation induced changes of the vacancy density and the chemical ordering of the crystal structures. Numerical simulations for TiNi SMAs have shown that upon irradiation the austenite finish temperature A_f can be raised slightly at the beginning stage. After that, both A_f and the martensite start temperature M_s will decrease strongly and reach some stable values after extensive irradiations. Both the dose rate and the temperature of irradiations will affect the changes of M_s and A_f. According to the simulation, the increase of A_f at the beginning of irradiations is resulted from the irradiation induced point defect production that increases mainly the non-chemical term in the Gibbs free energy change of the martensitic transformation. The irradiation induced chemical disordering reduces both the chemical and non-chemical terms in the Gibbs free energy change and leads to the strong decreases of M_s and A_f.     

Effects of ageing on bainitic and thermally induced martensitic transformations in ductile Cu–Al–Mn-based shape memory alloys

The effects of ageing at 473–573 K on the hardness, microstructure and thermoelastic martensitic transformation in Cu–Al–Mn-based shape memory alloys were investigated. It was found that hardness was dramatically increased by ageing due to the formation of fine bainitic plates and that the volume fraction of the bainite phase with ageing time can be described by the Austin–Rickett equation. The martensitic transformation temperatures decreased with the formation of bainite plates, mainly due to the composition change of the β-matrix. Moreover, the growth of thermally induced martensite plates was disturbed by the existence of bainite plates. Consequently, the transformation intervals (M_s–M_f and A_f–A_s) and transformation hystereses (A_f–M_s and A_s–M_f) increased with the progress of bainitic transformation.     

Time effect of martensitic transformation in Ni43Co7Mn41Sn9

Effect of thermal arrest on the L2₁-tetragonal martensitic transformation in a NiCoMnSn shape memory alloy was investigated. The phenomenon was studied by interrupted heating/cooling in differential scanning calorimetry analysis. The experimental evidence indicates that the forward martensitic transformation continued to completion during cooling arrest between M_s and M_f. The same behavior was also observed for the reverse transformation on heating. These observations demonstrate that the L2₁-tetragonal martensitic transformation in the Ni₄₃Co₇Mn₄₁Sn₉ alloy is time dependent at the finite cooling rate.     

Study of the phase transformation of cobalt

The allotropic transformation of cobalt was studied with a differential scanning calorimeter. The temperatures of the fcc to hep transformation, A_s, and fcc to hcp transformation, M_s, exhibit a linear dependence on the heating and cooling rates: A_s(K) = 723.2 + 0.28R and M_s(K) = 671.4-0.28R whereR is the rate in K/min. A_s, M_s, and the associated enthalpies of the phase changes were observed to increase with increasing number of cycles through the transformation. The M_s reaction was observed to proceed slowly isothermally while exhibiting athermal behavior when cooled rapidly.     

Effect of thermocycling on the temperatures of phase transformations,structure, and properties of the equiatomic alloy Ti<Subscript>50.0</Subscript>Ni<Subscript>50.0</Subscript>

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti₅₀Ni₅₀ in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (M_s, M_f) decrease by 2–3 K, and the temperatures of the reverse transformation (A_s, A_f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.     

Orientation dependence of superelasticity in ferromagnetic single crystals Co<Subscript>49</Subscript>Ni<Subscript>21</Subscript>Ga<Subscript>30</Subscript>

Dependence of the amount of reversible deformation on the orientation of the crystal axis and testing temperature has been studied using [001] and [$ \bar 1 $ \bar 1 24] single crystals of the Co₄₉Ni₂₁Ga₃₀ (at %) alloy upon compression. It has been shown that in the [001] crystals with T ≤ M _s (M _s is the temperature of the onset of the forward martensitic transformation upon cooling) the reversible deformation is equal to 5.5–6.5% and consists of the deformation connected with the shape-memory effect (SME) equal to 4–4.2%, and of “ferroelastic” deformation equal to 1.5–2.2%, which is reversible upon unloading. The total reversible deformation exceeds the lattice deformation ɛ₀ observed upon the B2-L1₀ martensitic transformation, which is equal to 4.5%. At T > A _f (A _f is the finish temperature of the reverse martensitic transformation upon heating), the reversible deformation in [001] crystals is equal to 6.5%. It has been shown electron-microscopically that the reversible deformation equal to 1.5–2.2% in the temperatures range of T = 77−300 K is connected with the development of mechanical twinning in the L1₀ martensite on (110) _L10 planes, which proves to be reversible in the [001] crystals and can be partly irreversible in the [$ \bar 1 $ \bar 1 24] crystals. Upon heating, the (110) _L10 twins of the stabilized L1₀ martensite pass into the ($ \bar 1 $ \bar 1 12) _B2 twins of the B2 phase.     

Study of Thermal Scanning Rates on Transformations of Ti-19Nb-9Zr (at.%) by Means of Differential Scanning Calorimetry Analysis

Differential scanning calorimetry (DSC) thermal analysis is a well-accepted technique used to measure the transformation temperatures of shape memory alloy and its thermoelastic transformation energies. In this study, both forward and reverse transformation temperatures of a nickel-free Ti-19Nb-9Zr (at.%) SMA were investigated using DSC technique with different cooling and heating scanning rates in a range of 10 to 100 °C/min. The results showed that the transformation temperature intervals vary substantially with respect to the thermal scanning rates. It is found that the martensitic start (M _s) temperature decreases with decreasing the cooling rates. The optimal scanning rate was found to be 40 °C/min for obtaining the maximum thermoelastic transformation energies stored between the forward and the reverse martensitic transformations. It is believed that the thermoelastic transformation energy increases with the increase in the volume fraction of martensite. Based on these measurements, these thermoelastic transformation energies between the forward and the reverse martensitic transformations were estimated to be ~21 and ~27 J/g, respectively. The appropriate selection of scanning rate for SMA analysis will be discussed.     

Effect of magnetic field on the morphology and fine structure of low-temperature martensite phase in a ferromagnetic Ni<Subscript>2.08</Subscript>Mn<Subscript>0.96</Subscript>Ga<Subscript>0.96</Subscript> alloy

The morphology and fine structure of high-temperature austenitic and low-temperature martensitic phases in a ferromagnetic Ni_2.08Mn_0.96Ga_0.96 alloy and the effect of magnetic field on the peculiarities of the martensite formation have been studied. The Ni_2.08Mn_0.96Ga_0.96 alloy in the initial cast and annealed states was found to undergo martensitic and magnetic phase transformations at temperatures M _s = 280 K, M _f = 265 K, A _s = 280 K, A _f = 295 K, and T _C = 375 K, respectively. Above M _s, the alloy is in a metastable premartensitic state, which leads to a characteristic diffuse scattering and tweed contrast when studying by electron microscopy. An applied magnetic field of up to 600 kA/m was found to affect the martensitic transformation in the alloy. The field application leads to changes in the morphology and fine structure of martensite due to orientation along the field direction of the magnetizations of the energetically advantageous (in terms of the directions of the magnetization vectors) c domains of the existing orientation variants of the martensite crystals having a packet pairwise-twinned morphology. The martensitic structure of the Ni_2.08Mn_0.96Ga_0.96 alloy already formed previously during cooling is not affected by an external magnetic field with a strength of up to 600 kA/m.     

Determination of the equilibrium transformation temperature (T0) and analysis of the non-chemical energy terms in a CuAlNi single crystalline shape memory alloy

A method for the determination of the equilibrium transformation temperature (T₀) in CuAlNi single crystalline alloys, by traditional uniform heating and cooling of the specimen under constant uniaxial applied stress, σ, is presented and the T₀(σ) functions are constructed. Above a certain stress level the phase transformations, even in a multiple interface mode, can be driven in such a way that the thermal hysteresis loops have a rectangular part, from which T₀ can be determined via the well-known relation: T₀ = (M_s + A_f)/2, where M_s and A_f are the martensite start and austenite finish temperatures, respectively. At low stress values the heating up branch of the hysteresis is different; it starts with a vertical part showing that the beginning of the austenite formation is free of the release of the elastic energy (this takes place only in the second part of this branch). Here the T₀ temperature can be determined as the arithmetic mean of M_s and the austenite start temperature, A_s. Using the experimentally determined stress dependence of the transformation strain, the T₀ vs. σ function was also constructed from the Clausius–Clapeyron relation and this curve fitted very well to the points obtained from the above relationships at high and low stress levels, respectively.The stress dependence of the non-chemical energy contributions to the phase transformation are also determined. It is shown that integrals of the differential values (the derivatives of the energy contributions by the transformed fraction) give self-consistent results with the (integral) quantities directly measured in differential scanning calorimetry (DSC) experiments or obtained from the area of the hysteresis loops.     

Influence of heating rate on the laser surface hardening of a medium carbon steel

The finite difference method was applied to analyse the influence of heating rate on the transformation phenomenon in laser surface hardening of a medium carbon steel. The implicit scheme of this method was adopted to improve the accuracy of the numerical analysis since the very high heating and cooling rate and the small hardened zone, which characterize the process investigated, need a very detailed mesh generation. The calculated cooling rate was high enough for all the material that undergoes transformation into austenite to be transformed into martensite. From the calculated temperature change the heating rate of the transformation zone boundary could be estimated to be of the order of 10⁴ °C s^-1, which causes a delay in the austenite transformation and consequently a shift in the transformation temperature. Considering a heating rate of 10⁴ °C s^-1, the A_s (start of austenitic transformation) temperature of the investigated medium carbon steel with 0.45 wt.% C should be approximated to be about 830 °C and the A_f (finish of austenitic transformation) temperature about 950 °C. Experimental results obtained by irradiating the test specimen with a 1 kW CO₂ laser showed better agreement with the hardened zone sizes predicted by the modified A_s temperature than with those predicted by the equilibrium A_s temperature. From simulations it appeared that the occurrence of surface melting and the size and shape of the hardened zone are strongly dependent on process parameters such as beam spot diameter and traverse speed.     

Size dependence of martensite transformation temperature in nanostructured Ni-Mn-Sn ferromagnetic shape memory alloy thin films

In the present study, we report the influence of grain size on structural and phase transformation behaviour of nanostructured Ni-Mn-Sn ferromagnetic shape memory alloy thin films synthesized by dc magnetron sputtering. With increase in substrate temperature, the structural phase changes from austenite with L2₁ cubic crystal structure to martensite with monoclinic structure. In addition, field-induced martensite-austenite transformation is observed in magnetization studies using superconducting quantum interference device magnetometer. The martensitic transformation behaviour of these films depends critically on the microstructure and dimensional constraint. Both, the martensite start temperature (M_s) and austenite finish temperature (A_f) of these nanostructured films decreases with decreasing grain size. The excess free volume associated with grain boundaries has been observed to increase with decrease in grain size which in turn leads to an increase in the number of grain boundaries. It has been proposed that the grain boundaries impose constraints on the growth of the martensite and confine the transformed volume fraction in nanocrystalline structure. A martensite phase nucleated within a grain will be stopped at the grain boundaries acting as obstacles for martensite growth. The investigations revealed that below a critical grain size of 10.8 nm, the austenite phase is observed to be more stable than the martensite phase which leads to the complete suppression of martensitic transformation in these films.     

Thermoelastic transition kinetics of a gamma irradiated CuZnAl shape memory alloy

Many types of shape memory alloys have been used in nuclear reactors and aerospace applications where they are exposed to high levels of various kinds of radiation. The effect of gamma irradiation on the transformation kinetics of thermoelastic transformations in a shape memory CuZnAl alloy was investigated by differential scanning calorimetry (DSC) with heating/cooling rates of 10, 20, 30 and 40 °C/min. Irradiation doses of 10, 20, 30 and 40 kGy were applied to the samples of the alloy. Changes in Gibbs free energies, entropies and elastic energies were calculated. Reverse transition temperatures A _s and A _f systematically decreased with increasing doses, although forward transition temperatures M _s and M _f underwent a minimum value at a dose of 20 kGy. Hysteresis in the transition temperatures changed as an inverse parabolic function of the irradiation dose. The activation energies of transformations were calculated by using Kissinger and Osawa methods.     

Phase transformation behavior and mechanical properties of Ti50Ni49−x Fe1Co x shape memory alloys

In this article, the influence of Co addition on phase transformation behavior and mechanical properties of TiNiFe shape memory alloy was investigated extensively. Differential scanning calorimetry (DSC) measurements shows that martensitic start transformation temperatures (Ms ) decrease drastically with increasing Co content, while the R phase transformation start temperatures (Rs ) vary slightly. Nevertheless, the substitution of Ni with Co does not exert substantial influence on the two-stage transformation behavior of the TiNiFe alloy. The results from stress-strain curves indicate that higher critical stress for stress-induced martensitic transformation (rSIM ) has been obtained because of Co addition. In such cases, the Ti50Ni48Fe1 Co1.0 alloy maintains a good shape memory effect, and a maximum recoverable strain of 7.5 % can be obtained.     

Finite-element simulation of quenching incorporating improved transformation kinetics in a plain medium-carbon steel

An experimental and numerical investigation was conducted to acquire more precise continuous cooling transformation kinetics for the quenching simulation of a plain medium-carbon steel by improving the conversion model of transformation strains to phase fractions. Previous conversion models have limited applications in plain medium-carbon steels with variable bainite start temperatures (B_s) and martensite start temperatures (M_s), which are cooling rate dependent and cannot be distinguished on dilatometric curves. Therefore, we propose new methods for determining variable B_s and M_s. More accurate diffusive transformation kinetics models were developed based on transformation kinetics data converted from continuous cooling dilatometric curves and considering variable B_s. The martensitic transformation kinetics model was also improved using the quantitative relationship between the variable M_s and the enriched carbon content in residual austenite during diffusive transformations. Finite-element simulation of quenching incorporating the improved transformation kinetics was performed to predict the temperature, microstructure, residual stress and distortion of a plain medium-carbon steel cylinder. The pearlite, bainite, and martensite fractions, axial and hoop stresses near the surface, and the degree of distortion calculated using the improved transformation kinetics exhibited markedly better agreement with the measured results than those calculated using the previous transformation kinetics with B_s and M_s held constant. Furthermore, a comparison of the two simulation results provided a detailed understanding of the effects of transformation kinetics on residual stress and distortion.     

Investigation of Fe content in Cu–Al–Ni Shape Memory Alloys

Polycrystalline Cu–Al–Ni–Fe-based shape memory alloys with different chemical composition were produced in an arc-melting furnace under an argon atmosphere. Homogenized and aged specimens were prepared for multiple analyses. The temperatures of reversible martensitic transformations, namely A_s, A_f, M_s, M_f, A_max and ΔH enthalpy values were determined by a DSC device. The phase transition analysis from the room temperature to 850°C was undertaken by DTA. To characterize the lattice structure, an XRD analysis was conducted, the results of which were confirmed by microstructure images obtained from optical microscope observations.     

Effect of dilution on fatigue behaviour of welded joints produced by low-transformation-temperature fillers

ABSTRACT

Low-transformation-temperature (LTT) fillers with various martensitic transformation start (M_s) temperatures were used to produce fillet welds. In comparison with conventional welds, the fatigue strength of the LTT fillet welds offers a significant improvement, with a minimum increase of 145%. Owing to the substantial dilution of base metal, the Cr and Ni alloying elements in the LTT weld metals decrease, resulting in an increase of the M_s temperatures. Therefore, the fillet welds produced using the LTT filler material with the lower M_s temperature (92°C) exhibit a larger compressive residual stress and higher fatigue strength.     

Influence of anisotropy, the latent heat and the thermal history of alloy on martensitic transformation strain in Ni3Ta single crystal

Transformation characteristics in the single crystal of Ni₃Ta shape memory alloy were studied by the dilatation measurement in the temperature range of room temperature up to 500 °C. The transformation strains were positive in the direction of the b-axes and the c-axes and negative in the direction of a-axes. The martensitic phase transformation takes place without volume change of the sample. Thermal diffusivity of the single crystal measured in two directions b-axes and a-axes was higher than that for polycrystalline material.The latent heat of the martensitic phase transformation influences the temperature distribution inside sample. Absorption (releasing) of the latent heat during heating (cooling) leads to cooling (heating) of the sample in place where the phase transformation takes place. This decrease (increase) of the temperature in the interface between both phases leads to stopping of the phase transformation. This effect is visible on the temperature dependence of the dilatation characteristics. The martensitic phase transformation in Ni₃Ta single crystal took place with hysteresis of 30 °C. This hysteresis changes depending on the thermal history of the sample. Hysteretic behaviour of the Ni₃Ta single crystal was analyzed and compared with behaviour of Ni_53.6Mn_27.1Ga_19.3 alloy where no hysteresis was found.     

Two-way shape memory effect and alternating current driving characteristics of a TiNi alloy spring

Two-way shape memory effect (TWSME) was induced into the TiNi shape memory alloys (SMAs) spring by thermomechanical training after annealing treatment, which has promising application in micro-actuating fields. The TWSME spring can contract upon heating and extend upon cooling. The results show that there is an increase of the recovery ratio up to a maximum TWSME of 45%. During the training procedure, transformation temperatures and hysteresis were measured by different scanning calorimetry (DSC). The results show that As (reverse transformation start temperature) and Af (reverse transformation fmish temperature) shift to lower temperature after training. The intervals of ArAs and Ms-Mf (Ms and Mf are the martensite start and finish temperatures, respectively) increase and the heat of transformation decreases after training. The electrothermal driving characteristics of the TWSME springs were also investigated with alternating current density of 3.2-14.7A/mm^2. It is found that the time response and the maximum contraction ratio greatly depend on the magnitude of the electrical current density.     

Highly Porous NiTi with Isotropic Pore Morphology Fabricated by Self-Propagated High-Temperature Synthesis

Highly porous NiTi with isotropic pore morphology has been successfully produced by self-propagating high-temperature synthesis of elemental Ni/Ti metallic powders. The effects of adding urea and NaCl as temporary pore fillers were investigated on pore morphology, microstructure, chemical composition, and the phase transformation temperatures of specimens. These parameters were studied by optical microscopy, scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC). Highly porous specimens were obtained with up to 83% total porosity and pore sizes between 300 and 500 μm in diameter. Results show pore characteristics were improved from anisotropic to isotropic and pore morphology was changed from channel-like to irregular by adding pore filler powders. Furthermore, the highly porous specimens produced when using urea as a space holder, were of more uniform composition in comparison to NaCl. DSC results showed that a two-step martensitic phase transformation takes place during the cooling cycles and the austenite finish temperature (A _f) is close to human body temperature. Compression test results reveal that the compressive strength of highly porous NiTi is about 155 MPa and recoverable strain about 6% in superelasticity regime.