Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Effect of aging on martensitic transformation behavior of Ti48.8Ni50.8V0.4 alloy

In this article, the effect of aging on martensitic transformation of Ti_48.8Ni_50.8V_0.4 alloy was investigated. The results show that the martensitic transformation of the solution-treated shape memory alloy is a typical single-stage transformation process. The transformation temperatures of the samples aged at different temperatures for 0.5 h were lower as compared to that of the solution-treated alloy. With the increase of aging temperature, the transformation temperatures increase. After aging at 500 °C, the samples exhibit a multiple-stage transformation. The samples after aging at 500 °C for more than 5 h resulted in the transformation sequence of A → R→M1 and A → M2 upon cooling and M2 → A and M1 → A upon heating.     

Influence of Heat Transfer on Determination of Transient Temperatures for Ni<Subscript>53.6</Subscript>Mn<Subscript>27.1</Subscript>Ga<Subscript>19.3</Subscript> Shape Memory Alloy from Dilatometric Data

The martensitic phase transformation in a Ni_53.6Mn_27.1Ga_19.3 shape memory alloy is an athermal phase transformation that starts practically, immediately after reaching a certain transient temperature. The final temperature is given at each point of the sample by two processes: heat conduction and phase transformation. Both processes take place in tandem. The thermal expansion and calculation of the temperature fields in a dilatometer are used to determine the transient temperatures and to study the transient temperature ranges.     

Microstructure and phase transformations in a Ni<Subscript>50</Subscript>Mn<Subscript>29</Subscript>Ga<Subscript>16</Subscript>Gd<Subscript>5</Subscript> alloy with a high transformation temperature

A Heusler Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy with a high transformation temperature has been obtained by substituting 5 at% Gd for Ga in a ternary Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloy. The microstructure and phase transformations in the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy have been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. It is shown that the microstructure of the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy consists of matrix and hexagonal Gd (Ni,Mn)₄Ga phase, which indicates a eutectic structure composed of these two phases. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy exhibits a martensite transformation start temperature up to 524 K, approximately 200 K higher than that of Ni₅₀Mn₂₉Ga₂₁ alloy. At room temperature, non-modulated martensite with twin substructure is observed in Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy.     

Ni24.7Ti50.3Pd25.0 high temperature shape memory alloy with narrow thermal hysteresis and high thermal stability

High temperature shape memory alloys with operating temperatures above 100 °C are in demand for use as solid-state thermal actuators in aerospace, automobile and other engineering applications. The present study deals with transformation behaviour and thermal stability of Ni_24.7Ti_50.3Pd_25.0 (at.%) high temperature shape memory alloy, in cast and homogenized condition. The martensite finish temperature and transformation hysteresis of the alloy were determined to be 181.0 °C and ∼8.5 °C respectively. The alloy showed high stability upon stress-free thermal cycling, variation in transformation temperatures being ±1 °C. The narrow thermal hysteresis and high thermal stability of the alloy upon transformation cycling has been discussed and correlated with its microstructural features, activation energy and elastic strain energy of thermoelastic martensitic transformation. The alloy exhibited modulus of ∼82 GPa and hardness of ∼4.7 GPa in martensite phase.     

Effect of Hydrogen on the Crystallization of Amorphous Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> Titanium Alloy

We describe effects unknown earlier in the physics of interaction between hydrogen and amorphous metallic alloys: an increase in the crystallization temperature of hydrogen-containing Ti₅₀Ni₂₅Cu₂₅ alloy in the course of its heating and suppression of the inverse martensitic transformation B2 → B19 after crystallization. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 3, pp. 66 – 70, May – June, 2005.     

Phase transformation,thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni₅₀Mn₃₀Ga_20−xCu_x alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement.     

Martensitic transition and structural modulations in Ni<Subscript>51</Subscript>Fe<Subscript>24</Subscript>Ga<Subscript>25</Subscript> ferromagnetic shape-memory alloy

The structure of near-stoichiometric Ni₅₁Fe₂₄Ga₂₅ Heusler alloy synthesized by a melt-spinning technique has been studied using transmission electron microscopy. The main phase possessed a local, well-defined L2₁ high-atomic order structure, and some fcc structural γ phase and lamellar twin structures were also present. At room temperature, a rich variety of micro-modulated domains in the parent phase was observed. The domain structures were aligned along the 〈110〉, 〈100〉, 〈211〉, and ~14° off 〈110〉 directions, resulting in a complex tweed contrast. These are possibly the precursors of martensitic transformation (MT). Below the MT temperature, Ms, the cubic parent phase transformed into modulated martensitic variants, which were composed of lamellar structures with predominantly 10 and 14 M modulated structures along the 〈110〉 directions.     

Transformation start lines in TiNi and Fe-based shape memory alloys after incomplete transformations induced by mechanical and/or thermal loads

The transformation start condition in the stress-strain or strain-temperature planes is examined in a TiNi polycrystalline shape memory alloy and an Fe-based polycrystalline shape memory alloy under mechanical and/or thermal loads. The martensite start and austenite start stresses in the TiNi alloy are revealed to be almost constant during isothermal loading at the pseudoelastic temperature range, being independent of the extent of prior transformations. In the Fe-based alloy, however, both the martensite start stress and the austenite start temperature are strongly dependent on the extent of prior transformations. The alloy performance is fully contrary to the prediction of the thermodynamic theory of 1. Miller and co-workers (I. Müller, 1989, Cont. Mech. Thermodyn. 1, 125; I. Müller and H.B. Xu, 1991, Acta Metall. Mater. 36, 263) and B. Raniecki et al. (1992, Arch. Mech. 44, 261)     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

Martensitic transformation and shape memory properties of Ti-Ta-Sn high temperature shape memory alloys

The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti-Ta-Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti-Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti-Ta-Sn alloys. A Ti-20Ta-3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti-Ta binary alloy showing similar martensitic transformation temperature.     

Thermal Transport Properties of Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Mixed Crystals Measured by Means of the Photopyroelectric Method

The concentration dependence of the thermal conductivity and thermal diffusivity were determined for Cd_1-x Mg _x Se mixed crystals in the temperature range between 20 ^◦C and 40 ^◦C. To determine the thermal transport properties, the photopyroelectric setup in the back detection configuration was constructed. In the concentration range 0< x <0.36, both thermal conductivity and thermal diffusivity were found to decrease with increasing magnesium concentration as well as with increasing temperature. The observed concentration dependence is discussed in the framework of the Adachi model.     

Burst character of thermoelastic shape memory deformation in ferromagnetic Ni-Fe-Ga-Co alloy

The shape memory deformation in single crystals of Ni₄₉Fe₂₇Ga₁₈Co₆ alloy under uniaxial compression conditions has been studied. It was found that samples in the martensite state deformed in the [110] and [100] directions exhibit strongly anisotropic properties. Despite virtually equal residual strain and the work of deformation in the two cases, the subsequent behavior of samples during shape recovery on heating at a constant rate is principally different. In the former case, the shape memory deformation exhibits a burst character and takes place at a temperature that is significantly higher than the martensite transformation temperature in the initial (undeformed) crystal. In contrast, the shape recovery in the latter case proceeds smoothly at a temperature that is only slightly higher than the martensite transformation temperature in the initial crystal. Due to the burst character of shape memory deformation, the crystal standing on a solid base acquires a velocity above 20 m/s. The results are quantitatively analyzed in terms of the theory of diffuse phase transitions.     

Martensite-austenite transformation in maraging steel alloys during tensile deformation and thermal cycling

The effect of simultaneous additions of tungsten on the martensite (M) ⇌ austenite (γ) transformation, taking place during tensile deformation under different constant stresses and thermal cyclic rates for Fe-Ni-Co based maraging steel alloys was studied. The strain rate sensitivity parameterm was found to be 1.0 and 0.6 for the M →γ andγ → M transformations, respectively. The interpretation of deformation results implied a preponderantly diffusional mechanism in the M →γ transformation and a dislocation mechanism in theγ → M transformation. The increase of the lattice parameters of maraging steel alloys indicated that the hardening element, which is tungsten, was dissolved after tensile deformation.     

Investigating the effects of bulk supercooling and rapid solidification on Co–Ni–Ga ferromagnetic shape memory alloys

In this study, Electromagnetic Levitation (EML) technique was utilized to explore the effect of bulk supercooling and rapid solidification in alloys with Co₄₆Ni₂₇Ga₂₇ and Co₄₈Ni₂₂Ga₃₀ (at.%) compositions. The effects of γ + β on the martensitic and austenitic transformation temperatures and magnetic properties were investigated. The presence of γ phase was found to suppress the martensitic and austenitic transformations to below room temperature. Bulk supercooling and rapid solidification led to the formation of homogeneous martensitic phase from the hyperperitectic Co₄₆Ni₂₇Ga₂₇ alloy. In contrast with pure martensite phase in Co₄₈Ni₂₂Ga₃₀, the hyperperitectic martensite in supercooled Co₄₆Ni₂₇Ga₂₇ sample showed no grain boundaries microsegregation and embrittlement that caused deep cracks along grain boundaries. The sample had a high Curie temperature about 400 K and good directional magnetic properties.     

Use of time history speckle pattern and pulsed photoacoustic techniques to detect the self-accommodating transformation in a Cu–Al–Ni shape memory alloy

Continuous and pulsed electromagnetic radiation was used to detect the self-accommodation mechanism on a polycrystalline Cu–13.83 wt.%Al–2.34 wt.%Ni shape memory alloy. Rectangular samples of this alloy were mechanically polished to observe the austenite and martensite phases. The samples were cooled in liquid nitrogen prior to the experiments to obtain the martensite phase. Using a dynamic speckle technique with a continuous wave laser we obtained the time history of the speckle pattern image and monitored the surface changes caused by the self-accommodation mechanism during the inverse (martensitic to austenitic) transformation. Using a photoacoustic technique based on a pulsed laser source it was also possible to detect the self-accommodation phenomena in a bulk sample. For comparison purposes, we used differential scanning calorimetry (DSC) to detect the critical temperatures of transformation and use these as reference to evaluate the performance of the optical and photoacoustical techniques. In all cases, the same range of temperature was obtained during the inverse transformation. From these results, we conclude that time history speckle pattern (THSP) and pulsed photoacoustic are complementary techniques; they are non-destructive and useful to detect surface and bulk martensitic transformation induced by a temperature change.     

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy were investigated according to martensitic transformation kinetics, morphology, magnetism of both austenite and martensite phases and also in terms of martensitic transformation start temperatures (M_s) for different austenite grain sizes of alloy. Kinetics of the transformation was found to be athermal. Also only lenticular martensite morphology was observed during investigations. On the other hand, Mössbauer spectra revealed a paramagnetic character for austenite phases and a ferromagnetic character for thermally induced martensitic phases. Determined M_s temperatures were found to be at − 128 °C for large grained samples and − 135 °C for small grained samples.     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

Microstructure of TiNi shape-memory alloy synthesized by explosive shock-wave compression of Ti–Ni powder mixture

Cylinders of TiNi shape-memory alloy were synthesized from mixtures of equiatomic fine irregular titanium and nickel powders by explosive-wave compression with a detonation velocity of about 6500 m s^-1. B2 type parent phase, R phase, B19′ type martensite, Ti₂Ni, Ti₃Ni₄ and Ti₂Ni₃ phases were observed in this as-synthesized material. In the B2 matrix high density dislocations existed. The Burgers vectors of many dislocations were determined to be parallel to directions. The R phase variants formed (0 0 1) B2 twinning structure. The substructure of the B19′ martensite was (0 0 1) B19′ type I twin and stacking faults on the (0 0 1) B19′ plane. When increasing the temperature of the as-synthesized material in a differential scanning calorimeter, no B19 ′ → R → B2 transitions were observed on the temperature range −50 to 100 °C. However, B2 → B19′(R) transitions occurred during the cooling cycle. After heat treating the specimen at 800 °C for 1 h and then ageing at 400 °C for 10 min, both B2 → R → B19′ and B19′(R) → B2 phase transitions were observed. This revised version was published online in November 2006 with corrections to the Cover Date.