Martensitic Transformation Range Research Articles

Исследование превращений переохлажденного аустенита при ступенчатой закалке стали 20Cr2Mn2SiNiMo

Currently, step quenching of steels in the temperature range of martensitic transformation, including quenching – partitioning, has found wide application in the automotive industry. Step quenching technology is successfully used to increase a set of properties, which most often include temporary tensile strength and relative elongation. The authors carried out a dilatometric study of the supercooled austenite transformations occurring in the 20Cr2Mn2SiNiMo steel, when implementing various options of step quenching with holding in the martensitic region. It was found that after single-stage quenching, single-stage quenching followed by tempering, and two-stage quenching, primary martensite, isothermal bainite, and secondary martensite are formed in various quantitative ratios. Using X-ray diffraction phase analysis, the amount of residual austenite was determined during step quenching. It has been shown that two-stage quenching makes it possible to stabilise up to 14 % of residual austenite, in the structure of the studied steel, at room temperature. Research has revealed that 20Cr2Mn2SiNiMo steel is characterised by a decrease in the crystal lattice parameter of the residual austenite, with an increase in its content in the steel structure. Uniaxial tensile and impact bending tests were carried out, and the values of the mechanical properties were determined. It has been found that during two-stage quenching, higher strength and elongation values, with lower values of relative contraction and impact strength are achieved compared to oil quenching and low-temperature tempering. The study showed that, with regard to the structural reliability of machine-building parts, step quenching is not the optimal heat treatment mode for the steel under study. The best combination of strength, ductility and impact hardness is achieved after quenching and low-temperature tempering.

Magnetocaloric Effect in a Ni2.25Mn0.75Ga0.93Si0.07 Alloy

The results of a study of the magnetocaloric effect (MCE) in Ni2.25Mn0.75Ga0.93Si0.07 alloy are presented in the cast state and in the state after multi-axial isothermal forging (MIF) at 700°C and true degree of deformation e = 3.19. It is shown that as a result of MIF, the initial equiaxed microstructure is transformed into a bimodal one in which large grains 100–200 μm in size are surrounded by a layer of fine-grained microstructure.As a result of MIF, the range of martensitic transformation is slightly shifted to the region of low temperatures by about 5°C. The analysis of phase transformations in the region of room temperatures shows that the intervals of martensitic and magnetic phase transformations are superimposed on each other. The MCE value in a magnetic field of 1.8 T is 0.59ºC in the initial cast state, and as a result of forging it decreases to 0.55°C.

Формирование микроструктуры типа «ожерелье» методом ковки в сплавах Гейслера системы Ni-Mn-Ga-Si как способ повышения механических свойств

The standard technique for melting Heusler alloys involves melting in an argon-arc furnace on a water-cooled copper crucible. As a result of intensive crystallization of the ingot, large elongated crystals are formed in the structure. The thermo-mechanical treatment of such a structure is ineffective, since microcracks are formed along the boundaries of large crystals as a result of internal stresses and martensitic transformation, along which then, during processing, cracks develop and the workpiece is destroyed. It is shown that additional heat treatment by the vacuum induction remelting makes it possible to eliminate this effect. As a result of the treatment, an equiaxed granular microstructure with a grain size of about 200 μm is formed. However, as a result of remelting in a quartz crucible, silicon atoms diffuse into the bulk of the ingot. Its content varies in the range of 1-2% depending on the duration of exposure in the molten state. A uniform distribution of silicon, without the formation of additional phases is shown by energy-dispersive analysis. Thus, the Ni54.1Mn19.6Ga24.6Si1.7, Ni56.2Mn18.8Ga23.2Si1.8, Ni57.4Mn18.2Ga22.7Si1.7 and Ni56.5Mn20.1Ga22.3Si1.2 alloys were obtained. A partially recrystallized structure of the "necklace" type was formed in the alloys by the multiaxial isothermal forging at 950-973 K and the true degree of deformation e=1.9...3.9. The initial large grains with a size of 100-200 microns are surrounded by a layer of fine-grained structure. The thickness of the interlayer at the periphery of the workpiece is about 5 grains. In the center of the workpiece, the proportion of the fine-grained structure may prevail over the coarse-grained structure. The study of cyclic and fatigue strength was carried out by the method of three-point bending during cyclic tests in the temperature range of martensitic transformation using the example of the Ni54.1Mn19.6Ga24.6Si1.7 alloy in the initial state and the Ni57.4Mn18.2Ga22.7Si1.7 alloy in the forged state. It is shown that the “necklace” type microstructure, compared to the equiaxed microstructure, demonstrates a twofold advantage in cyclic strength and a fivefold advantage in fatigue strength.

Latent heat storage of Nickel Titanium shape memory alloys and its application to solid-state heat storage device

Nickel Titanium shape memory alloy was proposed for heat storage material. When the shape memory alloy is in contact with a heat source, the heat flow from the heat source to the alloy was enhanced by the latent heat associated with the reverse martensitic transformation of shape memory alloy, and vice versa. This study performed both theoretical and computational analyses. First, the enthalpy curve in the temperature range of martensitic transformation was determined from the DSC curves obtained in experiment, which can describe the emission/absorption of latent heat during the martensitic transformation as the function of temperature. Next, a metal matrix composite of NiTi shape memory wires embedded in aluminum block was designed and the heat transfer with a heat source was observed by three-dimension finite element heat conduction analysis with ANSYS. We focused on the effects of the configuration of SMA wires and the heat resistance between the wire and aluminum matrix on the performance of the heat storage.

Investigation of thermal-structural characteristics at two-way shape memory effect of rapid quenched laminated amorphous-crystalline ribbons of Ti50Ni25Cu25 alloy

The work is devoted to the study of processes of structural changes leading to the realization of the two-way shape memory effect (TWSME) in the rapid quenched amorphous-crystalline alloy Ti50Ni25Cu25. The object of the study was a rapid quenched laminated amorphous-crystalline Ti50Ni25Cu25 alloy ribbon with a thickness of about 39 um and with a thickness of surface crystal layer about 10 μm was present on the non-contact side relative to the quenching wheel. In the ribbon under study, without any additional heat treatments the TWSME with bending deformation from a rectilinear shape during heating and cooling in the range of martensitic transformation is manifested. The samples were characterized by means: scanning and transmission electron microscopy; X-ray diffraction analysis; differential scanning calorimetry; measuring the characteristics of the crystalline layer during the implementation of TWSME and the temperature dependence of the form change. It was shown that the formation of a laminated amorphous-crystalline composite occurs due to the martensitic transformation of B2∼B19 in the crystal layer and the accompanying TWSME as a result of which the crystal layer is reduced.

Two-Way Shape Memory Effect in Rapidly-Quenched High-Copper TiNiCu Alloys Deformed in the Martensitic State

The effect of martensitic deformation on the two-way shape-memory effect (SME) of quasi-binary high-copper TiNi–TiCu alloys (with 25–40 at % Cu) is studied. The alloys are prepared by rapid quenching from the melt in an amorphous state in the form of ribbons 30–50 μm in thickness. The alloys crystallize during isothermal annealing, the time of which is varied from 100 to 300 s, and during a single current pulse 10 ms in duration. The relatively low bending strains (1.5–3%) are shown to cause two-way SME in TiNiCu alloys; as the strain of martensite increases, the reversible strain in the martensitic transformation range increases markedly. An increase in the copper content is found to cause a substantial decrease in the value of two-way SME, which is related to the formation of the brittle TiCu phase. At the same time, a decrease in the heat treatment time causes an increase in the reversible strain, which reaches the maximum value (2.35%) for the alloy with 25 at % Cu subjected to high-rate electric pulse treatment.

Microstructure and mechanical properties of the ultrafine-grained TiNi alloy after multiple martensitic transformations and subsequent aging

In this paper, we investigate the effect of preliminary thermal cycling in the temperature range of martensitic transformation on the aging processes in the ultrafine-grained Ti49.0Ni51.0 alloy, using as an example the examination of the microstructure, mechanical properties and fractography. Our studies show that preliminary thermal cycling leads to a significant increase in the density of dislocations that promote the intensification of the aging process and thereby enhance the strength and functional characteristics of the TiNi alloy in the ultrafine-grained state.

The influence of the temperature conversion rate on the deformation processes of the shape memory effect and transformation plasticity

The influence of the temperature conversion speed on the mechanical characteristics of metals with shape memory is mentioned in connection with the dissipative properties of this materials class. This paper presents the different cooling and heating regimes research results of ring-shaped force elements and spiral drives demonstrating the possibility of this parameter influence on their deformation characteristics in the temperature range of martensitic transformation.

Reversible elastocaloric effect at ultra-low temperatures in nanocrystalline shape memory alloys

Superelastic shape memory alloys (SMAs) exhibit a reversible elastocaloric effect that originates from a release/absorption of latent heat associated with a stress-induced martensitic phase transformation. In typical SMAs, the conventional elastocaloric effect will vanish when the operating temperature falls below the temperature range in which martensitic phase transformation can be triggered by stress. We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of ∼90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change ΔTad of +3.4 K is measured upon rapid release of tensile stress. The measured ΔTad are larger and extend over a wider temperature span than the existing electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials. We show that such low temperature elastocaloric effect originates from an entropic elasticity associated with large non-linear elastic deformations of the nanocrystalline microstructure at ultra-low temperatures. Our study suggests a new avenue to cool ultra-low temperature ambients.

Tunable Magnetocaloric Effect in Ni-Mn-Ga Microwires

Magnetic refrigeration is of great interest due to its high energy efficiency, environmental friendliness and low cost. However, undesired hysteresis losses, concentrated working temperature interval (WTI) and poor mechanical stability are vital drawbacks that hinder its practical application. Off-stoichiometric Ni-Mn-Ga Heusler alloys are capable of giant magnetocaloric effect (MCE) and tunable transformation temperatures. Here, by creating Ni-Mn-Ga microwires with diameter of 35–80 μm using a melt-extraction technique, negligible hysteresis and relatively good mechanical stability are found due to the high specific surface area (SSA) that reduces incompatibility between neighboring grains. The high SSA also favors the element evaporation at high temperatures so that the transformation temperatures can be feasibly adjusted. Tunable magnetocaloric effect owing to different magneto-structural coupling states is realized by (i) composition design and subsequent tuning, which adjusts the temperature difference between the martensite transformation (MT) and the magnetic transition, and (ii) creation of gradient composition distribution state, which manipulates the MT range. Magnetic entropy change ΔSm ~−18.5 J kg−1 K−1 with relatively concentrated WTI and WTI up to ~60 K with net refrigeration capacity ~240 J kg−1 at 50 kOe are demonstrated in the present Ni-Mn-Ga microwires. This criterion is also applicable for other small-sized materials.

On the microstructural origins of martensitic transformation arrest in a NiCoMnIn magnetic shape memory alloy

The martensitic transformation arrest phenomenon in Ni45Co5Mn36.6In13.4 meta-magnetic shape memory alloy (MMSMA) single crystals was investigated as a function of secondary annealing heat treatments, using thermo-magnetometry and transmission electron microscopy (TEM). Dark-field images of the austenite phase at room temperature revealed the long range L21 and B2 ordered microstructural landscape with different morphologies in the annealed single crystals. Their measured thermomagnetic responses demonstrated full transformation arrest after certain heat treatments and unique microstructural morphologies. Martensitic transformation hysteresis, range, and enthalpy were measured in the annealed partially- or non-arrested single crystals. With the data, herein, we found that the samples with long range L21 ordering exhibit martensitic transformation at higher temperatures than some of the crystals of the same composition showing predominantly B2 order. This finding opposes the previous reports on the effect of annealing on the martensitic transformation characteristics of MMSMAs. We provide evidence that long range order promoted with high temperature annealing is not the only microstructural feature capable of influencing the martensitic transition in NiCoMnIn MMSMAs. Data suggests that quenched in vacancies, in addition to long range order, also influence the transformation characteristics.

Simulation of plastic strain accumulation during thermal cycling of TiNi alloy

The Volkov and Evard microstructural model describing the functional properties of TiNi shape memory alloys was modified to simulate plastic strain variation during thermal cycling of the TiNi alloy under constant stress through the temperature range of the martensitic transformation. It was found that a good agreement between the experimental and simulated data was observed if the law for the variation in the critical force for plastic slip have taken into account the non-linear dependence between the variation in critical force and the decrease in this value in previous thermal cycles due to relaxation. It was shown that using a modified Volkov-Evard model allowed us to simulate plastic strain variation during thermal cycling under constant stress through a different temperature range of the forward martensitic transformation.

Softening process during reverse martensitic transformation in TiNi shape memory alloy

Strain variation in a Ti50Ni50 shape memory alloy subjected to 30 thermal cycles under a constant stress of 50 MPa through the temperature range of incomplete reverse martensitic transformation was studied. It is found that the rate of plastic strain accumulation depends on a fraction of the reverse transformation temperature range realized on heating of the Ti50Ni50 alloy during thermal cycling. The larger the fraction of the reverse transformation temperature range, the higher the rate of plastic strain accumulation was observed during thermal cycling. If the fraction of the reverse transformation temperature range is 0.75 or less, strain hardening results in a full arrest in the plastic strain accumulation at the stationary stage. Otherwise, the alloy accumulates the plastic strain with a constant rate at the stationary stage. It was shown that the strain hardening incompletely arrests the plastic strain accumulation at the last quarter of the reverse transformation temperature range due to the realization of a relaxation process on heating, which results in alloy softening. This is caused by an increase in dislocation mobility owing to a rise in sample temperature hence, this process is thermally activated. Completion of the softening process is controlled by the maximum temperature attained by heating during thermal cycling.

Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films

Magnetic shape memory nanostructures have a great potential in the field of the nanoactuators. The relationship between dimensionality, microstructure and magnetism characterizes the materials performance. Here, we study the martensitic transformation in supported and free-standing epitaxial Ni47Mn24Ga29 films grown by sputtering on (0 0 1) MgO using a stoichiometric Ni2MnGa target. The films have a Curie temperature of ~390 K and a martensitic transition temperature of ~120 K. Similar transition temperatures have been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films (with longer deposition time) present a wider martensitic transformation range that can be associated with small gradients in their chemical concentration due to the high vapour pressure of Mn and Ga. The magnetic anisotropy of the films shows a strong change below the martensitic transformation temperature. No features associated with variant reorientation induced by magnetic field have been observed. Annealed films in the presence of a Ni2MnGa bulk reference change their chemical composition to Ni49Mn26Ga25. The change in the chemical composition increases the martensitic transformation temperature, being closer to the stoichiometric compound, and reduces the transformation hysteresis. In addition, sharper transformations are obtained, which indicate that chemical inhomogeneities and defects are removed. Our results indicate that the properties of Ni–Mn–Ga thin films grown by sputtering can be optimized (fixing the chemical concentration and removing crystalline defects) by the annealing process, which is promising for the development of micromagnetic shape memory devices.

Features of the Internal Friction in the Temperature Range of Martensitic Transformation in TiNi

The influence of the external parameters (deformation amplitude and oscillation frequency) on the components of the internal friction peak during a thermoelastic martensitic transformation in TiNi has been considered. Models describing the behavior of the transition components of the IF peak have been analyzed. On the basis of the best model experimental dependence of the transformed volume fraction on temperature has been established.

Structural changes of the B2 phase in the surface layer of titanium nickelide after pulsed electron-beam treatment

The structural and phase state of titanium-nickelide surface layers after low-energy, high-current electron-beam application is studied by X-ray structural analysis and by optical microscopy and also scanning and transmission electron microscopy. In electron-beam treatment, a nonequilibrium single-phase state is formed in the recrystallized layer (thickness 8–10 μm). This state is characterized by a distorted structure based on B2 phase, textured close to the 〈410〉 direction. The layer at a depth of 10–20 μm contains not only B2 phase with slight lattice distortion but also a small proportion (up to 5 vol %) of phase with martensite B19′ structure. As a result of electron-beam treatment, the chemical composition of the modified layer is changed, with enrichment by titanium on account of the melting of Ti2Ni particles. The range of martensitic transformation is shifted to higher temperatures. Thus, even at room temperature, we may expect that the basic structural state in this layer will be martensite. Transmission electron microscopy indicates that no martensite phase is seen in the modified layer. A hypothesis explaining the structural and phase state at the titanium-nickelide surface after electron-beam treatment is outlined.

Accumulation of Residual Strain in TiNi Alloy During Thermal Cycling

Residual strain accumulation during thermal cycling of the Ti50Ni50 alloy under a constant stress of 200 MPa through the temperature range of complete and incomplete forward martensitic transformation was studied. The temperature range of the forward martensitic transformation during thermal cycling was chosen as 25, 50, 75, or 100% of the M s-M f interval measured in the first cycle. It was shown that intensive accumulation of residual strain took place in the last stage of the forward transformation. It was observed that resistivity increased more rapidly with an increase of the fraction of the temperature range of forward martensitic transformation.

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.

Unusual Multistage Martensitic Transformation in TiNi Shape Memory Alloy after Thermal Cycling

Variation in kinetics of martensitic transformation, in an equiatomic TiNi shape memory alloy, during thermal cycles was investigated. Samples annealed at 500 °C for 1 hour were subjected to repeated thermal cycles, through the temperature range of martensitic transformation. Unusual 4-stage martensitic transformation during cooling after 30 thermal cycles was observed. Moreover, a new unusual phenomenon was found in the preliminary thermal cycled TiNi alloy. It was observed that variation in the highest temperature Th of thermal cycles temperature interval resulted in the redistribution of released heat among four calorimetric peaks, observed on cooling. It was found that if the Th temperature did not exceed 240 °C the variation in kinetics was repeatable, and determined only by the value of Th. It was assumed that the defect structure induced on preliminary thermal cycling changes reversibly on cooling and heating.

Effect of thermal cycling on martensitic γ↔ε-transformation in alloy Fe – 22% Mn – 3% Si

Methods of mechanical spectroscopy and differential scanning calorimetry demonstrate the effect of thermal cycling for a specimen of alloy Fe – 22% Mn – 3% Si through the martensitic transformation range on martensitic transformation parameters: points for the start and end of transformation, amount of thermal hysteresis, and transformation development kinetics.