Temperature Range Of Martensitic Transformation 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.

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.

Transformation temperatures, mechanical properties and residual stress of two low-transformation-temperature weld metals

Two low-transformation-temperature weld metals, 12Cr6Ni and 8Cr10Ni weld metals, were devised and their martensite start temperatures were 245°C and 225°C, respectively. Despite a small difference of 20°C in martensite start temperature, the martensite finish temperature of the former was 100°C, while the one of the latter was below room temperature. The difference in martensite finish temperature was due to the stacking fault energy. Lower stacking fault energy accelerated martensitic transformation, thus elevating martensite finish temperature. Owing to difference in martensite finish temperature, the microstructures, mechanical properties and residual stress of low-transformation-temperature weld metals varied correspondingly. Furthermore, experimental and finite-element study indicated that volume expansion strain and martensitic transformation temperature range strongly affected residual stress.

Microstructure and Mechanical Properties of Martensitic High-Strength Engineering Steel

A promising martensitic steel with good hardenability is studied. In the cooling rate range 0.1–30°C/sec and the only transformation recorded by a dilatometer starts at an Ms temperature of 355 ± 10°C. Microstructure and mechanical properties of the steel studied are analyzed after various heat treatment regimes: cooling from the austenitizing temperature at various rates (from 0.02 to 20°C/sec), austempering at 280–380°C, and oil quenching and tempering at 200–600°C. It is shown that a 1000-fold change in the cooling rate has a weak effect on steel strength. Bainitic transformation in the steel in question occurs in the martensitic transformation temperature range, which leads to a reduction in impact strength compared with a martensitic microstructure both after slow cooling and isothermal hardening.

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.

Effect of restraint stress on martensite transformation in low transformation temperature weld metal

In this study, martensite transformation behavior of low transformation temperature (LTT) weld metals under unrestrained and restrained conditions was analyzed by thermal simulation technology. It is found that martensite transformation start (Ms) temperature and martensite transformation temperature range of LTT weld metal under restrained condition increased by 85 K and 50 K, respectively, compared with that of under unrestrained condition, which is attributed to the significant increase Gibbs free energy produced by restraint stress for martensite transformation. The increasement of Ms due to the effect of restraint stress is quantitatively described by Fe–X–C ternary regular solution-elastic energy model. Based on the transformation kinetics curve, the effect of restraint stress on martensite transformation of LTT weld metal can be divided into three stages, including stress-induced nucleation, transition and without effect stages. In initial stage, the martensite nucleates at a relatively high temperature with a low phase transformation rate due to the directional effect of restraint stress; in second stage with martensite transformation fraction range of 0.2–0.6, the elastic energy caused by restraint stress decreases significantly, and the chemical free energy of martensite transformation under restrained condition is lower than that under unrestrained condition, which results in the martensitic transformation rate of LTT weld metal under restraint condition being lower than that under unrestrained condition; in final stage, the small value of restraint stress has little effect on martensite transformation of LTT weld metal, and the transformation rate of martensite mainly depends on the content of austenite. This study provides a theoretical base for better understanding phase transformation and microstructure of LTT weld metal under restrained condition.

Effect of Si doping on microstructure and martensite transformation in Ni-Mn-Sb ferromagnetic shape memory alloys

We have recently reported that the quaternary Ni49.0Mn38.4Sb11.7Si0.9 alloy showed enhanced refrigeration capacity due to the wide working temperature interval and narrow magnetic hysteresis loss via Si-doping. Here we systematically investigate the microstructure, martensite transformation and magnetic transition behaviors of Ni50Mn38Sb12-xSix (x = 0, 1, 2, 3) alloys. Ni50Mn38Sb12-xSix (x = 0, 1, 2) alloys exhibited a full solid-solution state with a four-layer modulated orthorhombic (4O) martensite structure. However, three phases formed in Ni50Mn38Sb9Si3 alloy, and lath-like martensite twins existed only in α phase at room temperature. The martensite transformation temperature decreased with the increasing Si content when the Si atomic content increased from 0% to 2%, but significantly increased when Si content reached to 3% because of the increase of the valence electron concentration (e/a) of matrix phase (α phase). The martensitic transformation also occurred in the β phase below room temperature in Ni50Mn38Sb9Si3 alloy. Furthermore, the martensite transformation temperature range enlarged and the thermal hysteresis narrowed with the addition of Si atomic content from 0% to 2%, of which Ni50Mn38Sb10Si2 alloy had the widest martensite transformation temperature range and narrowest thermal hysteresis, which was supposed to have great magnetocaloric effect (MCE). However, when x = 3, the thermal hysteresis increased significantly and the transition character changed to a second-order phase transition above room temperature; a magneto-structural coupling of the first-order martensitic transformation and second-order magnetic transition also existed below room temperature, which all would result in low MCE.

A large reversible room temperature magneto-caloric effect in Ni-TM-Co-Mn-Sn (TM = Ti, V, Cr) meta-magnetic Heusler alloys

Herein, we achieved a large reversible room temperature magneto-caloric effect (MCE) through synergic tuning of martensitic transformation (MT) temperatures and transition entropy change (ΔStr) via micro-alloying with transition metals (Ti, V, and Cr) in Ni45Co5Mn40Sn10 meta-magnetic Heusler alloys (MHAs). By the minor addition of TM, MT temperatures were brought down to below room temperature and ΔStr was reduced while maintaining narrow MT temperature range (ΔT) and large difference in magnetization (ΔM) of Ni45Co5Mn40Sn10 MHA. In particular, Ni43.8Cr1.2Co5Mn40Sn10 MHA exhibited a very large reversible room temperature magnetic entropy change (ΔSM) of 24.5 J/kg·K with a broad operating temperature window of ∼11 K at 5 T. Indeed, the MHA exhibited a very effective refrigeration capacity (RCeff) of 276 J/kg for 5 T, which is the largest value among the reported Ni-Mn-based MHAs. The decrease of ΔStr reduces the magnetic field required for completely reversible MT and accelerates the saturation of ΔSM, which leads to maximum RCeff value in the composition of MHA. Thus, we can conclude that smaller ΔStr with narrow ΔT and large ΔM is a key variable to develop MHA with reversible MCE under low magnetic field, which will ultimately give us a guideline for the tailor-made design of high-performance magneto-caloric materials.

Correlation between defects and magneto-structural properties in Ni-Mn-Sn metamagnetic shape memory alloys

The effect of combined mechanical and thermal treatments in the magnetostructural properties was studied in Ni-Mn-Sn metamagnetic shape memory alloys, in which the extraordinary high stability of the L21 structure precludes the variation of atomic order by means of conventional thermal treatments. A Ni50Mn35Sn15 alloy has been mechanically milled and then annealed at different temperatures in order to produce different microstructural states. The evolution of both the internal stresses and the crystallite size upon annealing has been quantified and correlated to the evolution of the martensitic transformation features and the magnetic properties. It is found that the relaxation processes brought by annealing leads to recovery of the martensitic transformation and the enhancement of the magnetism at both macroscopic and local level. In particular, the density of non-magnetic inclusions (defects) and their stress field decrease upon annealing, thus leading to an increase of the saturation magnetization and a decrease of the martensitic transformation temperature range, respectively, which results in a higher magnetocaloric effect. The obtained results confirm that, once the transition temperature has been fixed by the composition, the modification of the microstructure through thermomechanical treatments appears as the best way to tune the functional properties of these alloys.

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.

Effect of grain constraint on the field requirements for magnetocaloric effect in Ni45Co5Mn40Sn10 melt-spun ribbons.

The influence of grain constraint on the magnetic field levels required to complete the isothermal martensitic transformation in magnetic shape memory alloys has been demonstrated for a NiCoMnSn alloy, and the magnetocaloric performance of an optimally heat treated alloy was quantified. Ni45CoxMn45-xSn10 melt spun ribbons with x = 2, 4, 5, and 6 were characterized. The x = 5 sample was determined to exhibit the lowest transformation thermal hysteresis (7 K) and transformation temperature range during transformation from paramagnetic austenite to nonmagnetic martensite, as well as a large latent heat of transformation (45 J kg-1 K-1). For this composition, it was found that increasing the grain size to thickness ratio of the ribbons from 0.2 to 1.2, through select heat treatments, resulted in a decrease in the magnetic field required to induce the martensitic transformation by about 3 T due to the corresponding reduction in the martensitic transformation temperature range. This decrease in the field requirement ultimately led to a larger magnetocaloric entropy change achieved under relatively smaller magnetic field levels. The giant inverse magnetocaloric effect of the optimized alloy was measured and showed that up to 25 J kg-1 K-1 was generated by driving the martensitic transition with magnetic fields up to 7 T.

Magnetostructural coupling and magnetocaloric effect in Ni-Mn-Ga-Cu microwires

Ni-Mn-Ga-X microwires were produced by melt-extraction technique on a large scale. Their shape memory effect, superelasticity, and damping capacity have been demonstrated. Here, the excellent magnetocaloric effect was revealed in Ni-Mn-Ga-Cu microwires produced by melt-extraction and subsequent annealing. The overlap of the martensitic and magnetic transformations, i.e., magnetostructural coupling, was achieved in the annealed microwires. The magnetostructural coupling and wide martensitic transformation temperature range contribute to a large magnetic entropy change of −8.3 J/kg K with a wide working temperature interval of ∼13 K under a magnetic field of 50 kOe. Accordingly, a high refrigeration capacity of ∼78 J/kg was produced in the annealed microwires.

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.

Thermomechanical treatment to achieve stable two-way shape memory strain without training in Ti-49.8 at.% Ni alloy

Two-way shape memory (TWSM) strain for the Ti-49.8 at.% Ni alloy after rolling (40 % thickness reduction) at temperatures 470 \(\div \) 870 K is studied. It is found that rolling at 470 and 570 K produces samples showing the dilatation jump in the rolling direction of 0.9 and 1.3 %, respectively, that does not change upon thermal cycling through the martensite transformation temperature range. The dilatation jump in the direction normal to the rolling plane is even two to three times higher. The TWSM dilatation jump demonstrated by the samples rolled at higher temperatures stabilizes only after 30 cycles. Stability of the TWSM strain of the samples rolled at 470 and 570 K is explained from the consideration of the alloy microstructure. Material which is sufficiently large and stable with respect to thermal cycling dilatation jump is ideal for the use in actuators.

Relation between martensitic transformation temperature range and lattice distortion ratio of NiMnGaCoCu Heusler alloys

In order to study the relation between martensitic transformation temperature range ΔT (where ΔT is the difference between martensitic transformation start and finish temperature) and lattice distortion ratio (c/a) of martensitic transformation, a series of Ni46Mn28−xGa22Co4Cux (x = 2–5) Heusler alloys is prepared by arc melting method. The vibration sample magnetometer (VSM) experiment results show that ΔT increases when x > 4 and decreases when x < 4 with x increasing, and the minimal ΔT (about 1 K) is found at x = 4. Ambient X-ray diffraction (XRD) results show that ΔT is proportional to c/a for non-modulated Ni46Mn28−xGa22Co4Cux (x = 2–5) martensites. The relation between ΔT and c/a is in agreement with the analysis result obtained from crystal lattice mismatch model. About 1000-ppm strain is found for the sample at x = 4 when heating temperature increases from 323 K to 324 K. These properties, which allow a modulation of ΔT and temperature-induced strain during martensitic transformation, suggest Ni46Mn24Ga22Co4Cu4 can be a promising actuator and sensor.

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.

Exchange Bias and Inverse Magnetocaloric Effect in Co and Mn Co-Doped Ni2MnGa Shape Memory Alloy

Exchange bias effect observed in the Ni1.68Co0.32Mn1.20Ga0.80 alloy confirms the coexistence of antiferromagnetic and ferromagnetic phases in the martensite phase. A large inverse magnetocaloric effect has been observed within the martensitic transformation temperature range, which is originated from modified magnetic order through magnetic-field-induced phase transformation from partially antiferromagnetic martensite to ferromagnetic austenite. The magnetic entropy change is 16.2 J kg−1 K−1 at 232 K under ΔH = 60 kOe, with the net refrigerant capacity of 68 J kg−1. These properties indicate Co and Mn co-doped Ni2MnGa alloy is a multifunctional material potentially suitable for magnetic refrigeration and spintronics applications.

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.