Austenite Transition Research Articles

Magneto-optical Kerr effect of a Ni2.00Mn1.16Ga0.84 single crystal across austenite and intermartensite transitions

We carried out magneto-optical Kerr effect (MOKE) and magnetization measurements on a single crystal of Ni2.00Mn1.16Ga0.84, which is a magnetic shape memory material with application potential for actuator devices or for energy recuperation. Up to the time of our study, there had been reports of MOKE measurements in polar geometry. Against earlier predictions, we show that surface magnetic states of the martensite and the austenite can be also probed efficiently via longitudinal MOKE. A single-variant magnetic state prepared at room temperature is characterized by square-shaped ferromagnetic hysteresis loops yielding coercive fields, which are key material properties for future applications. Temperature dependencies of Kerr rotation were found to be linearly proportional to magnetization for martensitic phases. After passing through an inter-martensitic structural transition below room temperature in zero magnetic field, the coercive fields are more than doubled in comparison with the room temperature values. Above room temperature where an austenite structure is formed, MOKE signals are dominated by quadratic contributions and the magnitude of Kerr rotation drops due to changes in the electronic and magnetic domains structure.

Abrasive Wear Properties of Fe-Based Alloys Designed for Mining Applications

AbstractThe purpose of this study was to determine experimentally the wear properties of 5 groups of iron-based alloys used in the mining and transport machines exposed to the action of a hard abrasive material. The groups of materials to be examined included austempered ductile irons (ADI), steels and cast steel designed for quenching and tempering and for surface hardening, hard-wearing hardened steels and structural steels. The wear tests were carried out on a disc-on-disc test rig. The test samples were examined under conditions of sliding mating, while the leading destructive process was microcutting of the surface with loose corundum grain. The loss of mass of the examined samples was measured as a parameter characterizing the wear. Base on it, other wear coefficients were determined, for example the volume loss, the intensity of wear and the wear rate. The volume loss values determined were presented as a function of the strength and the initial hardness. Based on the results obtained, it was found that the hardened steel and ADI had comparable wear properties, while the ADI surface was strengthened probably as a result of the transition of austenite into martensite and the impact of the deformation of the graphite contained in ADI on the abrasive wear of the surface.

Proton-irradiation-induced structural and magnetic property changes in Ni–Mn–Ga high-temperature shape memory films

Ni51.90Mn28.86Ga19.24 alloy films were irradiated by 120keV protons at different fluences to investigate the effect of irradiation on their crystal structure, phase-transition temperature, and magnetic properties. The results show not only that a nanoscale amorphous layer may have been induced on the surface, but also that non-modulated (NM) martensite was produced. Moreover, differential scanning calorimetry peaks of the martensitic and austenitic transition temperatures are shifted to higher temperatures after irradiation; in particular, two endothermic and two exothermic peaks were observed for a proton fluence of 2.0×1016/cm2. Further, the formation of NM martensite could produce an extra shoulder on the M–T curves of the irradiated films. Regarding the magnetic properties, a gradual decrease in the Curie temperature TC can be caused by irradiation at a higher proton fluence. The changes in the saturation magnetization Ms with the proton fluence seem to be related to the reduction in TC.

Successive magnetic phase transitions and magnetocaloric effect in the MnNiFeGe alloy

The magnetic system possessing successive phase transitions is expected to achieve a broad refrigeration temperature region. In present paper, the magnetocaloric effect is investigated in a MnNiFeGe alloy, in which the martensitic transformation and the ferromagnetic phase transition of austenite are in adjacent temperature regions. Under a low magnetic field of 0.9T, this alloy shows a conventional and an inverse magnetocaloric effect simultaneously while only a conventional effect is observed under a magnetic field of 2.0T, which is due to the magnetic-field-induced metamagnetic transition in the martensite. Metamagnetic phase transitions and potential application in magnetic refrigeration for the MnNiFeGe alloy are discussed.

Inverse magnetocaloric effect and magnetoresistance associated with martensitic transition for Cu‐doped Ni‐Mn‐In Heusler alloy

In this study, we reported the influence of replacing Cu for Ni on the crystal structure, martensitic transformation, and magnetic properties for polycrystalline Ni50−xCuxMn37In13 (x = 5 and 7) quaternary alloys. It was found that the studied alloys undergo a first‐order martensitic transformation from ferromagnetic austenite to paramagnetic martensite near room temperature. With increasing Cu content, the martensitic transformation significantly shifts toward a lower temperature, and fully splits off from the Curie transition of austenite, which enhances the difference in magnetization between the two phases for the alloy with x = 7. Moreover, both inverse magnetocaloric and magnetoresistance effects during reverse martensitic transformation were also studied in these two alloys. In comparison to Ni50−xCuxMn37In13 with x = 5, an optimized refrigerant capacity (∼100 J kg−1) and a considerable magnetoresistance (47%) were obtained in Ni43Cu7Mn37In13 alloy for the magnetic field change of 30 kOe. In addition, the magnetic measurements with hydrostatic pressure demonstrate that the magnetoresistance in this system mainly arises due to the electron‐lattice scattering triggered by structural transition.

Influence of Cu Doping on Martensitic and Magnetic Transitions in Ni-Mn-Sn Alloys

In this work, the fourth element Cu was introduced to substitute Ni in polycrystalline Ni-Mn-Sn alloys. It was shown that Cu doping did not change the crystal structure of the martensite in Ni50-xCuxMn39Sn11 (x=0, 1, ......,7) alloys, but resulted in the decrease of martensitic transformation temperatures. Due to the higher atomic radius of Cu with respect to that of Ni, the lattice volume of martensite unit cell increases with the gradual substitution of Ni by Cu. For the alloys with the Cu content of 0-4%, the martensitic transformation is from weak magnetic (paramagnetic) austenite to weak magnetic (paramagnetic) martensite. When the Cu content is higher than 4%, the paramagnetic to ferromagnetic transition of austenite was introduced. The temperature interval between magnetic transition and structural transformation was enlarged with the increase of Cu content. Due to the relatively smaller magnetization difference between austenite and martensite, the field induced inverse martensitic transformation behavior is not significant in the present Cu-doped alloys.

Upset Resistance Welding of Carbon Steel to Austenitic Stainless Steel Narrow Rods

Effects of welding current (at the range of 2-4 kA) on the microstructure and mechanical properties of upset resistance welds of AISI-1035 carbon steel to AISI-304L austenitic stainless steel rods were investigated. The results showed that the joint strength first increased by raising the welding current up to 3 kA and then decreased beyond it. Increasing trend was related to more plastic deformation, accelerated diffusion, reduction of defects and formation of mechanical locks at the joint interface. For currents more than 3 kA, decrease in the joint strength was mainly caused by formation of hot spots. Using the optimum welding current of 3 kA, tensile strength of the joint reached to ~76% of the carbon steel base metal strength. Microstructural observations and microhardness results confirmed that there was no hard phase, i.e., martensite or bainite, at the weld zone. Moreover, a fully austenitic transition layer related to carbon diffusion from carbon steel was observed at the weld interface.

Analytical modelling of Rolamite mechanism made of shape-memory alloy for constant force actuators

This article analyses the Rolamite architecture exploiting shape-memory alloys as power element to obtain a solid-state actuator. The Rolamite mechanism was discovered in the late 1960s, initially as precision and low friction linear bearing. The most common Rolamite configuration consists of a flexible thin metal strip and two rollers mounted between two fixed parallel guide surfaces. The system can roll back and forth without slipping guided by the plates along its so-called sensing axis. The system presents another relevant advantage in addition to low friction coefficient, which is the possibility to provide force generation in a quite simple way. In the original literature works, the force was provided, thanks to cut-outs of various shapes in the strip, although this method does not allow the Rolamite to be considered a proper actuator, but only a force generator. In this article, we developed the idea of exploiting the shape-memory alloy as Rolamite power element, and therefore, to use the shape-memory effect to change the elastic properties of the strip and to provide the actuation force. The mechanical analysis, where the martensite–austenite transition is modelled in a simplified way, shows that this application is feasible, mainly thanks to the initial precurvature of the shape-memory alloy strip. The discussion of the results highlights some important merits of this architecture such as long stroke, constant force and compactness.

The α↔γ transformation of an Fe1−xCrx alloy: A molecular-dynamics approach

Using molecular dynamics (MD) simulation, we study the temperature-induced [Formula: see text] phase transformation of an Fe[Formula: see text]Cr[Formula: see text] alloy. We find that the austenitic transition temperature is increased with respect to that of an Fe[Formula: see text]Ni[Formula: see text] alloy containing the same concentration of impurity atoms. During the austenitic transformation, heterogeneous nucleation of close-packed (cp) nuclei leads to a polycrystalline structure. The microstructure formed closely resembles that found in pure Fe and in FeNi alloys.

Impact of the B2 ordering behavior on the mechanical properties of a FeCoMo alloy

A Fe - 25at% Co - 9at% Mo alloy can be hardened by nm-sized (Fe, Co)7Mo6 µ-phase precipitates which is accomplished by solution annealing in the austenite region followed by rapid quenching to room temperature and subsequent aging below the austenite transition temperature. In overaged condition the Mo-content in the remaining matrix drops towards zero and, therefore, the matrix consist of 71at% Fe and 29at% Co. The binary Fe-Co system shows a disorder-order, A2↔B2 transition at a critical ordering temperature between 25at% and 72at% Co. It is expected that the remaining matrix of an overaged Fe - 25at% Co - 9at% Mo alloy also exhibits such an ordering reaction. It will be demonstrated that the formation of a B2 ordered FeCo phase can be delayed or completely prevented by rapid quenching from temperatures above the critical ordering temperature. This has a strong impact on the mechanical properties of this alloy which have been studied by means of tensile, impact toughness and hardness testing. The evidence for a disorder-order transition in this alloy has been given by neutron diffraction as well as high resolution transmission electron microscopy.

EFFECT OF METASTABLE AUSTENITE ON MECHANI-CAL PROPERTY AND MECHANISM IN CRYOGENICSTEEL APPLIED IN OCEANEERING

With the development of marine resources and its emerging markets, the marine equipments such as offshore drilling platform, subsea oil and gas transportation pipeline, and storage equipment of oil and gas are developing actively. It is urgent to develop a new type of steel with low cost and excellent toughness to satisfy the demand of marine equipment. In this work, the morphology of austenite and phase transition process in medium manganese steel during quenching and tempering (QT) and quenching, lamellarizing and tempering (QLT) heat treatments were investigated by using EBSD, HRTEM and thermal dilatometer. The results show that the discrepancy of stability in austenite caused by its shape, size and location leads to the more excellent toughness during QLT heat treatment compared to QT. It has been found from thermodynamical and dynamical viewpoint that the formation rates of reverted austenite during QLT heat treatment are significantly larger relative to QT due to the partition process of C and Mn elements in the lamellarizing stage. Moreover, two growth models of film-type reverted austenite are distinct during two heat treatments: the unidimensional two-sided growth mode during QT and unidimensional one-sided growth mode during QLT.

Manifestation of martensitic phase transformation and magneto-caloric properties in high quality magnetron sputtered Ni–Mn–In//Si ultra thin films

Ultrathin films of Ni–Mn–In with thickness ranging from 100nm down to 10nm deposited on Si (100) were investigated for thickness dependence of martensitic phase transformation and their magnetic properties. X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) revealed the (220) oriented austenitic cubic phase growth of 10–80nm Ni–Mn–In thin films. In contrast a signature of martensitic phase along with austenite phase was observed in 100nm sputtered coated Ni–Mn–In thin films. A systematic magnetization versus temperature (M–T) study of Ni–Mn–In thin films implies that martensitic transformation is possible down to 30nm. It also suggests the reduction in martensitic transformation temperatures (Mf &As) along with an increase in magneto-structural transition temperature course with decreasing film thickness. The magnetocaloric curves evaluated from isothermal magnetization study showed an increase in magnetic entropy change (ΔSm) with increasing film thickness, a maximum attainable ΔSm=0.114mJ/cc.K was observed for 100nm thin film at 240K as Ni–Mn–In endures a transition from martensitic phase to austenitic phase with a concomitant magnetic ordering. A shift in ΔSm maxima towards higher temperature with increase in film thickness was also observed from magneto-caloric graphs. The R–T measurements performed to investigate the martensitic to austenitic phase transition were in good agreement with thermo–magnetic curves. The tuning of martensitic phase transformation of Ni–Mn–In opens up a new possibility for the wide range working of many spintronics devices like magneto-electric random access memory (MERAM).

Magnetostructural transitions in Mn-rich Heusler Mn–Ni–In melt-spun ribbons with enhanced magnetocaloric effect

The magnetostructural transition behavior and magnetocaloric effect of Mn-rich Heusler Mn–Ni–In melt-spun ribbons have been investigated in the present study. Experimental results showed that the martensitic transition temperatures decreased by substituting small amounts of Ni with Mn in Mn49+xNi42−xIn9 (x=0,1,2). Within a temperature range from 100K to 380K, the Mn49Ni41In9 underwent a martensitic transformation from a paramagnetic L21-type cubic austenite to a ferromagnetic modulated fourteen-layered monoclinic (14M) martensite, followed with a ferromagnetic→weak-magnetic transition in martensite upon cooling. While for both Mn50Ni41In9 and Mn51Ni40In9, the paramagnetic→ferromagnetic transition in austenite occurred prior to the martensitic transformation upon cooling. Under a magnetic field change of 30kOe, the maximum magnetic entropy changes of the Mn50Ni41In9 melt-spun ribbons were found to be 5.7J/kgK and −2.3J/kgK in the vicinity of martensitic transformation and magnetic transition of austenite, respectively. An enhanced total effective refrigeration capacity as high as 184.2J/kg was obtained in the Mn50Ni41In9 melt-spun ribbons.

The phase diagrams with influence of biquadratic exchange coupling on martensitic–austenitic transformations for core–surface nanoparticles

Recently, origin of the martensite–austenite transitions in core–surface type magnetic nanoparticles has been investigated theoretically and it has been indicated that repulsive biquadratic exchange coupling (K<0.0) causes the coexisting martensite and austenite phases. In the present paper, the phase diagrams of homogeneous and composite nanoparticles in the kBT/J0−D/J0 plane are studied for the presence and absence of attractive biquadratic exchange interaction in addition to repulsive biquadratic exchange interaction. Significant changes in the phase diagram points are discussed in the presence of martensitic and austenitic transformations. Four regions in the phase diagrams are found as second-order, martensitic–austenitic, TCid and first-order phase transition regimes.

Enhanced magnetocaloric effect in Heusler Ni–Mn–Sn unidirectional crystal

A unidirectional crystal of Heusler Ni–Mn–Sn alloy with L21(220) preferential orientation was fabricated using a modified high-pressure optical zone-melting furnace in the present work. When a magnetic field was applied parallel to the crystal growth direction, the maximum values of magnetic entropy change in the vicinity of the martensitic transformation and the magnetic transition of austenite were found to be 5.58J/kgK and −2.87J/kgK, respectively. The total effective refrigeration capacity reached 127J/kg under a magnetic field change of 30kOe. The magnetic properties almost remained constant when the magnetic field was applied a direction perpendicular to the growth direction. Such magnetocaloric effect was ascribed to the relatively lower magnetic anisotropy along both directions, which is advantageous for practical utilization of refrigeration.

Effect of aging and ball milling on the phase transformation of Ni50Mn25Ga17Cu8−xZrx alloys

This study investigated phase transformation of Ni50Mn25Ga17Cu8-xZrx (x = 0, 4, 8) alloys after aging and ball milling. For the Cu4Zr4 and Zr8 dual phase alloys, aging has enhanced magnetic susceptibility and magnetic exchange of the matrix, resulting in an increase of the Curie temperature of austenitic matrix. The decrease of martensitic transformation temperatures for the Cu4Zr4 alloy and the increase of martensitic transformation temperatures for the Zr8 alloy after aging should be related to the dissolution and precipitation of the second phase in the matrix, respectively. Ball milling is effective to smash the Cu4Zr4 and Zr8 alloys to fine particles, but cannot fracture the Cu8 alloy to particles, indicating an inherent high ductility and strength of the Cu8 alloy. Therefore, the macroscopic brittleness of the polycrystalline Cu8 alloy was mainly caused by the weak grain boundaries. For the Cu4Zr4 and Zr8 particles, the martensitic transformation and Curie transition of austenitic matrix disappeared and the Curie transition of second phase remained after ball milling. After post-annealing at 800 °C, the Curie transition of austenite was recovered due to the restoration of atomic order, but the martensitic transformation cannot be retrieved which might be caused by the grain refinement of the austenitic matrix after ball milling.

The Role of Operational Factors in Shaping of Wear Properties of Alloyed Austempered Ductile Iron Part I. Experimental Studies Abrasive Wear of Austempered Ductile Iron (Adi) in the Presence of Loose Quartz Abrasive/ Rola Czynników Eksploatacyjnych W Kształtowaniu Właściwości Zużyciowych Stopowych Żeliw Sferoidalnych Ausferrytycznych Część I. Eksperymentalne Badania Zużycia Sciernego W Obecności Lużnego Scierniwa Kwarcowego Żeliw Adi

Abstract The paper presents the results of the wear tests of alloyed austempered ductile iron performed on a specially designed test rig simulating real operating conditions for chain wheels which were selected for testing due to their complex rolling and sliding form of contact between elements. The chain wheels subjected to tests were operated with the use of loose quartz abrasive and without it. The studies involved the determination of strength and plastic properties, hardness distributions, microstructure and linear wear of the ADI in question considered as a function of the austempering temperature. Based on the results obtained, the following was found: phase transition of austempered ductile iron can take place for combinations typical of industrial applications, a non-linear increase in the wear as a function of the austempering temperature occurs, an increase in the hardness of all the analysed types of ADI takes place during the operation without the presence of abrasive, the phase transition of austenite into martensite in ADI with a lower content of austenite requires higher loads as compared with cast iron containing 40% of austenite.

Magnetic, magnetocaloric and critical properties of Ni50−xCuxMn37Sn13 rapidly quenched ribbons

Magnetic, magnetocaloric and critical properties of Ni50−xCuxMn37Sn13 (x=0, 1, 2, 4 and 8) rapidly quenched ribbons have been studied. The substitution of Cu for Ni clearly effects on magnetic transitions, magnetocaloric effects and magnetic orders of these alloy ribbons. With increasing the Cu-concentration, the martensitic–austenitic phase transition shifts to lower temperature, from 265K (for x=0) to 180K (for x=4), and disappears with x=8, while the Curie temperature, TC, is almost unchanged. Both the conventional and inverse magnetocaloric effects are observed. The obtained values for the maximum inverse magnetic entropy changes, |ΔSm|max, of the ribbons are relatively large, 5.6Jkg−1K−1 (for x=0) and 5.4Jkg−1K−1 (for x=4) with the external magnetic field change ΔH=12kOe. The critical parameters (TC, β, γ and δ) of the ribbons are determined from the static magnetic data at the second order ferromagnetic–paramagnetic transition by using both the Arrott–Noakes and Kouvel–Fisher methods. The results reveal that the long-range ferromagnetic order in the alloy is tendentiously dominated by increasing the Cu-concentration.

Closed-form solutions for inhomogeneous states of a slender 3-D SMA cylinder undergoing stress-induced phase transitions

This paper considers the isothermal stress-induced phase transitions of shape memory alloys (SMAs), based on a two variant model with an internal variable. More specifically, we study analytically the inhomogeneous states of a slender SMA cylinder under uniaxial tension/extension in a three-dimensional (3-D) setting. By utilizing two small quantities, the complex 3-D mechanical system is reduced to three linear systems for three different regions (austenite, martensite and phase transition regions). Then, we concentrate on the inhomogeneous states with only one phase transition region which is far away from the two ends. For the case of planar and general interfaces among three regions, we construct closed-form solutions and semi-analytical solutions, respectively. Theoretically, the present analytical study generalizes the classical Ericksen’s 1-D results. First, unlike the 1-D case, for which there are infinitely many equally stable states, the current 3-D problem has only one optimal state (by the energy criterion). Second, in the optimal state the average axial stress on the cross section is still the 1-D Maxwell stress, although there is a nonuniform distribution. Physically, the analytical solutions clearly reveal the roles of material and geometrical parameters. The localized inhomogeneous deformation is due to the structural effect and the strain softening behavior, and an explicit formula for the width of the transition region is deduced. We also make a comparison of our 3-D analytical results with those of a 1-D gradient model, which leads to a plausible choice of the gradient parameter.

Magnetic properties and magnetocaloric effect in Ni–Mn–Sn alloys

Abstract Magnetic and magnetocaloric properties in Ni50Mn50−xSnx alloys with wide range of the Sn-concentration (x=0–40) were investigated. The alloys were prepared by arc-melting and subsequently annealing at 850 °C for 4 h. The X-ray diffraction analyses manifest the formation of the crystalline phases (Ni2MnSn, NiMn, Ni3Sn2, Mn3Sn, and MnSn2) in the alloys with various compositions and fabrication conditions. With increasing x, the saturation magnetization first increases from near zero (at x=10) to above 40 emu/g (at x=20) and then decreases to below 10 emu/g (at x=40) for both the as-melted and annealed cases. The martensitic–austenitic transition was observed in the alloys with a narrow range of x (13–15). The magnetic transitions in the alloy can be controlled by changing Sn-concentration. The alloy reveals both the positive and negative entropy changes with quite large magnitude (∆Sm>1 J/kg K with ∆H=12 kOe) with appropriate compositions and annealing conditions.