High Temperature Austenite Phase Research Articles

A Combinatorial Study for Interdiffusion, Crystallography and Mechanical Behavior of Ni-Mn-Ga Alloys

The ferromagnetic shape memory and magnetocaloric properties of NiMnGa alloys are closely related to the martensitic transformation from high temperature austenitic phase to low temperature martensitic phase. The transformation temperature and the resulting microstructure and crystallography of the martensites can be very complex, but are crucial to the optimization of the material performance. A combinatorial study with a series of solid-to-solid diffusion couples and various characterization techniques, including scanning electron microscopy, focused ion beam, transmission electron microscopy, electron probe microanalysis, and nanoindentation, was carried out to investigate the microstructural and crystallographic development, and mechanical properties in NiMnGa alloys. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear interphase boundary. Crystallographic variations in martensitic phase, including non-modulated (NM) martensite and modulated (5M or 7M) martensite, were found in the diffusion couples. All martensitic microstructure consists of variants with different orientations and the twinning relationship. A decrease of reduced elastic modulus (Er) was observed with Ni substituting for Ga in the austenitic phase. However, an opposite trend of an increase in Er was found in the martensitic phase. The softening of the elastic constants near the vicinity of martensitic transformation contributed to a sharp decrease in Er near the interphase boundary. The measured Er had a larger scatter for the martensitic phase than that for the austenitic phase.

Austenitic Nickel- and Manganese-Free Fe-15Cr-1Mo-0.4N-0.3C Steel: Tensile Behavior and Deformation-Induced Processes between 298 K and 503 K (25 °C and 230 °C)

The high-temperature austenite phase of a high-interstitial Mn- and Ni-free stainless steel was stabilized at room temperature by the full dissolution of precipitates after solution annealing at 1523 K (1250 °C). The austenitic steel was subsequently tensile-tested in the temperature range of 298 K to 503 K (25 °C to 230 °C). Tensile elongation progressively enhanced at higher tensile test temperatures and reached 79 pct at 503 K (230 °C). The enhancement at higher temperatures of tensile ductility was attributed to the increased mechanical stability of austenite and the delayed formation of deformation-induced martensite. Microstructural examinations after tensile deformation at 433 K (160 °C) and 503 K (230 °C) revealed the presence of a high density of planar glide features, most noticeably deformation twins. Furthermore, the deformation twin to deformation-induced martensite transformation was observed at these temperatures. The results confirm that the high tensile ductility of conventional Fe-Cr-Ni and Fe-Cr-Ni-Mn austenitic stainless steels may be similarly reproduced in Ni- and Mn-free high-interstitial stainless steels solution annealed at sufficiently high temperatures. The tensile ductility of the alloy was found to deteriorate with decarburization and denitriding processes during heat treatment which contributed to the formation of martensite in an outermost rim of tensile specimens.

Direct Measurements of Magnetocaloric Effect in a Single Crystalline Ni<sub>2.13</sub>Mn<sub>0.81</sub>Ga<sub>1.06</sub> Heusler Alloy

Magnetocaloric effect (MCE) in the vicinity of first order martensitic transformation and second order magnetic transition in a single crystalline Ni2.13Mn0.81Ga1.06 Heusler compound was studied by a direct method. The obtained results revealed that, for the applied magnetic field strength μ0H = 1.9 T, MCE is irreversible in the vicinity of the first order martensitic transformation only when the MCE measurements are performed under cooling protocol. Plot of the experimentally measured adiabatic temperature change ΔTad as a function of temperature T indicated that ΔTad has a negligible benefit from the magnetic field-induced conversion of the high-temperature austenitic phase into the low-temperature martensitic phase and is mainly determined by the paraprocess of the austenitic phase around both direct and reverse martensitic transformations.

Deciphering M–T diagram of shape memory Heusler alloys: reentrance, plateau and beyond

We present our recent results on temperature behaviour of magnetization observed in Heusler alloys. Three regions can be distinguished in the M–T diagram: (I) low temperature martensitic phase (with the Curie temperature K), (II) intermediate mixed phase (with the critical temperature K) exhibiting a reentrant like behavior (between and ) and (III) high temperature austenitic phase (with the Curie temperature K) exhibiting a rather wide plateau region (between and ). By arguing that powerful structural transformations, causing drastic modifications of the domain structure in alloys, would also trigger strong fluctuations of the order parameters throughout the entire M–T diagram, we were able to successfully fit all the data by incorporating Gaussian fluctuations (both above and below the above three critical temperatures) into the Ginzburg–Landau scenario.

Thermoelastic martensitic transformations in ternary Ni50Mn50–z Ga z alloys

We have studied the effect of gallium alloying on the structure, phase composition, and physical properties of ternary alloys of the Ni50Mn50–z Ga z (0 ≤ z ≤ 25 at %) quasi-binary section in a broad temperature range. Dependences of the type of crystalline structure of the high-temperature austenite phase and martensite, as well as the critical temperatures of martensitic transformations on the alloy composition, are determined. A phase diagram of the structural and magnetic transformations is constructed. Concentration boundaries of the existence of tetragonal L10 (2M) martensite and martensitic phases (10M and 14M) with complex multilayer crystalline lattices are found. It is established that the predominant martensite morphology is determined by the hierarchy of packets of thin coherent nano- and submicrocrystalline plates with habit planes close to {011} B2, pairwise twinned along one of 24 equivalent {011} B2 twinning shear systems.

Direct measure of giant magnetocaloric entropy contributions in Ni–Mn–In

Off-stoichiometric alloys based on Ni2MnIn have drawn attention due to the coupled first order magnetic and structural transformations, and the large magnetocaloric entropy associated with the transformations. Here we describe calorimetric and magnetic studies of four compositions. The results provide a direct measure of entropy change contributions at low temperatures as well as at the first-order phase transitions. Thereby we determine the maximum possible field-induced entropy change corresponding to the giant magnetocaloric effect. We find a large excess entropy change above that of the magnetic moments, but only in compositions with no ferromagnetic order in the high-temperature austenite phase. Furthermore, a molecular field model corresponding to magnetic order in the low-temperature phases is in good agreement, giving an entropy contribution nearly independent of composition, despite significant differences in overall magnetic response of these materials.

In Situ Measurement and Prediction of Stresses and Strains During Casting of Steel

Modeling the thermo-mechanical behavior of steel during casting is of great importance for the prediction of distortions and cracks. In this study, an elasto–visco–plastic constitutive law is calibrated with mechanical measurements from casting experiments. A steel bar is solidified in a sand mold and strained by applying a force to bolts that are embedded in the two ends of the bar. The temporal evolutions of the restraint force and the bar’s length change are measured in situ. The experiments are simulated by inputting calculated transient temperature fields into a finite element stress analysis that employs the measured forces as boundary conditions. The thermal strain predictions are validated using data from experiments without a restraint. Initial estimates of the constitutive model parameters are obtained from available mechanical test data involving reheated steel specimens. The temperature dependence of the strain rate sensitivity exponent is then adjusted until the measured and predicted length changes of the strained bars agree. The resulting calibrated mechanical property dataset is valid for the high-temperature austenite phase of steel. The data reveal a significantly different mechanical behavior during casting compared to what the stress–strain data from reheated specimens show.

Microstructure and corrosion behavior of laser processed NiTi alloy.

Laser Engineered Net Shaping (LENS™), a commercially available additive manufacturing technology, has been used to fabricate dense equiatomic NiTi alloy components. The primary aim of this work is to study the effect of laser power and scan speed on microstructure, phase constituents, hardness and corrosion behavior of laser processed NiTi alloy. The results showed retention of large amount of high-temperature austenite phase at room temperature due to high cooling rates associated with laser processing. The high amount of austenite in these samples increased the hardness. The grain size and corrosion resistance were found to increase with laser power. The surface energy of NiTi alloy, calculated using contact angles, decreased from 61 mN/m to 56 mN/m with increase in laser energy density from 20 J/mm(2) to 80 J/mm(2). The decrease in surface energy shifted the corrosion potentials to nobler direction and decreased the corrosion current. Under present experimental conditions the laser power found to have strong influence on microstructure, phase constituents and corrosion resistance of NiTi alloy.

Effect of compositional and antisite disorder on the electronic and magnetic properties of Ni–Mn–In Heusler alloy

A systematic study has been done on the electronic and magnetic properties of metamagnetic Ni–Mn–In Heusler alloy with compositional and structural (anti-site) disorder at high temperature austenite phase. The electronic structure calculation shows an increasing Mn–Ni hybridization which occurs due to the decrease in Mn–Ni bond length as the system approaches martensite phase. The results obtained from magnetic moment calculations follow a similar trend to the previous experimental and theoretical results. The magnetic coupling parameters, Jij, obtained from the ab initio calculation explains the presence of competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions in the system and the dominating AFM interactions nearer to the martensite phase.

Structural and magnetic transformations in Ni51 − x Mn36 + x Sn13 alloys

The structural and magnetic phase transitions in Ni-Mn-Sn-based alloys have been studied. The temperature dependences of the martensitic and magnetic phase transformations in Ni51 − x Mn36 + x Sn13 alloys (0 ≤ x ≤ 4) have been determined in the case where manganese atoms substitute for nickel atoms. For the studied alloys, the concentration phase diagram has been constructed and the temperature regions of the existence of the high-temperature austenite phase L21 and the low-temperature martensite phase in different magnetic states have been determined.

Magnetocaloric effect, magnetostructural and magnetic phase transformations in Ni50.3Mn36.5Sn13.2 Heusler alloy ribbons

Thermomagnetic properties and magnetocaloric effect (MCE) in Ni50.3Mn36.5Sn13.2 Heusler alloy ribbons are reported. Large magnetocaloric response has been obtained for μ0H=3T in both the reversible adiabatic change in temperature ΔTad (−6.3 and +4.7K) and the isothermal change in the total entropy ΔST (+11.8 and −2.2JK−1kg−1), respectively at the temperature Tstr=271K at which the magnetostructural phase transition from the low-temperature Martensite phase (MP) to the high-temperature Austenite phase (AP) takes place, and at TCA=311K at which the magnetic phase transition in the AP occurs. The MCE has been studied through the Stoner’s magnetocaloric parameter ξS (Stoner EC. Phil Mag 19 (1935) 565), and a generalized expression for the equations of state for magnetic materials is presented. The phase coexistence of the MP and AP has been studied via their respective sublattice with opposite-aligned magnetic moments by means of the effective magnetic anisotropy density eA induced by the supercooled (superheated) secondary AP (MP) in the MP (AP). Magnetic fields higher than the critical value of μ0HR=0.5T produce alignment of the two magnetic moments and hence a change in the magnetic ordering in the MP from antiferromagnetic to ferromagnetic, resulting in a thermomagnetic behavior in which the field-cooling and field-heating magnetizations become coincident, the induced anisotropy vanishes eA=0 and the metastability disappears. New tentative methods to obtain the spontaneous magnetization of both MP and AP and their respective fractions in the phase coexistence range are reported.

Calorimetric and magnetic study for Ni50Mn36In14 and relative cooling power in paramagnetic inverse magnetocaloric systems

The non-stoichiometric Heusler alloy Ni50Mn36In14 undergoes a martensitic phase transformation in the vicinity of 345 K, with the high temperature austenite phase exhibiting paramagnetic rather than ferromagnetic behavior, as shown in similar alloys with lower-temperature transformations. Suitably prepared samples are shown to exhibit a sharp transformation, a relatively small thermal hysteresis, and a large field-induced entropy change. We analyzed the magnetocaloric behavior both through magnetization and direct field-dependent calorimetry measurements. For measurements passing through the first-order transformation, an improved method for heat-pulse relaxation calorimetry was designed. The results provide a firm basis for the analytic evaluation of field-induced entropy changes in related materials. An analysis of the relative cooling power (RCP), based on the integrated field-induced entropy change and magnetizing behavior of the Mn spin system with ferromagnetic correlations, shows that a significant RCP may be obtained in these materials by tuning the magnetic and structural transformation temperatures through minor compositional changes or local order changes.

Role of magnetic degrees of freedom in a scenario of phase transformations in steel

The diversity of mesostructures formed in steel at cooling from a high-temperature austenite ("gamma") phase is determined by the interplay of shear reconstructions of crystal lattice and diffusion of carbon. Combining first-principles calculations with large-scale phase-field simulations we demonstrate a decisive role of magnetic degrees of freedom in the formation of energy relief along the Bain path of "gamma"-"alpha" transformation and, thus, in this interplay. We show that there is the main factor, namely, the magnetic state of iron and its evolution with temperature which controls the change in character of the transformation. Based on the computational results we propose a simple model which reproduces, in good agreement with experiment, the most important curves of the phase transformation in Fe-C, namely, the lines relevant to a start of ferrite, bainite, and martensite transformations. Phase-field simulations within the model describe qualitatively typical patterns at these transformations.

Dynamical instability and Fermi surface topology inNi2FeGa from first principles

The phonon spectrum of stoichiometric Heusler alloy ${\text{Ni}}_{2}$FeGa is calculated for the high-temperature cubic austenite phase by using first-principles density functional perturbation theory. We also compute the elastic constants of the alloy from the initial slopes of the acoustic phonon branch. The ${\text{TA}}_{2}$ phonon branch along [110] direction shows softening with a minimum dip at $\ensuremath{\zeta}=0.58$ which indicates the possibility of modulated phases prior to martensitic transformation. We also map the Fermi surface of this alloy both in 3D and 2D to check the presence of any nesting vectors. The observed nesting parameter is in good agreement with the above value of the wave vector in the [110] direction where phonon softening occurs.

Neutron diffraction evidence for kinetic arrest of first order magneto-structural phase transitions in some functional magnetic materials

Neutron diffraction measurements, performed in the presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of glass-like arrested states in both compounds. The cooling and heating under unequal fields protocol has been used to establish phase coexistence of metastable and equilibrium states, and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field–temperature dependent kinetic arrest line TK(H), through the transformation of the arrested phase to the equilibrium phase. This transformation has been observed isothermally in reducing H, as also on warming in constant H. TK is seen to increase as H increases in both cases, consistent with the low-T equilibrium phase having lower magnetization.

Phase Transitions and Microstructural Processes in Shape Memory Alloys

Shape memory alloys exhibit a peculiar property, shape memory effect that is the result from the structural changes in microscopic scale. These alloys return to previously defined shapes when they are subjected to variation of temperature after deformation of the low temperature phase. Shape-memory effect is based on martensitic transformation, with which the material changes its internal crystalline structure. The ordered structure or super lattice structure is essential for the shape memory effect of the material. Copper based alloys exhibit this property in the β-phase field, which possesses the simple bcc-structure at high temperature austenite phase. As the temperature is lowered, austenite phase undergoes martensitic transition following two ordering reactions, and microstructural changes in microscopic scale govern this transition. In the present work, Cu alloys were investigated by transmission electron microscope, TEM, and x-ray diffraction techniques.

Microstructure and magnetic properties of as-cast Ni2MnGa alloys processed by twin roller melt spinning

The magnetism and the microstructure of Ni2MnGa alloys, processed for the first time by twin roller melt spinning at tangential wheel speeds of 10m/s (V10), 15m/s (V15) and 20m/s (V20) are investigated. At room temperature, the major phase in the as-cast alloys is the cubic L21 Ni2MnGa ordered austenitic phase, with a lattice parameter only ∼0.1% larger than the tabulated value. The order domain size in the austenitic phase decreases from (40±1) nm in samples V10 to (19±1) nm in V20. Mn(S,Se) small precipitates are also found uniformly embedded in the ribbons with mean size of (26±2) nm (V10) and (7±2) nm (V20), exhibiting a definite orientation relation with the austenitic matrix, <100>P// <100>a. The as-cast alloys transform to an intermediate cubic phase I at about 220–230K depending on the quenching rate and to a martensitic phase at about 130K. The high temperature austenitic phase and the low temperature martensitic phase are ferromagnetic; in both cases the saturation polarization is lower in samples quenched at higher rates. The demagnetization curves measured from saturation in the martensitic state show two marked steps: a first one for positive fields in V15 and V10 (∼46mT) and a larger second one for relatively large inverse fields (∼130–250mT) in all the samples. These steps are likely to arise from a demagnetization mechanism involving a field induced twin boundary motion in the few martensite variants selected by the crystallographic texture and the quenched stresses in the ribbons.

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − xSix (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220–280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150–200 μΩ cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.

Structure and magnetic properties of highly dispersed Ni-Mn-Ga powders prepared by spark-erosion

Highly dispersed powders of Ni50,3Mn30Ga19,7 were prepared by spark-erosion in ethanol, water, and kerosene. Powder particles have mostly spherical shape and broad size distribution, with a maximum around 1 μm. In the as-prepared state, two series of peaks are observed by x-ray diffraction. They are associated with the disordering of two ordered phases, existed in bulk Ni-Mn-Ga: the high-temperature L21 austenitic phase and the low-temperature L10 martensitic one. Annealing decreases the half-width of the peaks and increases the L10/L21 intensity ratio. Magnetically as-prepared powders demonstrate a clear superparamagnetic behavior that changes to ferromagnetic one after annealing. These properties are quite different from the ones of mechanically dispersed powders and rapidly quenched ribbons of the same composition. The spark-erosion conditions, particularly cooling rates up to 109 K/s, lead to the formation of the mixed phase state, much higher atomic disorder, and to the appearance of different types of structural inhomogeneities.

Thermomechanical training and characterization of Ni–Ti–Hf and Ni–Ti–Hf–Cu high temperature shape memory alloys

Nickel–titanium (NiTi) is the most commonly used shape memory alloy (SMA) for actuator applications, though its usefulness is limited to temperature ranges below 100 °C. High temperature SMAs are formed by adding ternary elements to NiTi, but their usefulness as actuators is still in question. The purpose of this research was to characterize and train two high temperature SMAs, NiTi29.7Hf20 and NiCu5Ti29.7Hf20, to determine their effectiveness as linear actuators. Low temperature martensitic phase and high temperature austenitic phase stress–strain tests were performed to characterize the materials’ behavior followed by temperature cycling under constant stress. Temperature cycling under constant stress is known as thermomechanical training and resulted in small amounts of plastic strain growth and the development of two-way shape memory (TWSM). The results from these tests support the conclusion that hafnium distorts slip planes within the martensitic material phase, and that (Ti,Hf)2Ni and (Ti,Hf)3Ni4 particulates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously, which develops a strong internal stress field during training within the first few cycles. The internal stress field develops TWSM, but limits further plastic growth. The particulate formation also embrittles the material. The transformation temperatures of both alloys were below creep and annealing temperatures making them ideally suited for high temperature actuators.