Austenite Phase Research Articles

Enhanced metamagnetic shape memory effect in Heusler-type Ni37Co11Mn43Sn9 polycrystalline ferromagnetic shape memory alloy

Polycrystalline Ni-Co-Mn-Sn based ferromagnetic shape memory alloys (FSMAs) show promise as actuator materials, but their practical application involving magnetic field induced strain (MFIS) is often limited by three factors: the requirement for high magnetic fields (> 5 T), martensitic transition temperature away from room temperature, and limited recovery of pre-strain applied to the martensite phase. Current work investigates the martensitic transition and shape memory effect under the application of magnetic field for bulk polycrystalline Ni37Co11Mn43Sn9 alloy. The outcome of the study reveals a metamagnetic transition from the martensitic phase to the austenitic phase at a low field of 2.8 T at 300 K which results 0.25 % spontaneous MFIS. Interestingly, 1.3 % pre-strained specimen registers a 100 % recovery with the application of magnetic field of 4.5 T. Furthermore, the pre-strained specimen exhibited a two-way shape memory effect between a strain value of 1.0 % to 1.55 % during the field loading and unloading sequences. Notably, this study demonstrates, for the first time to the best of our knowledge that the spontaneous MFIS and pre-strain add together. This finding paves the way for achieving a giant MFIS by pre-straining a Ni-Mn-Sn/In class of FSMAs which shows large spontaneous MFIS.

Effect of heat-treatment on microstructure evolution of Stainless steel 316L developed using metal fused filament fabrication technique

The Metal fused-filament-fabrication (MFFF) technique has demonstrated fabulous latent for the low-cost consignment production. However, the polymer-based binders and their impacts, including pores and cavities, restrict the application of MFFF printing. In the current study, Stainless steel (SS)316L parts were developed using the MFFF technique, followed by debinding and sintering. The influence of post treatment was examined on the microstructure of the MFFF-produced SS316L at different temperatures, such as 700, 1050, and 1350 °C. Electron Backscatter Diffraction (EBSD) analysis revealed the progress of cellular structure at a temperature of 1350 °C, which confirmed the existence of austenite phase in the printed SS316L. The results demonstrate the efficacy of thermal treatment in refining the microstructure and residual stress of MFFF-processed SS316L parts.

Unveiling the recrystallization mechanism and kinetics parameters in cold-rolled dual-phase stainless steel using differential scanning calorimetry

The present work aims to assess the Recrystallization mechanism and Kinetics parameters in Cold-Rolled Dual-Phase stainless steel. For this purpose, Differential Scanning Calorimetry was used over a range of 5–20 K/mn of heating rate, to identify the classical kinetic parameters such as recrystallization temperature, activation energy, and growth mechanism following 90 % reduction in thickness. The findings revealed that the recrystallization temperature increased with rising heating rates within the range [706.8–732] for the ferrite phase, as well as [1282.0–1360.3] K for the austenite phase. The estimated activation energies for ferrite and austenite recrystallization fell within the range of [207.2 ± 1.4–220.1 ± 2.5] kJ/mol and [220.2 ± 2.0–241.1 ± 4.0] kJ/mol, respectively. The Avrami parameter approached half-unity for both ferrite and austenite recrystallized phases for which results can be interpreted to be chiefly responsible for a general mechanism involving the growth of new grains with appreciable initial volume through a diffusion-controlled process.

The role of TiN inoculant on microstructure and properties in wire arc directed energy deposition of super duplex stainless steels

The challenges in Wire Arc Directed Energy Deposition (Wire-DED) of super duplex stainless steels (SDSS) are the growth of large columnar grains and undesirably high austenite content. This results in a detrimental effect on the material properties. Here we report the additively manufactured SDSS with the addition of TiN inoculant via Wire-DED. And the effectiveness of incorporating TiN inoculant into additively manufactured SDSS was explored. The addition of trace amounts of TiN reduced the content of the austenite phase from 74.0% to 61.5%. Additionally, the introduction of TiN facilitated deoxidation and slag formation, leading to decreased inclusion content and a shift in inclusion type, which acted as nucleation sites for ferrite, promoting grain refinement. This results in weakened anisotropy of tensile properties and enhanced corrosion resistance. The anisotropy of the ultimate tensile strength and yield strength reduced from 9% to 3% and from 5% to 2%, respectively. This study demonstrates the potential of TiN inoculation in fabricating Wire-DEDed SDSS with tailored microstructures and improved performance, making them suitable for a wide range of practical applications.

Interplay between the reversibility of the inverse magnetocaloric effect and kinetic arrest in Ni-Co-Mn-Sn Heusler alloys

The nature of martensitic phase transitions and its modification as a function of composition and application of magnetic field have been studied in detail for Ni40Co10Mn40Sn10-xGax (x=0, 0.3, 0.4 and 0.5). The phase transition temperature (TM) increases with increasing x. The kinetic arrest (KA) of the austenitic phase has been observed for x=0 and 0.3, which disappeared for x>0.3. The KA results in a decrease in the change of magnetization (ΔM) across TM. However, the isothermal entropy change ΔS varies almost linearly with [dTM/d(μ0H)]−1 (field-induced shift in TM) irrespective of the composition variation and thermal cycling. This behavior indicates that the variation of dTM/d(μ0H) is the dominant factor determining the magnitude of the ΔS rather than ΔM. Consequently, the effect of KA on ∆M has a negligible influence on ΔS in the studied Heusler alloys. The reduced field-sensitivity in TM associated with the temperature-induced decrease of magnetization in the ferromagnetic austenitic phase leads to a decrease of the thermal hysteresis and an increase of ΔS. As a result, a large reversible (|ΔSrev|=21 J/kg K for Δμ0H=5 T) has been observed for x=0.5, which exceeds previously reported values for Sn-based Heusler alloys.

The Influence of Cr Addition on the Microstructure and Mechanical Properties of Fe-25Mn-10Al-1.2C Lightweight Steel

The influence of Cr addition on the microstructure and tensile properties of Fe-25Mn-10Al-1.2C lightweight steel was investigated. The characteristics of the microstructures and deformation behavior were carried out through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and room temperature tensile testing. Fe-20Mn-12Al-1.5C steel without Cr exhibited a fully austenitic single phase. With the addition of Cr, the volume fraction of ferrite continuously increased. When the content of Cr exceeded 5 wt%, the precipitation of Cr7C3 carbides was observed. In the steel with 5 wt% Cr, the quantity of κ carbides remarkably decreased, indicating that the addition of 5 wt% Cr significantly inhibited the nucleation of κ-carbides. As the Cr content increases from 0 wt% to 5 wt%, the austenite grain sizes were 8.8 μm and 2.5 μm, respectively, demonstrating that Cr alloying is an effective method of grain refinement. Tensile strength increased slightly while elongation decreased with increasing Cr content. As the Cr content exceeded 5 wt%, the yield strength increased but the elongation drastically decreased. The steel with 2.5 wt% Cr achieved a synergistic improvement in strength and ductility, exhibiting the best tensile performance.

Correlated high throughput nanoindentation mapping and microstructural characterization of wire and arc additively manufactured 2205 duplex stainless steel

Duplex stainless steels (DSS) in wire and arc additive manufacturing (WAAM) have attracted significant research attention due to their mechanical properties and corrosion resistance. This study uses conventional and nanomechanical testing methods to compare the mechanical and microstructural behaviors at macroscopic and microscopic length scales. Macro hardness (HV10) testing yielded 259 and 249 in low and high heat input (HI) samples, respectively, while ferrite content averaged 52.7 and 48.5%. However, these results fail to provide conclusive insight into the potential influence of microstructural variations at the macroscopic level, likely due to the composite response of the material. To overcome this limitation, the mechanical response of the DSS samples is assessed at the grain level via high throughput nanoindentation mapping with image processing to track the location of each indent. This approach enabled differentiating the indents landing on ferrite and austenite phases as well as those landing on the interfaces. The results showed that the austenite phase had higher hardness (4.30 and 4.35 GPa) than the ferrite phase (3.89 GPa and 4.03 GPa) for high and low HI samples, respectively. The observed differences in hardness between the phases can be attributed to higher nitrogen content in the austenitic phase.

Post-Processing Effect on the Corrosion Resistance of Super Duplex Stainless Steel Produced by Laser Powder Bed Fusion.

This study examines the microstructural characteristics and corrosion resistance of super duplex stainless steel (SDSS) produced through laser powder bed fusion (LPBF). The analysis shows that the as-printed samples mainly exhibit a ferritic microstructure, which is due to the fast-cooling rates of the LPBF technique. X-ray and microstructure analyses reveal the presence of minor austenite phases in the ferritic matrix. The process of solution annealing led to a more balanced microstructure. Analyses of corrosion resistance, such as potentiodynamic polarization tests and EIS, indicate that heat treatment has a significant impact on the corrosion behavior of SDSS. Solution annealing and stress relieving at 400 °C for 1 h can improve corrosion resistance by increasing polarization resistance and favorable EIS parameters. However, stress relieving at 550 °C for 5 h may reduce the material's corrosion resistance due to the formation of chromium nitride. Therefore, stress relieving at 400 °C for 1 h is a practical method to significantly enhance the corrosion resistance of LPBF-printed SDSS. This method offers a balance between microstructural integrity and material performance.

Different phase transformation behaviors of B2-CuZr crystalline phase and their associated mechanical properties by molecular dynamics using different potentials

Metastable CuZr-based alloys have attracted much attention due to their high glass-forming ability and shape memory effect. Many experiments have underscored the pivotal role of the B2-CuZr phase as an effective inclusion to improve both the strength and ductility of metallic glass matrix composites. The improved ductility can be attributed to the dilatation induced by martensitic transformation, while the strengthening effect is owed to the higher strength and hardness of the martensite phase compared to the austenite phase. Well-designed atomistic simulations can predict the mechanical behavior of materials before experiments and offer sufficient information on the atomic scale. The selection of an appropriate interatomic potential is a primary concern in any atomistic simulation and needs to be carefully considered to ensure reliable results. The uniaxial tensile behavior of B2-CuZr crystalline phase is investigated by molecular dynamics simulations using five typical potentials, namely the potentials developed by Mendelev-2007, Mendelev-2009, Mendelev-2016, Mendelev-2019 and Cheng-2009. The phase transformation behavior during tension, Young’s modulus, and yield strength of the simulated samples exhibit variations when different potentials are employed. Notably, only by employing Mendelev-2019 and Cheng-2009 potentials, the strength and Young’s modulus of the transformed phase are higher than the untransformed austenite phase, corresponding well with experimental results. This work emphasizes the impact of different interatomic potentials on phase transformation behaviors within a specific simulation context.

Effect of Secondary Phase on Passivation Layer of Super Duplex Stainless Steel UNS S 32750: Advanced Safety of Li-Ion Battery Case Materials.

Aluminum, traditionally the primary material for battery casings, is increasingly being replaced by UNS S 30400 for enhanced safety. UNS S 30400 offers superior strength and corrosion resistance compared to aluminum; however, it undergoes a phase transformation owing to stress during processing and a lower high-temperature strength. Duplex stainless steel UNS S 32750, consisting of both austenite and ferrite phases, exhibits excellent strength and corrosion resistance. However, it also precipitates secondary phases at high temperatures, which are known to form through the segregation of Cr and Mo. Various studies have investigated the corrosion resistance of UNS S 32750; however, discrepancies exist regarding the formation and thickness of the passivation layer. This study analyzed the oxygen layer on the surface of UNS S 32750 after secondary-phase precipitation. The microstructure, volume fraction, chemical composition, and depth of O after the precipitation of the secondary phases in UNS S 32750 was examined using FE-SEM, EDS, EPMA and XRD, and the surface chemical composition and passivation layer thickness were analyzed using electron probe microanalysis and glow-discharge spectroscopy. This study demonstrated the segregation of alloy elements and a reduction in the passivation-layer thickness after precipitation from 25 μm to 20 μm. The findings of the analysis aid in elucidating the impact of secondary-phase precipitation on the passivation layer.

Enhancement of hardness and tribological properties of AISI 321 by cathodic cage plasma nitriding at various pulsed duty cycle

AISI 321, an austenitic stainless steel, has high corrosion and temperature resistance, making it useful for aerospace and chemical processing. Cathodic cage plasma nitriding (CCPN) is a versatile technique that enhances material surface characteristics. This study aims to investigate the effect of the pulsed duty cycle from 10 % to 60 % on microhardness and tribological properties through CCPN treatment. The X-ray diffraction (XRD) analysis indicates a noticeable shift in the γ-phase towards the lower 2θ angle originating from the expanded austenite phase γN by the lattice expansion at the lowest duty cycle. Nitriding at the lowest duty cycle reduced the average crystallite size from 68 nm (untreated) to 4.5 nm, while increasing the duty cycle led to a decrease in micro-strain. The surface microhardness of the sample that underwent the lowest duty cycle improved to 1288 HV0.01 compared to the untreated sample (210 HV0.01). Moreover, the lowest wear rate is observed for the sample nitrided at a low-duty cycle, which agrees with hardness and strain behavior. The results show that the variable pulsed duty in CCPN can improve the surface hardness and tribological properties of AISI 321, particularly at low-duty cycles.

Zr and V-doped effect on Martensitic transformations and magnetic properties of NiMnIn shape memory alloy

In this study, the elements vanadium (V) and zirconium (Zr) were doped separately as a fourth element to the ternary NiMnIn Heusler magnetic shape memory alloy and the alloys were produced as ingots by arc-melting method. The shape memory properties, microstructures, magnetic and microstructural changes of these new alloys were investigated in detail. The vanadium (V) and zirconium (Zr) elements doped into the alloy caused a decrease in the transformation temperature values of the alloy, but did not cause a significant change in its magnetic properties. The master sample, which has low hysteresis, undergoes a phase transition from the martensite phase to the austenite phase due to an increase in the nucleation of austenite phases at the twinning boundaries. Here, the vanadium and zirconium doping of the alloy caused an increase in the strength and ductility values of the alloy and this caused the formation of the second phase. Due to the increase of this phase formation in the alloy, it can be said that there is a slight decrease in the shape retention of the alloy.

Cyclic stability in NiTi and NiTiCu thin films: Role of precipitates in low- and high-cycle regimes

This study examines the influence of precipitates on the cyclic behavior of submicron-thick NiTi and NiTiCu thin films by conducting tensile and fatigue experiments. X-ray diffraction confirmed that both materials contain austenitic phases at room temperature. NiTi has exhibited superior cyclic stability in the low-cycle regime when compared to NiTiCu, while the cyclic stability of NiTiCu surpasses that of NiTi in the high-cycle fatigue regime. The enhanced cyclic stability of NiTi films in the low-cycle regime is attributed to the homogeneous distribution of semi-coherent Ni4Ti3 precipitates within the grain interior. In contrast, the reduced cyclic stability of NiTiCu films can be directly correlated to the presence of incoherent (Ni,Cu)2Ti precipitates at grain boundaries. Conversely, NiTiCu exhibited enhanced cyclic stability under rising stress ratio, attributable to a notable reduction in hysteresis area.

Thermomechanics of phase transformation induced localization in NiTi tubes. Part I experiments

This two-Part study aims to illustrate and elucidate the effects of thermomechanical interactions in pseudoelastic NiTi structures that arise in the course of the reversible transformations between the austenitic and martensitic phases. Transformation leads to inhomogeneous deformation with the two phases coexisting and to latent heat-induced local heating/cooling. Part I first presents the results of displacement-controlled isothermal tension and compression experiments on NiTi tubes covering the pseudoelastic temperature regime. The experiments are conducted in a constant temperature circulating bath which suppresses thermomechanical interactions so the transformation front velocity is governed strictly by the applied displacement rate. The load/unload hystereses traced, quantify the transformation stresses and strains and will be used to calibrate the constitutive model of Part II. A set of isobaric experiments in which NiTi tubes are taken through a cool/heat cycle under constant stresses of different levels follow. The experiments are conducted in a custom small-scale environmental chamber with a circulating air stream run by a feedback temperature controller, coupled to load-controlled mechanical loading. The evolution of helical bands of the alternate phase is captured using digital image correlation. The results demonstrate that under such loading histories a strong interaction develops between the evolving inhomogeneous deformation, the latent heat and the surrounding environment. The speed of propagation of the helical fronts slows down or accelerates so as to match the rate at which heat is removed/added to the specimen by the airflow. Thus, here the evolution of localized deformation is strongly coupled to the interaction between the latent heats and heat transfer from the environment. The effect of the stress level on these events is examined through eight isobaric experiments, which provide a rich data set for evaluating constitutive models and structural analyses of these experiments such as those in Part II.

Intrinsic mechanism of grain size effect and grain boundary misorientation angle effect on crack propagation in martensitic steels

Grain size and grain boundary misorientation angle as two important parameters in the polycrystalline system have a significant influence on the mechanical properties of advanced martensitic steels. A phase field model is used to investigate the intrinsic mechanism of the effects of grain size and grain boundary misorientation angle on the crack propagation process which considers the coupling damage behavior between grains and grain boundaries in martensitic steels by means of the finite element finite method. The transgranular (through grain boundaries) and intergranular (along grain boundaries) failure can be simulated by this model through constructing a grain boundary energy function and using a specific order parameter to track crack propagation in the grain boundary area. This model can directly display the crack propagation path inside the grain (martensite and austenite phases) and grain boundary area with the stress distribution. The preferential cleavage planes in austenite and martensite phases are 111γ and 100α in grains with different orientations, respectively. Moreover, the simulation results show that the crack mainly bifurcates near the trifurcated grain boundary. The crack tip tends to induce transgranular fracture in low-angle grain boundaries and intergranular fracture in high-angle grain boundaries when it reaches the grain boundary area. Consequently, this model will be a powerful tool to investigate the influence of grain size and grain boundary misorientation angle on the mechanical properties of polycrystalline martensitic steels.

An In-Depth Investigation of the Effects of Tungsten Inert Gas Welding Process Parameters on Hardness and Corrosion Resistance of 2205 DSS Weldments: New Design of Experiment Parametric Studies and Optimization

The aim of this work is to examine and analyze, using response surface methodology, how the TIG welding process parameters of welding current (WC), welding speed (WS), and N2 with argon as shielding gas affect the hardness and corrosion resistance of 2205 DSS weldments. Due to the equal amounts of ferrite and austenite phases and alloying elements, duplex stainless steel DSS offers good mechanical properties and corrosion resistance. The mechanical characteristics and resistance to corrosion of the weld zone, as well as the heat-affected zone of the DSS, are, however, disturbed as a result of the welding process since it changes the distribution of these two phases and also the alloy is thermally disturbed. Therefore, in this work, an in-depth investigation of the effects of the above-mentioned parameters on the DSS quality has been performed. Results showed that increasing welding current while decreasing welding speed, which corresponds to the highest heat input, led to lower critical pitting potential and weld zone hardness but higher heat-affected zone hardness. The same results were obtained for decreasing welding current while increasing welding speed, which correspond to the lowest heat input. However, the addition of a small percent (%) of N2 to argon shielding gas resulted in increasing the critical pitting potential and decreasing the hardness in welds and heat-affected zones. Numerically, the RSM planned experimental results showed that an optimum welding current of 175A, welding speed of 170 mm/min, and 10% N2 with argon as shielding gas maximized the critical pitting potential up to 318 mV and optimized the hardness of the weld and heat-affected zone to about base metal hardness of 288 and 286 HV, respectively.

Influence of Nb on the hydrogen-assisted cracking behaviour in multiphase stainless steel: Phase fractions, stacking fault energies, and nanosized NbC precipitation

Nb alloying has been reported to deteriorate the hydrogen embrittlement resistance of multiphase steels, which contrasts to the case of single-phase metallic materials. This study clarifies the impact of Nb alloying on the hydrogen embrittlement behaviour of a novel multiphase stainless steel. The findings revealed a 17 % reduction in the hydrogen embrittlement sensitivity upon Nb incorporation. It was found that higher austenite fractions in the Nb-bearing steels impeded hydrogen ingress. The austenite phase in such Nb-bearing steels possesses a higher stacking fault energy, thereby delaying martensitic transformation and crack initiation. The precipitation of nanometre-sized NbC in the matrix obstructed hydrogen diffusion, which indirectly inhibited crack initiation and propagation.

Investigation of the magnetic and structural properties in the non‐stoichiometric Heusler alloy Ni50Mn25+xSn25‐x; x = 13, 14

AbstractMemory shape magnetic alloys, especially Heusler alloys, are important materials in replacing conventional cooling with magnetic systems. In the present study off stoichiometric Heusler alloys with nominal composition Χ50Υ25+xΖ25‐x (X = Ni; Y = Mn; Z = Sn; x = 13, 14) were prepared by arc melting followed by thermal treatment. Structural properties were analyzed with X‐ray diffraction at room temperature (RT) and at elevated temperatures, above the martensite—austenite transition area, to determine the relevant crystallographic parameters and observe the transition. Martensite stabilization at RT appears to be a challenge, coexistence of martensite—austenite phases were observed and calculated for both 38–12 and 39–11 (16% and 12% austenite, respectively). Magnetization measurements versus temperature and field were recorded in the areas of interest where 1st order transitions were expected (355 K for x = 13 and 408 K for x = 14), and the magnetic entropy's changes (ΔSm) were determined [0.4 (J/kgK) for x = 13 and 0.3 J/(kgK) for x = 14; Hmax = 1 T]. The complex character of the magnetic properties and their dependence on Mn‐Sn ratio and on the distance between Mn atoms is discussed. The structure and the lattice parameters were determined using an anisotropic strain broadening model; stress and strain were detected in the structure due to crystal phase coexistence.

Effect of annealing process on microstructure, residual stress, and fatigue behaviour of AISI 2205 duplex stainless steel

ABSTRACT This study investigated the microstructure, residual stress, and fatigue behavior of duplex stainless steels that have been annealed with different annealing processes. A cold-rolled AISI 2205 Duplex Stainless Steel (DSS) sample was annealed in three ways for this purpose. In modes 1 and 2, samples were annealed at 500°C for 6 and 8 hours, respectively. The sample was annealed for 6 hours at 950°C for mode 3.The Optical Microscopy (OM) technique was used to determine the crystallographic texture and microstructural characteristics. An X-ray Diffraction (XRD) analysis was used to determine the residual stresses in austenite and ferrite phases on pre- and post-heat-treated samples. The Scanning Electron Microscope (SEM) technique was utilized to assess fatigue cracks. The results showed that the texture microstructure of the specimens using mode 1 and 2 were remained; the grain had a smaller fragmentation. The elongation grains of the specimen after mode 3 were smaller, relatively spheroidal, and more homogeneous. There was a reduction in residual stress after mode 3 annealing process. The fatigue strength of the specimens that under applied this process was 11.1% higher than that of specimens in the as-received state.

VHCF behavior and defect tolerance of modified bainitic 100Cr6 with a high retained austenite content

Since the fatigue life of high strength steels, e.g., roller bearing steel 100Cr6, depends on microstructural defects, an improvement of their defect tolerance can enhance the performance of components. To improve the defect tolerance, the retained austenite content can be increased. Thus, in this work the high cycle and very high cycle fatigue behavior of two modified laboratory variants of 100Cr6, which have an increased content (1.5 wt%) of Al and Si, respectively, were analyzed since these alloying elements lead to higher fractions of austenite. Moreover, an industrially available variant with 0.6 wt% Si was analyzed as a reference. All steels were investigated in the bainitic condition and had austenite contents of about 20 vol-%. However, the distribution and chemical composition of the austenite differed. The presented results demonstrate that the effect of retained austenite on defect tolerance is caused by i) the increased ductility and ii) the local deformation-induced phase transformation. To strengthen the effects of the austenitic phase, a finer distribution as well as a lower austenite stability are useful. Consequently, to evaluate the effect of retained austenite on the fatigue behavior the content, the distribution and the chemical composition of the austenitic phase must be considered.