Austenite Phase Research Articles

Laser Rescanning for Enhancing Mechanical Properties of Laser-Directed Energy-Deposited High-Manganese Steels.

This study investigates the effects of laser deposition and laser rescanning (LR) on the microstructure and mechanical properties of high-manganese steel (HMnS) deposited by laser-directed energy deposition (L-DED) comprising 24 wt.% Mn. Four types of laser deposition and LR strategies were investigated: unidirectional L-DED scanning without laser rescanning, L-DED scanning with 90° alterations in the laser scanning path on each layer without laser rescanning, unidirectional L-DED with laser rescanning in the same direction, and L-DED with laser rescanning with 90° alterations in the laser scanning path. The L-DED-processed HMnS had only a few small pores and exhibited a microstructure without any serious defects such as cracks. Additionally, a strong fibrous texture along the <101>/building direction of the fully austenite phase was found. The mechanical properties (microhardness and tensile strength) of HMnS were improved by the LR with a grain refinement effect and fine solidification cell size due to the significantly faster solidification rate in LR than that in L-DED.

Frictional shear stress-induced incomplete martensitic transformation in mono-crystalline B2-CuZr spherulites

An austenitic B2-CuZr phase, when deformed, can undergo a martensitic transformation (MT) that produces a martensitic B19′-CuZr phase, resulting in remarkable transformation-induced plasticity (TRIP) and shape memory effects in B2-structured alloys. However, the shear mechanism as well as the crystallographic characteristics, which underlie the MT and, in turn, the TRIP and shape memory effects, remain unclear and somewhat controversial. Here we report that a single shear on {110}B2<001>B2, along with {1 1 5.5}B2 being the habit plane, are responsible for the MT in B2-CuZr, where {110}B2 and <001>B2 are the shear plane and the shear direction, respectively. These were revealed by determining unambiguously the orientation relationship between B2 and B19′, co-existing in individual spherulites contained in a B2-CuZr-containing bulk metal glass-matrix composite (BMGC). This exotic microstructure, featuring a core-shell like distribution in each single spherulite in which all the 12 possible martensite (B19′) variants constitute the core and the parent B2 forms the shell, was made from an incomplete MT in originally mono-crystalline B2 spherulites of the as-cast BMGC when subjected to moderately applied frictional shear stresses upon mechanical grinding. Finite element analysis suggests that a core-shell like shear stress distribution inside a B2 spherulite, caused by the confinement of the stiffer glass matrix, may account for it. These results provide direct experimental evidence to elucidate the B2→B19′ MT and its variant selections under various stress states, and pave the way for developing ductile B2-structured alloys, including but not limited to, shape memory intermetallics, high entropy alloys and B2-containing composite materials.

Controlling plasma-based surface modifications of an austenitic alloy by thermochemical and athermal diffusions

Under nitrogen plasma immersion ion implantation, the energy density per pulse EPulse/A was found to be proportional to the amount of the N-expanded austenite phase (γN) in a superaustenitic stainless steel, as previously seen for an austenitic-ferritic alloy. Since γN occurs in a range of stoichiometries, the parameter was also contrasted with the ion fluence Γ, which affects the retained dose. Γ varies inversely with EPulse/A and influences oppositely the properties of the modified surfaces. The temperature was the same among the treatments (320 °C) to rule out thermal diffusion as an extra variable. The increase of EPulse/A (for applied voltages 6.2–10.4 kV) produces thicker layers (1.4–2.2 µm), as an increase in lattice defects and stresses enhances N-diffusion, while high Γ values favor the N-saturation at the top surface for the opposite reason. The lowest Epulse/A (6.2–9.5 kV) result in brittle cases with a possible thin nitrides layer on top, whereas γN prevails in the ductile layer produced by the highest one (10.4 kV). The N-concentration governs the strength against plastic deformation: hardness varies up to 32% from the highest (6.2 kV) to the lowest Γ treatment (10.4 kV). In summary, the correlation of Epulse/A with Γ is indispensable for controlling properties of the modified surfaces for performance purposes.

Climbing the Hydrogen Evolution Volcano with a NiTi Shape Memory Alloy.

Alkaline water electrolysis is a sustainable way to produce green hydrogen using renewable electricity. Even though the rates of the cathodic hydrogen evolution reaction (HER) are 2-3 orders of magnitude less under alkaline conditions than under acidic conditions, the possibility of using non-precious metal catalysts makes alkaline HER appealing. We identify a novel and facile route for substantially improving HER performance via the use of commercially available NiTi shape memory alloys, which upon heating undergo a phase transformation from the monoclinic martensite to the cubic austenite structure. While the room-temperature performance is modest, austenitic NiTi outperforms Pt (which is the state-of-the-art HER electrocatalyst) in terms of current density by ≤50% at 80 °C. Surface ensembles presented by the austenite phase are computed with density functional theory to bind hydrogen more weakly than either metallic Ni or Ti and to have binding energies ideally suited for HER.

An atomistic study of effects of temperature and Ni element on the phase transition and wear behavior of NiTi shape memory alloy

Phase transition and tribological behavior of NiTi alloys have drawn great attention. Molecular dynamics simulation was adopted to investigate the effects of temperature on the mechanical behavior and variant formation of NiTi, as well as its wear behavior. Thermally and stress-induced phase transition behavior of NiTi alloys were studied. It is found that residual austenite phase boundaries during martensitic phase transformation could inhibit martensitic phase transition, changing the phase transition temperatures, and the mechanical properties of NiTi. Wear behavior of single-crystal NiTi with a wide temperature domain and content was studied. The optimal wear resistance of NiTi was at 300 K during cooling. Appropriate improvement in Ni content is beneficial to the formation of martensite variants and enhancement of the wear resistance.

Impact of austenite nanograins induced by sliding friction on interfacial strength during ultrasonic welding

This study investigated the effect of nanograin formation on joint strength through sliding friction during ultrasonic welding (USW). The nanograined austenite phase would induce the surface strengthening effect, potentially impeding the bonding process. A bonding strength exceeded 2000 N within a short welding duration of 2.5 s. Interestingly, the strength was attributed to ultrafine-grained interfacial microstructure resulting from the plastic flow during processing. This unique nanograined microstructure formed from dynamic recrystallisation unexpectedly facilitated the bond formation rather than hindering the bonding process. This study gained insights into the critical interfacial microstructure required for the USW of metals and alloys with high melting temperatures.

Improvement of shape recovery by controlling the microstructure and mechanical properties in a thermomechanically-treated Fe-based alloy

In this research, the effects of thermomechanical treatment including heavy cold rolling and post-deformation annealing on the improvement of shape recovery in an Fe–9Ni–7Mn (at.%) martensitic alloy were investigated. Experimental results showed that the 80 % cold rolling produced a little fraction of strain-induced austenite phase in the martensitic matrix due to the strain concentration in the microstructure and the temperature rise under deformation. During post-deformation annealing at 590 °C in a dual phase (α+γ) region, a greater fraction of the austenite was introduced in the martensitic matrix such that its amount first increased by increasing the annealing time to 1.8 ks and then decreased at a more time of reversion. The microstructural evolution revealed that the austenite reversion under post-deformation annealing initially occurred by a shear nature at the early steps of reversion and then continued by diffusion. Tensile testing represented a good mixture of a relatively high ultimate tensile strength of ∼780 MPa and proper ductility of ∼8.5 % in the post-deformation annealed (PDA) sample for 0.9 ks. In addition, the shape memory properties were examined under the conditions of cold rolling and post-deformation annealing. The results showed a maximum shape recovery rate of ∼23 % for the cold rolled specimen which increased to ∼50 % after post-deformation annealing for 0.9 ks. The presence of a relatively high amount of retained austenite with proper stability and strength in the microstructure of the PDA sample for 0.9 ks was the reason for achieving the highest value of shape recovery.

Microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 overlays on 304L stainless steel weld used in nuclear fuel reprocessing plant

Weld overlays are considered the most cost-effective surface modification technique to mitigate corrosion in the nuclear industry. Considering this, an attempt to overlay the 304L weld metal at some critical weld locations of the nuclear fuel reprocessing plant with Inconel alloy was initiated. The work focuses on the microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 weld overlays on 304L stainless steel (SS) weld metal in a nitric acid and chloride medium. Scanning electron micrographs of the 304L SS weld metal revealed a duplex microstructure containing δ-ferrite as a minor phase in the austenite matrix. The microstructure of the Inconel 82 and Inconel 625 weld overlays was found to be fully austenitic, with Nb-rich laves phases at the inter-dendritic region. The hardness of the Inconel weld overlays was found to be higher than that of the 304L SS weld metal. X-ray diffraction measurements showed austenite and δ-ferrite in the 304L SS weld metal and a single austenite phase in the Inconel weld overlays. Potentiodynamic anodic polarization experiments carried out in 4M and 8M nitric acid indicated superior passive film stability in the Inconel weld overlays when compared to the 304L SS weld metal. Comparatively, Inconel 625 weld overlay was found to be much superior to Inconel 82 weld overlay in nitric acid medium. The 304L SS weld metal exhibited inferior pitting resistance in chloride medium by virtue of the high δ-ferrite present in the weld fusion zone. The pitting resistance of the Inconel 82 weld overlay marginally improved, while a substantial enhancement occurred in the Inconel 625 weld overlay.

Microstructural and Hall–Petch Analysis of Additively Manufactured Ferritic Alloy Using 2507 Duplex Stainless Steel Powder

The powder bed fusion–laser beam (PBF-LB) process, a method of additive manufacturing (AM), was used to print duplex stainless steel (DSS) using commercial-grade 2507 powders. While conventionally processed DSS has a two-phase microstructure consisting of 50% austenite and 50% ferrite, the PBF-LB-printed 2507 alloy was nearly 100% ferrite. Optimal processing conditions that minimized porosity were determined to be 290 W laser power and 1000 mm/s scan speed, and grain size, texture, and phases were characterized as a function of laser power and scan speed. Grain size increased with increasing laser power but decreased with increasing scan speed. A &lt;100&gt; texture diminished with increasing scan speed from 1000 mm/s to 1400 mm/s. No austenite phase was detected. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) characterization revealed nanoscale chromium nitride precipitates in the ferritic matrix (incoherent hexagonal close-packed (HCP) precipitates at grain boundaries and coherent body-centered cubic (BCC) precipitates within the grains) and a high density of tangled dislocations. Tensile tests of as-printed alloys showed a yield strength of 570 MPa, an ultimate tensile strength of 756 MPa, and an elongation to failure of 10%. The tensile properties were analyzed based on the observed microstructure considering grain size, nanoscale precipitates, and the high density of dislocations.

Evaluation of microstructures and mechanical properties of deltha trip steel with different vanadium contents

This research studies the effect of adding 0.12 and 0.25 wt% vanadium on the microstructure and mechanical properties of a delta-TRIP steel. Optical microscopy and scanning electron microscope (SEM) were employed to investigate the microstructure of the steel. In addition, hardness measurements and tensile testing were used for the evaluation of the mechanical properties. The X-ray diffraction (XRD) analysis was also applied to analyze the present phases. The structure of the three studied steels before applying the heat treatment cycle consisted of delta-ferrite, allotriomorphic ferrite, martensite, and pearlite. The heat treatment cycle led to the stability of the austenite phase. The addition of 0.12 wt% V resulted in the enhancement of the mechanical properties so that the best combination of strength (866 MPa) and elongation percentage (41 %) was achieved in this steel. Nonetheless, the addition of 0.25 wt% V deteriorated the mechanical properties since the increase in the vanadium content promoted the formation of martensite, decreased the percentage of the retained austenite, and weakened the mechanical properties. However, adding 0.12 wt% V improved the mechanical properties since it increased the strength through solid-solution strengthening and precipitation hardening; no brittle martensite was formed, and lamellar δ-ferrite was achieved. The steel containing 0.12 wt% V, exhibiting a formability index of 35.7 GPa%, is in the range of the third-generation advanced high-strength steels.

Deformation Behavior and Texture Evolution of Ferrite and Austenite Phases in Lean Duplex Stainless Steel

To avoid cracks during hot rolling, a short process of direct cold rolling following solution treatment of the casting billet for preparing lean duplex stainless steel plates is proposed. The effect of cold rolling reduction on the microstructure, texture, and mechanical properties of UNS S32101 is investigated. The results show that dislocation slip is the main character in ferrite, which leads to dislocation tangles, dislocation cells, high‐density dislocation walls, and deformed microbands. However, twinning and strain‐induced martensite transformation (SIMT) occur in austenite, and the SIMT mechanism of austenite follows the classical model γ → ε → α′ and γ → α′, with the orientation relationship of Kurdjumov–Sachs ( and ) and Nishyama–Wassermann (N–W) ( and ). Meanwhile, at 50%, there is a transition from Cu texture to Brass texture in the austenite phase. At 12.5%, the yield strength is 344 MPa higher than that of the traditional hot rolling process, and the elongation remains about 35%. With the increasing cold rolling reduction, the elongation decreases while the strength rises significantly. Strengthening dislocations, fine‐tuning grains, and SIMT are the primary contributors to the improvement in strength.

Effect of elastic tensile stress on the pitting corrosion mechanism and passive film of 2205 duplex stainless steel

The effects of elastic tensile stress on the pitting corrosion mechanism and passive film of 2205 duplex stainless steel (DSS) in an NaCl solution containing sulfide ions are investigated. The results reveal that as the elastic tensile stress increases, the pitting corrosion sensitivity of 2205 DSS first decreases and then gradually increases, which is attributed to the changes in the chemical state, thickness, and micro-defects of passive film. Furthermore, the initial pits occur at the austenite phase under low elastic stress but at the ferrite phase under higher elastic stress. The corrosion pits grow rapidly along the pit bottom direction and the perpendicular direction of tensile stress.

Annealing-induced abnormal strength-ductility synergy in 316L stainless steel fabricated by hybrid in-situ rolled wire-arc additive manufacturing

The influence of annealing on 316L stainless steel prepared by hybrid in-situ rolled wire-arc additive manufacturing is investigated. The as-deposited steel consists of austenite and dendritic δ-ferrite phases. Upon increasing the annealing temperature, both the grain size and the type, shape, and content of the second phase undergo complex changes. Particularly, an annealing-induced abnormal strength-ductility synergy is observed after annealing at 1050 ℃. This is primarily attributed to the formation of dispersed and equiaxed σ phases, which not only cause significant strengthening but also inhibit the initiation and coalescence of parallel cracks.

Fully analytic approach to study bifurcation phenomenon for a shape memory alloy beam under temperature variation and lateral white-noise excitation

The main aim of this article is to obtain the probability density function of the response of an SMA beam under lateral white-noise excitation and study the bifurcation phenomenon through a fully analytic approach. Firstly, an efficient constitutive stress–strain relation for a shape memory alloy material supporting both martensite and austenite phases is considered. Secondly, the governing equation of motion for a typical SMA simply supported beam is derived using some dimensionless parameters. Thirdly, the corresponding probability density function of the response is computed analytically using the powerful Fokker–Planck–Kolmogorov equation, and finally, the bifurcation phenomenon for parameter variations of the beam is investigated. The results show how geometric (beam aspect ratio), force (the mean value of the white-noise excitation), and environmental (working temperature) factors can affect the nonlinear behavior and response bifurcation of the beam. A numerical validation is also done that guarantees the correctness of the method and results.

Comparison and analysis of ProTaper, ProTaper Next, and ProTaper Gold using finite element analysis

Aim: The aim of this study is to compare three endodontic files: ProTaper Universal (PTU), ProTaper Next (PTN), and ProTaper Gold (PTG) under bending forces using finite element analysis. Methods: This study utilizes finite element analysis conducted with ANSYS software. 3D models of the three endodontic files were prepared using ANSYS. A non-linear material model was applied, idealizing the stress-strain curve for Nickel-Titanium alloy (NiTi), the material of these endodontic files. Static and transient analyses were performed to identify three different material phases: austenitic phase, transition phase, and martensitic phase. Results: Finite element analysis reveals that the PTU file exhibited a maximum von Mises stress magnitude of 1249.7 MPa, PTN showed a maximum von Mises stress magnitude of 798.16 MPa, whereas the PTG file showed a minimum von Mises stress magnitude of 465.68 MPa. PTN showed more bending compared to the other file models for the same load magnitude. PTN remained in the austenitic phase until a tip displacement of 7.3657 mm, while PTU reached a maximum displacement of 4.5633 mm in the austenitic phase. Conclusion: The study concludes that the PTU file is the most rigid among the three file models. PTU generates higher stress magnitudes compared to the other files; thus, it is more likely to fracture under lesser bending forces. The study also concludes that PTG exhibits higher displacement with the lowest von-Mises stresses, making it the most flexible file among the three models.

One-step additive manufacturing of Ni–Mn–Sn alloys with a large elastocaloric effect

The one-step laser powder bed fusion method, without the need for heat treatment, successfully produces a microstructure containing austenite phase in NiMnSn samples. An impressive elastocaloric temperature change of 6.7 K was obtained.

Thermal aging effects on Tensile and Metallurgical characteristics of Stainless steel weld joint

A girth austenitic stainless steel (say, SS) weld joints are invariably used for joining piping components in typical light water based nuclear power reactor. To avoid hot cracking during the welding of SS, some 5-8% delta ferrite is required in the filler material, which remains in the metastable phase in weld pool. Under prolonged exposure at operating temperature (300 ℃), these delta ferrite which remains in weld metal may get transformed to brittle phases that leads to reduction in its ductility and toughness. Hence, for prolonged safe operation of these piping components in power reactors, it is important to quantify the amount of degradation in toughness and ductility because of thermal aging. Therefore, present work aims at quantifying the reduction in toughness in SS weld joint by carrying out accelerated thermal ageing and thereafter carrying out micro-structural examinations, hardness and tensile tests on specimens machined from different regions of weld joint. Austenitic stainless steel (SA312 Type 304LN) pipe weld joints have been prepared using Gas Tungsten Arc Welding (GTAW) for root passes and Shielded Metal Arc Welding (SMAW) for filling passes. Full-scale pipe weld were subjected to thermally ageing for 10,000 and 20,000 hours at temperature 400 ℃. The effects of thermal aging on the structural morphology, hardness at macro and micro-scale and tensile characteristics of different zones of these joints have been quantified. Micro-structural examination reveals vermicular and lathy morphology in the ferrite and austenite phase. It has been found that, thermal aging leads to significant increase in ferrite hardness and reduction in ductility for the weld specimen from SMAW compared to that of the GTAW and no effect in heat affected zone.

МАГНИТНЫЕ И СТРУКТУРНЫЕ СВОЙСТВА СПЛАВОВ ГЕЙСЛЕРА MN-NI-TI, ПОЛНОСТЬЮ СОСТОЯЩИХ ИЗ ПЕРЕХОДНЫХ МЕТАЛЛОВ

The paper presents a theoretical study of the effect of different atomic and magnetic orderings on the structural and magnetic properties of Mn2Ni1+xTi1-x alloys, which are composed entirely of transition metals. Using the density functional theory, we predict the structural ground states and magnetic reference states of compounds with x = 0, 0,25, 0,5, and 0,75 in both cubic austenite and tetragonal martensite phases. Partial substitution of Ti atoms with Ni leads to an increase in the energy barrier between structural phases, to a change from a layered atomic ordering to an alternating staggered order, and to a change from antiferromagnetic to ferromagnetic spin alignment in the cubic phase. All compounds with tetragonally distorted structures reveal the out-ofplane spin configuration and easy axis magnetocrystalline anisotropy except tetragonal L10 phase of Mn2NiTi. For the latter structure, easy-plane magnetic anisotropy is observed. The calculated values of anisotropy are comparable with those of tetragonal L10-FeNi.

Investigation on AISI 253 Stainless Steel by Gas Nitriding Process for Sustainable Applications

Austenitic stainless steel is highly valued in many industries due to its unique properties, which make it appropriate for various sustainable applications like wind turbine blades, solar panel components, biomass converter equipments. AISI 253 stainless steel were chosen for this research and gas nitrided were underwent at a low temperature of about 550°C. The samples were nitride for 8 hours of nitriding, the sample was found to move from austenitic to expanded austenite phase. Extending the nitriding time to 16 hours resulted in the formation of ferrites and an increasing concentration of nitrogen in succeeding layers. The specimens were carried out with nitriding for 24 hours. Cr-N phase were formed was This new phase contributes to increase in surface hardness. Wear loss and volume loss were analyzed from the wear test. In this work , there was considerable improvement in the wear resistance after 24hours of nitriding. The samples nitrides for 24 hrs also had high hardness of 1080Hv and had negligible wear, volumetric wear loss of the specimens. The microstructure and surface morphology analysis of the nitrided samples was carried out using scanning electron microscopy which helped in establishing the microstructural behaviour of the AISI 253 stainless steel.

Impact of process parameters and gas atmospheres on density and microstructure of super duplex stainless steel produced by Laser Powder Bed Fusion

Super duplex stainless steel (2507 SDSS) is an alloy renowned for its excellent corrosion resistance and mechanical properties. This unique combination arises from a duplex microstructure characterized by austenite and ferrite. However, when processed using the laser powder bed fusion (LPBF) process, this steel contains only the ferritic phase due to the rapid cooling rates inherent in LPBF, which suppresses the formation of the austenitic phase. This study explores a parameter process optimization for this steel in two different atmospheres, Ar and N, with the latter being an austenite-stabilizing element intended to enhance the formation of the austenitic phase. A comparison of the optimal process windows between the two atmospheres is conducted, and the amount of austenitic content is evaluated as a function of the process parameters and the shielding gas employed. Findings demonstrate that utilizing a nitrogen atmosphere, in conjunction with process parameters that reduce normalized enthalpy and material evaporation, increases austenite content post-manufacture. Additionally, nitrogen as the processing gas effectively minimizes and stabilizes nitrogen content within the samples.