Austenite Martensite Research Articles

Effects of deep cryogenic treatment on the microstructure and mechanical properties of an ultrahigh-strength TRIP-aided bainitic steel

Deep cryogenic treatment (DCT) provides an effective and convenient pathway to improve the comprehensive mechanical properties of multiphase steels. However, the transformation characteristics of retained austenite (RA) during the DCT and its effect on the precipitation behavior and bainitic transformation at the subsequent tempering stage are still unclear. In the present work, DCT is applied to a 1500 MPa grade ultrahigh-strength bainitic steel subjected to quenching and austempering treatment (QAT). The same region before and after DCT is selected and characterized by electron backscatter diffraction (EBSD), and the observations show that blocky RA located at the boundaries of primary martensite and prior austenite undergoes martensitic transformation. Dilatometry and transmission electron microscopy (TEM) characterization methods were used to elucidate the effects of tempering temperature and DCT on the competitive relationship between bainitic transformation and nanoscale carbide precipitation during tempering. Compared with QAT steel, the yield strength of QAT-C-T steels is significantly enhanced by 199–292 MPa, depending on the tempering temperature, due to the precipitation of nanoscale carbides and the elimination of unstable RA. EBSD characterization of RA at different tensile strains reveals that large blocky RA at prior austenite grain boundaries and fine RA between the bainite laths transform to martensite during uniform deformation and the necking stage, respectively. The evolution of RA during DCT and tensile strain demonstrates the correlation of its thermal and mechanical stabilities. • Effects of DCT and tempering on structure-properties of bainitic steel are studied. • DCT eliminates 5.5% blocky RA distributed at PAGB as observing same region via EBSD. • DCT affects both carbide precipitation and bainitic transformation during tempering. • DCT increases yield strength by eliminating unstable RA and promoting precipitation. • Thermal and mechanical stabilities of RA are interdependent in bainitic steel.

Excellent strength-toughness synergy in metastable austenitic stainless steel due to gradient structure formation

The effect of cold swaging and subsequent annealing on the structure and mechanical properties of a 321-type metastable austenitic stainless steel was studied. Cold swaging to a strain of 90% produced a gradient structure with a gradual decrease in the α-phase content from the edge to the center of the bar. Globular and lamellar regions of both strain-induced martensite and retained austenite were found in the center, meanwhile, a completely globular austenite–martensite dual-phase structure was formed at the edge. After annealing at 500 °C, (~30° below the reverse martensite-to-austenite phase transformation temperature) microstructure refinement due to simultaneous development of recrystallization and reverse martensite-to-austenite transformation was observed. Both ultimate tensile strength and yield strength increased noticeably after the annealing. Besides, notch toughness of the annealed specimen doubled in comparison to that of the cold-swaged condition and almost reached the level of a coarse-grained condition.

Long- and short-range order in the Ni52Co2Fe20Ga26 ferromagnetic Heusler alloy

The crystalline structure and Fe local environment in a Co-doped Ni–Fe–Ga Heusler alloy, prepared by the melt-spinning technique, were investigated by X-ray diffraction (XRD) and EXAFS at room and low temperatures. The characteristic temperatures of the austenite–martensite phase transitions were determined by differential scanning calorimetry via cooling and heating cycles of the alloy ribbons. As shown by room-temperature XRD, the austenitic phase of the alloy has the chemically ordered L21 Heusler structure. This was confirmed by EXAFS, although this technique was not able to conclusively distinguish between the L21 and B2 structures of the austenite for the analyzed alloy. The low-temperature martensitic phase and its structural evolution towards austenite with increasing temperature were studied by high-energy X-ray diffraction, which evinced the martensite modulation. However, the Fe environment could be fitted by EXAFS with the tetragonal L10 structure of the non-modulated martensite. This proves that the martensite modulation has structural effects on a long-range scale, without significant changes in the short-range order around the atoms. The changes in the local structure around iron on martensitic transformation were correlated with changes in the electronic structure, described by XANES spectroscopy at the Fe K edge.

A multiscale magneto-thermo-mechanically coupled model for ultra-low-field induced magneto-elastocaloric effect in magnetostrictive-shape memory alloy composite system

Recent experimental results show that apparent magneto-elastocaloric effect can be induced in the magnetostrictive-shape memory alloy composite system under an ultra-low magnetic field. In this paper, a multiscale theoretical model is constructed to predict the magneto-thermo-mechanically coupled response of such a composite system by considering the multi-field interactions among the heterogeneous constituent elements in the grain, polycrystalline aggregate and macroscopic scales. In the grain scale, for the magnetostrictive alloy (MEA), the fluctuations of stress and magnetic fields caused by the interactions among the domains are addressed by the self-consistent homogenization scheme. Adopting a probabilistic domain switching criterion based energetic analysis, a constitutive model of MEA is established. For the shape memory alloy (SMA), a crystal plasticity based thermo-mechanically coupled constitutive model is constructed in the framework of irreversible thermodynamics. The interactions among austenite phase and martensite variants are considered by the Mori-Tanaka's homogenization scheme. Thermodynamic driving force for martensite transformation and the internal heat production originated from transformation latent heat and inelastic deformation dissipation are derived from the dissipative inequality and the conservation of energy, respectively. In the polycrystalline aggregate scale, to estimate the interactions among the grains and predict the overall responses of the polycrystalline aggregates of MEA and SMA, a unified incremental magneto-thermo-mechanically coupled self-consistent homogenization scheme is developed. In the macroscopic scale, the magneto-thermo-mechanical interaction equations among the MEA rod, SMA cuboid and Al alloy frame in the composite system are derived by considering the conditions of deformation compatibility, force balance and thermodynamic equilibrium. The capability of the proposed multiscale model to describe the magneto-elastocaloric effect of MEA-SMA composite system is validated by comparing the predictions with the existing experimental data. Moreover, the influences of geometric dimension, pre-load, frame's stiffness and crystallographic orientation on the magneto-elastocaloric effect of the composite system are discussed. The proposed model provides a theoretical guidance for the optimization design of solid-state refrigeration devices in both the microscopic and macroscopic scales.

Controllable Martensite Transformation and Strain-Controlled Fatigue Behavior of a Gradient Nanostructured Austenite Stainless Steel.

Gradient nanostructured (GNS) surface layer with a controllable martensite fraction has been synthesized on 316L austenitic stainless steel by means of surface mechanical rolling treatment (SMRT) with temperature being controlled. The mean grain size is in the nanometer scale in the near-surface layer and increases gradually with depth. In addition, the volume fraction of martensite decreases from ~85% to 0 in the near-surface layer while the SMRT temperature increases from room temperature to 175 °C. Fatigue experiments showed that the strain-controlled fatigue properties of the GNS samples are significantly enhanced at total strain amplitudes ≥0.5%, especially in those with a dual-phase surface layer of austenite and pre-formed martensite. Analyses on fatigue mechanisms illustrated that the GNS surface layer enhances the strength-ductility synergy and suppresses the formation of surface fatigue defects during fatigue. In addition, the dual-phase structure promotes the formation of martensite and stacking faults, further enhancing fatigue properties at high strain amplitudes.

Stress-Strain State and Structural-Phase Transformations of Metal Articles Obtained by Laser Shaping

The article analyses the effect of temperature distribution at different points of the irradiation zone on the nature of leakage of structural-phase transformations. The results of the X-structural analysis are given, which shows the presence of martensite and residual austenite in the treatment zone. The peculiarities of structural-phase transformations during laser heating are studied and their effect on strain value during laser shaping is determined. The results of experimental studies are given, according to which, if the mechanisms for forming the temperature gradient and polymorphic transformations (for 65G steel) operate in parallel, the amount of deformation is one third of the sample for which only the temperature gradient mechanism works (12Х18N10Т steel).

Effect of Welding Heat Input on the Microstructure and Impact Toughness of HAZ in 420 MPa-Grade Offshore Engineering Steel

In the present work, the effect of the welding heat input on the microstructure, martensite–austenite (M–A) constituents, and impact toughness of the coarse-grained heat-affected zone (CGHAZ) in offshore engineering steel with Ca deoxidation is studied. With the heat input increased from 50 to 100 kJ/cm, the HAZ toughness decreased rapidly, while the measured microhardness decreases steadily. The grain sizes are increased from 52 to 132 μm, and the width of bainite lath increased from 0.4 to 2 μm. The area fraction of lath bainite (LB) decreased, while the area fraction of granular bainite (GB) increased. The average width of M–A constituents grows from 0.3 to 0.6 μm, and the average length grows from from 0.5 to 0.9 μm. Its area fraction is increased from 5.3 to 8.6% and then decreased to 6.1%, and its number density decreased from 0.7 to 0.2 μm−2. The morphologies of M–A constituents change from dot-like to slender and blocky, which are deleterious to impact toughness. The fracture mechanism changes from ductile to quasicleavage and cleavage as the heat input is increased. As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm.

3D spatial reconstruction of macroscopic austenite–martensite transition zones in NiTi wires induced by tension and twisting using diffraction/scattering computed tomography

Macroscopic austenite–martensite transition zones in thin superelastic NiTi wires subjected to stretching, twisting and their combination were reconstructed via diffraction/scattering computed tomography. The method allows to visualize spatial distributions of both phases from a series of X-ray diffraction scans. A cone-shaped, radially symmetric transformation front was observed in stretched wire both with and without applied pre-twist. Twisting of a pre-stretched wire (with a cone-shaped transformation band front already developed) led to the formation of a multi-prong localization morphology recognizable on the surface. Transition zones in these cases are highly heterogeneous complex 3D objects exhibiting distinct phase gradients; the longitudinal size of the zones is comparable with the wire diameter. In contrast, localized martensitic strips parallel with wire axis and with a wedge-like cross-section appeared on the surface of the wire which was twisted only. We discuss the observed localization patterns with respect to current computational simulations and micromechanical models.

A Mechanism for Carbon Depletion at Bondline of High-Frequency Electric-Resistance-Welded X70 Pipeline Steel

The bondline of electric-resistance-welded (ERW) linepipe steel, often etched white (i.e., ferrite) in optical microscopy, is generally believed to be carbon depleted. The mechanism for the carbon depletion, however, is not fully understood by researchers. To this end, atom probe tomography (APT) was used to measure elemental segregation of the as-welded and post-weld heat-treated bondline regions of X70 linepipe welds. The thin vertical features at the bondline in the as-welded condition were identified as carbon-rich martensite-austenite (M-A) constituents, and the majority ferrite phase in the bondline was identified as carbon-depleted ferrite. Following the post-weld normalization, all alloying elements, except Nb and Mo, are homogenized across the bondline and heat-affected zone. The carbon depletion in the ERW bondline was accurately measured. A new mechanism for carbon depletion has been proposed using Scheil calculations of elemental partitioning during weld formation. Segregation of elements in the heat-affected zone was shown to follow the negligible partitioning local equilibrium (NPLE) kinetics for bainite transformation.

Microstructure and Magnetic Properties of Selected Laser Melted Ni-Mn-Ga and Ni-Mn-Ga-Fe Powders Derived from as Melt-Spun Ribbons Precursors

Selective Laser Melting was successfully used as a fabrication method to produce Ni-Mn-Ga and Ni-Mn-Ga-Fe ferromagnetic shape memory alloys. The starting material in a powder form with an average particle size of about 17.6 µm was produced by milling of as melt-spun ribbons. The microstructure, phase composition, and martensitic transformation behavior of both powder precursors and laser melted alloys were investigated by several methods, including high energy X-ray diffraction, electron microscopy, and vibrating sample magnetometry. The as laser melted materials are chemically homogenous and show a typical layered microstructure. Both alloy compositions have a duplex structure consisting either of austenite and 10M martensite (Ni-Mn-Ga) or a mixture of 14M and NM martensitic phases (Ni-Mn-Ga-Fe), contrary to the as milled powder precursors showing fcc structure in both cases. The forward martensitic transformation takes place at 336 and 325 K for Ni-Mn-Ga and Ni-Mn-Ga-Fe, respectively, while the magnetic response is much stronger for Ni-Mn-Ga than for the quaternary alloy. The results show that Selective Laser Melting allows for producing of good quality, homogenous materials. However, their microstructural features and consequently shape memory behavior should be tailored by additional heat treatment.

The dominating role of austenite stability and martensite transformation mechanism on the toughness and ductile-to-brittle-transition temperature of a quenched and partitioned steel

The effect of austenite stability and martensite transformation (MT) mechanism on the tensile and impact properties has been investigated for Fe-0.3C-2.7Mn-1.7Si (wt.%) quenched and partitioned (Q&P) steel. The stability of metastable retained austenite (γM) in the Q&P steel samples, which were subjected to different quenching temperatures (20, 120, and 240 °C), was quantitatively evaluated according to the Msσ temperature. Results suggest that the stability of γM was mainly related to the carbon content, grain size/morphology, and stress shielding effects provided by the surrounding martensite. In addition, in contrast to the consistently positive transformation-induced plasticity effect (TRIP) of γM on the tensile properties, MT had a two-side effect on the dynamic impact properties, depending on the MT mechanism and the related Msσ temperature. At high temperatures, the strain-induced MT in the QP120 and QP240 samples relieved the local stress concentration and impeded the crack initiation and propagation by the TRIP effect, whereas the stress-assisted MT at low temperatures resulted in a low crack initiation, propagation energy, and intergranular fracture mode, thereby resulting in a lower impact toughness of the QP240 sample than that of the almost austenite-free QP20 sample. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the Q&P samples showed an almost linear relationship with their Msσ temperature. Finally, an excellent combination of large product of strength and elongation, low DBTT, and high toughness was obtained in the QP120 sample, which was mainly attributed to the good stability of the film γM during the tensile or impact tests.

Aachen-Heerlen annotated steel microstructure dataset

Studying steel microstructures yields important insights regarding its mechanical characteristics. Within steel, microstructures transform based on a multitude of factors including chemical composition, transformation temperatures, and cooling rates. Martensite-austenite (MA) islands in bainitic steel appear as blocky structures with abstract shapes that are difficult to identify and differentiate from other types of microstructures. In this regard, material science may benefit from machine learning models that are able to automatically and accurately detect these structures. However, the training process of the state-of-the-art machine learning models requires a large amount of high-quality data. In this dataset, we provide 1.705 scanning electron microscopy images along with a set of 8.909 expert-annotated polygons to describe the geometry of the MA islands that appear on the images. We envision that this dataset will be useful for material scientists to explore the relationship between the morphology of bainitic steel and mechanical characteristics. Moreover, computer vision researchers and practitioners may use this data for training state-of-the-art object segmentation models for abstract geometries such as MA islands.

Microstructural refinement mechanism and its effect on toughness in the nugget zone of high-strength pipeline steel by friction stir welding

High-strength pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 400–700 rpm, and the grain refinement mechanism of the nugget zone (NZ) was determined. The thermo-mechanical process during FSW in the NZ was simulated by multi-pass thermal compression, thereby achieving the austenitic non-recrystallization temperature (Tnr). The austenitic non-recrystallization in the NZ at the lowest rotation rate of 400 rpm caused a significant grain refinement. Furthermore, the reduced rotation rate also resulted in the formation of a high ratio of island-like martensite-austenite (M-A) constituent. The toughness of the NZs was enhanced as the rotation rate decreased, which is attributed to the fine effective grains and homogeneously distributed fine M-A constituents dramatically inhibiting crack initiation and propagation.

Metamagnetic transition and magnetocaloric properties of Ni45Mn42In13 Heusler alloy

ABSTRACT We report a study of the metamagnetic transition and magnetocaloric effect in the ternary Ni45Mn42In13 Heusler alloy system. We also investigate the structural and magnetic properties of the alloy. The crystal structure exhibited a mixture phase of martensite and austenite at room temperature which are typical for Heusler alloys. The thermomagnetization curves have martensitic and magnetic transition near room temperature and with a quite large magnetization value under 5 T applied external magnetic field. The obvious reversible metamagnetic behavior was seen between 285 and 295 K temperatures. The isothermal M–H curves also confirmed the metamagnetic transition and provided the magnetocaloric effects by Maxwell relations. This system underwent an austenite–martensite phase transition and the change in magnetic entropy was found to be quite large across this martensitic transition, so this led to an inverse magnetocaloric effect. Significant maximum magnetic entropy was obtained under 5 T applied magnetic field.

Phase transformation and mechanical properties of sintered Fe-Mo-Si-C-(Cu) alloys

Sintered Fe-Mo-Si-C alloys, prepared from pre-alloyed Fe-0.85Mo powder, fixed 4wt. % silicon carbide powder and varied graphite powder contents, showed microstructures similar to those of ductile cast irons, i.e., the microstructural feature consisted of a black particle enveloped with matrix consisting of ferrite halo and pearlite. The varied added-graphite content caused morphological change from black nodular to black vermicular particles. With increasing added-graphite content resulted in the decrease of black nodular/vermicular particle fraction, the increase of pearlite fraction and the slight change of ferrite fraction. The black nodular particles were either graphite or Fe-Mo-Si-C/graphite core-shell particles whereas black vermicular particles were totally composed of carbon. With 2 wt.% copper addition, the morphologies of black nodular and vermicular particles in sintered Fe-Mo-Si-C-Cu alloys were not affected, but one component of the matrix changed from pearlite to duplex structure consisting of bainitic ferrite (BF) and martensite-austenite (M-A) constituent. The change of matrix component from pearlite to duplex BF/M-A structure led to drops of ultimate tensile strengths and elongation values but small effects on yield strengths and hardness values. No benefits of tensile properties were gained from the duplex structure due to coarse BF/M-A size.

Multiple Wavelengths Texture Measurement Using Angle Dispersive Neutron Diffraction at WOMBAT

In contrast to conventional angle dispersive neutron diffractometers with a single-tube detector or a small-size linear position-sensitive detector, the WOMBAT diffractometer of the Australian Nuclear Science and Technology Organisation (ANSTO) is equipped with a large-area curved position-sensitive detector, spanning 120° for the scattering angle 2θ and 15° for the azimuth η, respectively. Here, WOMBAT was employed in establishing a texture measurement environment for complex textured samples, through measuring neutron diffractograms at two selected wavelengths on a typical reference sample of martensite–austenite multilayered steel sheet. All neutron patterns were simultaneously Rietveld analyzed using the software, Materials Analysis Using Diffraction (MAUD). The shorter wavelength (λ1 = 1.54 Å, k1 = 4.08 Å−1) enabled collecting the martensite reflections α-110, α-200, α-211, α-220, α-310, and α-222, as well as the austenite peaks γ-111, γ-200, γ-220, γ-311, γ-222, and γ-331 simultaneously, by pre-setting the detector range to 2Θ = 30~150°. The longer wavelength (λ2 = 2.41 Å, k2 = 2.61 Å−1) enabled separating the overlapping strong martensite α-110 and austenite γ-111 Laue–Bragg interferences more reliably. Moreover, the detector panel division along the vertical direction has a good stereographic coverage in the azimuthal angle η,. Such a combination of multiple-wavelength neutron diffraction combined with simultaneous Rietveld texture analysis was confirmed as being very valuable for realizing high precision measurements for complex textured samples at an orientation distribution graticule of 5°, and in a much shorter beam time than the conventional angle dispersive method.

Microstructure Refinement by Low-Temperature Ausforming in an Fe-Based Metastable High-Entropy Alloy

The effects of ausforming in an Fe30Mn10Cr10Co high-entropy alloy on the microstructure, hardness, and plastic anisotropy were investigated. The alloy showed a dual-phase microstructure consisting of face-centered cubic (FCC) austenite and hexagonal close-packed (HCP) martensite in the as-solution-treated condition, and the finish temperature for the reverse transformation was below 200 °C. Therefore, low-temperature ausforming at 200 °C was achieved, which resulted in microstructure refinement and significantly increased the hardness. Furthermore, plasticity anisotropy, a common problem in HCP structures, was suppressed by the ausforming treatment. This, in turn, reduced the scatter of the hardness.

Phase Transformations, Microstructure and Shape Memory Effect of NiTiAg Alloy with Different Atomic Percentages (at. % Ag) Manufactured by Casting Method

In this paper, shape memory alloys (SMAs) (NiTi-based) have been manufactured by casting with a different atomic percentage of a silver element (0, 1, 2 and 3 at. % Ag) using a Vacuum Arc Remelting (VAR) furnace. The silver element is added to the binary alloys due to its excellent properties such as (anti-corrosion, anti-bacterial and high electrical conductivity), which make these alloys using in wider applications. These alloys with different atomic percentages (Ni55Ti45Ag0, Ni55Ti44Ag1, Ni55Ti43Ag2 and Ni55Ti42Ag3) have been manufactured. The successful manufacturing process has been achieved and proved via examinations and tests. The FESEM microscopic examinations show that the silver element has been distributed uniformly and homogeneously in the NiTi matrix. Moreover, the emergence of austenite phase, martensite phase and little amount impurities. Regarding the XRD examination, showed that there is an increase in the number of peaks of Ag phase with an increase in the atomic percentage of the silver element, as well to emergence of phase (Ti2Ni) upon heating, phase (Ti 002) upon cooling, and phase (Ni4Ti3) is not desired. The starting and finishing of the phase transformations have been determined for all samples by the DSC test. The Shape Memory Effect (SME) for the alloy (Ni50Ti42Ag3) is measured to be about 89.99%.

Low-carbon cast microalloyed steel intercritically heat-treated at different temperatures: microstructure and mechanical properties

In this study, dual-phase (DP, ferrite + martensite) microstructures were obtained by performing intercritical heat treatments (IHT) at 750 and 800 °C followed by quenching. Decreasing the IHT temperature from 800 to 750 °C leads to: (i) a decrease in the volume fraction of austenite (martensite after quenching) from 0.68 to 0.36; (ii) ~ 100 °C decrease in martensite start temperature (Ms), mainly due to the higher carbon content of austenite and its smaller grains at 750 °C; (iii) a reduction in the block size of martensite from 1.9 to 1.2 μm as measured by EBSD. Having a higher carbon content and a finer block size, the localized microhardness of martensite islands increases from 380 HV (800 °C) to 504 HV (750 °C). Moreover, despite the different volume fractions of martensite obtained in DP microstructures, the hardness of the steels remained unchanged by changing the IHT temperature (~ 234 to 238 HV). Applying lower IHT temperature (lower fraction of martensite), the impact energy even decreased from 12 to 9 J due to the brittleness of the martensite phase. The results of the tensile tests indicate that by increasing the IHT temperature, the yield and ultimate tensile strengths of the DP steel increase from 493 to 770 MPa, and from 908 to 1080 MPa, respectively, while the total elongation decreases from 9.8 to 4.5%. In contrast to the normalized sample, formation of martensite in the DP steels could eliminate the yield point phenomenon in the tensile curves, as it generates free dislocations in adjacent ferrite.

Tempering and Austempering of Double Soaked Medium Manganese Steels

The addition of a tempering or austempering step to the double soaking of a 0.14C–7.17Mn (wt pct) steel was investigated in the present contribution. The double soaking heat treatment is a two-step intercritical annealing heat treatment, which generates microstructures of athermal martensite, retained austenite and ferrite when applied to medium manganese steels. Microstructures following double soaking and (aus)tempering contained a combination of retained austenite, athermal or tempered martensite, and blocky or bainitic ferrite. X-ray diffraction, dilatometry and transmission Kikuchi diffraction were utilized to investigate microstructural changes which occurred during tempering or austempering. The resulting mechanical properties were measured using uniaxial tensile testing. The double soaking plus tempering heat treatment was shown to generate an ultimate tensile strength of 1,340 MPa in combination with 28 pct total elongation while the double soaking plus austempering heat treatment resulted in an ultimate tensile strength of 1,675 MPa and total elongation of 22 pct. Overall, both novel heat treatments produced a combination of strength and ductility desired for the third generation of advanced high strength steels.