Transformation-induced Plasticity Steel Research Articles

Multimodal assessment of mechanically induced transformation and damage in TRIP steels using X-ray nanotomography and pencil-beam diffraction tomography

A comprehensive multimodal measurement environment integrating X-ray computed tomography (CT) and special X-ray diffraction (XRD) using a collimated beam has been developed and was applied for the first time to investigate martensitic transformation and damage behaviour in transformation-induced plasticity (TRIP) steels. The transformation and damage behaviour of numerous γ grains were analysed individually to understand the behaviours and to identify the factors controlling these processes.A transition was observed from a stress-assisted transformation in the early loading phase, controlled by mechanical driving forces, to a strain-induced transformation controlled by dislocation density. A characteristic reversal in the transformation rate was observed whereby γ grains with a crystallographic orientation that initially undergo rapid transformation due to high mechanical driving force underwent a sudden deceleration of transformation. Conversely, γ grains with an orientation that initially underwent gradual transformation rapidly completed transformation. The preceding hard phase inhibited the transformation of neighbouring γ grains through the triple effects of reducing stress triaxiality, relaxing stresses and constraining plastic deformation.The growth of pre-existing voids was modest, and material damage was dominated by voids that were newly formed during deformation from α′ grains. The initiation and progress of damage were influenced by the combined effects of the γ grain morphology and their interaction with surrounding α′ grains. The complexity of the local morphology governed the martensitic transformation and damage behaviour, and geometrical parameters were identified as highly sensitive indicators for describing this phenomenon. Finally, industrial measures to design damage-resistant TRIP steels based on these research findings are discussed.

Deformation-induced martensitic transformation kinetics in TRIP-assisted steels and high-entropy alloys

In this study, the sigmoidal kinetics of deformation-induced martensitic transformation (DIMT) in the transformation-induced plasticity (TRIP)-assisted alloys, including metastable high-entropy alloys (HEAs), austenitic stainless steels, and advanced high-strength steels (AHSSs) such as low-alloyed TRIP steels, medium-Mn steels, and quenching and partitioning steels, were studied using various models. To tune mechanical stability, the influences of stacking fault energy (SFE), chemical composition, austenite grain size, deformation temperature, strain rate, and stress state were considered. The novel Fe47Co30Cr10Ni5V8-xSix (x = 3, 4, and 6 at.%) alloys were the main investigated HEAs in the present work, in which silicon addition effectively led to lowering SFE and faster DIMT kinetics by promoting the formation of body-centered cubic α΄-martensite. The Olson-Cohen, Guimaraes (based on Johnson-Mehl-Avrami-Kolmogorov analysis), Shin, and Ahmedabadi models were analyzed and critically discussed. Moreover, a Hill-based sigmoid model was formulated as fα' / fsat = 1 – p / (p + εq) with the parameters p and q characterizing the stability of austenite, and fsat representing the saturated volume fraction of α΄-martensite. It was demonstrated that χ = p × q can be considered the sole austenite stability parameter. It was concluded that the proposed Hill-based model, demonstrating the highest accuracy among the studied models, can effectively simulate the kinetics of α΄-martensite formation.

The effect of double intercritical annealing on micromechanical behavior of a medium Mn transformation induced plasticity steel investigated by in situ high energy X-ray diffraction

The micromechanical behavior of a medium Mn transformation induced plasticity (TRIP) steel processed by intercritical annealing (IA) and double intercritical annealing (DIA) has been investigated by using in situ high energy X-ray diffraction during uniaxial tensile deformation. The results show that the ultimate tensile strength (UTS) and total elongation are 777.1 MPa and 39.6 % for IA and 953.6 MPa and 59.1 % for DIA, and that the product of UTS and total elongation is 30.8 GPa·% for IA and 56.4 GPa·% for DIA, respectively. Discontinuous phase transformation from austenite to martensite leads to Portevin-Le Châtelier (PLC) band propagation for both IA and DIA. The pronounced TRIP effect takes place in DIA step by step during deformation and contributes to higher strain hardening, which results in almost doubling the product of UTS and total elongation. The stepwise phase transformation highly corresponds to the lattice strain evolution of ferrite and austenite, indicating that load partitioning occurs during the deformation. The change of grain structure generated by different heat-treatments correlate with the load partitioning, thereby inducing large discrepancy in micromechanical behavior. The present investigation provides a deeper fundamental understanding of the effect of double inter-critical annealing on micromechanical behavior and effective engineering strategies for improving the product of UTS and total elongation for the medium Mn TRIP steels.

Development of TRIP phenomenon using power refining of retained austenite by niobium

AbstractThe illustrations above demonstrate that as the percentage of niobium increases, the grain size decreases, leading to an increase in the stability of transformation induced plasticity steel. This, in turn, results in improved mechanical strength. The optimal volume fraction of retained austenite for a significant transformation induced plasticity effect to occur is reported to be in the range of 10 vol. %–20 vol. %. The volume fraction of retained austenite directly determines the carbon content and grain size of the retained austenite, its two main stabilization factors. The stability of the retained austenite dictates when the strain‐induced martensite transformation (SIMT) occurs during straining of transformation induced plasticity high strength steel. Unstable retained austenite transforms almost immediately upon deformation, increasing work hardening rate and formability during the stamping process. At the appropriate stability of the retained austenite, the Strain‐Induced Martensite Transformation begins only at strain levels beyond those produced during stamping and forming, and the retained austenite is still present in the final part; it can transform into martensite in the event of a crash, providing greater crash energy absorption. The tensile strength level of micro‐alloyed transformation induced plasticity steels may exceed 1 GPa. Grain refinement mechanism is considered as the most applied mechanism used in increasing of steel strength without deterioration of ductility and toughness. Recently, it was proposed that the grain refinement could act positively on promoting the transformation induced plasticity effect of advanced high strength steel through improving the stability of retained austenite. In this research, quenching and partitioning heat treatment technique was applied to four grades of low carbon steel with different percentage of niobium. Then, the effect of niobium on grain refinement has been detected by microstructure observations. Mechanical properties and strain hardening properties of investigated steel have been determined. In addition, by using x‐ray diffraction, as well as a new electric resistance‐based sensor, it was possible to characterize the stability of the retained austenite through the plastic deformation of steel. Grain refinement by using of niobium has a great impact on promoting the stability of retained austenite against the plastic stress at the plastic deformation zone.

A new perspective on Lüders band formation in medium-Mn steels based on Lüders-strain-rate and dislocation evolution

Propagating Lüders bands with pronounced strain localization are often observed in medium-Mn transformation-induced plasticity (TRIP) steels, which adversely limits the application of such alloys in engineering structures. The present work aims to understand this phenomenon from a dislocation-based perspective, with a focus on the specific role of dislocation multiplication. The material studied is a typical dual-phase medium-Mn steel with a ferrite matrix and a second phase of retained austenite. A warm-rolling process was used as a pre-straining approach to adjust the microstructure of the material, such that materials with two different microstructural states can be obtained: as-received material and pre-strained material. They show distinct differences in initial dislocation density. Stress-controlled rather than conventional strain-controlled tensile testing was adopted, which allowed the Lüders band to develop in an unconstrained manner. In this way, the intrinsic Lüders-strain-rate can be obtained. The results show that the Lüders band-related mechanical parameters, including Lüders strain, Lüders-strain-rate and yield stress drop, all depend on the initial microstructure state and the dislocation evolution process during Lüders band formation. In particular, Lüders-strain-rate is a critical microstructure-sensitive factor. It is closely related to the dislocation multiplication in the formation of Lüders band, which can be characterized and quantified using scanning transmission electron microscope and synchrotron X-ray diffraction. Finally, a quantitative relationship between Lüders-strain-rate and dislocation multiplication rate is established, which allows providing a better understanding of the exceptional Lüders band phenomenon in medium-Mn steels.

Synergistic effect of fatigue and hydrogen-induced damage under asymmetric loading of TRIP steel

This study investigates the synergistic assessment of cyclic plasticity damage and hydrogen-induced cracking of transformation-induced plasticity (TRIP) steel subjected to asymmetric strain-controlled fatigue loading. The presence of hydrogen in TRIP steel results in cyclic softening throughout its life, suppressing the hardening behavior induced by martensitic transformation. The mild hardening observed in hydrogen-free specimens is attributed to the predominance of hardening induced by martensitic transformation over cyclic softening. The increase in mean strain leads to higher mean stress relaxation, suppressing the compressive stress developed during the deformation-induced martensitic transformation phenomenon. Geometrically necessary dislocation (GND) density map and fractographic images substantiate the hydrogen-enhanced decohesion failure mechanism, decreasing fatigue resistance for hydrogen-induced TRIP steel. The failure mechanisms of the fatigue damage due to the ingress of hydrogen atoms in the material are proposed for further insights.

Microstructure Evolution and Mechanical Properties of Laser Welded Dissimilar/Similar Joints Between Medium‐Mn Transformation‐Induced Plasticity Steel and DP980/22MnB5 Steels

In this article, the microstructure and mechanical properties of the Fe–0.31C–6.86Mn–3.53Al steel and DP980 and 22MnB5 steel dissimilar/similar joints by fiber laser welding are studied. Based on experimental results, fusion zone (FZ) of transformation‐induced plasticity (TRIP)‐welded joint mainly consists of 77 vol% martensite and 23 vol% ferrite. The FZ of TRIP‐DP980 and TRIP‐22MnB5 welds mainly consist of martensite. The hardness profile of weld cross‐section exhibits significant asymmetry, and softening zones hardness of TRIP‐DP980 and TRIP‐22MnB5 accounts for 82% and 64% of DP980 and 22MnB5 base materials (BM), respectively. However, no softening zone is observed on TRIP steel side, primarily due to the lack of martensite in TRIP BM. The tensile results showed that welding efficiency of TRIP‐TRIP reached 99%, but elongation of welded specimen is only 50% of TRIP BM due to the high hardness and brittleness of FZ region. TRIP‐DP980 and TRIP‐22MnB5 joints did not fail in softening zone, because the constraint stress is generated on both sides of softening zone to inhibit the plastic deformation in softening zone during tensile process. The tensile stress is distributed on the surface of tensile specimens and concentrated in BM on the side with low yield strength (YS), resulting in the final fracture.

Discovery of microsegregation-aided transformation and twinning-induced plasticity in low Mn steel through directed energy deposition of functionally graded materials

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316 L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1 wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused phase stability and stacking fault energy heterogeneity, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960 MPa, yield strength of 415 MPa, total elongation of 26 %, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Triplex steel powder design to avoid hot cracking in laser-powder bed fusion using computational thermodynamics

High manganese steels offer exceptional combinations of high strength and ductility, resulting in weight reduction when utilized in structural applications. Nevertheless, the conventional manufacturing routes of these steels is hindered by many production problems. Additive Manufacturing (AM) has emerged as a reliable solution to fabricate thin or complex shape compounds using these steel grades. Indeed, several studies have demonstrated the success to fabricate high manganese twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) steels by laser-powder bed fusion (L-PBF). However, a recent study on a high manganese triplex steel composition has revealed the occurrence of both hot cracking and micro segregation during rapid solidification in L-PBF, highlighting potential processability issues of this family of high alloy steels. In this study, the hot cracking susceptibility of different triplex steels is evaluated, focusing on the impact of the composition on the solidification paths and final microstructures. A Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) approach is employed to predict alloy-dependent hot cracking susceptibility so as to establish guidelines for preventing hot cracking and provide insights into alloy design for AM. Microstructural observations are used to determine the accuracy of CALPHAD predictions in terms of elemental segregation, phase formation and hot cracking susceptibility. To this end, scanning transmission electron microscopy is used to evaluate elemental segregation at grain boundaries, while the local distribution of phases and their relative amount is measured by electron backscattered and X-ray diffraction, respectively. Hot cracking susceptibility is experimentally evaluated by measuring the length of cracks (when present) in the cross section of printed specimens.

Optimizing strength & ductility combination of medium-carbon TRIP steel via multi-stage work hardening

The effect of the retained austenite (RA) stability on mechanical properties of a medium-carbon transformation-induced-plasticity (TRIP) steel at different deformation temperatures was studied. Two steels were obtained by adjusting the annealing temperature from 780 °C to 820 °C and controlling the isothermal bainitic process, and the carbon content of RA in the steels was 1.2% and 1.4%, respectively. At 25 °C, ultimate tensile strength, yield strength and total elongation of the steel after intercritical annealing at 780 °C were 1390 MPa, 870 MPa and 40%, respectively. When the deformation temperature increased to 200 °C, yield strength nearly kept constant (860 MPa) while total elongation significantly increased to 73%. The stability of RA had a significant influence on the deformation mechanisms of the steel, hence leading to the distinct variation of the mechanical properties. The RA stability increased with increasing deformation temperature to 200 °C, reducing the rate of transformation from RA to martensite and making the TRIP effect more persistently. Meanwhile, due to low solubility of Cu atom and the large number of dislocations acting as fast diffusion channels for Cu and C atoms in transformed martensite, the joint action of the increasing temperature and the continuous TRIP effect promoted the formation of numerous nanosized precipitates including η carbides and Cu particles in the newly formed martensite. As a result, the work hardening rate was significantly increased, improving the ductility without sacrificing the strength. This study provides a strategy to control the deformation mechanisms of medium-carbon TRIP steel, which can tailor the stability of RA according to the requirement of application, so as to obtain a good combination of strength and ductility.

Micromechanical simulation of interfacial fracture behavior using cohesive zone modeling for TRIP steel composite with ceramic particles

ABSTRACT In this work, ceramic particle and metal matrix interfacial delamination in transformation-induced plasticity steel composite reinforced with magnesium partially stabilized zirconia particles is investigated using a parametric modeling approach. The global behavior of the composite is modeled using elastic and Johnson-Cook plasticity models for the ceramic particles and the austenite matrix. Interfacial degradation is implemented through a cohesive zone model with a traction-separation law. Both perfect and damaged models are considered in the global stress-strain curve analysis. In the damaged model, the plastic region is characterized by softening and hardening stages, corresponding to unstable and stable crack propagation, respectively. To comprehensively identify the interfacial evolution, parameters such as normal contact strength, normal separation and stiffness degradation are evaluated along the particle/matrix interface. From a statistical perspective, the mechanical behavior of the system is analyzed through the kernel distribution plots for both the particles and the matrix. As the strain level increases, right- and left-skewed distributions are observed in the particles and matrix, respectively, particularly under high-strain conditions. Consequently, in the plastic hardening region, the median value exceeds the mean value, indicating that relying solely on the average stress value results in an underestimation during significant delamination.

Investigation of the post-expansion collapse strength of solid expandable tubulars made of different steels

At present, there are different ideas about on the main factor influencing post-expansion collapse strength of solid expandable tubulars (SETs). In this study, three SETs with the same dimensions, which were made of 20 (Chinese grade), a twinning-induced plasticity (TWIP) and a transformation-induced plasticity (TRIP) steels, were expanded so the inner diameter increase by about 14%. The test results showed that the SETs made from high-strength steels (a TWIP and a TRIP) had poor resistance to collapsing after expansion. Through a combination of finite element method (FEM) simulations and experimental tests, the post-expansion collapse strength of three SETs were comparatively analyzed to find out the main influencing factor. After isolating the influences of geometric factors on the collapse strength, the influences of the Bauschinger effect and the residual stress on the collapse strength were studied. There was no correlation between the obvious degree of the Bauschinger effect of the three steels and the post-expansion collapse strength of the three SETs, which suggested that the Bauschinger effect was not the main factor influencing the post-expansion collapse strength. This is obviously different from the conclusion for SETs made from only one steel. The post-expansion collapse strength calculated by FEM simulations differed greatly from that of the actual measurement if the residual stress after expansion was ignored, though both were very close when the residual stress after expansion was considered. This clearly indicates that the residual stress after expansion has a significant effect on the post-expansion collapse strength and it is the main factor influencing the post-expansion collapse strength.

Stabilizing austenite in flash-annealed lean steel via laminate chemical heterogeneity

Retained austenite (RA) is especially valued in advanced high strength steels for its transformation-induced plasticity (TRIP) effect during deformation. However, the stabilization of RA in lean steels (C < 0.2, Mn < 2 wt%) is challenging due to the lack of austenite stabilization elements. In this study, a flash austenitization heat treatment approach was used to shorten the low-carbon TRIP steel processing time. This achieved a dual-phase microstructure composed of mass retained austenite and fine-grained ferrite in place of traditional bainite. Blocky, lamellar, and granular RAs were observed in the microstructure. Mn heterogeneity in the initial pearlitic cementite and ferrite was found to prevail in the retained austenite. Following the designed annealing route, phase-field simulations revealed that the ferrite transition kinetics accelerated in the Mn-depleted region which induced efficient carbon enrichment in austenite. The pronounced chemical heterogeneity of C and Mn co-resulted in the nonuniformity of the phase transition driving force from austenite to bainite and martensite, thus aiding the stabilization of austenite.

Liquid Metal Embrittlement Cracking in Uncoated Transformation-Induced Plasticity Steel during Consecutive Resistance Spot Welding

In the automotive production line, a single pair of electrodes is employed to produce hundreds of consecutive welds before undergoing dressing or replacement. In consecutive resistance spot welding (RSW) involving Zn-coated steels, the electrodes undergo metallurgical degradation, characterized by Cu-Zn alloying, which impacts the susceptibility to liquid metal embrittlement (LME) cracking. In the present investigation, the possibility of LME crack formation in uncoated TRIP steel joints during consecutive RSW (involving 400 welds in galvannealed and uncoated TRIP steels) was investigated. The results have shown that different Cu-Zn phases were formed on the electrode surface because of its contamination with Zn from the galvannealed coating. Therefore, during the welding of the uncoated TRIP steel, the heat generated at the electrode/sheet interface would result in the melting of the Cu-Zn phases, thereby exposing the uncoated steel surface to molten Zn and Cu, leading to LME cracking. The cracks exhibited a maximum length of approximately 30 µm at Location A (weld center) and 50 µm at Location B (shoulder of the weld). The occurrence and characteristics of the cracks differed depending on the location as the number of welds increased due to the variation in Zn content. Type A cracks did not form when the number of welds was less than 280. Several cracks with a total length of approximately 30 μm were suddenly formed between 280 and 400 welds. On the other hand, type B cracks began to appear after 40 welds. However, the number and size of these exhibited inconsistency as the number of welds increased. Overall, the results have shown that small LME cracks can form even in uncoated steels during consecutive welding of Zn-coated and uncoated steel joints.

Unstable stress-triaxiality development and contrasting weakening in two types of high-strength transformation-induced plasticity(TRIP) steels: Insights from a new compact tensile testing method

The impact of high stress triaxiality on work hardening in transformation-induced plasticity (TRIP) steel has been widely acknowledged, particularly through measurements of the austenite fraction. Understanding this TRIP behavior is crucial for predicting material fracture in press-forming processes. However, the actual flow stresses under high-stress-triaxiality conditions remain largely undetermined. To address this gap, we developed a new tensile testing method using tiny notched round bars to investigate stress-triaxiality-induced work hardening in TRIP steels. The specimens were analyzed using two-dimensional micrometry to allow finite element analyses to identify the flow stress. Additionally, we conducted in situ tensile tests in which their crystal lattice stresses were monitored using synchrotron X-ray diffraction (XRD) to realize mechanism analyses of the unexpected work-hardening behavior identified by the developed tensile testing method. Our combined approach revealed a mutual, unstable increase in the flow stress and stress triaxiality in the TRIP-aided bainitic ferrite steel, which reduced the hardening exponent coefficients and thus induced a higher stress triaxiality. In contrast, the TRIP-aided martensitic steel exhibited a weakening behavior, characterized by a significant decrease in the hardening exponent coefficients in the case of the sharpest notch. XRD analyses showed that microstructural heterogeneity led to an extraordinarily high hydrostatic stress in the austenite phase, accounting for these contrasting behaviors. This finding challenges the established consensus on TRIP steels and suggests the need for a revised framework for their application in press-forming, taking into account stress-triaxiality conditions.

A Combined Numerical–Analytical Study for Notched Fatigue Crack Initiation Assessment in TRIP Steel: A Local Strain and a Fracture Mechanics Approach

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

Tailoring microstructure evolution and austenite stability of TRIP steels by Rare-Earth micro-alloying

The microstructure evolution and austenite stability of Rare Earth (RE)-alloyed Transformation Induced Plasticity (TRIP) steels subjected to intercritical annealing and isothermal bainitic transformation have been investigated for superior strength-ductility performance. Structure morphology, austenitizing temperature and phase proportion during thermomechanical process were carefully compared, confirming that minor RE additions indeed refined the microstructure, and affected the thermodynamics of austenite and bainite transformation. Austenitizing ranges of TRIP steels were lifted with increasing RE content then stabilized when exceeding 360 ppm; the intercritical austenite reduced but dissolved more CMn atoms, hence the bainite transformation in RE-alloyed TRIP steels was significantly inhibited, which elevated the retained austenite quantity and thermal stability simultaneously. Therein, more austenite nucleated from the structure fragments with RE-refined lamellar morphologies and strictly grew into acicular counterparts or aggregated into clusters. The aggregated austenite, composed of several subgrains with [011¯] coaxial relations or Σ3 60°〈111〉 misorientation but coherent interfaces, were engulfed or being swallowed up by coalescing ferrites in sluggish growth, guaranteeing the higher mechanical stability than intergranular austenite. In this study, the product of strength and elongation of TRIP steels after optimized RE-alloying was increased by 38% as compared to that without RE additions. The micro-alloying effect of RE elements in TRIP steels highlights the innovations of new-generation automotive steels.

Influence of Laser Power on Microstructure and Mechanical Properties of Laser Welded Medium Manganese Transformation-Induced Plasticity Steel

Influence of Laser Power on Microstructure and Mechanical Properties of Laser Welded Medium Manganese Transformation-Induced Plasticity Steel

Quantitatively evaluating respective contribution of austenite and deformation-induced martensite to flow stress, plastic strain, and strain hardening rate in tensile deformed TRIP steel

Transformation-induced plasticity (TRIP)-assisted steels exhibit an excellent combination of strength and ductility due to enhanced strain hardening rate associated with deformation-induced martensitic transformation (DIMT). Quantitative evaluation on the role of DIMT in strain hardening behavior of TRIP-assisted steels and alloys can provide guidance for designing advanced materials with strength and ductility synergy, which is, however, difficult since the phase composition keeps changing and both stress and plastic strain are dynamically partitioned among constituent phases during deformation. In the present study, tensile deformation with in situ neutron diffraction measurement was performed on an Fe-24Ni-0.3C (wt.%) TRIP-assisted austenitic steel. The analysis method based on stress partitioning and phase fractions measured by neutron diffraction was proposed, by which the tensile flow stress and the strain hardening rate of the specimen were resolved into factors associated with each phase, i.e., the austenite matrix, deformation-induced martensite, and the transformation rate of DIMT after differentiation, and then the role of each factor in the global strain hardening behavior was discussed. In addition, the plastic strain partitioning between austenite and martensite was indirectly estimated using the dislocation density measured by diffraction profile analysis, which constructed the full picture of stress and strain partitioning between austenite and martensite in the material. The results suggested that both the transformation rate and the phase stress borne by the deformation-induced martensite played important roles in the global tensile properties of the material. The proposed decomposition analysis method could be widely applied to investigating mechanical behavior of multi-phase alloys exhibiting the TRIP phenomenon.

Influence of BaO and MgO on viscosity, crystalline phase, and structure of CaO–Al2O3–10%SiO2–20%CaF2-based system

In the current study, the low SiO2 mold flux for the continuous casting of the transformation-induced plasticity steel was studied to avoid the interface reaction between the dissolved aluminum in the molten steel and silica in the mold flux. The viscometer, Raman spectra, and X-ray disproportion were employed to study the viscosity, melt structure, and crystalline phase of the mold flux, respectively. The viscosity of the mold flux decreased with the replacement of MgO by BaO due to the balance of the melt structure and the precipitation phase. The aluminosilicate and silicate structures were depolymerized to simpler units with the replacement of MgO by BaO in the mold flux since a preference that Ba2+ tended to act as a charge compensator. The viscosity of the mold flux roughly increased with the addition of the total BaO and MgO content due to the formation of the MgAl2O4 phase in the pure liquid phase. The substantial precipitation of the MgAl2O4 phase in the liquid melt caused a sharp increase in the viscosity of the mold flux, which should be avoided by lowering the MgO content to less than 3 wt%.