A novel thermomechanical approach to produce a fine ferrite and low-temperature bainitic composite microstructure

A novel thermomechanical processing was developed in the present study to produce a unique microstructure consisting of fine ferrite grains (i.e. ～4 μm on average) and low-temperature bainite in a relatively low-carbon steel with a modest hardenability. The thermomechanical route consisted of warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to the bainitic transformation regime (i.e. 400–200 °C). The low-temperature bainite consisted of high dislocation density bainitic laths and very fine retained austenite films. This microstructure offered a high work hardening rate leading to a unique combination of ultimate tensile strength and elongation. This was due to the presence of ductile fine ferrite grains and hard low-temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy, electron backscatter diffraction and atom probe tomography techniques.     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

Carbon supersaturation of ferrite in a nanocrystalline bainitic steel

The extremely slow transformation kinetics of a nanocrystalline bainitic steel allows the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography as the bainite transformation progresses at 200 °C. A high level of carbon, well above that expected from para-equilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.     

Calculations of charge transfer in Nb–17Al and V–50Al alloys,using the Auger parameter

Conventional and high energy X-ray photoelectron spectroscopy (XPS) with Al K_α and Cr K_β radiation, respectively, were used to calculate the Auger parameters of the elements in the V–50 at% Al and Nb–17 at% Al alloys. The shifts of the Auger parameters of the elements of interest between unalloyed and alloyed conditions were used to calculate values of charge transfer occurring upon alloying. The results were related to thermodynamic predictions and the microstructures of the two alloys. The ordering tendency in the Nb–17 at% Al alloy, shown by thermodynamic modelling and the microstructural studies, was attributed to substantial electron transfer from Al to Nb. The small charge transfer from Al to V in the V–50 at% Al alloy was attributed to the lack of ordering in this alloy.     

Pearlite to austenite transformation in an Fe–2.6Cr–1C alloy

The mechanism of austenite formation, the kinetics of cementite lamellae dissolution and the crystallography of the austenitization from pearlite have been studied in an Fe–2.6 wt% Cr–0.96 wt% C alloy. Austenite grains nucleate after rather long incubation time both at pearlite colony boundaries and at the ferrite/cementite interfaces within a pearlite. Characteristic morphologies of transformation products were observed at various stages of transformation. Particular attention was paid to the structural evolution close to the α/γ interfaces. No indications of the diffusion ahead of the α/γ interface were found. The kinetics of austenite growth is controlled initially by carbon diffusion but in later stages by chromium diffusion. The results are discussed by assuming local equilibrium at the moving interfaces with the software and database ThermoCalc. The method involves the driving force determination for the diffusion of carbon and substitutional element during austenitization. The orientation relationships between ferrite, martensite and cementite were also determined. The possible austenite orientations were evaluated assuming the α/θ, α/γ and γ/θ orientation relationships so far obtained.     

Multi-phase microstructure design of a low-alloy TRIP-assisted steel through a combined computational and experimental methodology

The multiphase constitution of a transformation-induced plasticity (TRIP)-assisted steel with a nominal composition of Fe–1.5Mn–1.5Si–0.3C (wt.%) was designed, utilizing a combination of computational methods and experimental validation, in order to achieve significant improvements in both strength and ductility. In this study, it was hypothesized that a microstructure with maximized ferrite and retained austenite volume fractions would optimize the strain hardening and ductility of multiphase TRIP-assisted steels. Computational thermodynamics and kinetics calculations were used to develop a predictive methodology to determine the processing parameters in order to reach maximum possible ferrite and retained austenite fractions during conventional two-stage heat treatment, i.e. intercritical annealing followed by bainitic isothermal transformation. Experiments were utilized to validate and refine the design methodology. Equal channel angular pressing was employed at a high temperature (950 °C) on the as-cast ingots as the initial processing step in order to form a homogenized microstructure with uniform grain/phase size. Using the predicted heat treatment parameters, a multiphase microstructure including ferrite, bainite, martensite and retained austenite was successfully obtained. The resulting material demonstrated a significant improvement in the true ultimate tensile strength (～1300 MPa) with good uniform elongation (～23%), as compared to conventional TRIP steels. This provided a mechanical property combination that has not been exhibited before by low-alloy first-generation high-strength steels. The developed computational framework for the selection of heat treatment parameters can also be utilized for other TRIP-assisted steels and help design new microstructures for advanced high-strength steels, minimizing the need for cumbersome experimental optimization.     

Experimental evidence and thermodynamics analysis of high magnetic field effects on the austenite to ferrite transformation temperature in Fe–C–Mn alloys

The non-isothermal decomposition of austenite into ferrite and pearlite in Fe–xC–1.5 wt.% Mn steels with x = 0.1, 0.2 and 0.3 wt.% C is investigated by in situ dilatometry and microstructure characterization in magnetic fields up to 16 T. The global shift towards higher temperatures of the respective austenite, ferrite + austenite and ferrite + pearlite stability regions is experimentally quantified. A systematic increase in the ferrite area fraction and proportional reduction of the Vickers hardness values with the magnetic field intensity are also reported. Moreover, the steels’ magnetizations, measured up to 3.5 T and 1100 K, are used to calculate the magnetic contribution to the free energy of the transformation and to account thermodynamically for the field dependence of the transformation temperature. The impact of magnetic field is found to be greater with increasing carbon content in the steels.     

Thermodynamic analysis of two-stage heat treatment in TRIP steels

In this work, we present a detailed thermodynamic analysis of the two-stage heat treatment (intercritical annealing (IA) and banite isothermal transformation (BIT)) necessary to stabilize retained austenite in transformation-induced plasticity (TRIP) assisted steels. Through a set of experiments on alloys with nominal composition Fe–0.32C–1.42Mn–1.56Si (wt.%), we monitored the evolution of the volume fraction of retained austenite at room temperature as a function of the IA and BIT temperatures. We also investigated the thermodynamic limit for the bainitic transformation during BIT under the displacive (partitionless) transformation assumption. The fraction of retained austenite at the end of the two-stage heat treatment was calculated by taking into account the corresponding start of the martensitic transformation (T_Ms). Comparisons with experiments suggest good qualitative agreement in the fraction of retained austenite when considering the effect of the IA temperature. On the other hand, the analysis of the effect of BIT on the amount of retained austenite showed qualitative disagreement with the observations. To further analyze this discrepancy, we utilize a modified thermodynamic analysis with empirical observations as input, and conclude that the assumption of thermodynamic equilibrium at IA is not valid at lower IA temperatures. Moreover, the unexpected high carbon enrichment in retained austenite indicates the importance of the kinetic effects. We conclude that the thermodynamic limit for the bainitic transformation can be used at least to provide a lower bound to the expected fraction of retained austenite under specific IA + BIT treatment schedules.     

Modeling austenite decomposition into ferrite at different cooling rate in low-carbon steel with cellular automaton method

The austenite decomposition into ferrite during continuous cooling in low-carbon steel has been investigated with a two-dimensional cellular automaton (CA) approach. In this model, the growth of ferrite grain is controlled by both carbon diffusion and γ–α interface dynamics. In order to predict the growth kinetics of ferrite grain, the coupled carbon diffusion behavior in untransformed austenite and γ–α interface dynamics are numerically resolved. The simulation provides an insight into the carbon diffusion process in retained austenite and microstructure evolution during the transformation. The predicted ferrite growth kinetics and average grain size at different cooling rates are compared with experimental results in the literature and the simulated results show that the final grain size and newly formed ferrite fraction vary with cooling rate. The γ–α interface is stable in the studied cooling rate range (up to 58 °C s⁻¹) in this work, so the simulated morphology of ferrite grain is almost equiaxed, which is not influenced by the anisotropy of the hexagonal mesh in this CA model.     

The dynamic transformation of deformed austenite at temperatures above the Ae3

Recent observations regarding the dynamic transformation of deformed austenite at temperatures above the Ae₃ are reviewed. Experimental results obtained on four different steels over the temperature range from 743 to 917 °C and at strains up to ε = 5 are described. It is shown that there is a critical strain for the formation of superequilibrium ferrite and that the volume fraction of transformed ferrite increases with the strain. The structures observed are Widmanstätten in form and appear to have nucleated displacively. The effect of deformation on the Gibbs energy of austenite is estimated by assuming that the austenite continues to work-harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve by making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100 °C above the Ae₃. The C diffusion times required for the dynamic formation of the cementite particles observed are estimated. These range from ～25 to 100 μs and are therefore consistent with the times available during rolling. The Gibbs energy calculations suggest that growth of the Widmanstätten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher carbon levels.     

Atomic scale observations of bainite transformation in a high carbon high silicon steel

A fine-scale bainitic microstructure with high strength and high toughness has been achieved by transforming austenite at 200 °C. X-ray diffraction analysis showed the carbon concentration of these bainitic ferrite plates to be higher than expected from para-equilibrium. Atom-probe tomography revealed that a substantial quantity of carbon was trapped at dislocations in the vicinity of the ferrite–austenite interface. These results suggest that the carbon trapping at dislocations prevents the decarburization of super-saturated bainitic ferrite and therefore alters the carbide precipitation sequence during low-temperature bainite formation.     

Direct measurement of carbon enrichment during austenite to ferrite transformation in hypoeutectoid Fe–2Mn–C alloys

Carbon enrichment in untransformed austenite at the end of Mn partitionless growth of ferrite for Fe–2Mn–(0.05, 0.14)C (mass%) alloys isothermally transformed in the temperature range 873–998 K was measured using field-emission electron probe microanalysis to reveal its dependence on the transformation temperature, nominal carbon content and prior austenite grain size. The PLE/NPLE model gives much better predictions than the PE model for carbon enrichment in untransformed austenite at the end of partitionless growth. Carbon enrichment could be increased by reducing the prior austenite grain size. Furthermore, carbon enrichment shifted from the PLE/NPLE transition line to the T₀ line on lowering the transformation temperature. This shift is probably attributed to the solute drag effect and/or to the finite interface mobility, both of which vary with the transformation temperature.     

Effects of boron on phase transformation of high Nb containing TiAl-based alloy

The effect of 0.7 at% boron on the phase transformation of Ti–46.5Al–8Nb during a range of different continuous cooling rates from the alpha single phase region has been investigated. In addition the microstructure of Ti–46.5Al–8Nb and Ti–46.5Al–8Nb–0.7B during water quenching at a range of different temperatures near the alpha transus temperature (T_α) has been examined. It has been found that the trend of the variation of microstructures with different cooling rates is the same in the two alloys varying from massive gamma (γ_m) to lamellar, but boron addition increases the cooling rate to obtain these microstructures. During water quenching at different temperatures (T₁ ≅ T_α, T₂ < T_α, and T₃ ≪ T_α) boron addition has a very strong effect on the trend of the variation of microstructures.     

Effect of annealing temperature on microstructural modification and tensile properties in 0.35 C–3.5 Mn–5.8 Al lightweight steel

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α + γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795 °C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980 °C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830 °C or 880 °C annealed steels.     

Austenite–ferrite transformation kinetics under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → ferrite (α) massive transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and ferrite in the temperature range of the transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α transformation and a decrease of the overall transformation time. A phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase transformation due to the applied stress.     

An investigation of the Al–Pd–Rh phase diagram between 50 and 100 at% Al

Partial 1100, 1000, 900 and 790 °C isothermal sections of the Al–Pd–Rh phase diagram were studied. The isostructural binary AlPd and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. The Al–Pd and Al–Rh ε-phases form another continuous range. The C–Al₅Rh₂ phase dissolves up to 13 at% Pd, Al₉Rh₂ and Al₇Rh₃ are extended up to 3 at% Pd. Two ternary phases: cubic C₂ (a=1.5483 nm) and hexagonal C₃ (a=1.09159, c=1.3386 nm) were revealed. The former extends along about 65 at% Al from 4 to 27 at% Pd.     

Effect of small aluminum additions on microstructure and mechanical properties of molybdenum di-silicide

The effect of Al alloying on the microstructure and properties of MoSi₂ was investigated. MoSi₂–2.8 and 5.5 at% Al (1.5 and 3 wt %, respectively) alloys showed a two phase microstructure comprising of α-Al₂O₃ and MoSi₂–Al alloy, while the MoSi₂–9 at% (5 wt %) Al alloy possessed a 3 phase microstructure: MoSi₂–Al alloy, Mo(Si,Al)₂ and α-Al₂O₃. Al additions to MoSi₂ reduced the SiO₂ phase, which is detrimental for high temperature strength of MoSi₂, forming Al₂O₃. The surplus Al entered into solid solution with the MoSi₂ and altered the lattice parameters. The Al in solid solution was 0.5 at% in MoSi₂–2.8 at% Al alloy, 2.5 at% in MoSi₂–5.5 at% Al alloy and 3.1 at% in the MoSi₂–9 at% Al alloy. The fracture toughness underwent only a moderate improvement in MoSi₂–2.8 at% and 5.5 at% Al alloys, but exhibited a significant 49% rise in the MoSi₂–9 at% Al alloy. Replacement of SiO₂ by Al₂O₃ in MoSi₂–Al alloys led to a significant improvement in the high temperature yield strength between 1100°C and 1250°C.     

Time-dependent synchrotron X-ray diffraction on the austenite decomposition kinetics in SAE 52100 bearing steel at elevated temperatures under tensile stress

We have studied the decomposition kinetics of the metastable austenite phase present in quenched-and-tempered SAE 52100 steel by in situ high-energy synchrotron X-ray diffraction experiments at elevated temperatures of 200–235 °C under a constant tensile stress. We have observed a continuous decomposition of austenite into ferrite and cementite. The decomposition kinetics is controlled by the long-range diffusion of carbon atoms into the austenite ahead of the moving austenite/ferrite interface. The presence of a tensile stress of 295 MPa favours the carbon diffusion in the remaining austenite, so that the activation energy for the overall process decreases from 138–148 to 82–104 kJ mol^?1. Before the austenite starts to decompose, a significant amount of carbon atoms partition from the surrounding martensite phase into the metastable austenite grains. This carbon partitioning takes place simultaneously with the carbide precipitation due to the over-tempering of the martensite phase. As the austenite decomposition proceeds gradually at a constant temperature and stress, the elastic strain in the remaining austenite grains continuously decreases. Consequently, the remaining austenite grains act as a reinforcement of the ferritic matrix at longer isothermal holding times. The texture evolution in the constituent phases reflects both significant grain rotations and crystal orientation relationships between the parent austenite phase and the newly formed ferritic grains.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.     

DTA/TG study of tungsten oxide and ammonium tungstate reduction by (Mg + C) combined reducers at non-isothermal conditions

In this work the reaction mechanism in the WO₃–Mg/C systems and ammonium paratungstate (APT)–Mg/C systems are studied. As reducer magnesium, carbon or combinations of both are explored. It is shown that in the WO₃–Mg system the reduction undergoes by solid–solid mechanism before melting Mg, where metallic tungsten and MgO are formed. Unlike this system, in the WO₃–C system mainly WO_x (< 880 °C) and WO₂ (> 960 °C) and small amount W is formed. In the WO₃–Mg–C ternary system reduction temperature shifts to higher temperature range and depends on amount of carbon. Similar to WO₃–Mg system, APT–Mg reaction starts and completes in the solid state. Thus, firstly the APT decomposes, then reduction of formed WO₃ takes place at ~ 600 °C yielding W and MgO. Likewise to WO₃–Mg system adding carbon into APT–Mg mixture shifts reduction temperature to even higher temperature zone which can exceed melting point of Mg and further reduction undergoes with molten magnesium. It is shown that the reduction products are MgO and W.