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This paper investigates the influence of niobium on the thermal stability of ultra-fine bainite microstructures and their mechanical properties by means of an isothermal transformation at 340 °C, followed by tempering in the range of 200–700 °C for two medium-carbon, carbide-free bainitic steels with and without 0.018 wt. % Nb. The results reveal that ultra-fine bainite microstructures, especially film-like retained austenite, is refined, and the average carbon content of retained austenite is also improved as a result of the addition of Nb element. Both of the bainitic steels in this paper exhibit good thermal stability at low temperatures (≤400 °C) and precipitation hardening arising from carbide precipitation following tempering at 500 °C. The precipitation strengthening compensates for the decrease in strength induced by the decrement in the dislocation density and coarsening of the bainitic ferrite plates during tempering process. M23C6- and MC-type carbides are precipitated in Nb-free and Nb steels, respectively, when the tempering temperature exceeds 400 °C. The further coarsening of the carbides and bainitic ferrite plates, and the decrement in the dislocation density in the latter, relate to the decrease in the mechanical properties for both of the tested bainitic steels following tempering at 500 and 600 °C. Compared to those in the bainitic steel without Nb, the larger size and superior thermal stability of MC-type carbides results in greater strength and elongation of the bainitic steel containing 0.018 wt. % of the Nb element, but does harm to its impact toughness after tempering at 700 °C.

The effects of isothermal and pre-transformation temperatures on the microstructure and mechanical properties of ultrafine bainitic steels are investigated and theoretically analyzed by using optical microscopy, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analyses in this study. The results show that the QBT (quenching followed by isothermal transformation of bainite) samples exhibit a multiphase microstructure containing lamellar ferrite, lenticular martensite, retained austenite. The isothermal and pre-transformation temperatures have an effect on the morphology, quantity of bainite and the stability of retained austenite. As the pre-transformation temperature is decreased from 245 °C to 210 °C, the content of premartensite increases significantly from 21% to 56%. The premartensite not only refines the bainite structure, but also provides nucleation points for the bainite transformation and accelerates the bainite transformation. Correspondingly, the pre-martensite has a good contribution to the strength of ultrafine bainitic steels, when premartensite transformation temperature is 210 °C, the martensite content is 56%, ultrafine bainitic steels reach the highest tensile strength of 1857 MPa. With the increase of isothermal temperature of bainite, the width of bainite lath increases accordingly, and the retained austenite changes from thin film to blocky, which results in the decrease of strength and improvement of plasticity of ultrafine bainitic steels. However, when the bainitic isothermal temperature is too high, the transformation of bainite gradually stops because of the enrichment of carbon in austenite.

Phase transformations in high-carbon steel has been investigated by dilatometry during continuous cooling and isothermal transformations, following austenizing at temperatures of 900 °C–1200 °C (1173–1473 K). Optical microscopy and high-resolution scanning electron microscopy techniques revealed the presence of martensite, complex bainite, pearlite and metastable retained austenite under a variety of heat treatments. Magnetic measurements proved to be a useful technique to determine the fraction of retained austenite. Quantitative image processing with ImageJ software and magnetic techniques were used to calculate phase fractions. The rate of phase transformations increased by an increase in austenizing temperature for transformations occurring during continuous cooling and isothermal transformations. A mere increase in cooling rate from 3 °C (276 K) per second to 5 °C (278 K) per second changed the relative phase fractions and hardness significantly. The hardness of isothermally cooling specimens was a strong function of temperature and microstructure. Although isothermal holding at 500–525 °C (773–798 K) resulted in a mixture of bainite and pearlite, the hardness decreased significantly by decreasing the isothermal transformation temperature from 525 °C (798 K) to 500 °C (773 K). These outcomes provide valuable guidelines for the development of novel microstructure in high carbon steel in industrial practice.

There are overwhelming experimental observations indicating intermetallic (IM) phase formed in the intermediate temperatures in high entropy alloys. In this study, we proposed a model to show how kinetics could intervene the thermodynamic determination of phase formation. The model offers a good explanation for the prevalent formation of IM formation in the intermediate temperature range in high entropy alloys available in the literature. To further demonstrate the kinetic effect, we selected the equiatomic CrMnFeCoNi alloy (i.e., Cantor Alloy) as a surrogate material and employed differential scanning calorimetry (DSC) at different heating rates to investigate IM formation in this alloy. In this case, we found the presence of four IM compounds, BCC-Cr, L10-NiMn, B2-FeCo, and Cr-rich σ phases, in the temperature ranging from about 200 to 900°C, above which the alloy was a complete solid solution. From the DSC measurements, we were also able to build a quasi-equilibrium TTT diagram for Cantor Alloy. It was the first time that such a TTT diagram was established for a high entropy alloy.

The effects of ausforming temperature on bainite transformation kinetic and plastic deformation mechanism were evaluated by thermal simulation method and warm rolling process. Results showed that entire process of bainite transformation austempered at 300 °C was notably accelerated by ausforming due to the increased nucleation sites, and by diminishing ausforming temperature as well. However, ausforming would increase undercooled austenite stability, leading to the decrease of maximum attainable volume fraction of bainitic ferrite. Compared with traditional isothermal transformation, ausforming process could effectively refine microstructure, as well as improve mechanical properties. And with decreasing ausforming temperature, strength, hardness and ductility were all increased, which attributed to the thinner thickness of bainite lath and smaller dimension and proportion of blocky retained austenite, by contrast, the change trend of impact toughness was virtually opposite resulted from variant selection. When ausforming at 300 °C, the ultra-fine bainitic steel exhibited the best comprehensive mechanical performance, which was almost 1850 MPa and with up to 23% for ultimate tensile strength and total elongation, respectively. Keywords: Ultra-fine bainitic steel, Ausforming, Retained austenite, Plastic deformation, Mechanical properties

The quenching temperatures below Ms are difficult to uniformly control during large-scale processing of quenched and partitioned (Q&P) steels, which results in significant variation in tensile properties for the entire coil. The present study is aimed at obtaining a stable elongation for a wide range of quenching temperatures in hot rolled Q&P steels through microstructural design. Instead of cold rolling and annealing processes, a novel process involving hot rolling and air cooling followed by cooling in furnace was applied to a newly designed 0.25C-3Mn-2Al (wt.%) steel. The results indicated that a stable amount of retained austenite (RA) ∼ 19–23% was obtained during air-cooling to finish temperatures in the range of ∼280–360 °C, resulting in no loss in elongation at low quenching temperatures. The RA that was promoted by the transfer of carbon from ferrite/bainite reached up to 1.346 μm, whereas the RA embedded in martensitic laths was ∼40 nm. The two types of RA provided continuous TRIP effect during tensile deformation and the ultra-fine martensite lath of ∼100 nm ensured tensile strength greater than 1091 MPa. Finally, a stable total elongation of ∼19.2–20.5% in combination with tensile strength of ∼1091–1196 MPa was obtained over a wide quenching temperature range of 80 °C, which broadened the processing window for the production of hot rolled Q&P steels. In addition, the isothermal transformation during the initial partitioning stage was the underlying reason to obtain stable amount of RA. The CCE (Constrained Carbon Equilibrium) model combined with isothermal transformation was used to quantitatively calculate the volume fraction of RA.

Pearlitic steels with three different inter-lamellar spacing ranging from 79 to 120 nm have been developed in bulk by austenitization and subsequent isothermal transformation with varying degree of under-cooling. The colony size was found to decrease with reduction of transformation temperature. However, the pearlitic nodule size had a similar range for all the specimens. Charpy impact energy of the specimens has been determined for all the transformation conditions at different test temperatures. At 25 °C, the variation of impact energy was negligible with the variation of inter-lamellar spacing and the fracture was completely brittle with clear cleavage facets. The facet size had a strong correspondence with the pearlitic nodule size. However, as the test temperature was raised to 125 °C and 250 °C, the impact energy was enhanced and the pearlite with the finest inter-lamellar spacing fractured after absorbing the highest energy. At higher temperatures, a region of ductile fracture was observed in the fracture initiation region which expanded with an increase in test temperature and decreasing lamellar spacing. At lower testing temperatures, the crack seemed to be cutting through the colonies without any noticeable deformation of cementite lamellae. However, the cementite for all the specimens bent significantly before breaking at higher temperatures. The bent in the direction of crack propagation increased with refinement of inter-lamellar spacing. The study shows that although nodule size determines the impact toughness at lower testing temperatures, inter-lamellar spacing influences it at higher temperatures.

This work aims at broadening water quenching window for production of hot rolled quenched and partitioned (Q&P) steels. Comparing with conventional researches, a low density steel 0.25C–3Mn–2Al (wt%) was designed to increase the amount of retained austenite (RA) through quenching and non-isothermal partitioning processes. The results indicated that stable amount of RA greater than 19.3% was obtained in the quenching temperature range of ∼280–360 °C, resulting in excellent plasticity ∼19.2–20.5% in combination with high tensile strength of ∼1091–1196 MPa. The isothermal transformation was responsible for the stable amount of RA and the CCE model combining with isothermal transformation was used to quantitatively calculate RA.

The properties of Cu-0.2 wt%Zr-0.15 wt%Y alloy aged at 400 °C, 450 °C, 500 °C, 550 °C for 0 h, 0.25 h, 0.5 h, 1 h, 2 h, 4 h and 6 h after 0%, 20%, 40%, 60% cold rolling were investigated systematically by the transmission electron microscopy (TEM) and selected area electron diffraction (SAED) in this study. The hardness and electrical conductivity were measured under different conditions. The optimum ageing process for the alloy was determined. After solution treated at 900 °C for 1 h, the studied alloy with 60% cold rolling can obtain the best properties after aging treatment at 450 °C for 1 h. It means the conductivity can reach to 90.6 %IACS and the hardness reach to 145.0 HV. The ageing hardening behavior was also analyzed according to the transmission microstructures. A large amount of Cu10Zr7 precipitates were identified by TEM analysis in the copper matrix. The phase transformation kinetics equations for Cu-0.2 wt%Zr-0.15 wt%Y alloy was established under different aging temperature after 60% cold rolling. The kinetics curves of aging precipitation and isothermal transformation were obtained. In addition, the small addition of Y can promote the precipitation of the second phase.

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9.10Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500°C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates, forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships were observed between ferrite and cementite in the nodular bainite. Microhardness of nodular bainite in the sample transformed at 500°C (214 ± 4 VHN) was higher than that in the sample transformed at 550°C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.

Hysteresis is an important phase transformation property to understand and control in shape-memory materials. While a detailed understanding of hysteresis has been developed in shape-memory alloys (SMAs), it is far less understood in emerging shape-memory ceramics (SMCs). Here, we systematically study thermal hysteresis of the tetragonal-monoclinic martensitic transformation in ZrO2-CeO2 SMCs. We present calorimetry measurements of thermal hysteresis, which reveal a non-monotonic trend with CeO2 content as well as significant rate dependence, and correlate these findings with the transformation crystallography measured by in situ X-ray diffraction. We quantitatively analyze these results in the framework of martensite nucleation theory and conclude that the initial decrease in hysteresis with CeO2 content can be attributed to improving interface compatibility (λ2) for coherent nucleation whereas the sharp increase in hysteresis at higher CeO2 contents (below temperatures of ~773 K) is a result of interface friction consistent with thermally-activated interactions with the Peierls barrier of the crystal. Our analysis gives an activation energy of ~136 kJ/mol, which is consistent with previously reported activation energies of isothermal transformation in these materials. These findings clarify contradicting reports in the ZrO2 literature and confirm that the same controlling factors of hysteresis in SMAs, namely λ2 and thermal friction, are also operative in SMCs.

International audience; This paper presents a coupled approach able to describe γ precipitation evolution and associated yield strength after various heat treatments in Inconel 718 alloy. The precipitation state is modeled via the implementation of classical nucleation and growth theories for plate-shaped particles. The precipitation model is validated through small-angle neutron scattering and transmission electron microscopy experiments. The precipitation size distribution serves as an input parameter to model the yield strength using a micromechanical model based on shear and bypass mechanisms accounting for the particular shapes of the precipitates. Results are in good agreement with measured yield stresses for various precipitation states. A complete simulated TTT diagram of the γ phase with the associated yield strength is proposed. The coupled model is finally applied to a series of non-isothermal treatments representative of welding (or additive manufacturing) from the peak aged state.

A simple mathematical model for establishing isothermal transformation kinetics from continuous cooling transformation data is presented. A new regression function of k, which is a reaction parameter of the Johnson-Mehl-Avrami equation is proposed. The isothermal transformation kinetics of austenite-pearlite transformation in a eutectoid steel is converted from continuous cooling transformation data using the regression function in combination with the inverse additivity rule. The converted isothermal transformation kinetics excellently agrees with experimental results irrespective of temperature, showing a big improvement compared with a previous model.

By means of laser melting and following isothermal transformation (LMFIT), the shear band and α′-martensite in mid-carbon alloy steel layer induced by stress were analyzed. The residual stress, microstructures and nanohardness were analyzed by X-ray stress analyser, X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. The results indicate that the carbide-free baninte layers with high hardness are formed on the surface of mid-carbon and high-carbon alloy steels. By LMFIT, the hardness of mid-carbon alloy steel layer can be increased by the carbon supersaturation in bainite ferrite (BF). Meanwhile, shear band and α′-martensite can be induced by thermal stress in mid-carbon alloy steel layer to further improve the hardness. The gradient change of residual stress in the bainite layer leads to that of the nanohardness in bainite layer.

Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system.

A simple and completely predictive model has been developed to predict whether a multicomponent equiatomic alloy will form a single phase BCC, FCC, HCP or a combination of two or more solid solution phases or intermetallic compounds (IM) or an amorphous phase. This approach is based on the viscosity of alloys as a function of temperature, utilising the viscosities of its constituting elements, and suitably incorporating the crystal structure information. Some other parameters affecting viscosity of an alloy like atomic size of constituting elements, packing density of the unit cell, etc., are suitably incorporated into the model. The temperature-time-transformation (TTT) diagrams were generated with the help of the viscosity data of five widely experimentally examined alloys, CoCrCuFeNi, CoCrFeMnNi, AlCoCrFeNi, AlCuMgMnZn and ZrTiCuNiBe. The chance of formation of preferable lower order alloys has also been considered. In this regard, all the possible binary to quinary alloys that can form from the constituting elements have been studied. The formation of the single phase BCC, FCC, or formation of multi phases, IMs or an amorphous phase in these alloys has been excellently predicted by the model. It has also been revealed that AlCuMgMnZn alloy prefers to form a number of IMs with a rare HCP phase, which matches excellently with the experimental evidence. The most important part of the present work is that it acts as an efficient guide about the processing route that should be used to form an intended phase in a particular alloy via the critical cooling rate Rc obtained through the predicted TTT diagrams. Further, two alloys (AlCoCrFeNi and CoCrFeMnNi) could not be vitrified even via melt spinning route as predicted by the model. Almost all the equiatomic alloys found so far ranging from ternary to octanary according to literature have been studied by the present model. The phase formation in most of the alloys has been predicted correctly by the model.

The thermodynamic properties and crystallization kinetics of the Co90Sc10 amorphous alloy prepared by rapid quench technique were investigated by X-ray diffractometer (XRD), differential scanning calorimeter (DSC), and simultaneous thermal analyzer (STA). In the isochronal heating process, the activation energy of crystallization is 388 kJ/mol, which was calculated by Kissinger method. The Johnson-Mehl-Avrami (JMA) model was used to describe the isothermal transformation kinetics. The variation of the activation energy of the isothermal process indicated that tiny Co particles precipitate out in the supercooled liquid region and become the nuclei of crystallization that reduce the activation energy at the beginning of crystallization process. Comprehensive results show that a-Co90Sc10 is a promising soft magnetic material for various applications.

International audience; The industrial importance of carbonitriding is owed to the exceptional wear and fatigue resistance it imparts to treated steel parts. This resistance is related to microstructural changes occurring during the enrichment treatment and upon cooling. Here, the effects of interstitial contents, in particular nitrogen, and transformation temperature were investigated in 23MnCrMo5 steel. Samples were homogeneously enriched in the austenitic phase and the isothermal transformation of the enriched austenite between 750 °C and 600 °C was studied. CrN was found to precipitate during the enrichment treatment. During subsequent isothermal holding, CrN precipitate as fine platelets in nitrogen containing samples. At equal carbon content, ferrite formed faster and in finer grains in presence of nitrogen. Preexisting CrN facilitate ferrite nucleation resulting in more numerous ferrite grains. The intense nitride precipitation is the main origin for enhanced hardness in nitrogen-enriched alloys. The exact mechanism leading to the observed microstructures could not be determined and remains under investigation. In particular, the high nitrogen supersaturation of ferrite required to produce the observed fraction of CrN has to be explained.

The phase transformation kinetics of Cu-Be-Co-Zr alloy during aging treatment was investigated. Based on the relationship between electrical conductivity and volume fraction of precipitates, the phase transformation ratios of Cu-Be-Co-Zr alloy aged at 480 °C for different holding time (0, 30, 60, 120, 180, 240, 360, 480, and 600 min) were calculated. The kinetics equation of phase transformation and the electrical conductivity equation were derived. The isothermal transformation kinetics S-curves of Cu-Be-Co-Zr alloy was drawn. By the integral equation of solid thermal decomposition reaction mechanism, the phase transformation mechanism of Cu-Be-Co-Zr alloy is controlled by three dimensions of diffusion.

The polymorphism of glycolide has been investigated. Form 1 and Form 2 of glycolide have been identified and characterized by X-ray powder diffraction (XRPD). Differential scanning calorimetry (DSC) and thermogravimetry (TG) analyses were carried out to study the thermodynamic stability relationships and the transition of the two forms of glycolide. Based on the heat of transition rule, Form 1 and Form 2 were determined as an enantiotropic system. The solubility measurements were done at 310.15 K under atmospheric pressure (0.1 MPa) by using the isothermal gravimetrical method for form 1 in this work. According to the solubility data of glycolide Form 1 and Form 2 in ethyl acetate, 1-propanol, 2-propanol, ethanol and 1-butanol, the transition temperature was then estimated by solubility extrapolation method, heat of transition (ΔHtr) method and confirmed by isothermal transformation method at (308.35 ± 0.5) K. Among these three methods, the isothermal transformation method provided the most accurate transition temperature with the help of suspended-transformation experiments monitored by online Raman spectroscopy.

With austenite under 50% compression using a dilatometer, ferrite transformation in a low-carbon Ti-V-bearing steel was sequenced through the isothermal transformation at 650 °C for various holding times (10 s, 20 s, 30 s). Bimodal grain size distribution was observed in the prolonged holding conditions. Corresponding measurements of Vickers and nano-indentation hardness were carried out; the tiny ferrite grains possessed lower hardness (303 HV and 4.8 GPa) as compared to larger ferrite grains (336 HV and 5.6 GPa). Transmission electron microscopy provided strong evidence attributing this discrepancy to interphase precipitation taking place in the larger-grained ferrite, instead of in the tiny-grained ferrite.

The kinetics of sigma phase precipitation at the temperature range of 750–950 °C in a rapidly solidified 15Cr-2Ni-2Al-11Mn resource-saving duplex stainless steel was investigated. After fitting the experimental results with the Avrami equation, the TTT diagram of sigma phase was established. It is found that the precipitation rate of sigma phase maximizes at about 850 °C and that the precipitation hardening effect sharply peaks at about 1.5 vol% sigma phase content, which is obtained by aging at 850 °C for 180 min. With the further increase of sigma phase content from 1.5 vol%, the strength reduces and the ductility increases again.

The influence of tempering at 400 and 500 °C on two bainitic ferrite microstructures obtained via pure isothermal transformation and ausforming followed by the isothermal transformation was studied in a medium C-Si rich bainitic steel. To understand the dilatometric behaviour and mechanisms of carbide-free bainite decomposition, extensive microstructural characterization using scanning electron microscopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tempering at 400 °C was negligible for both microstructures. Tempering at 500 °C led to a remarkable contraction in dilatometric signal in an ausformed sample, while a pure isothermal sample showed slight contraction. The main mechanism governing microstructural changes during tempering at 500 °C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austenite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 °C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 °C led to a thickening of the bainitic ferrite plates for both testing conditions. However, the thickening was restricted in the ausformed bainite, which was related to the pinning effect of very small cementite precipitates, which moderate the tempering effects on hardness reduction.

In this study, we developed two novel multi-step phase transformation processes by adjusting the order of bainitic isothermal transformation and martensitic transformation, (i.e., B-M and M-B-M) to successfully improve the ductility and toughness of a modified 9Cr–2WVTa steel by employing the finer bainitic/martensitic (B/M) multiphase microstructure. In particular, compared with the traditional normalizing-tempering (N-T) treatment, the M-B-M process significantly improved the total elongation/impact toughness of the steel from 25.1%/72.0 J/cm−2 to 32.1%/104.6 J/cm−2 at room temperature, but reduced the ultimate tensile strength/yield strength of the steel from 863 MPa/711 MPa to 706 MPa/567 MPa. The increase in ductility is attributed to the lower dislocation density in the matrix and more subgrains within the bainite laths, and the increase in toughness is due to the presence of the secondary cracks and the lower residual stress. The microstructure evolutions and the relationship between the mechanical properties and microstructure features for the different processes have been comparatively discussed.

The isothermal transformation kinetics, microstructure, and mechanical properties of a 0.23C high silicon steel, which is isothermally treated both below and above the martensite start temperature (Ms) are investigated. Results show that the small amount of athermal martensite (AM) in below Ms treated sample gives the additional nucleation sites for bainitic transformation, which accelerates the kinetics of bainitic transformation. The nucleation rate accelerates by two orders of magnitude at below Ms compared to above Ms. The microstructural analysis identifies the isothermal decomposition product formed in below and above Ms treated sample as bainitic ferrite. Furthermore, as compared to above Ms, below Ms isothermal treatment shows higher stability of austenite, giving rise to much improved ductility. Moreover, the below Ms isothermal treatment refines the bainitic lath more effectively.

Hysteresis is an important phase transformation property to understand and control in shape-memory materials. While a detailed understanding of hysteresis has been developed in shape-memory alloys (SMAs), it is far less understood in emerging shape-memory ceramics (SMCs). Here, we systematically study thermal hysteresis of the tetragonal-monoclinic martensitic transformation in ZrO2-CeO2 SMCs. We present calorimetry measurements of thermal hysteresis, which reveal a non-monotonic trend with CeO2 content as well as significant rate dependence, and correlate these findings with the transformation crystallography measured by in situ X-ray diffraction. We quantitatively analyze these results in the framework of martensite nucleation theory and conclude that the initial decrease in hysteresis with CeO2 content can be attributed to improving interface compatibility (λ2) for coherent nucleation whereas the sharp increase in hysteresis at higher CeO2 contents (below temperatures of ~773 K) is a result of interface friction consistent with thermally-activated interactions with the Peierls barrier of the crystal. Our analysis gives an activation energy of ~136 kJ/mol, which is consistent with previously reported activation energies of isothermal transformation in these materials. These findings clarify contradicting reports in the ZrO2 literature and confirm that the same controlling factors of hysteresis in SMAs, namely λ2 and thermal friction, are also operative in SMCs.

The stress development in the iron oxide scale formed on pure Fe during isothermal oxidation at 700 degrees C followed by isothermal transformation at 500 or 380 degrees C was measured by in situ high-temperature X-ray diffraction with the sin(2)psi method. The eutectoid transformation resulted in compressive stress generation in the Fe3O4 and Fe in the eutectoid structure. This compressive stress was relaxed during the isothermal heat treatment after the eutectoid reaction. The stress generation was ascribed primarily to volume changes associated with the oxidation and/or reduction of the iron oxide at the interfaces, Fe3O4 precipitation, and the eutectoid reaction.

Analytical models of the diffusion controlled pearlite-to-austenite transformation during isothermal and uniform heating processes are presented. The solute elements diffusing through the volume of austenite and the interfaces between austenite and pearlite were both considered synchronously. The austenite formation from pearlite of a Fe-1C-1.44Cr steel was calculated with the present model and the results were compared to the experimental results. Calculation results show that the transformation is Cr diffusion-controlled and C diffusion-controlled below and above the transition temperature of austenite growth (so called PNTT), respectively. The theory about PNTT is quantitatively proved to be right.

Correlative physical simulation, synchrotron x-ray diffraction and laser dilatometry were used to characterize the surface and volumetric austenite reversion kinetics and stability in a Ti-stabilized supermartensitic stainless steel. A fast heating rate of 500 °C s−1 was used to minimize any martensite to austenite reversion related to the heating stage. This allowed the characterization of the austenite reversion kinetics and its corresponding thermal stability on cooling for tempering temperatures between 600 and 700 °C. In all cases, a soaking time of 9000 s and a cooling rate of 5 °C s−1 were used. The isothermal transformation was divided in two regimes: At and above 625 °C, the kinetics of the transformation was faster and the austenite equilibrium volume fraction was reached. Below 625 °C, the transformation was slower and incomplete. The reverted austenite was stable during cooling after tempering at and below 610 °C, partially stable for temperatures between 625 and 650 °C, and unstable for temperatures between 670 and 700 °C. The austenite Ni content should be higher than 8 wt % in order to effectively stabilize austenite at room temperature. Correlated bulk (dilatometry) and surface (diffraction) analyses showed very good agreement during the isothermal stage. However, martensitic transformation at the sample surface was evidenced at higher temperatures related to the bulk due to the free surface effect. A reversion TTT diagram and the austenite stability curve were constructed from the in situ x-ray diffraction data, providing tools for microstructural and performance optimization of this material.

The different β↔α phase transformation behaviors in Ti-55531 alloys with three types of preset microstructure were studied at isothermal conditions in the temperature range of 450–750°C. The dilatometry was employed to record the length change of the specimen. TTT diagrams for alloys with different microstructure were plotted based on the phase fractions of transformed α at different temperatures. With the help of Johnson–Mehl–Avrami theory, the calculated kinetic curves were in good agreement with experimental results. The β→secondary α transformation in the alloy with preset ‘β-quenched’ microstructure took place at higher nose temperature with a shorter incubation time in comparison with others, but exhibited lower steady transformation rate than that with preset ‘β+αequiaxed’ microstructure. With local composition analyses, the high nose temperature was interpreted by the difference in β stability and supersaturation. The alloy with preset ‘β+αequiaxed’ microstructure showed higher transformation rate than that with preset ‘β+αlamellar’ microstructure, which might be associated with the richness of defects. Keywords: Ti-55531, Phase transformation, Microstructure, Isothermal kinetics, TTT diagram, JMA theory

The kinetics of the isothermal product formed below the M S temperature in 0.32C-1.78Mn-0.64Si-1.75Al-1.20Co (wt%) steel has been analyzed. It has been shown that the diffusion controlled growth rate of pearlite using Zener-Hillert model is too slow to explain the observed kinetics at such a low temperature. The isothermal product cannot be identified as the eutectoid phases. However, an existing model of carbide precipitation from supersaturated ferrite correctly predicts the precipitation observed in bainitic ferrite plate. The results indicate that bainitic ferrite is supersaturated with carbon when it first forms and therefore indicated a displacive mechanism of transformation.

Bainite isothermal transformation kinetics for Fe-(1.5% and 3%)Si-0.4%C alloys (mass%) was investigated at 400–500 °C and incomplete transformation phenomenon (ICT) of bainite transformation was observed at 450 °C for the 3Si alloy and at 400 °C for the two alloys. Unlike to the ordinary ICT reported in other alloy systems, cementite precipitation with Si partitioning took place from the beginning of ICT. Carbon enrichment in austenite during ICT was measured by three-dimensional atom probe and was found to be higher than T0 or T0′ prediction while significantly deviates from NPLE limits and PE predictions to lower carbon content. Theories for bainite transformation, such as T0 limit, solute drag and WBs limit, were examined based on the experimentally measured carbon content in austenite during ICT. T0′ limit theory is difficult to rationalize the much larger measured carbon content than T0′ prediction. In addition to solute drag effect and spike development in the NPLE mode, solute drag theory should incorporate dissipations caused by other sources in order to account for the estimated 1250–1700 J/mol deviation from PE predictions. In addition, WBs limit theory gives good descriptions on the carbon enrichment in austenite during ICT stage.

In the present work, the compatibility of additivity rule with the crystallization behavior of an Au 50 Cu 25.5 Ag 7.5 Si 17 glass forming alloy on the conditions of both isothermal and isochronal transformations is investigated. Taking advantages of the ultrafast heating and cooling rates of the Flash DSC technique, we verified the additivity rule in a solidification process for the first time. It is proved to be effective in predicting the beginning and the end of isochronal crystallizations during continuous heating and cooling from the kinetic analysis of corresponding experimental isothermal transformation. The predicted isochronal curves are fairly in accordance with the experimental results, especially for the incubation and crystallization processes during rapid solidification of the glass forming liquid. The compatibility of additivity rule enables a conversion between isothermal and isochronal transformations for the crystallization of supercooled liquid even during solidification from the melt.

Low temperature bainite is a mixture of ferrite and austenite with a high dislocation density and nanoscale precipitates produced by isothermal transformation of the austenite in high-carbon high-silicon steels. The mass balance for carbon is systematically unsuitable when considering only ferrite and austenite forming the structure, but no attempt has been made to evaluate the amount of carbon located at linear defects and precipitates. Additionally, bainitic ferrite has been recently shown to have a tetragonal crystal structure, allowing greater amounts of carbon in solid solution than those expected by the paraequilibrium phase boundaries. In order to quantify the contribution of all the carbon sinks, we have followed the evolution of carbon in ferrite and austenite, along with the precipitation of cementite and η–carbide, during the isothermal bainitic transformation at 220 and 250 °C by means of in-situ synchrotron high energy X-ray diffraction and complementary transmission electron microscopy (TEM) and atom probe tomography (APT) analyses. This is the first time that the mass balance for carbon is successfully achieved by considering all the transformation products together with an estimation of the carbon segregated to linear defects., APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. The synchrotron experiments were conducted in December 2014 under the MA2305 grant at the ESRF in Grenoble which is fully acknowledged. The authors thank the support of LabEx DAMAS (ANR-11-LABX-0008-01) from Lorraine. The authors would also like to show their gratitude to Dr. J.C. Hell for his valuable assistance when conducting the experiments at ESRF. This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of a Coordinate Project (MAT2016-80875-C3-1-R); and the Research Fund for Coal and Steel under the Contract RFSR-CT- 2014-00019.

The effect of nitrogen content on the martensite-austenite (M-A) constituent in low-carbon Mo-V-N steels produced by an isothermal transformation, γ→α+γ′ (metastable austenite), at 550 °C and air-cooling was investigated via optical microscope, transmission electron microscopy, electron probe microanalysis, and DICTRA software. An increased nitrogen content from 0.0030wt% to 0.0165 wt% decreased (increased) the carbon (nitrogen) diffusion during α→γ′; meanwhile, the amount of M-A constituent and the initial martensite transformation temperature (Ms) increased. Furthermore, the inner structure of the M-A constituent varied from twin-type M-A to lath-type M-A.

The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 °C followed by tempering within 240–450 °C. Results show that the sample tempered at 340 °C occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 °C and 360 °C with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 °C. When the temperature is increased to 450 °C, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 °C, followed by decreasing with further increase in the temperature. This behavior is related to the competition between retained austenite further transforming into bainite and decomposing into carbide during tempering.

Present work deals with the study of structural changes of medium carbon micro alloyed steel during two heat treatment cycles as isothermal transformation and thermomechanical processing followed by isothermal processing. In isothermal transformations, the steel immediately after austenitization at 1250°C for 20 minutes was transferred a to salt bath at a preselected temperature of 365, 400, 435, 470 or 500°C and after holding for 35 minutes followed by their quenching in water. In the other heat treatment cycle, after austenitization at 1250°C for 20 minutes the steel was subjected to thermomechanical treatment by hot rolling at a finish temperature of 750°C followed by isothermal treatment under the similar conditions as mentioned above.The third heat treatment, Two Step Cooling (TSC), involved ϕ30 mm forged bar of 38 MnSiVS5 soaked at 1200 °C (30 min) → hot rolled at finish rolling temperature of 800 °C in 6 to 8 passes to ϕ 17 mm → air cooling up to 700 °C followed by water quenching (Two Step Cooling) → annealed at 400-550 °C (in steps of 50 °C) for 90 minutes. The effect of microstructure on the fracture toughness behavior of heat treatment routes has been studied and related to their microstructures. The cost effectiveness of this steel for critical aeronautical field is discussed.

Ultrahigh-strength low alloy 42CrMo4 steels, widely used in the automobile industry, were heat treated by two different cycles, namely: conventional austempering above the martensite start temperature MS and low-temperature isothermal transformation below MS, 604 K (331 °C). Metallurgical and mechanical properties of the heat-treated alloys were investigated carefully by various analysation techniques. The results of tensile testing indicated that the proof strength and ultimate tensile strength could be significantly improved by isothermal transformation below MS at 583 K (310 °C) for 3.6 ks without degrading the ductility. The ultrafine-grained/nanostructured mixed lower bainite/martensite microstructure created by quenching and partitioning martensite, and the unique fracture morphology were found to be responsible for this improvement. To further confirm the experimental results, a micromechanical model was employed to analyse the strengthening mechanisms.

International audience; This paper deals with the results of an investigation of low-temperature martensitic transformation in a metastable PuGa 1 at.% alloy. The kinetics of this process were studied experimentally via in situ XRD characterizations performed during isothermal holds. This revealed a double-C Time–Temperature–Transformation diagram similar to that reported in the literature for enriched alloys. The originality of our results lies in the demonstration of the existence of different martensite morphologies corresponding to the upper and lower parts of the TTT diagram. These microstructural variations seem to be the result of different accommodation mechanisms.The experimental nucleation rate study has suggested an autocatalytic character of the isothermal martensitic nucleation process as well as a large sensitivity to elastic and plastic strain. Indeed, the incubation period and subsequent increase in nucleation rate could be related to the appearance of new embryos rather than the growth of existing plates, while the partial character of the transformation may be attributed to interactions between plates that are being formed in the two-phase delta + alpha′ alloy. These hypotheses have been confirmed by the classical heterogeneous nucleation approach by Pati and Cohen, which takes into account the autocatalytic character of the transformation. This approach has also indirectly revealed the existence of restraining forces that reflect a high sensitivity of the energy barrier for nucleation to the alpha′ phase amount.

Large-strain ausforming and low-temperature bainite transformation were carried out in a medium C - Si rich alloy steel on a thermomechanical simulator. Then, temperature–time–transformation curves of the bainite transformation after ausforming at different temperatures were obtained by dilatometry. The effects of ausforming on transformation kinetics, microstructure, and hardness of samples were studied. Results show that the entire process of bainite transformation can be accelerated by ausforming. With decreasing ausforming temperature, the transformation rates greatly increased at the initial transformation stage but slightly decreased at the final stage. Compared with non-ausformed sample, not only was the maximum growth rate in the ausformed samples larger, but the maximum growth rate also appeared earlier. In addition, the maximum rate of the ausformed bainite transformation was increased and the time of reaching the maximum rate was shortened with decreasing the ausforming temperature. The effect of ausforming on bainite transformation kinetics depended not only on the ausforming temperature and strain but also on the isothermal transformation temperature. The reduction in transformation time was increased under decreased isothermal transformation temperature. Ausforming refined the bainite plates and increased the fraction of retained austenite formed in the subsequent low-temperature isothermal transformation. Consequently, nanostructured bainite with enhanced hardness was prepared in the medium C - Si rich alloy steel.

The B2 → B19′ martensitic transformation in a quenched Ni-rich NiTi alloy was found to proceed under isothermal conditions at temperatures both within and outside of the transformation temperature range as observed during continuous cooling measurement. The variation in the volume fraction of the martensite that appeared during isothermal holding of quenched Ni-rich NiTi alloy at different temperatures T∗ was measured. It was found that the maximum volume fraction of the martensite (45%) appeared during holding at Ms temperature for 30 min. It was found that the kinetics of the isothermal transformation was different for T∗ > Ms and T∗

This work addresses the fundamentals of inter-critical austenite reversion in a 12Cr-6Ni-2Mo supermartensitic stainless steel, combining thermodynamic and experimental assessments. The calculation of the temperature and composition at which ferrite and austenite phases have the same free energy, i.e. T0 and C0, is discussed as a methodology to understand the austenite reversion and stabilization mechanisms. An ultra-fast heating rate of 500 °C.s-1 provided isothermal nucleation and growth from a fully solubilized martensite, allowing direct comparison with the compositional tie-lines and the transformation paths described by the free energy calculations. Isothermal transformation temperatures below and above T0 (625 °C) were used. Below T0, massive reversion was suppressed since it would imply a free energy increase. The opposite occurred above T0, since the critical Ni concentration for austenite reversion (C0-Ni) was lower than the solubilized case. Nevertheless, transmission electron microscopy and Atom Probe Tomography evidenced that, in all cases, lath growth occurred by local equilibrium partitioning of Ni, along with co-segregation of ferrite-stabilizing elements (Cr and Mo) at the advancing interface. The complex interaction between Cr, Ni and Mo on the energy gain upon nucleation of austenite revealed that Cr segregation can be beneficial while the adverse effect of Mo can be quickly outbalanced by Ni. The concentration of Ni along the reverted laths was rather inhomogeneous due to isothermal coalescence and lack of redistribution. However, the most stable laths were obtained for transformations at least 15 °C below T0, with average austenite/martensite Ni partitioning factors higher than 2.0.

Ausforming treatments at different temperatures were applied to a medium C[sbnd]Si rich carbide free bainitic steel, and the role of transformation plasticity on the evolution of the microstructure is discussed. Three ausforming temperatures were selected prior to isothermal bainite transformation at 325 °C. Deformation was applied above the bainite start temperature, B, (600 °C), below B (400 °C), and at the isothermal transformation temperature (325 °C). A combination of high-resolution dilatometry, XRD, SEM, and EBSD was used to investigate phase changes, microstructure evolution, and variant selections for the different ausforming conditions. The results indicated that the transformation rate was enhanced for all deformation conditions compared with the pure isothermal condition. Ausforming above B did not modify the morphology of the carbide free microstructure, while ausforming below B led to an asymmetrical morphology in the specimen. The alignment of the bainite plates can be at a random or a specific angle, depending on the supercooled austenite condition prior to the bainitic transformation. The thickness of the bainitic ferrite plates was refined to about 100 nm for the ausforming cycle at 325 °C resulting in a hardness of values of 540 HV. Transformation plasticity strains increased with increasing bainite transformation and were more intense with increasing microstructure alignment at the two lower ausforming temperatures. Texture studies revealed that both pure isothermal and ausforming at temperatures above B resulted in an almost random texture, while a strong texture was obtained when ausforming below B, a which was attributed to variant selection., This research is part of a Ph.D. scholarship awarded by the Ministry of Higher Education and Scientific Research in Libya in cooperation with the Canadian Office for International Education (CBIE).

This work investigates the correlation between microstructure and mechanical properties of a nano-precipitation strengthened ultra-low carbon (NPS- ULC) Ti–Mo–Nb steel. Two types of matrix microstructures (ferrite and martensite) with nano-precipitates were obtained through hot rolling and isothermal transformation in the case of ferrite, or by quenching and tempering, for martensite. The martensitic microstructure showed increases in both YS and UTS by ~110 and ~100 MPa, respectively, over the ferritic microstructure without sacrificing tensile ductility. All ferritic and martensitic specimens exhibited a two-stage work hardening behavior with different work hardening rates at high strain levels. Quantitative analysis of the strengthening contributions confirms that the increase in yield stress of the NPS-ULC specimens with a martensitic matrix was a result of the fine martensitic laths as well as the higher dislocation density and nano-precipitates.

The Fe-Ni system has seen a resurgence of interest for the development of high magnetocrystalline anisotropy alloys. The kinetics of the chemical order-disorder phase transformation of the ferromagnetic test-bed compound L1 2 FeNi 3 were quantified using differential scanning calorimetry data obtained from annealed meltspun Fe-75 at% Ni ribbons. The isothermal transformation kinetics are found to be well described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation with a determined Avrami exponent of unity, consistent with one-dimensional growth of the chemically-disordered A1 phase from the chemically-ordered L1 2 FeNi 3 parent phase. The determined activation energy for the transformation from the chemically-ordered L1 2 structure to the disordered A1 structure in FeNi 3 , 3.1 ± 0.1 eV, is equal to that reported for the L1 0 -to-A1 order-disorder phase transformation of the FeNi phase that is found in meteoritic specimens. These results confirm that the L1 2 phase of FeNi 3 is a suitable model phase for investigation of formation aspects of the tetragonal L1 0 -type FeNi phase, also known as tetrataenite, which exhibits extremely sluggish formation kinetics but possesses appreciable magnetocrystalline anisotropy and excellent saturation magnetization that is of interest for sustainable permanent magnet applications.

A novel bainitic isothermal transformation plus quenching and partitioning (B-QP) process was developed for a 0.2C-1.5Si-1.8Mn bainitic steel. The microstructure and mechanical properties of the steel treated by different heat treatment process were investigated. The results show that the B3-QP (the subscript ‘3’ indicates the isothermal holding of 3 min for the bainitic transformation) samples exhibit a multiphase microstructure containing carbide-free bainite, martensite, film-like retained austenite. The microstructure is much finer than that of the Q&T samples with full martensite and the B30-QP samples with a bainite microstructure. Although the Q&T sample has the highest tensile strength (1455 MPa), the ductility and the product of strength and elongation (PSE) are lower. In contrast, the B3-QP and B30-QP samples exhibit better ductility and higher PSE values of 26.3 GPa% and 28.2 GPa%, respectively. Furthermore, the B3-QP sample exhibits maximum impact toughness value of 121 Jcm−2, which is about 20% and 55% higher than those of Q&T sample (101 J cm−2) and B30-QP sample (78 J cm−2) respectively.

We use ab initio molecular dynamics (AIMD) simulations to build an amorphous model of GeSb 2 Te 4 and nanosecond-long annealing AIMD simulations to construct its Time-Temperature-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams. The critical cooling rate for amorphous GeSb 2 Te 4 , derived from the “nose” of the TTT diagram, is in the range 470–1100 K/ns. The Kissinger analysis of the non-isothermal crystallization kinetics suggests that it follows a non-Arrhenius crystallization behavior in which the crystallization temperature is upshifted by the heating rates. These results, from unprecedentedly long AIMD simulations, provide a detailed picture of the complex crystallization of GST alloys at the extremely small time and length scales relevant to phase change memory devices.

Grain boundaries and other interfaces can undergo complexion transitions from one thermodynamic state to another, resulting in discontinuous changes in interface properties such as diffusivity, mobility, and cohesive strength. The kinetics of such complexion transitions has been largely overlooked until recently. Just as with bulk phase transformations, complexion transition kinetics can be represented on time-temperature-transformation (TTT) diagrams. An experimental complexion TTT diagram is presented here for polycrystalline Eu-doped spinel annealed at 1400–1800 °C. This material developed a microstructure with a bimodal grain size distribution, indicating that a complexion transition occurs within this temperature range. The time and temperature dependence of this complexion transition was analyzed and used to produce a grain-boundary complexion TTT diagram for this system. Complexion TTT diagrams have the potential to be remarkably useful tools for manipulating the properties of internal interfaces in polycrystalline metals and ceramics. The development of experimental complexion TTT diagrams is likely to have an important impact on the field of grain-boundary engineering, and hence the development of these experimental diagrams should be an intense area of focus in the coming years.

We have constructed a time-temperature-transformation (TTT) diagram for the martensitic transformation in a Ti-51.3Ni (at%) alloy. The transformation proceeds when the specimen is held at fixed temperatures between 110 K and 140 K, but not at other temperatures. The time required for the formation of 0.1% of the martensite phase is plotted as a TTT diagram. It exhibits a C-curve with a nose temperature near 130 K. Application of a thermal activation model to the TTT diagram suggests that the size of a nucleus of the martensite phase is about (2.2 nm)3.

The α-phase transformation kinetics of as-cast U - 8 wt% Mo below the eutectoid temperature have been established by in situ neutron diffraction. α-phase weight fraction data acquired through Rietveld refinement at five different isothermal hold temperatures can be modeled accurately utilizing a simple Johnson-Mehl-Avrami-Kolmogorov impingement-based theory, and the results are validated by a corresponding evolution in the γ-phase lattice parameter during transformation that follows Vegard’s law. Neutron diffraction data is used to produce a detailed Time-Temperature-Transformation diagram that improves upon inconsistencies in the current literature, exhibiting a minimum transformation start time of 40 min at temperatures between 500 °C and 510 °C. The transformation kinetics of U – 8 wt% Mo can vary significantly from as-cast conditions after extensive heat treatments, due to homogenization of the typical dendritic microstructure which possesses non-negligible solute segregation.