Effect of Nb on the transformation kinetics of low carbon(manganese) steel during deformation of undercooled austenite

The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 （80% reduction） in Nbfree and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel, the transformation incubation period of Nb-bearing steel was prolonged and the transformation kinetics curves parallelly moved to higher strain because of the solute Nb drag effect. Studies on kinetics also showed that the deformation-enhanced ferrite transformation （DEFT） of the two steels were composed of three stages, which can be expressed by the J-M-A equations individually. However, the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels, and the same in the third stage for both the steels corresponding to the nucleation Of retained austenite.

Transformation Kinetics of Pearlite to Austenite in Low Alloy Steel Containing Rare Earth

The phase transformation kinetics of pearlite to austenite in low alloy steel containing RE was studied by the methods of DSC. The results show that the apparent transformation activation energy of pearlite to austenite in the low alloy steel is 1141.04 kJ·mol -1, and the transformation activation energy of pearlite to austenite decreases with increasing of the volume fraction of transformation phase. Through which, the relationship curve between the volume fraction of transformation phase and the temperature were drawn.

A MODEL FOR BAINITIC TRANSFORMATION KINETICS WITH PARABOLIC NUCLEATION RATE

Based on the assumptions of parabolic variation of nucleation rate versus time and transformation kinetics depending mainly on nucleation rate, a different model for bainitic transformation kinetics in hypoeutectoid steels was established. And this model was proved to be effective in the description of bainitic transformation by comparison with the result of conventional Avrami equation.

MODELLING FOR PREDICTION OF CONTINUOUS COOLING TRANSFORMATION KINETICS OF HOT-DEFORMED AUSTENITE STEEL

On the thermodynamics basis of regular solution sub-lattice model and soperelement model, kinetics basis of Cahn's transformation kinetics theory, and according to Scheil's additivity rule and eoperimental results obtained by thermal dilation method,a prediction model of transformations from hot-deformed austenite to ferrite, pearlite and bainite in low alloy steels, which could be applied to continuoas cooling process, is developed. The calculated transformed junctions of each phase based on laboratory controlled rolling and controlled cooling conditions in a low alloy steel are in reasonable agreement with the measured ones.

Calculation of AF transformation kinetics in HSLA steel weld

It is of great importance for obtaining the perfect welding properties to control the acicular ferrite (AF) transformation behavior reasonably in steel weld. AF continuous transformation kinetics in the HSLA steel weld was calculated and modeled based on the direct growth on the inclusions inert interface. The simulation results are coincident with the experimental value well.

Introducingωand O'nanodomains in Ti-6Al-4V:The mechanism of acceleratingα→βtransformation kinetics via electropulsing

Conventional kinetics theory for diffusion-controlled phase transformation shows that the reverse transition should lag behind the temperature rise through rapid heating,i.e.,overheating is required.In this work,we found that theβ-transus temperature decreased by∼50℃ during studying theα→βtransformation in Ti-6Al-4V alloy via electropulsing treatment(EPT).The calculation suggests that the acceleration of transformation kinetics cannot be fully explained by Joule heat and athermal effects of the electromigration effect and electron wind theory.The microstructural evolution during EPT was systematically investigated utilizing scanning electron microscope(SEM),electron backscattered diffraction(EBSD),X-ray diffraction(XRD),transmission Kikuchi diffraction(TKD),and transmission electron micro-scope(TEM).Microscopic analysis shows that the nano-sizedωand O'phases formed in theβphase,which causes large numbers of lattice distortion regions.The defects are conducive to accelerating the bulk diffusion of alloying elements inβ.Moreover,the nanodomains limited the growth of martensite,therefore nanocrystalline martensite formed after quenching.These findings develop the understanding of the destructive effect of current on metallic crystal,which will help to guide microstructural regulation in titanium and other alloys.

Impact of anisotropic growth on kinetics of solid-state phase transformation

Based on the statistical analysis of blocking effect arising from anisotropic growth,the anisotropic effect on the kinetics of solid-state transformation was investigated.The result shows that the blocking effect leads to the retardation of transformation and then a regular behavior of varying Avrami exponent.Following previous analytical model,the formulations of Avrami exponent and effective activation energy accounting for blocking effect were obtained.The anisotropic effect on the transformation depends on two factors,non-blocking factor γ and blocking scale k,which directly acts on the dimensionality of growth.The effective activation energy is not affected by the anisotropic effect.The evolution of anisotropic effect with the fraction transformed is taken into account,showing that the anisotropic effect is more severe at the middle stage of transformation.

DSC STUDY OF MARTENSITIC TRANSFORMATION KINETICS IN A Cu－Zn－Al－Mn－Ni SHAPE MEMORY ALLOY

The kinetics of the reversible martensitic transformation in a Cu-Zn-Al-Mn-Ni shape memory alloy has been studied by means of differential scanning calorimetry.The apparent activation energy has been calculated and the kinetic equations of positive and adverse martensitic transformation have been established with the variations of temperature and time.

Effects of tantalum on austenitic transformation kinetics of RAFM steel

The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by aphase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D_0（pre-exponential factor for diffusion）and Q_d（activation energy for diffusion）,were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of A_（c1） and A_（c3） （onset and finish temperature of austenitic transformation,respectively）are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D_0.However,Q_d is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.

Mechanism and Kinetics of Phase Transformation in Two-phase TiAl-based Alloys

The aged and quenched microstructures of both alloys, Ti-42at-%Al and Ti-45at -%Al,homogenized in the disordered single phase field. were investigated And the results show that the quinched microstructure is a supersaturated single phase of ordered 22. When the supersaturated phase is aged in the two phase range at 1273 and 1373 K, it will transform to a lamellar microstructure of γ+α2. with a discontinuous decomposition mechanism in Ti-42at-%Al alloy and a semicontinuous decomposition mechanism in T1-45at-%Al alloy. With the methods of quantitative metallograph examination and X-ray diffraction analysis. the relationship between the amount of γ, phase precipitation and the time of isothermal transformation is agreed

Analytical Description for Solid-State Phase Transformation Kinetics:Extended Works from a Modular Model, a Review

Solid-state phase transformation plays an important role in adjusting the microstructure and thus tuning the properties of materials. A general modular, analytical model has been widely applied to describe the kinetics of solid-state phase transformation involving nucleation, growth and impingement; the basic conception for iso-kinetics which constitutes a physical foundation for the kinetic models or recipes can be extended by the analytical model. Applying the model, the evolution of kinetic parameters is an effective tool for describing the crystallization of enormous amorphous alloys. In order to further improve the effectiveness of this kinetic model, recently, the recipes and the model fitting procedures were extended, with more factors （e.g., anisotropic growth, soft impingement, and thermodynamic driving force） taken into consideration in the modified models. The recent development in the field of analytical model suggests that it is a general, flexible and open kinetic model for describing the solid-state phase transformation kinetics.

Computer Simulation of Ferrite Transformation during Hot Working of Low Carbon Steel

On the basis of transformation kinetics and thermodynamics, the austenite-ferrite transformation start temperature during deformation was predicted for several grades of low-carbon steels under different processing conditions. Results indicate that Ar3d temperature mostly depended on alloying composition and processing parameters. Ar3d increased as strain rate or strain increased for the same steel grade. In view of enhancement of deformation on transformation, the basic kinetics model was established to simulate deformation induced transformation behavior, using which the influence of the deformation stored energy and effective deformation ledge on the nucleation and growth can be considered. The simulated results are in good agreement with experiment results.

Characteristics of microstructural evolution during deformation-enhanced ferrite transformation in Nb-microalloyed HSLA steel

Microstructure evolution during deformation of undercooled austenite at 760℃ was investigated in Nb-microalloyed steel by using SEM （scanning electron microscope）, TEM （transmission electron microscope）, and EBSD （electron backscattered diffraction）. It is indicated that during deformation-enhanced ferrite transformation （DEFT） in Nb-microalloyed steel, the incubation period is prolonged, and the higher strain is needed to accomplish ferrite transformation. Therefore, the transformation kinetics curves move to high strain parallelly; and the transformation kinetics curves of Nb-microalloyed steel can be divided into three stages. At the fast stage, the solute drag effect of Nb and the consumption of strain energy for the dynamic precipitation of Nb（CN） led to a long incubation period, and at the second stage, ferrite transformation was accelerated significantly and fine Nb（CN） precipitates restrict the grain growth of ferrite effectively. The results also showed that DEFT in Nb-microalloyed steel is still a nucleation dominated process, and during the microstructure evolution the interchange of 〈001〉 and 〈111〉 texture was obtained.

Aging Kinetics in a CuNiSiCr Alloy

The aging precipitation behavior in solution treated Cu-Ni-Si-Cr alloy has been studied in terms of the analyses of the variations in electrical conductivity. On the basis of the linear relationship between the electrical conductivity and the volume fraction of the precipitates, the phase transformation kinetics equation was deduced from the Avrami empiricai formula. On the basis of this equation, transformation kinetics curves corresponding to 5% and 50% transformation were established.

MATHEMATICAL MODELLING OF PHASE TRANSFORMATION FROM HOT-DEFORMED AUSTENITE IN LOW CARBON STEELS

On the basis of superelement model, Cahn’s transformation kinetics theory and Scheil’s additivity rule, the CCT diagrams and transformation kinetics in low carbon steel were predicted considering both undeformed and deformed conditions. The influence of deformation on phase equilibria and transformation incubation period was evaluated quantitatively. The recrystallization kinetics and the evolution of dislocation density were calculated during continuous cooling. The results show deformation considerably shortens transformation incubation period, accelerates transformation kinetics and makes CCT curve shift leftwards. The calculated CCT diagrams and the volume fraction of each phase are in good agreement with measurements.

Grain Size Prediction after Continuous Cooling Transformation from Deformed Austenite to Ferrite

On the basis of transformation thermodynamics and kinetics theories,an algorithm for predicting ferrite grain size after continuous cooling transformation from deformed austenite to ferrite is suggested.The calculated results of computer simulation with the algorithm are in so good agreement with the measured ones in controlled rolling and controlled cooling experiments that the theoretical algorithm is feasible.

CALCULATION OF TRANSFORMATION VOLUME FRACTION TO HQ590 HOT ROLLED STRIP STEEL DURING CONTINUOUS COOLING

On the basis of phase transformation kinetics, the transformation of γ→α,P,B have been investigated through considering the effect of deformation. The calculation methods of volume fraction have also been given. Comparing with common method, the simulated results are in more agreement with experiment results.

Formation of Omega-like Nanocrystalline in the Melt-Spun Nd_(85)Al_(15) Alloy by Phase Transformation

Microstructure and subsequent phase transformations on heating of the melt-spun Nd85Al15 alloy have been studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The melt-spun Nd85Al15 alloy shows two-stage transformation processes as follows: amorphous+72 nm supersaturated bcc-Nd(AI) solid solution-7 nm omega-like phase-AINd3+hexagonal Nd. The activation energies for the first and second transformation were found to be 100 kj/mol and 188 kj/mol, respectively. The formation mechanism of nanoscale omega-like phase is discussed.

Transformation mechanism of nutrient elements in the process of biochar preparation for returning biochar to soil

Returning biochar to soil is a heavily researched topic because biochar functions well for soil improvement. There is a significant loss of nutrients, which occurs during biochar preparation before biochar is returned to soil,thereby seriously undermining biochar＇s efficacy. Therefore, the transformation mechanisms of biochar p H,mass, nutrients and metals during pyrolysis under different atmospheres and temperatures were studied such that the best method for biochar preparation could be developed. Several conclusions can be reached：（1） a CO2 atmosphere is better than a N2 atmosphere for biochar preparation, although preparation in a CO2 atmosphere is not a common practice for biochar producers;（2） 350 ℃is the best temperature for biochar preparation because the amount of nutrient loss is notably low based on the premise of straw transferred into biochar; and（3） transforming mechanisms of pH, N, P and K are also involved in the biochar preparation process.

Notably Accelerated Nano-Bainite Transformation via Increasing Undissolved Carbides Content on GCr15Si1Mo Bearing Steel

In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 ℃, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.