Equilibrium Transformation Temperature Research Articles

Phase transition temperature and non-isothermal kinetics in zirconium alloys by differential scanning calorimetry

In this study, zirconium alloys were analyzed for their phase transformation behavior and kinetics in the presence of non-isothermal conditions through differential scanning calorimetry. The phase transition temperature was examined on the differential thermal curves. Indeed, it was affected by heating rates, and the equilibrium transformation temperature of the alloy was extrapolated using a model approach. The α → β phase transformation activation energy was calculated using Kissinger–Akahira–Sunose as well as Flynn–Wall–Ozawa approach, and activation energy was related to conversion ratios, thus suggesting that phase transformation refers to the complicated multi-step reaction. Furthermore, the alloy transformation mechanism was discussed using the optimized Johnson-Mehl-Avrami.

Solid-Solid Phase Transformations and Their Kinetics in Ti-Al-Nb Alloys

The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO2 emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4–5 different heating rates (0.8 to 10 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti3Al to (αTi), ωo (Ti4NbAl3) to (βTi)o, and O (Ti2NbAl) to (βTi)o. The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures.

AUSTENITE – FERRITE TRANSFORMATION TEMPERATURES OF C MN AL HSLA STEEL

The article is aimed to investigate a shift of transformation temperatures of C-Mn-Al HSLA steel with different cooling rates. The transformation temperatures from austenite to ferrite have been determined by dilatometry using thermal-mechanical simulator Gleeble 1500D. To define the start and finishing temperatures of the austenite-ferrite transformation intersectional method was used. Effect of cooling rate on transformation temperature has been evaluated for 0.17, 1, 5, 10, 15, 20, 25°C.s-1. There was found out that rising the cooling rate results in moving transformation temperature range to lower temperatures. The transformation temperatures have been also compared with temperatures calculated using equations of several authors. Some of them have considered cooling rates only. Cooling rates have effect on final microstructure. The effect has been evaluated by measuring hardness (HV10) relating the cooling rates from 0.17 to 25°C.s-1. Increasing cooling rates resulted in increase of hardness. Moreover, Thermo-Calc software was used to determine the Ae3 and Ae1 equilibrium temperatures. Equilibrium transformation temperatures Ae3-Ae1 were higher than experimentally measured by dilatometric method using Gleeble 1500D.

Ultrafine Pearlitic Transformation Behavior of 2000 MPa Ultra‐High‐Strength Steel Wire Under Nb Microalloying

The influence of Nb in the transformation behavior of ultra‐high‐strength pearlitic steels is studied. The phase transition temperature is measured by thermal simulation device, and the continuous cooling transformation curve is established. Results show that with addition of Nb, the equilibrium transformation temperature of the test steel increases, and both size of the original austenite, pearlite colonies and pearlite lamellar spacing are refined. Pearlitic transformation mechanism has changed by the addition of Nb through the interaction of solid solution and precipitation. Nb microalloy delays the beginning and ending point of pearlite transformation when the cooling rate is greater than 1.00 and 0.10 °C s−1, respectively. In contrast, the transformation point is advanced when the cooling rate is less than 0.50 and 0.05 °C s−1. This variation is mainly due to the morphology and distribution of Nb‐containing precipitates at different cooling rates.

On the thermodynamic analysis of martensite stabilization treatments

It is shown that the broadenings of the forward and reverse martensitic transformations are proportional to the second derivatives of the elastic energy by the transformed martensite volume fraction. The shifts of the transitions, beside the effect of chemical contribution to the equilibrium transformation temperature, To, can depend on the change of the elastic and dissipative energies as well. The effect of mechanical stabilization (by making the surfaces roughened) in Ni49Fe18Ga27Co6 single crystal, where the transformation too place by single interface motion, can be well treated in thermoelastic approximation. In Ni50Mn28.5Ga21.5 single crystal (where the usual multivariant martensite structure developed) not only the dissipative energy, but the slope of the second derivative of the elastic energy for cooling has also been altered after mechanical stabilization and there can exist an elastic energy contribution to the width of the hysteresis too. Symmetry conforming short range ordering under uniaxial stress (SIM-aging) in Ni53Mn25Ga22 and Ni51Fe18Ga27Co4 single crystals caused an asymmetric change in the second derivatives of the elastic energy: the sharpening of the corresponding branches of the hysteresis was stronger for heating than cooling. The thermoelastic balance condition did not fulfill for the SIM-aged samples and the elastic energy contributions to the shift of the transformation temperatures could even compensate the increase of To (caused by the stabilization by symmetry conforming short range ordering). It is also illustrated that the dissipated energy per one cycle could decrease even if the area of the hysteresis has been increased.

A phase-field model for non-isothermal phase transformation and plasticity in polycrystalline yttria-stabilized tetragonal zirconia

We propose an elastoplastic phase-field (PF) model to investigate the mechanics of tetragonal-to-monoclinic phase transformation (TMPT) and elastoplastic deformation of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ). A Landau polynomial with non-vanishing chemical energy at the equilibrium temperature is introduced to account for the actual formation energies of the phases. The effects of different grain orientations, latent heat, and temperature on TMPT and deformation mechanisms are considered. The suppressive transformation effects of the grain boundaries (GBs) is modeled using an inhomogeneous kinetic coefficient in the bulk and GBs. The simulation results for single crystals demonstrate the capability of the model to reproduce the orientation-dependent compressive deformation of YSTZ similar to atomistic simulations and micropillar experiments. The single crystal with [100] crystallographic orientation along the loading direction (SC[100]) displays both TMPT and plasticity, SC[101] experiences only phase transformation, while SC[001] undergoes only plastic yielding. The TMPT induced by compressive loading exhibits shape memory effect (SME) below the equilibrium transformation temperature and pseudoelasticity above it, while the critical transformation stress increases with increasing loading temperature. The irrecoverable plastic strain is found to trap a part of the monoclinic phase, which prevents a complete reverse transformation. The polycrystalline cases also display SME and PE at low and high temperatures, respectively. Due to the orientation differences between grains and the stress concentrations at geometric nonlinearities, plastic deformation occurs in polycrystalline YSTZ for an applied load less than the yield stress. The results suggest a possible limitation of plasticity and an improvement of the shape recovery of YSTZ if one can control the orientation of the grains and/or increase the density of stacking faults at the GBs during material processing.

Dynamic Transformation During the Simulated Hot Rolling of an API‐X80 Steel

The occurrence of dynamic transformation is investigated during the simulated hot rolling of an API‐X80 steel. Torsion tests are carried out under continuous cooling conditions, with all deformation passes applied above the equilibrium transformation temperature. Three different interpass times are investigated: 1, 8, and 20 s. Samples are water‐quenched before and after the odd‐numbered passes, and the volume fraction of ferrite is evaluated metallographically. Dynamic transformation took place under all conditions and the volume fraction of retained ferrite increased with the retained strain. The increase in flow stress during simulated rolling is less than expected under all conditions, which is evidence for the dynamic formation of ferrite. The driving forces and energies barriers are evaluated, indicating that dynamic transformation is mechanically activated.

Nanoindentation into a metastable austenite triggers the martensitic phase transformation—An atomistic study

Indentation into a metastable austenite may induce the phase transformation to the bcc phase. We study this process using atomistic simulation. At temperatures low compared to the equilibrium transformation temperature, the indentation triggers the transformation of the entire crystallite: after starting the transformation, it rapidly proceeds throughout the simulation crystallite. The microstructure of the transformed sample is characterized by twinned grains. At higher temperatures, around the equilibrium transformation temperature, the crystal transforms only locally, in the vicinity of the indent pit. In addition, the indenter produces dislocation plasticity in the remaining austenite. At intermediate temperatures, the crystal continuously transforms throughout the indentation process.

Effect of Grain Size and Residual Strain on the Dynamic Transformation of Austenite under Plate Rolling Conditions

When austenite is deformed within the austenite phase field, that is, above the equilibrium transformation temperature Ae3, it partially transforms dynamically into ferrite. Here, rough rolling simulations are conducted at 1100 °C on an X70 steel to study the influence of grain size and residual strain on this type of transformation. Three simulated roughing passes are applied with pass strains of 0.4 at a strain rate of 1.0 s−1. The resulting stress–strain curves are recorded and analyzed. The volume fractions of ferrite and martensite (prior austenite) are determined on quenched samples by means of metallographic techniques. It is observed that dynamic transformation took place during the first roughing pass and that the volume fraction of transformed ferrite increases with the cumulative strain. Both the critical strain to the onset of dynamic transformation as well as the grain size decrease with pass number.

Effect of Cr on Lamellar Spacing and High-Temperature Stability in Eutectoid Steels

In order to clarify the refinement behavior of the lamellar spacing and high temperature stability of the lamellae in pearlite by the addition of Cr, the effects of supercooling, the diffusion coefficients of C and Cr and spheroidizing of cementite were investigating using Fe-0.8%C binary and Fe-0.8%C-Cr ternary steels.The equilibrium transformation temperature of pearlite was increased by the addition of Cr. In comparison with the Fe-0.8%C steel, at a fixed pearlite transformation temperature, the pearlite lamellar spacing of the Cr-added steel was refined by increasing supercooling, and at the same supercooling, the lamellar spacing was refined by increasing the Cr content. This is caused by a decrease of the diffusion coefficient of C in the austenite by addition of Cr.Spheroidizing of cementite in pearlite at high temperature was inhibited by increasing the Cr content. The spheroidizing rate was reduced by the addition of Cr. Spheroidizing of cementite in pearlite by increasing the Cr content was inhibited due to the increased contribution of Cr diffusion for the coarsening reaction of cementite.

Magnetic Field-Induced Reverse Martensitic Transformation and Thermal Transformation Arrest Phenomenon of Ni41Co9Mn39Sb11 Alloy

In order to investigate behavior of magnetic field-induced reverse martensitic transformation for Ni-Co-Mn-Sb, magnetization experiments up to a static magnetic field of 18 T and a pulsed magnetic field of 40 T were carried out. In the thermomagnetization curves for Ni41Co9Mn39Sb11 alloy, the equilibrium transformation temperature T0 was observed to decrease with increasing applied magnetic field, μ0H, at a rate of dT0/dμ0H = 4.6 K/T. The estimated value of entropy change evaluated from the Clausius-Clapeyron relation was about 14.1 J/(K·kg), which was in good agreement with the value obtained by differential scanning calorimetric measurements. For the isothermal magnetization curves, metamagnetic behavior associated with the magnetic field-induced martensitic transformation was observed. The equilibrium magnetic field, μ0H0 = (μ0HAf + μ0HMs)/2, of the martensitic transformation tended to be saturated at lower temperature; that is, transformation arrest phenomenon was confirmed for the Ni-Co-Mn-Sb system, analogous with the Ni(Co)-Mn-Z (Z = In, Sn, Ga, Al) alloys. Temperature dependence of the magnetic field hysteresis, μ0Hhys = μ0HAf − μ0HMs, was analyzed based on the model for the plastic deformation introduced by the dislocations. The behavior can be explained by the model and the difference of the sweeping rate of the applied magnetic field was well reflected by the experimental results.

Influence of Cooling Path on the Dynamic Transformation of Austenite-to-Ferrite in Low Carbon Manganese Steels

For an effective thermomechanical process control, knowledge of the start and finish temperatures of the austenite transformation is critical. Continuous Cooling uniaxial-Tension (CCT*) is a useful way to measure these values. Therefore, the dynamic transformation of austenite to ferrite (γ → α) was investigated in C-Mn steels to understand the hot ductility behaviour of these steels after varying the cooling paths i.e. simulating “hard” and “soft” cooling rates of the strand during continuous casting. Results show that hard cooling into the dual phase (γ + α) region significantly increases the dynamic transformation temperatures due to a higher driving force owing to double transformation and precipitation of AlN during the process. A comparison of dynamic versus static transformation (Ar3 and Ar1) temperatures and equilibrium transformation temperatures gives a better understanding of the contribution of strain to transformation during casting or hot deformation.

Free energies of austenite and martensite Fe–C alloys: an atomistic study

We investigate the influence of C interstitials on the phase stability of Fe–C crystals. We employ the Meyer–Entel interatomic interaction potential which is able to reproduce the austenite-martensite phase transition for pure Fe, and supplement it by a simple pairwise Fe–C interaction potential. Using two different thermodynamic methods, we calculate the free energies of the martensite and austenite phases. We find that C destabilizes the ground-state bcc phase. The decrease in the equilibrium transformation temperature with increasing C content parallels the one found in the experiment. This destabilization is found even if C is added for a potential in which only the bcc phase is stable until the melting point; here, for sufficiently high C addition, a stable fcc phase is established in the phase diagram.

Strain-Induced Austenitic Structure in Microalloyed Steels

Abstract Austenite morphology is one of the main factors determining austenite-ferrite transformation kinetic and effectively affects the final microstructure and properties. The basic criteria for proper assessment of the austenite transformation products, theirs refinement, is the relation between the nucleation to growth rates. The main factor accelerating both, the nucleation rate of austenite during heating, and ferrite during cooling is the presence of accumulated deformation energy. The primary aim of this work is to increase our knowledge of the effects of deformation - its accumulated energy on the austenite structure and properties. Two specific steel grades were selected for the present investigation, i.e. microalloyed and IF steel, essentially different in equilibrium transformation temperatures. Obtained austenitic microstructures were analyzed, first of all as a start point for the austenite-to-ferrite transformation. Specific case of this transformation was considered i.e. Strain Induced Dynamic Transformation SIDT. The characteristic feature of the SIDT is the strong dependence of theirs kinetic on the austenite morphology, especially grain size. Thermomechanical processing, that utilize the SIDT, is one of the most effective ways to produce ultrafine-grained steel. One of the main benefits of the austenite refinement, just before the γ→α transformation, is its significant effect on the microstructure evolution during subsequent thermomechanical processing. Experimental results clearly show how direct and positive influence the austenite grain refinement has on the composition and refinement of transformation products. Presented study was focused on Strain Induced Dynamic Reverse Transformation. It is proved that this kind of transformation is very efficient way to intensify thermomechanical processing of microalloyed steels. Dynamic transformation kinetics were analyzed based upon flow curves recorded during the SIDT process. The main effect of presented research is analyze of influence of prior microstructure on dynamically formed austenite morphology

Role of mechanical activation in the dynamic transformation of austenite

When austenite is deformed above the equilibrium transformation temperature Ae3, it is dynamically transformed into Widmanstätten ferrite by a displacive mechanism. On removal of the load it is slowly retransformed into austenite by diffusional processes. The forward transformation has recently been explained in terms of a thermodynamic model in which the lower free energy of austenite is raised above that of normally unstable ferrite as a result of the additional stored energy associated with the dislocations introduced by straining. This model is here shown to be unable to account for the initiation of transformation at critical strains of about 0.1, at which only low densities of dislocations are present. Of particular importance is the observation that dynamic transformation can be initiated at temperatures 100°C and more above the Ae3 and that the critical strain actually decreases with increasing temperature and increasing chemical free energy barrier. This discrepancy is removed by allowing for mechanical (stress-based) activation of the transformation. The latter provides the energy required to accommodate the shear of the parent austenite into Widmanstätten plates, as well as the volume change or dilatation accompanying ferrite formation. The work of dilatation and the shear accommodation work, omitted from the previous analysis, are introduced here as barriers to the transformation that are overcome by the applied stress. This modified approach is able to account for the very rapid forward (mechanically activated) transformation compared with the much slower reverse transformation that takes place in the absence of stress.

Effect of thermodynamics on transformation kinetics; analysis of recipes

For transformations occurring in the vicinity of equilibrium transformation temperature, the thermodynamic terms affect significantly the transformation kinetics and thus the results of kinetic analyzing. Accordingly, the effects of two types of thermodynamic terms (i.e., temperature- and transformed fraction-dependent terms) on the traditional kinetic recipe analyzing are evaluated. The results of the Kissinger plot and the Starink–Zahra plot are influenced by temperature-dependent thermodynamic term. The results of the Ozawa plot, the Avrami plot and the Starink–Zahra plot are affected by transformed fraction-dependent thermodynamic terms. The kinetic parameters of the hcp/bcc allotropic phase transformation in pure Zr as measured by non-isothermal differential scanning calorimetry are determined by the traditional kinetic recipe analyzing. The results further confirm the theoretical analysis.

Evaluation of the maximum transformation rate for determination of impingement mode upon near-equilibrium solid-state phase transformation

Considering the role of hard impingement mode in analyzing phase transformation kinetics, a maximum transformation rate analysis method was developed for pre-selection of impingement mode. Based on an extended analytical model, this method proposed for transformations with very large undercooling is reevaluated for transformations in the vicinity of equilibrium transformation temperature. Accordingly, a new peak maximum analysis method for both isothermal and non-isothermal transformations conducting near or far from the equilibrium state is proposed to determine the prevailing mode of impingement. The results obtained by application of the new method to numerically calculated transformations are in good agreement with the input values. Then, the method was applied to the isothermal ferrite/austenite transformation in Fe–3.28at.%Mn alloy as measured by dilatometry and non-isothermal hcp/bcc phase transformation in pure Zr as measured by differential scanning calorimetry.

Age-induced multi-stage transformation in a Ni-rich NiTiHf alloy

Abnormal multi-stage transformations have already been studied in binary Ni-rich NiTi alloys. In this research, this kind of transformation was investigated in a low supersaturated Ni-rich ternary NiTiHf high-temperature shape memory alloy by aging at intermediate temperatures for various durations. Meticulous examinations of the results of differential scanning calorimetry tests demonstrated the heterogeneous precipitation of (Ti,Hf)3Ni4 particles and the three-stage transformation (one-stage R and two-stage B19′) in the aged alloy. Aging provided a significant rise in transformation temperatures (TTs) until they reached their equilibrium states, corresponding to the equilibrium Ni content at each aging temperature. Equilibrium TTs were higher when aging was performed at a lower temperature. The remarkable increase in TTs was compared with those in aged Ni-rich NiTi alloys, and discussed based on the variation in valence electron concentration. A model was also proposed for the microstructural evolution during aging. Furthermore, aging provided enhanced hardness and strain recovery for the alloy. In particular, aging at a lower temperature resulted in a considerable improvement in hardness and shape recovery, which was discussed based on the microstructural changes in the aged alloy. Equilibrium Ni content at each aging temperature proved to be a crucial parameter in controlling the alloy properties, even in heterogeneous precipitations.

Calculation of Elastic Energy Contributions in Single Crystalline Cu-11.5wt%Al-5.0wt%Ni Shape Memory Alloy

Using our local equilibrium model of the martensitic transformation [ the elastic energy contributions, as the function of martensite volume fraction, ξ, in the phase transformation of single crystalline Cu-11.5wt%Al-5.0wt%Ni shape memory alloy were calculated from our measurements published earlier [. The derivative of the elastic energy δE/δξ=e (E is the total elastic energy stored/released during the austenite to martensite (AM) as well as MA transformation) could be calculated only irrespectively of the ST0 term (T0 is the equilibrium transformation temperature and S is the entropy change of phase transformation). But, since ST0 is independent of ξ, the functions obtained reflect the ξ dependence of e as well as E quantities. From the DSC curves measured at zero uniaxial stress (σ = 0) [, the ξ-T hysteric loop was constructed. Then the e (ξ) curves at fix σ as well as fix T were calculated. The E values obtained from the integral of e (ξ), fit well to the E(σ) as well as E(T) curves calculated from the strain-temperature and stress-temperature curves measured in [.

Two Novel Methods to Realize the Superplastic Forming of Ultrahigh Strength Steels

Two novel methods of obtaining microduplex structures, ferrite plus spherical carbides, in ultrahigh strength steels (~2000MPa) are introduced. One is through an adequate deformation just below the austenite-ferrite equilibrium transformation temperature (i.e. Ae3 temperature, ~983K) followed by water quenching. The adequate deformation directly leads to the formation of a (ferrite plus spherical carbides) microduplex structure. The microstructure evolution during the deformation includes pearlite transformation, cementite spheroidization and ferrite recrystallization. The other is through an adequate deformation above Ae3 temperature (~1003K) followed by water quenching to produce martensite firstly and then obtain a (ferrite plus spherical carbides) microduplex structure during warm deformation of martensite. Microstructural analysis on the microduplex structure shows that submicron carbides are located at ferrite grain boundaries while nanometer ones are dispersed inside ferrite grains. This kind of carbide distribution may suppress the coarsening of ferrite grains and form a dynamic equilibrium of ferrite grain size on a specific deformation condition. The strain rate sensitivity of the (ferrite plus spherical carbides) microduplex structures is about 0.4 at 973K and strain rate of 10-4s-1.