Phase Transformation Products Research Articles

Continuous Cooling Transformation Behaviour and Bainite Transformation Kinetics of 23CrNi3Mo Carburised Steel

In this study, the phase transformation behaviour of the carburised layer and the matrix of 23CrNi3Mo steel was comparatively investigated by constructing continuous cooling transformation (CCT) diagram, determining the volume fraction of retained austenite (RA) and plotting dilatometric curves. The results indicated that Austenite formation start temperature (Ac1) and Austenite formation finish temperature (Ac3) of the carburised layer decreased compared to the matrix, and the critical cooling rate (0.05 °C/s) of martensite transformation is significantly lower than that (0.8 °C/s) of the matrix. The main products of phase transformation in both the carburised layer and the matrix were martensite and bainite microstructures. Moreover, an increase in carbon content resulted in the formation of lamellar martensite in the carburised layer, whereas the martensite in the matrix was still lath. Furthermore, the volume fraction of RA in the carburised layer was higher than that in the matrix. Moreover, the bainite transformation kinetics of the 23CrNi3Mo steel matrix during the continuous cooling process indicated that the mian mechanism of bainite transformation of the 23CrNi3Mo steel matrix is two-dimensional growth and one-dimensional growth.

Mechanisms for influence of post-deformation annealing on microstructure of NiTiFe shape memory alloy processed by local canning compression

Mechanisms for the influence of post-deformation annealing on the microstructure of NiTiFe shape memory alloy (SMA) processed by local canning compression were investigated by means of experiment. The microstructure composition of the NiTiFe SMA processed by post-deformation annealing at lower temperature is different from that of binary NiTi SMA, where coarse grains are found in addition to the amorphized structure (AMS) and the nanocrystalline structure (NCS). The main reason for this phenomenon lies in the fact that the value of the deformation temperature relative to the martensite start temperature Ms of the NiTiFe SMA is much larger than that of binary NiTi SMA and some coarse grains are retained after the alloy is processed by severe plastic deformation (SPD). The mechanisms of structure changes in the NCS areas during post-deformation annealing of NiTiFe SMA were found to resemble those of binary NiTi SMA and the mechanisms of structure changes in AMS area of the alloy during post-deformation annealing were revealed for the first time. Texture analysis on the NiTiFe SMA processed by SPD and post-deformation annealing below recrystallization temperature reveals that the strong <111> texture is formed on the condition of post-deformation annealing at 600 ℃. The <111> texture may result from two approaches, where the first one is the <111> -oriented NCS or nano-sized crystalline inclusions (NCI) formed by SPD and the second one is the <111> preferred orientation of the grains nanocrystallized from the amorphous phase. {112} B2 austenite twin is a kind of deformation twin which is usually formed during the deformation above the critical temperature of stress-induced martensitic (SIM) transformation Md. However, this kind of twin was found for the first time in the NiTiFe SMA processed by local canning compression at room temperature and post-deformation annealing at 600 ℃. These {112} B2 austenite twins may be the reverse phase transformation product of the {113} martensite twins induced by the SPD and they grow up under the higher annealing temperature.

Phase Transformation Mechanism and Microstructure Evolution of TC4 Alloy during Continuous Cooling

The effects of cooling rate on the microstructure and phase transformation mechanism of TC4 alloy during the continuous cooling of β phase region were studied by non-isothermal expansion method combined with microstructure observation. The phase transformation feature points at different cooling rates were obtained by the expansion curve and its first derivative curve. The characteristics of the phase transformation products in the corresponding phase transformation feature points were observed by optical microscopy (OM). The results show that with the increase of cooling rate (excluding water cooling), the nonlinearity of the expansion curve caused by phase transformation changes from one place to two, and the temperature corresponding to the peak value of the phase transformation rate curve also showed different changes. As the cooling rate increases, the microstructure sequentially underwent lamellar α, massive αm, and fine-needle α′ martensite. The phase transformation conditions of massive αm and α′ martensite are significantly different, and showed a competition trend between them. According to the phase transformation feature points and their corresponding microstructure, the continuous cooling diagram for TC4 β solution treatment was plotted.

Influence of hot compressive parameters on flow behaviour and microstructure evolution in a commercial medium carbon micro-alloyed spring steel

Abstract The spring fastener that connects sleepers and rail in high-speed railway is with a high responsibility since its quality directly affects the safety of the railway system. This paper investigates the isothermal hot compression behaviour of a commercial medium carbon micro-alloyed spring steel (raw material) by a Gleeble 3500 thermal-mechanical simulator, which was used to produce spring fasteners. The dependency of the flow behaviour and microstructure evolution under different deformation conditions were systematically studied. The results revealed that the peak stress points are not necessarily recognised as the dynamic recrystallisation (DRX) behaviour because the partial occurrence of DRX may appear in flow stress curves with no stress peak. The metallurgical events that happen during the process of hot compression are close related to the phase transformation products. In the prior austenite grains, high dislocation density can promote the nucleation acicular ferrite. Nevertheless, the DRX behaviour consumed the stored energy, which is conducive to the transformation of bainite and martensite. Besides, the microstructure evoluted from prior austenite grains occupies a larger fraction of middle angle grain boundaries and higher mean local strain compared to the counterparts of DRX grains. The findings would provide useful references for the industrial optimisation in the hot forming process of a spring fastener using Si-Cr medium carbon spring steels.

Nucleation Analysis of Variant Transformed from Austenite with Σ3 Boundary in High-Strength Low-Alloy Steel

We elucidate here the effects of annealing twins on phase transformation products based on electron back-scatter diffraction analysis and corresponding microstructure visualization and quantification methods. Martensite and bainite were obtained by rapid continuous cooling and isothermal processing at different temperatures, respectively, which were designed to study variants formed in austenite with Σ3 boundary. The isothermal transformation near martensite start (Ms) temperature was most conducive in obtaining the highest content of twin-related V1/V2 variant pair. Based on the classical nucleation theory of martensite and bainite, respectively, the role of austenite Σ3 boundary in martensite and bainite transformation is illustrated.

Kinetics of Isothermal Phase Transformations in Premium and Standard Rail Steels

Recently published articles have enhanced the importance of understanding phase transformations in rail eutectoid steels in an attempt to improve their manufacturing and welding processes. Herein, the kinetics of isothermal phase transformations of three eutectoid steels used in railways around the world are evaluated. Computer simulations are performed to evaluate the effect of steel chemical compositions on phase transformations considering thermodynamic equilibrium conditions. Austenitizing temperatures under continuous heating and the time intervals of isothermal austenite decomposition are measured by dilatometry. The time–temperature–transformation diagrams are determined. Qualitative and quantitative microscopy techniques are applied to characterize the phase transformation products. It is shown that by applying a well‐planned heat treatment, it is possible to obtain microstructural characteristics for a standard steel as refined as those achieved for premium steels. Empirical equations proposed in the literature to predict the Bs temperatures are applied, and the experimental values are compared. Using the dilatometry data, it is possible to determine the global activation energy for austenite–bainite transformation. Results show that premium steels have higher overall activation energy for austenite–bainite decomposition than what occurs in standard. This article presents unpublished data useful for steel researchers as well for rail manufacturers and welders.

Toughening FeMn-based high-entropy alloys via retarding phase transformation

Various high entropy alloys (HEAs) with improved mechanical properties were developed by reducing the phase stability and then promote the phase transformation. The promotion of deformation-induced martensitic transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) mostly focuses on overcoming the trade-off of strength-ductility of HEAs at room temperature. However, the hcp phase is brittle at cryogenic-temperature, and thus the enhancement of cryogenic ductility of these HEAs still remains a challenge. Here, we present a concept to toughening Fe50Mn30Co10Cr10 HEAs at cryogenic-temperature via retarding phase transformation. The retarded but more persistent phase transformation at high strain level was realized via tailoring the grain size. To further verify the effect of phase transformation rate on ductility of HEAs, the mechanical properties of Fe40Mn40Co10Cr10 HEAs with higher stacking fault energy were tested at room and cryogenic temperature, respectively. The present study sheds light on developing high performance HEAs, especially for alloys with brittle phase transformation products.

The critical impact of intercritical deformation on variant pairing of bainite/martensite in dual-phase steels

We elucidate here the critical impact of intercritical deformation on variant pairing of bainite/martensite in dual-phase steels. Intercritical deformation was instrumental in significantly increasing the content of ferrite and accelerated the diffusion of carbon from ferrite to untransformed austenite, which finally transformed to martensite on quenching. In contrast, the untransformed austenite with lower enrichment of carbon transformed to bainite. Furthermore, the increase of carbon concentration in untransformed austenite, the length fraction of V1/V2 pair was increased to accommodate the strain associated with the bainite transformation. When the phase transformation product changed from bainite to martensite, all six variants with high fraction belonging to the same closed-packed (CP) group formed to accommodate the higher strain associated with martensite transformation. The steel with the lowest percentage of ferrite (25%) had the highest yield strength (583 MPa), while the yield strength (574 MPa) of steel with 58% ferrite continued to be high because of dislocation strengthening caused by intercritical deformation. Additionally, the uniform elongation was increased and yield strength to tensile strength ratio was reduced with the increase of content of ferrite in the studied steels.

Microstructural Characteristics and Mechanical Properties in the Laser Beam Welded Joints of High-Strength Microalloyed Steel

The laser beam welded joints of the high-strength microalloyed steel consist of the base metal (BM), the partially transformed heat-affected zone I (PTHAZ-I), the partially transformed heat-affected zone II (PTHAZ-II), the fully transformed heat-affected zone (FTHAZ) and the fusion zone (FZ). The martensite/austenite (M/A) islands of the bainites in the PTHAZ-II are dense versus those in the PTHAZ-I, and the martensites exist in the FTHAZ and the FZ together with the bainites under the slow welding speed. The M/A islands of the bainites and/or the martensites in the PTHAZ become denser under the high welding speed, and the martensites in the FTHAZ and the FZ predominate. The low-angle boundary density and the high local misorientation proportion are used to identify the constituents roughly, consistent with the microstructural characteristics in the welded joints. The misorientation angle distributions are employed to evaluate the degree of the phase transformation, reflecting the change law of the degree of the phase transformation in the welded joints. The hardness and the tensile tests indicate that the HAZ and the FZ are stronger than the BM, dependent on the microstructures in the welded joints. The strengthening in the HAZ and the FZ is mainly attributed to the products of non-equilibrium phase transformation with the high dislocation density.

Effect of hot deformation parameters on flow stress and microstructure in a low carbon microalloyed steel

Hot deformation behavior of low carbon microalloyed steel was investigated by single-pass compression tests using a thermomechanical simulator. After the specimens were subjected to different hot deformation processes, their microstructures were obtained by constant cooling rate and then examined using optical microscopy and electron backscattering diffraction (EBSD) technique. The results showed that the absence of stress peak in the flow stress curves does not necessarily indicate the absence of dynamic recrystallization (DRX) because in some cases the partial DRX phenomenon does occur when the stress peaks are absent in the flow curves. The metallurgical events that occur during hot deformation directly influence the nature of phase transformation products. High dislocation density in the deformed austenite can promote the intragranular nucleation of acicular ferrite, while the DRX austenite grains prefer to form lath bainite/martensite. The EBSD results confirmed that the microstructures that were transformed from prior pancaked austenite grains always contain higher fraction of middle misorientation angle boundaries and larger average local strain as compared to those with prior DRX grain shape.

Effect of Cu addition on hardness and microstructural features of low alloy white cast iron

The aim of this work is to investigate in details the effect of Cu on hardening variation and microstructural feature of low alloy white cast iron. For this purpose, a wide variety of iron melts consisting of different weight percent Cu were prepared with step by step addition of about 0.5%Cu to a constant base chemical composition of low alloy white iron. Hardness measurements were supplemented with x-ray diffraction, colored light metallography and electron microscopy to follow the relationship between hardness and microstructure of as-cast white irons. The experimental results show that as-cast hardness has been sequentially increased from 49HRC to the values of 52, 57 and 61HRC with step by step addition of only about 0.5%Cu to the chemical composition of low alloy base melt iron, respectively. The remarkable improvement of as-cast hardness has been rationalized to the formation of more and more displacive martensitic phase transformation products developed as a consequence of alloying partitioning within the associated austenite and carbide during solidification. The M3C eutectic carbide is just rich in Cr while the austenite phase, apart from a significant enrichment of Mn, Ni and Cu, has been surprisingly enriched from Mo as well.

Alpha-to-gamma reverse phase transformation of molten removal in alumina ceramics laser processing

Generally speaking, in the melting process of alumina ceramics, all the metastable transitional phase will be gradually transformed into the stable α phase with rising temperature due to an irreversible lattice reconstruction. However, under the action of pulsed laser cutting, the lattice structure of α-alumina needs to withstand the great stress from instantaneous high energy impact. Thus, a slip dislocation of 1/4 at. diameter is generated on the atomic plane of each second basal layer to produce γ-alumina. In this work, a ceramic plate comprised of α-alumina is taken as the object of study. The α-to-γ reverse phase transformation of the molten removal is verified by the observed microstructure from scanning electron microscope (SEM) and transmission electron microscope (TEM). Simultaneously, the X-ray diffraction (XRD) investigation shows that the basis of molten removal after laser cutting is still α-alumina. The micrographs of octahedral corners or hexagons are shown in different observation directions. There is no γ-alumina phase transformation product in the spherical granular removal at each vapor-to-melt ratio (rvmr). In contrast, γ-alumina is detected in the adhering slag with a micro-morphology of cubic phase. The mass fraction is increased with the rising of vapor-to-melt ratio. At the same time, θ-alumina also exists in some spattering removal as intermediate product between α and γ phase. It is indicated that the generation of α-to-γ reverse phase transformation occurs mainly in the melting process of temperature rise, and the maintenance of phase transformation results mainly occurs during cooling process of condensation.

Microstructural adjustments and mechanical properties of a cold-rolled biomedical near β−Ti alloy sheet

In this study, microstructural adjustments and mechanical properties of a cold-rolled near β-type alloy Ti−25Nb−3Zr−3Mo−2Sn (wt%) sheet were investigated. Microstructures and phase transformation products strongly depended on aging temperatures. Solution treatments within single β-phase field removed the stress-induced α″ martensites and produced a few new lath-shaped ones, but metastable β phase still dominated. This is exactly the reason why current alloy exhibits the lowest modulus (54 GPa) and best elongation to fracture (39 %), but the worst yield strength of only 340 MPa, at solution-treated state. A fairly large number of ellipsoidal ω phase nanoparticles precipitated throughout parent β phase during aging at 380 °C. These ω nanoparticles possess remarkable strengthening effect, but deteriorate ductility seriously. A novel post-aging process was proposed to remove brittle ω phase. By contrast, aging at 450 °C resulted in sufficient precipitation of fine needle-like α phase. This brought about the best combination of high yield strength (770 MPa) and moderate elastic modulus (75 GPa) and good elongation (15 %) for biomedical implants.

Phase transformations of siderite ore by the thermomagnetic analysis data

Thermal decomposition of Bakal siderite ore (that consists of magnesium siderite and ankerite traces) was investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory-built facility that allows automatic registration of sample magnetization with the temperature (heating/cooling rate was 65°/min, maximum temperature 650°C) at low- and high-oxygen content. Curie temperature gradually decreases with each next cycles of heating/cooling at low-oxygen content. Curie temperature decrease after 2nd cycle of heating/cooling at high-oxygen content and do not change with next cycles. Final Curie temperature for both modes was ~320°C. Saturation magnetization of obtained samples increases up to 20Am2/kg. The final product of phase transformation at both modes was magnesioferrite. It was shown that intermediate phase of thermal decomposition of Bakal siderite ore was magnesiowustite.

Rechargable xLi2MnO3·(1 − x)Li4/3Mn5/3O4 electrode nanocomposite material as a modification product of chemical manganese dioxide by lithium additives

Relatively simple preparation procedure of rechargeable Li-ion battery cathode material via manganese dioxide treatment with Li-containing additive and subsequent calcination has been demonstrated. X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and atomic force microscopy study were characterisation methods of modification products. Pyrolusite, Li0.3MnO2, layered Li2MnO3, and spinel Li4Mn5O12 phases were revealed as products of initial ramsdellite phase transformations at temperatures of heat treatment ranging from 360°C to 600°C. Optimal temperature of final heat treatment from the point of view of rechargeability and discharge characteristics was 450°C. Samples heat-treated at 450°C are characterized by the unique combination of Li4/3Mn5/3O4 and Li2MnO3 phase components due to their structural integration, a significant degree of disordering, and sizes of nanocrystallites with Li diffusion path, which is the most favourable for reversibility. The prepared nanocomposite cathode material delivers a capacity of 150mAhg−1 within 2.5–4.5V at 0.1mA discharge.

Nucleation Laws of Supercooled Austenite Transformation

It is significant theoretically to study the nucleation laws of the phase transformation products of the supercooled austenite. The different iron and steel materials were used to study the nucleation of pearlite, bainite and martensite by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. The results show that, from the high-temperature zone to the low-temperature zone, the nucleation of the transformation products of the supercooled austenite is a gradually evolutionary process from two-phase nucleation to single-phase nucleation. Pearlite nucleates in the austenitic grain boundary, bainite nucleates preferentially in the grain boundary and sometimes in the grain interior and martensite nucleates preferentially in the interface and generally in the grain interior. The preferential nucleation sites are the interface, and with the decrease of the transformation temperature, they gradually shift to the defects in the grain interiors, which accord with the general rules of the phase transformation nucleation.

Influence of Strain History and Cooling Rate on the Austenite Decomposition Behavior and Phase Transformation Products in a Microalloyed Steel

The effect of simple strain path changes as well as post-deformation continuous cooling rate during thermomechanical-controlled processing of microalloyed steel was studied using laboratory physical simulation. The phase transformation characteristics were directly analyzed by dilatometry under various cooling rates. The microstructures of the transformation products were characterized quantitatively using EBSD. The results have shown that while strain path changes impose a considerable influence on the hot flow behavior of the austenite, the cooling rate following hot deformation is the determining factor of the phase transformation mechanism and behavior which establishes the final transformation products and subsequent mechanical properties.

Stress-induced phase transformation characteristics and its effect on the enhanced ductility in continuous columnar-grained polycrystalline Cu–12 wt % Al alloy

The microstructure evolution and stress-induced phase transformation (SIPT) characteristics of continuous columnar-grained (CCG) polycrystalline Cu–12wt%Al alloy during the tensile deformation process were investigated to understand the ductility enhancement mechanism, which shows a tensile elongation of 28%, nine times as high as that of the conventional as-cast polycrystalline counterpart. The results show that the CCG alloy has highly-oriented 〈001〉β texture and straight low-energy grain boundaries (GBs) along solidification direction, which significantly promote grains compatibility during plastic deformation. Additionally, besides β1'→α1' transformation, β1'→γ1'→α1' SIPT was also observed simultaneously in the CCG polycrystalline Cu–12wt%Al alloy, which is different from single crystalline and conventional polycrystalline counterparts. As a consequence of the special phase transformations, high phase transformation plasticity (15–20%) and prominent subsequent plastic deformation (8–13%) of the phase transformation product α1' martensite can be obtained.

Modelling Kinetics of Phase Transformation for the Indirect Hot Stamping Process

To configure the indirect hot stamping process, a finite-element-based prediction of the parts geometry and mechanical properties is required. In case of indirect hot stamping, inhomogeneous cooling schedules cause different phase transformation points and products. The volume expansion caused by phase transformation of fcc into bcc leads to transformation induced stresses that are important for the calculation of overall stresses in press hardened components. To calculate theses stresses correctly, it is necessary to study the kinetics of phase transformation in consideration of the cooling path of an indirect hot stamping process. Dilatometer tests are employed to obtain the kinetics of phase transformation is determined in dilatometer tests. These results are used to identify the parameters for the phase transformation models implemented in the material model *MAT_244 [ that is implemented in the finite-element-code LS-DYNA [. In this context the material model parameters are identified by using evolutionary optimization strategies. Based on the identified parameters the predictive quality of the implemented phase transformation models will be studied in order to improve their prediction accuracy for the indirect hot stamping process.

Thermo-Mechanical Simulation of Ultra-High Strength Ferrite-Bainite Dual Phase Steel

In the present paper, thermo-mechanical simulation of ultra-high strength ferrite-bainite dual phase (DP) steel was performed using a thermomechanical simulator. Continuous cooling transformation (CCT) diagram was constructed for DP steel. The effects of composition and cooling rate on the kinetics and products of phase transformation and the form of the CCT diagram were investigated. The results have shown that the α→γ transformation in DP steel was found to be more sluggish due to the addition of alloying elements. The segregation of manganese and niobium at austenite grain boundaries is expected to cause a solute drag effect, thereby reducing the rate of γ→α transformation in DP steel. The pearlite transformation region disappeared for cooling rates from 0.1 to 20°C/s. The microstructure comprises of bainite and martenite was obtained at fast cooling rate. The present steel is expected to have a higher hardenability.