Eutectoid Temperature Research Articles

Ab initio study of the intermetallics in Nb–Si binary system

First-principles calculations were carried out for the Nb3Si, αNb5Si3, βNb5Si3 and NbSi2 intermetallic compounds and their constituent elements. The enthalpies of formation were computed for all the intermetallic compounds, as well as the eutectoid transformation temperature for Nb3Si → Nb + αNb5Si3, the transition temperatures of βNb5Si3 to αNb5Si3, and the range of stability of the NbSi2. There was good agreement between the results of the present study and the Nb–Si phase diagram. The elastic constants, bulk, shear and Young's moduli, Poisson's ratio and Debye temperature were also calculated for the intermetallic compounds and Nb and Si. The Cauchy pressures, Pugh's index and Poisson's ratio indicate that the high temperature phases (βNb5Si3 and Nb3Si) should be more ductile (with Nb3Si being the most ductile) than the αNb5Si3 and NbSi2 (with the latter being the most brittle).

Eutectoid Transformation in Zn-Al Alloys Solidified by Rapid Cooling

Rapid solidification represents a very attractive approach to develop new Al alloys in an economically convenient way. The lower segregation content, refined grains, higher ultimate tensile and yield strengths combined to a good ductile properties confer to these materials an interesting position also in the so critical automotive and/or aeronautical applications. The current paper presents results of an analysis concerning Zn-Al alloys with a new metastable microcrystalline structure, where Copper has been used as alloying element. With addition of elements as Ti and B modification of the microstructure has been reached. In order to study the influence of the cooling rate on the microstructure and structural transformations castings has been realized with melt spinning technique, in both steel and sand moulds. For morphological investigations optical and scanning electron microscopy has been employed. By dilatometric analysis and X-Ray diffraction technique the thermodynamic factors, the kinetic effects, phase transformations and the volume changes related to the transformations produced at the eutectoid temperature have been monitored. For the aforementioned field of applications the most favourable composition has been chosen: based on the up to date outcomes, by modifying the original alloy with some elements a quite homogeneous structure combined with good mechanical behaviour has been obtained.

Effects of high magnetic field on isothermal pearlite transformation and microstructure in a hypereutectoid steel

A high magnetic field was applied during the isothermal pearlite transformation of a hypereutectoid carbon steel at three different temperatures. It was found that the magnetic field applied can increase the transformation rate and the pearlite nodule growth rate, decrease the interlamellar spacing of pearlite and the amount of low angle boundary in ferrite lamella. The magnetic field effects are more obvious at high temperature. The mechanism of field-effect was discussed by analyzing the influences of high magnetic field on the eutectoid temperature, diffusion activation energy in austenite, Gibbs free energy of ferrite and dislocation mobility.

Phase behavior and superprotonic conductivity in the Cs1−xRbxH2PO4and Cs1−xKxH2PO4systems

The solid acid compound CsH2PO4 (CDP) adopts a cubic structure of high conductivity above 228 °C, rendering it attractive as a fuel cell electrolyte for intermediate temperature operation. This superprotonic phase is stable from the phase transition temperature, Ts, to the dehydration temperature, Td, where the latter depends on water vapor pressure (e.g. Td = 290 °C at pH2O = 0.8 atm). In this work we examine the possibility of modifying these temperatures and thereby, amongst other characteristics, fuel cell operating conditions by introduction of Rb and K as substituents for Cs in CDP. The phase behavior of the Cs1−xRbxH2PO4 and Cs1−xKxH2PO4 pseudo-binary systems are determined by in situ X-ray diffraction (XRD) and thermal analysis. It is found that RbH2PO4 (RDP) and CDP are entirely miscible both below and above the transition to the cubic phase. With increasing Rb concentration, Ts increases and Td decreases. In contrast, K has limited solubility in CDP, with a 27 at.% solubility limit in the cubic phase, and both Ts and Td decrease with K content. The eutectoid temperature in the Cs1−xKxH2PO4 system is 208 ± 2 °C and the K solubility decreases sharply below this temperature. In both systems, conductivity decreases monotonically with increasing substituent concentration. Furthermore, even after normalization for cation size, the impact of K is greater than that of Rb, suggesting local disruptions to the proton migration pathway, beyond global changes in unit cell volume. Although this investigation shows unmodified CDP to remain the optimal fuel cell electrolyte material, the study provides a possible framework for elucidating proton transport mechanisms in superprotonic conductors.

Influences of non-stoichiometry on thermodynamics and kinetics of iron oxides reduction processes

Non-stoichiometry influences both the thermodynamic and kinetic analyses of the iron oxide redox processes. The thermochemical data of iron oxide redox reactions in various textbooks are not consistent, and the kinetic characteristics are not well understood because of the non-stoichiometry. To clarify such confusions, some famous thermodynamic data are compared, and highly precise experimental work conducted for verification. It is shown that the thermodynamic data for the pure iron oxide reduction reactions from JANAF agree well with the experimental results; the eutectoid temperature of iron oxides was proven to be 576°C; Dieckmann’s defect model of magnetite was proven in good agreement with the experimental results only at high oxygen activities but not low oxygen activities; and the dependences of iron deficiency on Δwt-% (weight loss ratio) and Fe2+% (ferrous ratio) were calculated and experimentally verified in pure iron oxides reduction processes.

Continuous and Discontinuous αTi Layers Between Grains of β(Ti,Co) Phase

The microstructure of Ti-Co polycrystals with 1.6 and 3.2 at.% Co has been studied between 690 and 810 °C after long anneals (720-860 h) in the αTi+β(Ti,Co) two-phase area of the Ti-Co phase diagram. It has been observed that depending on the annealing temperature and GB energy, the αTi phase forms either chains of separated lens-like precipitates or continuous uniform layers along β(Ti,Co)/β(Ti,Co) GBs. In other words, β(Ti,Co)/β(Ti,Co)GBs completely or partially wetted by the αTi phase were observed. At 690 °C, slightly above eutectoid temperature Tet = 685 °C, the portion of the completely wetted β(Ti,Co)/β(Ti,Co) GBs is 25% for the Ti-1.6 at.% Co alloy and 60% for the Ti-3.2 at.% Co alloy. It increases with increasing temperature and reaches the maximum of 80% for the Ti-1.6 at.% Co alloy at 780 °C and of 75% for the Ti-3.2 at.% Co alloy at 750 °C. At 810 °C, i.e., close to the upper border of the αTi + β(Ti,Co) two-phase area of the Ti-Co phase diagram, the portion of the completely wetted β(Ti,Co)/β(Ti,Co) GBs drops down to 40% for the Ti-1.6 at.% Co alloy and 20% for the Ti-3.2 at.% Co alloy. Thus, it has been observed for the first time, that the portion of grain boundaries completely wetted by the layers of a second solid phase can non-monotonously depend on the temperature.

Thermomechanical process optimization of U-10 wt% Mo – Part 1: high-temperature compressive properties and microstructure

Nuclear power research facilities require alternatives to existing highly enriched uranium alloy fuel. One option for a high density metal fuel is uranium alloyed with 10wt% molybdenum (U–10Mo). Fuel fabrication process development requires specific mechanical property data that, to date has been unavailable. In this work, as-cast samples were compression tested at three strain rates over a temperature range of 400–800°C to provide data for hot rolling and extrusion modeling. The results indicate that with increasing test temperature the U–10Mo flow stress decreases and becomes more sensitive to strain rate. In addition, above the eutectoid transformation temperature, the drop in material flow stress is prominent and shows a strain-softening behavior, especially at lower strain rates. Room temperature X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy analysis of the as-cast and compression tested samples were conducted. The analysis revealed that the as-cast samples and the samples tested below the eutectoid transformation temperature were predominantly γ phase with varying concentration of molybdenum, whereas the ones tested above the eutectoid transformation temperature underwent significant homogenization.

Microstructural evolution and phase transition dependent on annealing temperature and carbon content for LaFe11.5Si1.5Cx compounds prepared by arc-melting

Three different parts of LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2, 1.0) ingots prepared by arc-melting were determined. The region of the sample close to the surface towards the arc was marked as part 1 and the region of the sample close to the water jacketed copper crucible was marked as part 3. Without affecting the microstructure of part 1, carbon doping could increase the amount of 1:13 phase of the LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2) ingots, which mainly existed in the part 3. The DSC curves of part 1 of LaFe11.5Si1.5Cx(x = 0, 0.2) ingots show a peak at 1445 K moved to 1402 K after carbon doping. The eutectoid reaction: LaFeSi → 1:13 phase + La5Si3 is believed to be around the peak. After annealing at 1353 K, 1443 K and 1473 K for 6 h, the microstructure of LaFe11.5Si1.5Cx(x = 0, 0.1, 0.2) compounds was investigated. Too high annealing temperature could induce the formation of La5Si3 phase and even the growth of α-Fe. The best annealing temperature should be slightly lower than the eutectoid reaction temperature, even if larger atomic diffusion rate can be obtained at higher temperature. Carbon doping could reduce the eutectoid reaction temperature. Energy can be saved for a low annealing temperature. In addition, carbon doping can accelerate the formation of the 1:13 phase by improving the nucleation rate.

Complex Widmanstätten plates consisting of cementite and ferrite, product phases of a eutectoid reaction, in an Fe–C–Mn alloy

Complex Widmanstätten plates of cementite and ferrite, product phases of a eutectoid reaction, in an Fe–10.6 Mn–0.64 C steel have been investigated. Different forms of proeutectoid M 3 C carbide appear in the austenite as grain boundary precipitates or Widmanstätten plates in the austenite at temperatures below 1048 K. Some Widmanstätten plates may consist of lamellar cementite and ferrite as complex Widmanstätten plates at temperatures below 1023 K. Aside from the complex plates, pearlite nodules exist at the austenite grain boundaries at temperatures below 973 K. Therefore, the lamellar cementite and ferrite in both complex Widmanstätten plates and pearlite nodules are all product phases from the eutectoid reaction. The upper temperature limit for the eutectoid reaction is between 1023 K and 973 K. The complex Widmanstätten plates have some characteristics similar to inverse bainites, but appear at much higher temperatures, near the eutectoid temperature of the steel. ► Complex Widmanstätten plates of M 3 C and ferrite in an Fe–C–Mn alloy were discovered. ► M 3 C appears in the austenite as grain boundary precipitates or Widmanstätten plates. ► Complex Widmanstätten plates have some characteristics similar to inverse bainite. ► Pearlite nodules exist at the austenite grain boundaries of the high manganese steel.

Study on the Fast Nitriding Process of Active Screen Plasma Nitriding

Abstract A new nitriding technology using active screen plasma nitriding (ASPN) was carried out in order to increase the nitriding speed of AISI 5140 steel. The fast nitriding process is based upon the different solubility and diffuse rate of nitrogen atoms in austenite and in ferrite of steel respectively. First, the nitriding samples were heated above the eutectoid temperature for a few minutes to dissolve a large amount of nitrogen and form a nitrogen-rich layer on the surface of the nitrided samples. Then, the nitriding temperature was decreased below the eutectoid temperature and kept the temperature for a long time to make the dissolved nitrogen in the nitrogen-rich layer diffuse into ferrite. The two different nitriding processes were carried on alternately. Experimental results indicate that the fast nitriding process not only enhances the nitriding speed remarkably, but also keeps the high hardness of the nitrided layer. The new fast nitriding technology with nitrogen-rich layer can be explained with “absorption- diffusion” model.

Magnetic Field-Induced Granular Pearlite at Early Stages of Phase Transformation

ffects of high magnetic fields (HMF) up to 19.81T on pearlite phase transformation are studied by examination of the microstructures of a Fe-0.47C-2.3Si-3.2Mn (wt %) alloy partially isothermally processed above the eutectoid temperature. The results show that granular pearlite (GP) can be obtained at earlier transformation stages. The evolution of the granular pearlite is always accompanied by the formation of lamellar pearlite. TEM analysis reveals the existence of sub-grain boundaries within GP colonies and indicates that the nucleation of ferrite matrix in GP belongs to multiple nucleation mechanism. Most of carbides at the early stage of pearlite formation are found to precipitate at the α/γ interface--the growing front of ferrite phases, and some of coarse carbides can further develop into thin lamellar cementite.

Phase Equilibria of the Zinc Oxide–Cobalt Oxide System in Air

Phase equilibria of the zinc oxide–cobalt oxide system were studied by a combination of X‐ray diffraction and in situ electrical conductivity and thermopower measurements of bulk ceramic specimens up to 1000°C in air. Rietveld refinement of X‐ray diffraction patterns demonstrated increasing solubility of Co in ZnO with increasing temperature, which is supported by the slight increase in wurtzite (Zn1−xCoxO) cell volume and lattice parameter a versus temperature determined for the phase boundary compositions. Similarly, the solubility of Zn in CoO increased with increasing temperature. In contrast, the spinel phase (ZnzCo3−zO4) exhibited retrograde solubility for Zn. Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co1−yZnyO into wurtzite and spinel is 894 ± 3°C, and the upper temperature limit of the stability of the spinel phase is 894°C–898°C for the compositions Co/(Zn+Co) = 0.82–1. The resulting composition‐temperature phase diagram is presented and compared to the earlier (1955) diagram by Robin.

Nitrocarburising of Low Alloyed Case Hardening Steels Applying Three Different Temperatures

Nitriding is one of the most important processes among surface technologies. The purpose of this process is to produce a wear resistant layer on the surface of components, to meet the complex requirements. Ferritic nitrocarburising was carried out at three different temperatures (520, 570 and 620°C) and with five different process times, to study the effects on process parameters . The experiments proved that in case of ferritic nitrocarburising, it is possible to produce larger diffusion zone during the same time. If the temperature is higher than the iron-nitrogen eutectoid temperature, the diffusion is deeper, but we should consider the softening of the core.

Thermo-Metallurgical Modeling of Nodular Cast Iron Cooling Process

A new numerical model to describe the microstructural evolution of a eutectic nodular cast iron during its cooling is presented. In particular, equiaxial solidification assuming an independent nucleation of austenite and graphite nodules is considered. In this context, the austenite has dendritic growth whereas the graphite grows with a spherical shape. After solidification occurs, the model assumes that the graphite nodules present in the cast iron continue growing since the carbon content in austenite decreases. Once the stable eutectoid temperature is reached, the alloy undergoes the austenite-ferrite transformation. The nucleation of the ferrite takes place at the contour of the spherical graphite nodules where austenite has low carbon concentration. A ferrite shell surrounding the graphite nodules is formed afterward by means of a process governed by carbon diffusion. Then, a ferrite-pearlite competitive transformation occurs when the temperature is below the metastable temperature. This thermo-metallurgical model is discretized and solved by means of the finite element method. The model allows the computation of cooling curves, fraction evolution for each component, and size and distribution of graphite nodules. The present numerical results are compared with experiments using standardized Quick-cup-type cups, and satisfactory numerical predictions of the final microstructure and cooling curves are achieved.

Eutectoid Al-51at%Zn Alloy Studied by Isothermal Mechanical Spectroscopy

sothermal mechanical spectroscopy measurements were performed in an Al-51 at % Zn alloy at various temperatures below and above the eutectoid transition temperature: during a heating the α-β eutectoid mixture changes into α solid solution at 550 K. Damping experiments were performed in a very large frequency range (10-5– 50 Hz) between room temperature and 673 K. Internal friction spectra performed between 200 K and 540 K, exhibit two thermally activated relaxation peaks (P1 and P2). P1 decreases and disappears with the increase of measurement temperature while P2 appears and increases. P2 totally disappears above the eutectoid transition temperature. Above 550 K, a new peak (P3) is evidenced at very low frequency. The relaxation parameters of P3 (limit relaxation time τ0= 9×10-7and activation energy H = 105 kJ/mole (1.1 eV)) allow to associate this peak with the motion of sub grain boundaries. P1 and P2 (τ0≈ 10-7and H ≈ 70 kJ/mole (0.75 eV) for both peaks) are associated with a thermally induced atom diffusion across the α-β interface.

Effects of a High Magnetic Field on Austenite Decomposition in High Purity Fe-1.1C (wt.%) Alloy

The effects of the 12T magnetic field on transformed morphology and crystallographic characteristics of high purity Fe-1.1C (wt.%) alloy during diffusional decomposition of austenite are studied by SEM/EBSD. With the applied field, the lamellar spacing of pearlite is greatly enlarged and the spheroidization tendency of the pearlitic cementite is enhanced. This can be attributed to the thermodynamic effect of the field on phase equilibrium by elevating the eutectoid temperature; hence the eutectoid transformation happens at higher temperature that favors carbon diffusion. As a consequence, the eutectoid cementite that is usually in lamellar shape spheroidizes to reduce its specific surface area. In addition, three orientation relationships between ferrite and cementite in pearlite, namely Isaichev ORs and two near Pitsch-Petch ORs (P-P1 and P-P2), are found both in non-field and field treated specimens. The magnetic field favors the occurrence of P-P2 OR due to the promotion of nucleation of the pearlitic ferrite. However, there is no obvious effect of the field on the crystallographic orientation of pearlitic ferrite, and that is probably related to the quick saturation of magnetization that results in the disappearance of magnetization anisotropy.

Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

Abstract The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al–Bi–Sb) and Ca–Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm −2 , despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.

Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature

Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42–45)Al-(3–5)Nb-(0.1–2)Mo-(0.1–0.2)B (in atom percent) are termed TNMTM alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/a2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro- and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature.

Stability of nanostructured W-C phases during carburization of WO 3

The formation and stability of nanostructured W-C phases during carburization of chemically synthesized WO 3 were investigated by quantitative X-ray diffraction using Rietveld structural refinement. It was observed that despite a greater thermodynamic stability of WC (hexagonal; P-6 m2) below the eutectoid temperature of 1523 K, WC 1− x (cubic; Fm3 m) was first to form followed by W 2C (hexagonal; P6 3 /mmc). However, WC 1− x and W 2C were metastable in the temperature range of 1123–1323 K and had a tendency to transform to W 2C or WC, respectively. The observed phase formation sequence was attributed to the kinetic factors involved in the reduction of WO 3 to nanostructured W (cubic; Im3 m) followed by the diffusion of C in the as reduced nanostructured W leading to C-deficient tungsten carbides (WC 1− x and W 2C) in the surface layer. The C-deficient W-C phases gradually transformed to stoichiometric WC at elevated temperature by bulk diffusion of C.

Effect of Divorced Eutectoid Transformation Temperature on Behaviour of Cementite Growth in GCr15 Steel

Cementite spheroidization of GCr15 bearing steel includes granulating and growth of cementite. The effects of divorced eutectoid transformation (DET) temperatures on growth of cementite were studied. It was found that the particles of cementite are fine and uniform when DET occurs at 690～720°С, and alloy elements mostly precipitate at different temperatures during cementite growth of GCr15 bearing steel; Cr and Si mostly precipitate in the eutectoid temperature upper limit, while Mn is in the eutectoid temperature lower limit.