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Phase equilibria in the Fe–Si binary system were investigated experimentally and thermodynamic assessment was carried out. The αFe (A2) + α"Fe3Si (D03) two-phase microstructures at 600°C and 650°C were obtained, whose grain sizes were sufficiently coarsened to be analyzed by FE-EPMA with a spatial resolution below 0.5 μm under the condition of 6 kV accelerating voltage. α'FeSi (B2) + α"Fe3Si (D03) two-phase equilibria above 700°C were detected for the first time and equilibrium compositions were determined by the diffusion couple method. The horn-shaped two-phase miscibility gap extends from the low temperature αFe + α"Fe3Si equilibrium along the B2/D03 second-order transition boundary and closes below 1000°C. Four-sublattice split compound energy formalism was applied to calculate the Gibbs energy of the bcc phases, A2(αFe), B2(α'FeSi) and D03(α"Fe3Si), and the thermodynamic parameters in the Fe–Si binary system were evaluated. Equilibrium relations in the binary system were well reproduced, especially the effect of the B2 and D03 ordering on the liquidus and solidus curves and the miscibility gap between bcc phases. Optimized thermodynamic parameters as well as the experimental results are expected to be helpful for developing higher multi-component systems for practical steels.

Mechanical properties of Fe-20Mn-(10-14)Al-(0-1.8)C (mass%) quaternary and Fe-20Mn-(10-14)Al-(0.75-1.8)C-5Cr (mass%) quinary alloys were investigated by hardness, cold-workability and tensile tests at room temperature. The γ(fcc) alloys in both quaternary and quinary systems with a low density of less than 7.0 g/cm 3 showed an excellent ductility and their hardness and tensile strength increased with increasing Al and C contents. The γ+α(bcc) duplex alloys also exhibited a high tensile strength by controlling the α volume fraction. TEM observation confirmed that high hardness and tensile strength of the alloys with high Al and C contents are caused by the precipitation of nano-size κ-carbide with perovskite structure during cooling from the annealing temperature. Fe-20Mn-11Al-1.8C-5Cr alloy with a density of 6.51 g/cm 3 showed a high specific strength of more than 180 MPa · cm 3 /g with a good tensile elongation of 40 %. The present Fe-20Mn-Al-C(-5Cr) alloys showed a higher specific strength than conventional steels.

The phase equilibria in the FeS–XS and MnS–XS (X: Ti, Nb and V) pseudo-binary systems were investigated by using an X-ray diffraction and electron probe micro analyzer. In the FeS–TiS and FeS–VS systems, a mono-sulfide with the NiAs structure form a complete solid solution, while two-phase separation of the mono-sulfide with the NiAs structure is observed in the FeS–NbS system. In all MnS–XS systems, two-phase region of a Mn-rich mono-sulfide and X-rich mono-sulfide exists widely. Solubility of each elements in the Mn-rich mono-sulfide and X-rich mono-sulfide is small, except that of Mn in Nb-rich mono-sulfide.

Solubility of sulfur in Fe–V and Fe–Nb alloys was experimentally determined using a diffusion couple technique. Solubility product of VS and NbS in ferrite (α) and austenite (γ) phases were determined as log(mass%V)(mass%S)α=4.76−10100/T, log(mass%V)(mass%S)γ=4.90−11000/T and log(mass%Nb)(mass%S)γ=4.70−10500/T in the temperature range from 1523 to 1573 K. A thermodynamic analysis was also carried out to evaluate the formation energy of VS and NbS. The estimated formation energy of VS and NbS are ΔGVSsulfide=−263100+61.0T (J/mol) and ΔGNbSsulfide=−260800+68.4T (J/mol), respectively.

Phase equilibria between the α (A2), α' (B2) and γ (A1 ) phases in the Fe-Rh binary system were investigated using electron probe micro-analysis (EPMA) and differential scanning calorimetry (DSC) techniques. The A2/B2 order-disorder transformation temperature was also examined by a transmission electron microscope (TEM) observations and high-temperature X-ray diffraction (HTXRD). It was confirmed that while the transition temperature from the α (A2) phase to the γ (A1 ) phase decreases with increasing Rh content up to about 20 at% Rh, the b.c.c. region is stabilized by further Rh addition in the composition range between about 20 and 50 at% Rh, and then again the stability of b.c.c. phase decreases in the composition region over 50 at% Rh. It was also shown by TEM observation and HTXRD examination that the compositions of the phase boundary of the A2/B2 ordering at 500°C and 600°C were determined to be 16.5 and 19.1 at% Rh, respectively. On the basis of those experimental results, a thermodynamic analysis was carried out. The results of the thermodynamic calculation suggest that the anomalous behavior of the stability of the b.c.c. phase is caused by the A2/B2 ordering.

The recrystallization and grain growth characteristics of pre-deformed Fe–1.9vol%Bi alloy specimens in the two-phase region of αFe and liquid Bi were investigated by microstructural observation. Precipitate-free zones (PFZs) were mainly formed in the vicinity outside of the curved grain boundaries. Furthermore, large Bi particles were also observed on the grain boundaries of the matrix along the PFZs. These results suggest that the intragranular liquid Bi particles were trapped and dragged by grain boundaries. The grain growth of the matrix phase was extremely retarded by the effect of this dragging.

Reactive diffusion between solid Fe and liquid Zn was experimentally examined using Fe/Zn diffusion couples prepared by an immersion technique. Using this technique, a pure iron sheet was immersed in a molten pure Zn bath with a constant temperature of T=723 K (450°C) for various times up to t=7.2×103 s (2 h). The microstructure of the cross section of the Fe/Zn diffusion couple was observed by scanning electron microscopy, and the chemical composition of each phase was determined by electron probe microanalysis. Interface concentrations of Fe for the ζ-FeZn13 and δ-FeZn7–10 phases at the ζ/δ interface, cζδ and cδζ, and for the liquid-Zn (L) phase at the ζ/L interface, cLζ, determined from the diffusion couples were found to be extremely higher than the corresponding equilibrium values, especially in the early stage of t

Effects of low temperature aging and the addition of Co on the martensitic and magnetic transitions in the Ni–Al–Fe β (B2-type structure) alloy system were investigated using differential scanning calorimetry, transmission electron microscopy and vibrating sample magnetometry. The martensitic transition temperatures of a Ni57Al25Fe18 specimen homogenized at 1 473 K for 1 h were decreased by aging after homogenization, the maximum decrease by aging being observed at 573 K. On the other hand, the substitution of Co or Fe for Ni induced a decrease of the martensitic transition temperatures and an increase of the Curie temperature, while the substitution of Co for Fe resulted in an increase of the martensitic transition and Curie temperatures. Co was thus shown to be an effective element by which both the martensitic and magnetic transition temperatures can be controlled.

The carbide dispersion carburizing process was applied to Fe-based high-speed steels, which contained multiple carbide-forming elements, Mo and V. Fine carbides precipitated during the carburization and the dispersed particles in the Fe–Mo–V–C alloys heat-treated at 1 200°C were identified as VC, (Mo, V)2C, and (Fe ,Mo)6C, which behave as effective inhibitors against the grain growth of austenite and as reinforcers resulting in increased hardness of the martensitic matrix. The maximum Vickers hardness of the Fe–10Mo–5V–2.05C alloy quenched from 1 200°C and tempered at 600°C achieved values of 1 100 and 1 000, respectively. Thermodynamic calculation of the Fe–Mo–V–C quaternary system proved to be useful for optimizing the composition of carbide forming elements, the carburizing conditions, and the microstructure of carburized and solution-treated alloys.

Martensitic transformations in the Fe-(24-30)Ni-(5-8)Si (mass%) alloys have been investigated by means of optical and transmission electron microscopy, differential scanning calorimetry and hardness-testing. The Ms temperature is decreased by Si addition and the morphology of martensite is mainly lenticular in the unaged specimens. However, a pronounced decrease in the Ms temperature and a change in the martensite morphology from the lenticular to the thin plate type are observed on ausaging at 400°C. The increase in austenite hardness, the decrease in the Ms temperature and the increase in the tetragonality of martensite after ausaging at 400°C are clarified as due to the formation of nanoscale particles of γ'-(Ni,Fe) 3 Si with the L12 structure during ausaging at 400°C. The Fe-Ni-Si alloys that form thin plate martensite show the shape memory effect, which arises from the reverse transformation of stress-induced martensite to austenite. Precipitation hardening of the austenite phase by fine y' particles during ausaging improves the degree of shape recovery.

The evolution of sulfide morphology in the Fe-(0.3-18)mass%Cr-(0.05-0.3)mass%S alloys during solidifi-cation and heat treatment was investigated by means of optical microscopy, scanning electron microscopy, analytical electron microscopy and X-ray diffraction. In situ observation of the formation of the fine particle sulfide was also conducted at elevated temperatures by confocal scanning laser microscopy. The morphology of sulfide in the Fe-Cr-S ternary alloys was found to change from a cell wall type to a globular type with increasing Cr content. Accompanying the phase transformation of the matrix from the δ phase to the γ phase, two types of transgranular fine particle sulfide were formed. One is a fine spherical sulfide formed from the FeS-rich liquid phase through the remelting reaction of δ→γ+Liq. in less than 5 mass% Cr alloys, and the other is a fine rod-like sulfide formed through the eutectoid reaction of δ→γ+sulfide in 5 to 13 mass% Cr alloys. The formation mechanism of various types of sulfide morphology was examined based on phase diagram information.

Thermodynamic calculations of the phase equilibria between austenite (γ) and complex carbonitrides containing Nb, Ti and V, (Nb, Ti, V)(C, N), were performed. The solubility product and formation energy of each carbide and nitride as well as the interaction parameters between components were evaluated. Experimental investigation on phase separation in the complex carbonitrides was also carried out using STEM-EDX and X-ray diffraction. The immiscibility of carbonitride was found to strongly affect the phase equilibria even when the content of microalloying elements in IF steels was very low.

Phase equilibria in the Fe-Cr-S system were analyzed on the basis of the thermodynamic evaluation of the Fe-FeS, Fe-Cr, Cr-CrS and Fe-FeS-CrS-Cr systems. The Gibbs energy of individual phase was approximated by the two sub-lattice model for describing the thermodynamic properties. Most of the experimental information was well described by the present set of thermodynamic parameters. In particular, the calculated liquidus surface in the Fe-rich portion and the solubilities of S in the fcc and the bcc phases were shown to be in excellent agreement with experimental results. Discrepancies observed in some experimental phase diagrams were explained on the basis of the calculated results.

The solubility product of TiN in an austenitic steel was experimentally determined using a diffusion couple technique. Diffusion couples consisting of Ti–N steels with various compositions were annealed at temperatures between 1 473 and 1623 K, taking special care to exclude the oxygen contamination during the annealing. The solubility limit of TiN was determined from the break points of iso-activity lines for nitrogen. A thermodynamic analysis was also carried out to arrive at solubility products by considering the wide range of temperature dependence of the TiN formation energy. Solubility products in austenite (γ), ferrite (α or δ) and liquid phases were determined as log(mass%Ti)(mass%N)γ=4.35–14 890/T, log(mass%Ti)(mass%N)α(δ)= 4.65–16310/T and log(mass%Ti)(mass%N)liq=4.46–13500/T respectively.

Effect of titanium addition on the size and dispersion of MnS inclusions in Fe-0.1 C-1 Mn-0.025(mass%) base alloys has been investigated by microscopy. Titanium addition brings about a marked decrease in the size of MnS inclusions which are otherwise large and globular in Ti-free specimens. The formation mechanism of MnS inclusions is discussed on the basis of phase diagram information and thermodynamics of particle engulfment during solidification. It is shown that the reduction in size of the MnS particles consequent on Ti addition is directly the result of successive nucleation events involving (Ti,Mn)O and MnS at the solid/liquid interface of iron.

The effect of alloying elements on the solubility of Cu in solid Fe has been investigated by using solid-liquid diffusion couples. An addition of Co, Ni, and Al results in an increase of the solubility of Cu, while that of V, Cr, Mn, Si, and Sn decreases the solubility. Thermodynamic calculation of the solid/liquid phase equilibria in the Fe-Cu-X ternary system has been also performed, where a set of thermodynamic parameters of solid and liquid phases has been evaluated. Furthermore, the equation that describes the change of the solubility of Cu in solid Fe with an addition of alloying elements has been derived by thermodynamic analysis of these experimental values.

The morphology of MnS inclusions in steel formed during solidification and the modification of this morphology by additions of alloying elements Al, Si, C and Ti have been investigated using optical and electron microscopy techniques. In sulfur-lean melts the morphology of the secondry MnS inclusion formed after the primary crystallization of the Fe phase during solidification can be classified under the following categories: i) Globular or droplet shaped MnS resulting from a monotectic reaction, ii) Rod like MnS formed through an eutectic reaction and iii) a fish-bone type MnS formed as a result of an irregular eutectic reaction. In sulfur-rich melts the primary crystallization phase is MnS and depending on the nature of additional elements present in the melt and the melt atmosphere, the morphology of these primary MnS inclusions can be classified as iv) spherical, v) dendritic or vi) angular. The formation mechanism of the different MnS morphologies is discussed on the basis of phase diagram information. It sis shown that the spherical morphology of the primary and secondary MnS inclusions is a result of metastable reactions, while the eutectic, dendritic and anglular morphologies are the products of stable reactions, with the high melting dispersed particles like TiN and Al2O3 acting as nucleants for the MnS crystals. It is also shown that the addition of C and Si increases the probability of stable reactions by increasing the temperature interval between the eutectic and the monotectic points.

Phase constitutions of Fe-(20-30)wt%Mn-(0-10)wt%Al-C alloys have been investigated by electron probe microanalysis and transmission electron microscopy. The phase relations between austenite and perovskite carbide, (Fe, Mn)3AlC in the temperature range of 900-1 200°C have been carefully examined. An L12-type ordered structure, which was reported to be formed in rapidly solidified alloys as a metastable phase has not been detected in specimens aged at temperatures above 600°C.

In order to develop a series of austenitic manganese nonmagnetic steels, a systematic investigation has been made on the effects of chemical compositions including impurities and also heat treatments on the magnetic permeabilities, mechanical properties, machinabilities, and weldabilities.The magnetic permeability depends on the austenite stability, that of strained specimen was expressed as linear function of Md temperature and the increase due to isothermal austenite decomposition was found to be suppressed by vanadium addition. A linear regression equation relating 0.2% proof stress to chemical composition indicated a great strengthening effect of vanadium comparable to that of carbon. Isothermal decomposition of austenite also lowers the Charpy impact toughness. It was found that the 0.7C-15Mn-1Ni steel was the optimum base composition. A modified grade of the base steel to possess both improved machinabilities and weldabilities was developed by lowering the phosphorus level and also adding free-machining elements as calcium and sulfur.