K C Hari Kumar

An attempt has been made to predict the segregation of carbon in continuously annealed hot-rolled low-carbon aluminum-killed steel (LCAK) through thermodynamic calculations. It is found that the surface carbon content calculated by the Guttmann model is comparable with the experimental results for inert atmosphere condition, whereas a large discrepancy is observed for the conventional continuous annealing furnace atmosphere. The main reason for this difference is the water-gas reaction that takes place on the surface of the steel sheet. It is proposed that this reaction, along with furnace atmosphere, determines the extent of carbon content on the sheet's surface. Based on this, the surface carbon content for various furnace conditions could be predicted. © 2010 Elsevier B.V.

The solidus and the liquidus of U-Zr system were determined using the “spot-technique” over the entire composition range. The experimental facility was fabricated in-house and installed in a glove box for investigating solid to liquid phase transitions in radioactive alloys. This equipment is capable of operating in the temperature range 1273–2273 K. U-Zr alloys of desired composition were prepared by arc melting the constituent elements in required proportion. These alloys were heated by radiofrequency (RF) induction under vacuum and the change in their optical reflectivity was monitored. Chemical analyses of U-Zr alloys indicate that the uncertainty in the composition is ±1.3 at.% Zr (max.). The accuracy of the temperature measurement was ascertained by measuring the melting points of high purity Au, Cu, Ni and Zr. The measured values of the melting points were accurate within ±3 K. This is the first time that the solidus and the liquidus of U-Zr system were determined by using a single experimental technique over the entire composition range.

Ultrafine grained (ufg) and nanocrystalline (nc) materials are widely researched due to significant improvements in yield and fracture strength. However, achieving a reasonable ductility in these materials is still a challenge. Recent results have shown that the combination of high strength and ductility could be achieved in precipitation hardening alloys through severe plastic deformation followed by annealing/ageing treatments. In the present work, the solutionised plates of an Al-Mg-Si alloy (modified AA6061 alloy) were subjected to severe cold rolling at room and liquid nitrogen temperatures to a true strain ∼1.6. The rolled sheets were aged to induce precipitation. The equilibrium second phase distribution for the above alloy was calculated using CALPHAD. The rolled and aged samples were analysed using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness and tensile tests. The stored energy obtained from DSC measurements was found to be independent of the rolling temperature. The volume fraction of S {1 2 3} 〈6 3 4〉 orientation is predominant (∼40%) in both the rolling conditions. The strength and ductility were simultaneously improved following ageing of the cryorolled (CR) and room temperature rolled (RT) samples. Transmission electron microscopy analysis revealed dislocation cell structures in the CR and RT conditions. Analysis of second phases revealed fine spherical Mn rich precipitates (most likely Al6Mn) following ageing. © 2009 Elsevier B.V. All rights reserved.

In the present study, an approach based on binary Gibbs energy-composition (G-x) plots is used for understanding the phase constitution in high-entropy alloys (HEAs). Equimolar HEAs CoCrMnNi, CoCrCuMnNi and AlCoCrMnNi were made using vacuum arc melting and heat treated for 24 h at 1000 °C. XRD and SEM were used for the characterization of constituent phases. A method based on the G-x plots of binary sub-systems was devised for the interpretation of the constituent phases in these alloys and a few other HEAs reported in the literature, namely CoCrFeMnNi and CoCuFeMnNi. The competition between phases and demixing tendencies were correlated with the G-x plots. These plots revealed that in CoCrMnNi and CoCrFeMnNi, tendency for formation of intermetallic phases and demixing were absent. This correlated well with their single-phase disordered FCC (A1) microstructure. On the other hand, the CoCrCuMnNi alloy had a duplex microstructure consisting of two FCC phases that can be attributed to the strong demixing tendency for FCC phase in the Cr-Cu system. The microstructure of the CoCuFeMnNi consisted only single FCC phase. Though, two of the constituent binaries showed tendency for demixing of this FCC phase, it is not strong enough to cause immiscibility. The AlCoCrMnNi alloy gave a duplex microstructure of an ordered (B2) and a disordered (A2) BCC phase. G-x plots showed that the competition from B2 phase in binaries is so strong that its formation inevitably occurred in this HEA. Such crucial insights into how strongly the thermodynamics of the binary sub-systems control the phase constitution in HEAs are offered by the present approach. Although the results can in principle be interpreted using comprehensive Calphad calculations, one requires reliable multicomponent Gibbs energy databases to do so. The approach presented here is rather simple, as it requires only Gibbs energy functions of relevant phases of the constituent binary systems.

The constitutional and thermochemical information of the Co-Zr system is modelled using the Calphad method to obtain a reliable thermodynamic description of the system. Appropriate sublattice models are chosen to describe the Gibbs energy of all stable phases of the system. The Gibbs energies of the disordered BCCA2 and the ordered Co-Zr (BCCB2) are coupled using the orderdisorder model within the framework of sublattice formalism. Results from ab initio calculations are used to aid the optimization of Gibbs energy descriptions of Co-Zr3, Co-Zr2, and BCCB2 phases. Calculated phase diagram and thermochemical data are compared with the experimental data. © 2010 Elsevier Ltd. All rights reserved.

CALPHAD approach has been used to predict the stable phases, their relative amounts and compositions in multicomponent equiatomic high entropy alloys. The results show a good match between the predictions and experimental results on the phase formation for two equiatomic high entropy alloys (CrCoCuNi and CrCuMnNi alloys) prepared by mechanical alloying, considering the kinetic constraints of the non-equilibrium processing route. © Indian Institute of Metals 2012.

The density of Al2O3–CaO–MgO–SiO2 system is calculated using a model for molar volume. The model is similar to the one used for enthalpy of a multicomponent solution in the CALPHAD approach. The expression for molar volume consists of two terms: one representing the volume contribution from pure components and the other the volume of mixing. The molar volume of mixing takes into account of the binary and the ternary interactions. A total of 565 experimental density data for the constituent unary, binary and ternary systems were collected from the literature. These were used as input to simultaneously optimise the model parameters describing the molar volume, after ensuring that the data belong to a fully molten state. During the optimisation 48 data points that showed significant deviation from the average trend were excluded. The optimised model parameters for the unary, binary and ternary subsystems were used to calculate the density of the quaternary system. The calculated results were compared with experimental data. Finally, it is shown that the volume of mixing of binary systems correlates well with the corresponding enthalpy of mixing.

We introduce a novel material based on microstructural engineering of intermetallics at an ultrafine scale. A unique microstructure was developed in a ternary alloy composition (Ni-12 at.% Al-11 at.% Zr) containing two coupled intermetallic phases (Ni3Al and Ni5Zr) consisting of colonies with interconnected lamellae that degenerate into irregular morphologies during growth. This architecture exhibits excellent high temperature microstructural stability, exceptional high strength with adequate tensile ductility at room temperature, and outstanding cyclic oxidation resistance.

Thermodynamic modelling of Ti-Zr-N system is performed using Calphad method coupled with ab initio calculations. The energies of formation of stable and metastable end-members of sublattice formulations of solid phases in Zr-N system and enthalpy of mixing of the mixed nitride (Ti, Zr)N (δ) are calculated using ab initio method. Phonon calculations are used to compute the Gibbs energies of stoichiometric ZrN and the mixed nitride δ. With the aid of experimental thermochemical and constitutional data from the literature along with the results of ab initio calculations, thermodynamic optimization is carried out to obtain the Gibbs energy model parameters.

Quenching and partitioning (Q&P) is one of the most promising heat treatment processes to produce microstructure in steels which contains martensite and high fraction of retained austenite. In the present study, three newly designed steels (A-C) were produced by casting with varying amounts of C, Mn, Si and Al and then subjected to the quenching and partitioning (Q&P) treatment. Microstructural investigation revealed that all the three steels were characterized by two phase microstructure comprising of lath martensite and austenite (interlath film and blocky morphology) as constituents. All the three steels contained almost similar volume fraction of retained austenite in it (0.15-0.18) irrespective of variation in the alloying content. The microstructure of the steel with lowest amounts of C, Si and highest amount of Mn (steel C) contained the finest martensite laths, high fraction of interlath austenite films with high austenite contiguity ratio in comparison with the other two steels (A and B). It also exhibited best strength (882. MPa)-toughness (188. MJ) combination. Austenite fraction analysis on the tensile tested samples suggested that in all three steels a significant percentage (35-50) from the total retained austenite undergo strain induced transformation to martensite. The study suggests that the Q&P steels are very sensitive to small variation in the chemical composition with respect to microstructural features and consequently mechanical properties, although fraction of microstructural constituents does not change significantly. © 2014 Elsevier B.V.