Hari Kumar

Indian Institute of Technology Madras | IIT Madras · Department of Metallurgical and Materials Engineering

Transition metal nitrides, especially tantalum nitrides, are pivotal for applications in extreme environments demanding excellent mechanical properties and thermodynamic stability. Among them, θ$\utheta$‐TaN, a high‐pressure polymorph of tantalum nitride with its exceptional bulk modulus (362 GPa) and hardness (31.7 GPa) promises to have many techn...

Single Crystal Elastic Constants (SECs) are pivotal for understanding material deformation, validating interatomic potentials, and enabling crucial material simulations. The entropy stabilized oxide showcases intriguing properties, underscoring the necessity for the determination of precise SECs to establish reliable interatomic potential and unloc...

Palladium-containing silicon oxycarbide (SiPdOC) ceramics were synthesized using polymethylsilsesquioxane modified with palladium acetate as a single-source precursor. Thus, pyrolysis in Argon at 1100 °C led to nanocomposites consisting of Pd2Si nanocrystallites dispersed in an amorphous SiOC matrix. Exposure of SiPdOC to higher temperatures result...

It is well known that the phase diagrams calculated using bulk Gibbs energy alone is insufficient when the dimensions of alloy particles are less than 100 nm. This is due to the surface energy contribution to the Gibbs energy, which increases as surface-to-volume ratio of alloy particles increases. In this work, we have used the CALPHAD approach to...

A thermodynamic description of the ternary system Co–Nb–Ta is obtained by combining the descriptions of the lower-order systems according to the CALPHAD method. The thermodynamic data set for the binary Co–Nb system is taken from [1], where a new thermodynamic model for the μ-phase was introduced. The μ-phase appears in several transition metal sys...

Mg-based blowing agents exhibit the potential to yield aluminum foams with better structure and properties than those achieved by using conventional blowing agents. However, all the studies to date used a high amount of such blowing agents (e.g., 15 wt pct Mg) for foaming aluminum. In this study, we investigate the minimum amount of Mg blowing agen...

Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive V...

The effect of temperature (350 °C < MS < 450 °C) on the partitioning mechanisms and the final microstructure evolution in a CMnSiAl quenching and partitioning (Q&P) steel was investigated. The microstructure of both the Q&P specimens, comprised of distorted BCC or pseudo tetragonal martensite structure with two different characteristics namely (i)...

Ablation behaviour of poly(hydridomethylsiloxane) derived open and closed porous structured SiOC ceramic foams was evaluated using oxy-acetylene flame at 1500 °C for various time durations. X-ray diffraction and scanning electron microscopy analyses of ablated SiOC ceramic foams revealed the formation of a thin protective SiO2 layer inhibiting furt...

Three different compositions of C–Mn–Si–Al type Q&P steels, A, B and C, were subjected to thermo-mechanically controlled processing (TMCP) at two different rolling temperatures viz. 1075 °C and 1150 °C, using two different levels of strain 0.51 and 1.1, given in single pass, followed by direct quench and partitioning (DQP) to produce a martensite –...

Alloys Fe-18Cr-9Ni-3Al, Fe-16Cr-18Ni-5Al, and Fe-20Cr-20Ni-5Al wt.% were prepared by vacuum arc melting and heat treated at 600 °C/514 days, 800 °C/56 days, 1000 °C/32 days, and 1100 °C/15 days. The microstructures of the samples were characterized to identify the equilibrium phases at each temperature. The compositions of the phases were measured...

Boron is known to influence the grain boundary microchemistry and thereby improve the mechanical performance of superalloys; however, the form in which boron exists at the grain boundary is debatable. We show from electron microscopy characterization that in a directionally solidified Ni-base superalloy boron is present in the form of nearly spheri...

The interdiffusion coefficients can be estimated now purely experimentally from multicomponent diffusion profiles utilizing the recently proposed pseudo-binary and pseudo-ternary diffusion couple methods. One can estimate the composition dependent main interdiffusion coefficients over the composition range and main intrinsic diffusion coefficients...

There is considerable research interest in developing medium Mn steels as part of the 3rd generation of advanced high strength steels, mainly due to the possibility to achieve high tensile strength-high ductility combination at an affordable cost. In the present work, we have designed a steel chemistry and its thermomechanical processing route base...

The main focus of the present study is to compare the effect of different alloying elements on Al–Mg alloy foams. Al–Mg15–X10 (X = Cu, Zn and Si) alloy foams were produced via powder metallurgy route by using Mg as a blowing agent. Macro- and microstructural characterisations of the foams were performed using X-ray tomography, X-ray diffraction and...

Phase diagram calculation of Fe–Ti system is obtained by coupling of CALPHAD with first principle methods. Single Gibbs energy model is used for FeTi to take care of order-disorder transition. First principle calculation shows that Laves phase is ferromagnetic nature with the magnetic moment of 1.417μB which is incorporated in the modelling. For ex...

The traditional method of establishing phase diagrams relied on experimental methods. An alternate approach based on thermodynamic principles, known as the CALPHAD method, paved way to establish phase diagrams of even complex materials efficiently and reliably. Gibbs energy modelling is the central theme of the CALPHAD method. A brief account of th...

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 mo...

The phase equilibria in the Fe-Cr-Ni-Al system was investigated between 800 and 1000 °C with diffusion couple technique. The experiments were carried out at 800, 900, 950, and 1000 °C using the single-phase terminal alloys (in wt%) Fe-25Cr-5Al (BCC, α) and Fe-30Ni (FCC, γ). The experiments showed the formation of β-NiAl phase in the diffusion coupl...

A few decades earlier, Kirkaldy and Lane proposed an indirect method of estimating the tracer and intrinsic diffusion coefficients in a ternary system (without showing experimental verification), which is otherwise impossible following the Kirkendall marker experiments. Subsequently, Manning proposed the relations between the tracer and intrinsic d...

Experimental determination of solidus and liquidus in reactive systems at very high-temperatures requires special equipment and is rather complex. In the present study, we describe setting up an experimental facility based on the “spot-technique.” It was demonstrated that this setup could be used to measure phase transformation temperatures involvi...

The problem of estimation of the tracer diffusion coefficients is solved by utilizing a novel tracer-interdiffusion couple technique even in the absence suitable radioisotope of one of the components and absence of reliable thermodynamic parameters. This is demonstrated by generating reliable and reproducible mobility data in the alloys of the Fe-G...

In this work, we report the results of ab initio calculations of thermochemical properties of several compounds in the Fe-Nd, B-Nd and B-Fe-Nd systems. We have performed DFT + U calculations to compute the enthalpy of formation of the compounds NdB6, NdB4, Nd2B5, Nd2Fe17 and Nd5Fe2B6. It was found that the values obtained with an effective Hubbard...

Grain boundary structure-property relationship was studied in a Ni-base 602CA coarse-grained alloy using a novel correlative tracer diffusion-analytical microscopy approach. Co-existence of several short-circuit contributions to tracer diffusion was distinguished at higher temperatures. These contributions were related to different families of high...

In this work, nanocomposites made of nanosized zirconia crystallized in situ in an amorphous silicon oxycarbo(nitride) (SiOC(N)) matrix have been designed through a precursor route for visible light photocatalytic applications. The relative volume fraction of the starting precursors and the pyrolysis temperatures not only influences the phase fract...

In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach ba...

Grain boundary tracer diffusion of Ni, Fe and Cr was studied in a Ni-base 602CA coarse-grained alloy. A co-existence of several short-circuit contributions was distinguished at higher temperatures in Harrison's B-type regime (773-873 K), which were related to different families of high-angle grain boundaries with distinct coverages by precipitates...

The requirement for an alloy-specific heat treatment of TiH2 for producing foams by the powder metallurgy route was demonstrated in this study. Three heat-treated TiH2 powders were used to produce foams from three Al–Si–Mg alloys having different liquidus temperatures. Dehydrogenation behaviour of heat-treated TiH2 was studied using mass spectrosco...

In this work we report the thermodynamic modelling of the Al-N, B-N and Al-B-N systems by the Calphad approach, making use of the ab initio calculations and the available experimental thermochemical and phase equilibria data. Gibbs energy description for the Al-B system is taken from the literature, with a modification for the model for the liquid...

In this work, we report the results of ab initio calculations of thermochemical properties of several compounds in the Fe-Nd, B-Nd and B-Fe-Nd systems. We have performed DFT+U calculations to compute the enthalpy of formation of the compounds NdB6, NdB4, Nd2B5, Nd2Fe17 and Nd5Fe2B6. It was found that the values obtained with an effective Hubbard U...

Gibbs energy model parameters for Si-Zr-N system were obtained using Calphad approach coupled with ab initio calculations. The enthalpies of formation of α and βSi3N4 in Si-N system and Zr5Si3N end-member in Si-Zr-N system were calculated using density functional theory (DFT). The finite temperature thermodynamic properties were calculated using qu...

Herein, we provide a detailed insight into the precursor chemistry, precursor-to-material transformation and characterization of nanocomposites made of a TiN nanophase and a Si-O-C-N ceramic. The polymer-derived ceramics (PDCs) route applied to synthesize these nanocomposites resulted in the formation of nanocrystals less than 4 nm in size and the...

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 constitue...

Electrolysis of molten fluorides is one of the promising methods for the recovery and recycling of rare earth metals from used magnets. Due to the dearth of phase equilibria data for molten fluoride systems, thermodynamic modelling of LiF-DyF3-NdF3 system using the CALPHAD approach was carried out. Gibbs energy modelling for LiF-NdF3 and LiF-DyF3 s...

Titania (TiO2) is considered to have immense potential as a photocatalyst, the anatase phase in particular. There have been numerous attempts to push the limits of its catalytic activity to higher wavelengths to harness the visible electromagnetic radiation. Most of the investigations till date have been restricted to fine-tuning the bandgap by dop...

Transition metal oxide ceramics have great potential in the field of semiconductor electronics, sensors and energy. However, as the quest for new materials with improved, advanced or novel properties is ever increasing, the possibility of developing multicomponent equimolar oxide ceramics become important with the prospect of synergising the proper...

One of the major challenges in high entropy alloy (HEA) research is to obtain single-phase solid solutions by proper selection of components and processing techniques. Often one encounters situations where topologically close-packed (TCP) phases are present in the HEA microstructures. TCP phases are a class of intermetallic phases that are in gener...

Yttrium monosilicate and disilicate are important materials for environmental barrier coatings. The two silicates were synthesized by sol-gel route and their phase selection upon calcination and thermal exposure was studied. First products of crystallization were the monosilicate and yttria. Amorphous silica precipitated out at 1300 °C as apatite p...

Ultrafine grained AA7075 aluminum alloy (Al–6Zn–2Mg–1Cu), was produced by cryorolling the solutionized samples followed by annealing and peak aging treatments. In the cryorolled and peak aged (CRPA) alloy, the presence of large amount of dislocations acted as nucleation sites for precipitation, resulting in finer and densely distributed precipitate...

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...

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...

The Z-phase is commonly found in ferritic-martensitic steels used in the fabrication of steam pipes of power plants. It is well established that the formation of coarse Z-phase precipitates under service conditions is responsible for reduction in creep strength of this type of steels. In order to address this problem, it is imperative to have a goo...

Nanocrystalline yttria powders were synthesized from deagglomerated yttrium oxalate. The precipitation of this oxalate was carried in two different modes viz., addition of aqueous oxalic acid into yttrium nitrate solution (forward strike) and vice versa (reverse strike) followed by ultrasonication in acetone and water. Nanocrystalline yttria was ob...

In this work we report the Gibbs energy model parameters of equilibrium phases of Fe–Ta system obtained by the Calphad approach. In order to assist the Gibbs energy modeling new constitutional and enthalpy increment data were obtained by experiments. Further, the energy of formation of intermediate phases and the energy of mixing of bcc solid solut...

In this paper we report the Gibbs energy functions for all stable phases in the Fe–Mo system obtained using Calphad method. Newly measured enthalpy increment data, tie-line data and liquidus data for selected compositions are used as input for the Gibbs energy modeling, along with carefully selected thermochemical and phase diagram data from litera...

Experimental investigations and Gibbs energy modelling of KCl–LiCl–UCl3 system employing CALPHAD method are reported. Gibbs energy modelling of the subsystems KCl–UCl3 and LiCl–UCl3 was carried out primarily using phase diagram data from the literature. For the Gibbs energy modelling of the KCl–LiCl subsystem, new phase boundary data corresponding...

Nanocrystalline yttria powders were synthesized from yttrium nitrate by the citrate gel-combustion technique. The auto-ignition of five different gels with fuel to oxidant (citric acid/nitrate) ratios 0.25, 0.5, 0.75, 1.0 and 1.1 was studied. All these mixtures yielded precursors which upon calcination in air at 1073 K yielded yttria. The bulk dens...

The rock salt-type transition metal carbides and nitrides are excellent refractory materials as well as important microstructural constituents in High Strength Low Alloy (HSLA) steels. Therefore, it is important to have knowledge on their elastic, thermophysical and thermochemical properties in order to gain deeper understanding of their role in th...

Using first-principle calculations employing density functional theory (DFT) the stabilityof a (3, 3) carbon nanotube (CNT) intercalated with lithium atoms, with respect to their position aswell as Li/C ratio, is studied. On varying the distance of a lithium atom from the axis of the CNT in theradial direction, through the center of a graphitic hex...

An attempt has been made to predict phase formation using a CALPHAD-based approach for a large number of compositions that are known to form FCC, BCC and a mixture of FCC and BCC phases. The stable phase is assumed to be the first phase that is formed upon cooling from liquid state with the highest driving force. The driving force for other phases...

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 allo...

The effect of microstructure on corrosion behaviour of an ultrafine grained (UFG) Al–4Zn–2Mg alloy produced by cryorolling was investigated. Both UFG and coarse grained (CG) alloys in peak-aged condition were subjected to potentiodynamic polarization and intergranular corrosion (IGC) tests in chloride solutions. UFG alloy exhibited increased corros...

Thermo-kinetic simulation of precipitate evolution during long-term thermal exposure in ferritic–martensitic heat-resistant power plant steels (P91 and P92) is carried out using MatCalc software, in combination with a Gibbs energy database and a mobility database for steels that come with MatCalc. MX and M23C6 are predicted to remain as major preci...

The effect of cryorolling on microstructural evolution and aging characteristics of an Al–4Zn–2Mg alloy is investigated using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and hardness measurements. The Al–4Zn–2Mg alloy in solutionized condition was cryorolled to achieve...

The effect of microstructure on corrosion behaviour of an ultrafine grained (UFG) Al-4Zn-2Mg alloy produced by cryorolling was investigated. Both UFG and coarse grained (CG) alloys in peak-aged condition were subjected to potentiodynamic polarization and intergranular corrosion (IGC) tests in chloride solutions. UFG alloy exhibited increased corros...

An Al-4Zn-2Mg alloy was subjected to cryorolling (CR) followed by short annealing. An average grain size of ~100nm was achieved. Cryorolled samples showed large reduction in grain size due to suppression of dynamic recovery and absence of annihilation of dislocations, as compared to room temperature rolled samples. Further, the ultrafine-grained (U...

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 discrep...

In this paper, the main achievements of the thermodynamic assessment of the Al-Cu-Mg-Si-Zn system are summarised and necessary key experiments identified in order to improve the thermodynamic description. A review of the main features of the thermodynamic models that are used for describing various non-stoichiometric intermediate phases, as well as...

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 BCC_A2 and the ordered CoZr (BCC_B2) a...

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This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume E `Refractory Metal Systems', of SpringerMaterials - Group IV `Physical Chemistry'.

This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume E `Refractory Metal Systems', of Landolt-Börnstein - Group IV `Physical Chemistry'.

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...

This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume D `Iron Systems', of Landolt-Börnstein - Group IV `Physical Chemistry'.

The Bi--In--Sn--Zn system is an important alloy system in lead-free soldering. Thermodynamic descriptions for the ternary systems Bi--In--Sn, Bi--In--Zn, Bi--Sn--Zn and In--Sn--Zn are optimized, using the CALPHAD method and combined to obtain a description of the quaternary Bi--In--Sn--Zn. All available experimental data from the literature are tak...

The Bi–In–Sn–Zn system is an important alloy system in lead-free soldering. Thermodynamic descriptions for the ternary systems Bi–In–Sn, Bi–In–Zn, Bi–Sn–Zn and In–Sn–Zn are optimized, using the CALPHAD method and combined to obtain a description of the quaternary Bi–In–Sn–Zn. All available experimental data from the literature are taken into consid...

Mechanically polished NiTi alloy (50 at% Ni) was subjected to heat treatment in air in the temperature range 300-800 degrees C and characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Thermogravimetry measurements were carried out to investigate the kinetics of oxidation. The res...

Fretting tests under dry unlubricated conditions (22 °C to 25 °C, 50 to 55 pct RH) were performed on monolithic TiB2, TiB2-based cermet with a Ni3(Al,Ti) binder, sialon-TiB2, and ZrO2-TiB2 composites to assess their relative wear performance against bearing-grade steel. Based on the measured friction and wear data, the relative ranking of the inves...

A thermodynamic description of the ternary system Co-Nb-Ta is obtained by combining the descriptions of the lower-order systems according to the CALPHAD method. The thermodynamic data set for the binary Co-Nb system is taken from [1], where a new thermodynamic model for the μ-phase was introduced. The μ-phase appears in several transition metal sys...

The B–Fe binary system has been optimized using the CALPHAD method, utilizing published experimental thermochemical and phase diagram data. Two models for the solid solubility of B in b.c.c. Fe and in f.c.c. Fe are presented, B as a interstitial, and B as a substitutional constituent. Thermodynamic calculations based on the interstitial and the sub...

The discussion on thermodynamic modelling of solution that started at the Ringberg meeting 1996 is continued with some revisions and some details. In particular the methods to implement chemical ordering for a multicomponent system, also on interstitial sublattices and the contributions due to strain energies are discussed.

Thermodynamic optimisation of phase diagrams is a procedure that requires considerable experience and skill. The purpose of this article is to furnish certain guidelines that might facilitate the work and improve the quality of the thermodynamic optimisation of phase diagrams using the Calphad method. Some particulars regarding experimental data, G...

The sublattice modelling is applied for the case of μ-phase taking into account the crystallographic information and solubility range data. Three sublattice models are proposed and applied for the μ-phase. The models are used in the thermodynamic optimisation of the Co-Nb system. The results obtained using the models are compared and some conclusio...

The Gibbs energy descriptions of the equilibrium phases of the Co–Nb system were obtained by a computerised least-squares optimisation of the experimental thermodynamic and phase diagram data from the literature. A review of the available experimental data is presented. The modelling of phases and the optimisation procedure are described. Calculate...