Subramanian Raju | Indira Gandhi Centre for Atomic Research (original) (raw)
Papers by Subramanian Raju
Metallurgical and Materials Transactions A, 2021
This study presents a comprehensive analysis of the entropy of condensed phases, its temperature,... more This study presents a comprehensive analysis of the entropy of condensed phases, its temperature, pressure, and composition dependence on a macroscopic correlative platform. Two principal contributions to total nonconfiguration entropy (ST) are outlined. They are: (i) the pure thermal (Sth) contribution arising from the isochoric temperature dependence of Gibbs energy (GT) and (ii) the elastic contribution (Sel) representing the dilatational volume effects. It is then argued that entropy variation among a group of alloy phases can be exclusively related to molar volume, only when both thermal pressure (pth) and thermal entropy terms assume common values for all members. This argument is extended to establish a linear relationship between transformation entropy (ΔStr) and transformation-induced volumetric strain (ΔVtr/V). The temperature and pressure dependencies of entropy have been discussed in terms of the complementing roles of Sth and Sel and simple approximations to these effects are suggested. A macroscopic power law relation for systematizing the standard entropy variation using a composite scaling parameter (MV2/3/Tm) has been proposed, and its validity is demonstrated for both solid and liquid metals. This power law correlation has been exploited to deduce the following outcome: (i) a simple approximation for the initial slope (dp/dTm) of p–Tm melting curve, (ii) self-consistent correlation of entropy with specific heat and Debye temperature, (iii) estimation of entropy of metastable phases, and (iv) correlating dilute solution entropy with volume effects of alloying.
Indian Welding Journal, 2013
Metallurgical and Materials Transactions A, 2019
The variation of bulk modulus with composition of alloy phases is a core issue in any thermodynam... more The variation of bulk modulus with composition of alloy phases is a core issue in any thermodynamic theory of alloy formation. Though it emerges from fundamental theory that elastic property, especially the bulk modulus is crucially dependent on interstitial electron density (q b) distribution and its variation with respect to alloy composition, a coherent thermodynamic treatment connecting bulk modulus with electron density together with its composition dependence, is still lacking. The present study addresses this issue for solid solution alloys. A phenomenological analysis of the composition dependence of bulk modulus (B T) of single phase alloys has been presented in terms of bonding charge density (q b) and its corresponding change with atomic volume (V). This link is developed using the fundamental interrelationship existing between bulk modulus, electron density, and molar volume. The change in bonding charge density (Dq b) with composition (x) has been modeled using an exponential scaling relation with respect to the corresponding change in atomic volume (DV). This scaling relation is based on the concept of the universal binding energy relation, which in turn results in a simple exponential variation of bulk modulus with composition-induced change of atomic volume. It is also shown that a common functional representation namely, B T (V) % B o exp{C 9 (DV)}, can be obtained for the temperature, pressure, and composition dependence of bulk modulus in terms of corresponding changes in volume (DV). The constant C takes context-dependent meaning and values. The applicability of this exponential relation towards representing the effect of composition on bulk modulus has been satisfactorily demonstrated for many substitutional alloy systems.
Materials Today: Proceedings, 2019
The defect recovery and recrystallization as well as the strain induced martensite formation in h... more The defect recovery and recrystallization as well as the strain induced martensite formation in heavily cold rolled 316L austenitic steel has been investigated using Impulse excitation technique. It has been observed that the amount of strain induced martensite is small even after 80% cold rolling and hence the martensite reversion do not alter the elastic modulus to any significant extent during annealing. On the other hand the stored energy of deformation as well as the subsequent annealing phenomenon like recovery and recrystallization has significant effect on the elastic properties of these alloys. The measured temperature variation of elastic moduli data during recovery and recrystallization has been analyzed using the empirical Kolmogorov-Johnson-Mehl-Avrami (KJMA) modelling framework to obtain the kinetic parameters in the steel.
Journal of Alloys and Compounds, 2008
The thermal properties of Ti4 wt.%Nb4 wt.%Zr alloy, namely the enthalpy increment and heat capa... more The thermal properties of Ti4 wt.%Nb4 wt.%Zr alloy, namely the enthalpy increment and heat capacity have been characterized as a function of temperature using drop and differential scanning calorimetry, respectively. The measured data clearly attested to the presence of a phase ...
Metallurgical and Materials Transactions A, 2016
The enthalpy of formation ΔoHf is an important thermodynamic quantity, which sheds significant li... more The enthalpy of formation ΔoHf is an important thermodynamic quantity, which sheds significant light on fundamental cohesive and structural characteristics of an alloy. However, being a difficult one to determine accurately through experiments, simple estimation procedures are often desirable. In the present study, a modified prescription for estimating ΔoHfL of liquid transition metal alloys is outlined, based on the Macroscopic Atom Model of cohesion. This prescription relies on self-consistent estimation of liquid-specific model parameters, namely electronegativity (ϕL) and bonding electron density (nbL). Such unique identification is made through the use of well-established relationships connecting surface tension, compressibility, and molar volume of a metallic liquid with bonding charge density. The electronegativity is obtained through a consistent linear scaling procedure. The preliminary set of values for ϕL and nbL, together with other auxiliary model parameters, is subsequently optimized to obtain a good numerical agreement between calculated and experimental values of ΔoHfL for sixty liquid transition metal alloys. It is found that, with few exceptions, the use of liquid-specific model parameters in Macroscopic Atom Model yields a physically consistent methodology for reliable estimation of mixing enthalpies of liquid alloys.
In present study, a two dimensional cellular automata (CA) simulation has been carried out to stu... more In present study, a two dimensional cellular automata (CA) simulation has been carried out to study the effect of nucleation mode on the kinetics of recrystallization and microstructure evolution in an austenitic stainless steel. Two different nucleation modes i.e. site saturation and continuous nucleation with interface control growth mechanism has been considered in this modified CA algorithm. The observed Avrami exponent for both nucleation modes shows a better agreement with the theoretical predicted values. The site saturated nucleation mode shows a nearly consistent value of Avrami exponent, whereas in the case of continuous nucleation the exponent shows a little variation during transformation. The simulations in the present work can be applied for the optimization of microstructure and properties in austenitic steels.
Journal of Chemical Sciences, 2010
Metallurgical and Materials Transactions A
Journal of Chemical Sciences
A brief review of the issues involved in modelling of the solid state transformation kinetics is ... more A brief review of the issues involved in modelling of the solid state transformation kinetics is presented. The fact that apart from the standard thermodynamic parameters, certain path variables like heating or cooling rate can also exert a crucial influence on the kinetic outcome is stressed. The kinetic specialties that are intrinsic to phase changes proceeding under varying thermal history are enumerated. A simple and general modelling methodology for understanding the kinetics of non-isothermal transformations is outlined.
Materials Science Forum, 2015
The feasibility of employing the indigenously developed Ferroboron alloy as an alternate neutron ... more The feasibility of employing the indigenously developed Ferroboron alloy as an alternate neutron shield material in combination with 9Cr-based ferritic steel clad in future Indian Fast Breeder Reactors (FBR), has been investigated from a metallurgical perspective. In this regard, extensive studies have been undertaken to estimate quantitatively the nature of interaction between Ferroboron and P91-ferritic steel at high temperatures. It is found that in the temperature range 550 to 600°C, 9Cr-based ferritic steel is fully compatible with Ferroboron. However, at higher temperatures, Feroboron interacts with ferritic steel; but the maximum estimated loss of clad thickness is restricted to about 250 μm for 60 years of service.
Materials Science and Technology, 2015
ABSTRACT The present paper presents the results of an extensive electron microscopy investigation... more ABSTRACT The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr-W-V-Ta reduced activation ferritic-martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.
Transactions of the Indian Institute of Metals, 2013
ABSTRACT The thermally-activated lattice defect relaxation kinetics in 90 % cold-rolled titanium ... more ABSTRACT The thermally-activated lattice defect relaxation kinetics in 90 % cold-rolled titanium has been investigated through differential scanning calorimetry, isothermal aging experiments, supplemented by metallography and theoretical modelling. Calorimetry results revealed two major softening thermal arrests at 755 and 944 K, respectively, marking the onset of recovery and recrystallization of deformed state. Adoption of higher heating rates resulted in overlapping of recovery and recrystallization; in fact, the excess unspent driving force due only to partial recovery accelerated the recrystallization. Standard isoconversional analysis of calorimetry data yielded 115 and 147 kJ mol−1 for apparent activation energies of recovery and recrystallization, respectively. These values agree well with the results of an independent analysis of time, temperature variation of hardness of isothermally annealed samples.
Transactions of the Indian Institute of Metals, 2013
ABSTRACT The thermo-kinetics aspects of phase transformations in U rich U–xZr binary alloys, with... more ABSTRACT The thermo-kinetics aspects of phase transformations in U rich U–xZr binary alloys, with x = 2, 5 and 10 wt% Zr have been investigated using dynamic calorimetry. The on-heating and cooling transformations at controlled scan rates in the range, 1–99 K min-1, have been monitored and the following transformation sequence is obtained at slow heating (3 K min-1) of a U–2Zr alloy: (i) a or a0 (distorted orthorhombic martensite) ? d(UZr2) ? a ? c2 (bcc phase enriched in Zr); (ii) a ? c2 ? b (tetragonal) ? c2; (iii) b ? c2 ? b ? c1 (bcc phase enriched in U); (iv) b ? c1 ? c; (v) c (bcc) ? liquid (melting). Similar transformation sequence for other compositions with varying enthalpy effects has been witnessed for 5 and 10 Zr alloys. The observed transformation characteristics are rationalized for the effect of Zr content and heating/cooling rate variations.
... P KUPPUSAMI, E MOHANDAS, S RAJU and VS RAGHUNATHAN* Metallurgy Division, Indira Gandhi Centre... more ... P KUPPUSAMI, E MOHANDAS, S RAJU and VS RAGHUNATHAN* Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Abstract. ... step processes, the initial and the final slopes of resistance curves of figure 2 were plotted as a function of I/kT. ...
Metallurgical and Materials Transactions A, 2021
This study presents a comprehensive analysis of the entropy of condensed phases, its temperature,... more This study presents a comprehensive analysis of the entropy of condensed phases, its temperature, pressure, and composition dependence on a macroscopic correlative platform. Two principal contributions to total nonconfiguration entropy (ST) are outlined. They are: (i) the pure thermal (Sth) contribution arising from the isochoric temperature dependence of Gibbs energy (GT) and (ii) the elastic contribution (Sel) representing the dilatational volume effects. It is then argued that entropy variation among a group of alloy phases can be exclusively related to molar volume, only when both thermal pressure (pth) and thermal entropy terms assume common values for all members. This argument is extended to establish a linear relationship between transformation entropy (ΔStr) and transformation-induced volumetric strain (ΔVtr/V). The temperature and pressure dependencies of entropy have been discussed in terms of the complementing roles of Sth and Sel and simple approximations to these effects are suggested. A macroscopic power law relation for systematizing the standard entropy variation using a composite scaling parameter (MV2/3/Tm) has been proposed, and its validity is demonstrated for both solid and liquid metals. This power law correlation has been exploited to deduce the following outcome: (i) a simple approximation for the initial slope (dp/dTm) of p–Tm melting curve, (ii) self-consistent correlation of entropy with specific heat and Debye temperature, (iii) estimation of entropy of metastable phases, and (iv) correlating dilute solution entropy with volume effects of alloying.
Indian Welding Journal, 2013
Metallurgical and Materials Transactions A, 2019
The variation of bulk modulus with composition of alloy phases is a core issue in any thermodynam... more The variation of bulk modulus with composition of alloy phases is a core issue in any thermodynamic theory of alloy formation. Though it emerges from fundamental theory that elastic property, especially the bulk modulus is crucially dependent on interstitial electron density (q b) distribution and its variation with respect to alloy composition, a coherent thermodynamic treatment connecting bulk modulus with electron density together with its composition dependence, is still lacking. The present study addresses this issue for solid solution alloys. A phenomenological analysis of the composition dependence of bulk modulus (B T) of single phase alloys has been presented in terms of bonding charge density (q b) and its corresponding change with atomic volume (V). This link is developed using the fundamental interrelationship existing between bulk modulus, electron density, and molar volume. The change in bonding charge density (Dq b) with composition (x) has been modeled using an exponential scaling relation with respect to the corresponding change in atomic volume (DV). This scaling relation is based on the concept of the universal binding energy relation, which in turn results in a simple exponential variation of bulk modulus with composition-induced change of atomic volume. It is also shown that a common functional representation namely, B T (V) % B o exp{C 9 (DV)}, can be obtained for the temperature, pressure, and composition dependence of bulk modulus in terms of corresponding changes in volume (DV). The constant C takes context-dependent meaning and values. The applicability of this exponential relation towards representing the effect of composition on bulk modulus has been satisfactorily demonstrated for many substitutional alloy systems.
Materials Today: Proceedings, 2019
The defect recovery and recrystallization as well as the strain induced martensite formation in h... more The defect recovery and recrystallization as well as the strain induced martensite formation in heavily cold rolled 316L austenitic steel has been investigated using Impulse excitation technique. It has been observed that the amount of strain induced martensite is small even after 80% cold rolling and hence the martensite reversion do not alter the elastic modulus to any significant extent during annealing. On the other hand the stored energy of deformation as well as the subsequent annealing phenomenon like recovery and recrystallization has significant effect on the elastic properties of these alloys. The measured temperature variation of elastic moduli data during recovery and recrystallization has been analyzed using the empirical Kolmogorov-Johnson-Mehl-Avrami (KJMA) modelling framework to obtain the kinetic parameters in the steel.
Journal of Alloys and Compounds, 2008
The thermal properties of Ti4 wt.%Nb4 wt.%Zr alloy, namely the enthalpy increment and heat capa... more The thermal properties of Ti4 wt.%Nb4 wt.%Zr alloy, namely the enthalpy increment and heat capacity have been characterized as a function of temperature using drop and differential scanning calorimetry, respectively. The measured data clearly attested to the presence of a phase ...
Metallurgical and Materials Transactions A, 2016
The enthalpy of formation ΔoHf is an important thermodynamic quantity, which sheds significant li... more The enthalpy of formation ΔoHf is an important thermodynamic quantity, which sheds significant light on fundamental cohesive and structural characteristics of an alloy. However, being a difficult one to determine accurately through experiments, simple estimation procedures are often desirable. In the present study, a modified prescription for estimating ΔoHfL of liquid transition metal alloys is outlined, based on the Macroscopic Atom Model of cohesion. This prescription relies on self-consistent estimation of liquid-specific model parameters, namely electronegativity (ϕL) and bonding electron density (nbL). Such unique identification is made through the use of well-established relationships connecting surface tension, compressibility, and molar volume of a metallic liquid with bonding charge density. The electronegativity is obtained through a consistent linear scaling procedure. The preliminary set of values for ϕL and nbL, together with other auxiliary model parameters, is subsequently optimized to obtain a good numerical agreement between calculated and experimental values of ΔoHfL for sixty liquid transition metal alloys. It is found that, with few exceptions, the use of liquid-specific model parameters in Macroscopic Atom Model yields a physically consistent methodology for reliable estimation of mixing enthalpies of liquid alloys.
In present study, a two dimensional cellular automata (CA) simulation has been carried out to stu... more In present study, a two dimensional cellular automata (CA) simulation has been carried out to study the effect of nucleation mode on the kinetics of recrystallization and microstructure evolution in an austenitic stainless steel. Two different nucleation modes i.e. site saturation and continuous nucleation with interface control growth mechanism has been considered in this modified CA algorithm. The observed Avrami exponent for both nucleation modes shows a better agreement with the theoretical predicted values. The site saturated nucleation mode shows a nearly consistent value of Avrami exponent, whereas in the case of continuous nucleation the exponent shows a little variation during transformation. The simulations in the present work can be applied for the optimization of microstructure and properties in austenitic steels.
Journal of Chemical Sciences, 2010
Metallurgical and Materials Transactions A
Journal of Chemical Sciences
A brief review of the issues involved in modelling of the solid state transformation kinetics is ... more A brief review of the issues involved in modelling of the solid state transformation kinetics is presented. The fact that apart from the standard thermodynamic parameters, certain path variables like heating or cooling rate can also exert a crucial influence on the kinetic outcome is stressed. The kinetic specialties that are intrinsic to phase changes proceeding under varying thermal history are enumerated. A simple and general modelling methodology for understanding the kinetics of non-isothermal transformations is outlined.
Materials Science Forum, 2015
The feasibility of employing the indigenously developed Ferroboron alloy as an alternate neutron ... more The feasibility of employing the indigenously developed Ferroboron alloy as an alternate neutron shield material in combination with 9Cr-based ferritic steel clad in future Indian Fast Breeder Reactors (FBR), has been investigated from a metallurgical perspective. In this regard, extensive studies have been undertaken to estimate quantitatively the nature of interaction between Ferroboron and P91-ferritic steel at high temperatures. It is found that in the temperature range 550 to 600°C, 9Cr-based ferritic steel is fully compatible with Ferroboron. However, at higher temperatures, Feroboron interacts with ferritic steel; but the maximum estimated loss of clad thickness is restricted to about 250 μm for 60 years of service.
Materials Science and Technology, 2015
ABSTRACT The present paper presents the results of an extensive electron microscopy investigation... more ABSTRACT The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr-W-V-Ta reduced activation ferritic-martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.
Transactions of the Indian Institute of Metals, 2013
ABSTRACT The thermally-activated lattice defect relaxation kinetics in 90 % cold-rolled titanium ... more ABSTRACT The thermally-activated lattice defect relaxation kinetics in 90 % cold-rolled titanium has been investigated through differential scanning calorimetry, isothermal aging experiments, supplemented by metallography and theoretical modelling. Calorimetry results revealed two major softening thermal arrests at 755 and 944 K, respectively, marking the onset of recovery and recrystallization of deformed state. Adoption of higher heating rates resulted in overlapping of recovery and recrystallization; in fact, the excess unspent driving force due only to partial recovery accelerated the recrystallization. Standard isoconversional analysis of calorimetry data yielded 115 and 147 kJ mol−1 for apparent activation energies of recovery and recrystallization, respectively. These values agree well with the results of an independent analysis of time, temperature variation of hardness of isothermally annealed samples.
Transactions of the Indian Institute of Metals, 2013
ABSTRACT The thermo-kinetics aspects of phase transformations in U rich U–xZr binary alloys, with... more ABSTRACT The thermo-kinetics aspects of phase transformations in U rich U–xZr binary alloys, with x = 2, 5 and 10 wt% Zr have been investigated using dynamic calorimetry. The on-heating and cooling transformations at controlled scan rates in the range, 1–99 K min-1, have been monitored and the following transformation sequence is obtained at slow heating (3 K min-1) of a U–2Zr alloy: (i) a or a0 (distorted orthorhombic martensite) ? d(UZr2) ? a ? c2 (bcc phase enriched in Zr); (ii) a ? c2 ? b (tetragonal) ? c2; (iii) b ? c2 ? b ? c1 (bcc phase enriched in U); (iv) b ? c1 ? c; (v) c (bcc) ? liquid (melting). Similar transformation sequence for other compositions with varying enthalpy effects has been witnessed for 5 and 10 Zr alloys. The observed transformation characteristics are rationalized for the effect of Zr content and heating/cooling rate variations.
... P KUPPUSAMI, E MOHANDAS, S RAJU and VS RAGHUNATHAN* Metallurgy Division, Indira Gandhi Centre... more ... P KUPPUSAMI, E MOHANDAS, S RAJU and VS RAGHUNATHAN* Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Abstract. ... step processes, the initial and the final slopes of resistance curves of figure 2 were plotted as a function of I/kT. ...