Saroja Saibaba

Measurement of high temperature phase stability and thermophysical properties of alloy 740

In the present study, the characterisation of phase stability and measurement of different thermophysical properties of alloy 740 has been carried out. The transformation temperatures including liquidus/solidus and corresponding enthalpy of transformation have been measured for different phase changes up to melting using dynamic calorimetry. Further, the enthalpy increment data have been measured in the temperature range of 473–1473 K to obtain the heat capacity using static calorimetry. The present calorimetric data have been analysed in corroboration with the results obtained using JMatPro and Thermo-Calc simulation. In addition, the temperature dependence of other thermophysical properties such as thermal expansivity, density, thermal diffusivity and thermal conductivity are also measured in the range of 300–1473 K using thermomechanical analyser and laser flash method.

A novel route of synthesis of aluminium nano-aggregates: an ion beam modification in metal-organic complex

A novel route for the formation of nano-aggregates of aluminium by ion irradiation of a spin-cast metal-organic thin film of aluminium acetyl acetonate [((CH3CO)2CH)3Al] is reported. Glancing angle x-ray diffraction studies on an irradiated sample show the formation of fcc aluminium clusters. Microstructural studies using transmission electron microscopy show the formation of aggregated branched nanoclusters of diameter ≈5-60 nm, which on increasing fluences interconnect through branches and form a network-like structure. Networked structures of aggregated Al clusters might have formed due to overlapping of multiple ion tracks at relatively higher fluences.

Mitigating Embrittlement in Structural Steels of Power Plants by Grain Boundary Engineering

The Cr-Mo alloyed steels used as structural materials of power plants, are susceptible to grain boundary embrittlement upon long-term exposure at service temperatures. The present study is aimed at characterization of grain boundary nature in the 9Cr-1Mo-0.1C steel, and identification of process treatment for mitigating the temper embrittlement problem. A new heat treatment of DNT (Normalizing by Double austenitization and Tempering) was designed to refine the prior-austenite to grain size from 25 to 12 μm. As compared to CNT (Conventional Normalizing & Tempering) treatment, the DNT treated steel exhibited lower DBTT (Ductile-to-Brittle Transition Temperature), comparable room temperature and high temperature mechanical properties, while the embrittlement effect after ageing at 773 K for 5,000 h was less pronounced.

A Thermodynamic Framework Bridging the Composition and Temperature Dependence of Bulk Modulus With Enthalpy of Mixing

AbstractThe intrinsic thermodynamic links that exist between thermochemical and thermophysical quantities, especially their temperature, pressure, and composition dependence, have seldom been analyzed in sufficient detail in literature. In this connection, an attempt is made to establish a thermodynamic bridge, relating ΔoHmix, the standard enthalpy of mixing with ΔoBT, the change in isothermal bulk modulus as a result of alloying and its composition and temperature dependence. In essence, by adopting the standard regular and subregular solution approximations to the composition dependence of mixing enthalpy; and furthermore, incorporating separately the configurational (ΔoSconf) and vibrational (ΔoSVib) entropy contributions to mixing Gibbs energy change (ΔoGmix), simple models have been derived for the composition and temperature variations of excess bulk modulus ΔBT. In particular, a regular or subregular solution analog of the composition variation of ΔBT is shown to be possible if ΔoHmix could be described likewise. The vibrational entropy contribution to ΔBT is found to be important only when the change in Grüneisen parameter during alloying turns to be significant. The practical utility of the theoretical framework developed in this study has been demonstrated by applying it to disordered fcc Cu1−xNix alloys, wherein it is shown that ΔoHmix and ΔoBT are linearly correlated, as predicted by the theory.

Structural characterisation of Y2Ti2O7 dispersoids in ODS alloys

Oxide dispersion strengthened (ODS) steels are being developed as promising core component structural materials for future generation fast breeder and fusion reactors because of their better high temperature thermal stability and neutron irradiation void swelling resistance as compared to currently used austenitic steels. The type of oxide dispersoids, their size distribution in metallic matrix and stability at adverse service conditions (such as high temperature along with fast neutron irradiation) governs the physical and mechanical properties of the steel. Yttria (Y2O3), is the most preferred oxide dispersoid being used in the ODS steels, because of its superior thermal and neutron irradiation stability. However there are reports that show that these oxides either dissolve or dissociate or even become amorphous during mechanical milling and reprecipitate as coarse particles during high temperature consolidation process, in absence of Ti in a model Fe-15Y2O3 system [1]. It is believed that Ti can inhibit the growth of nano-dispersoid during annealing by formation of Y-Ti-O complex oxides such as Y2Ti2O7 or Y2TiO5 or YTiO3 [2]. The Y2O3 to Ti weight ratio in the alloy is critical in determining the chemical composition of the dispersoid and the Y2Ti2O7 oxides are finer and most stable oxide among all combinations of Y-Ti-O complex oxides whose size which varies in the range of 2-15 nm in ODS steel. Synchrotron XRD is used to characterize the dispersoids in the ODS steel, with 0.35 wt% of yttria and 0.2 wt% of Ti, due to the low volume fraction. TEM has been found more suitable for complete characterization of the nano-sized (~2-5 nm) dispersoids w.r.t size, distribution, morphology, chemical composition and crystal structure. However, characterisation of samples prepared by conventional methods for TEM studies continues to be difficult owing to magnetic nature of ferritic steel. Hence FIB was employed to extract electron transparent samples which are of micrometer dimensions. In order to understand the structural evolution of the Y2Ti2O7 oxide in ODS steel, a concentrated alloys of Fe-15wt%Y2O3-Xwt%Ti (X=0, 5, 10, 15) were synthesized by mechanical milling and subsequently annealed. Figure 1(a) and (b) represents the typical bright field (BF) TEM micrographs of Fe-15wt%Y2O3-15wt%Ti model ODS after 60h of milling and subsequent annealing at 1273K respectively, the corresponding SAD patterns are shown as inset. The analysis of SADP reveals amorphisation of yttria upon milling and recrystallisation of Y2Ti2O7, in annealed alloy powder. Interestingly, it was observed when the Y2O3 to Ti weight ratio is 1:1, the oxide phase formed upon annealing is only Y2Ti2O7 and are very finer in size (varies in the range of 2-30 nm). Details of these studies will be presented in the paper. 1. Pradyumna Kumar Parida, Arup Dasgupta, K. Jayasankar, M. Kamruddin, S. Saroja, J. Nucl. Mater. 441 (2013) 331–336. 2. P.K. Parida, A. Dasgupta, K.G. Raghavendra, K. Jayasankar, S. Saroja, Trans IIM (2016) 1-7.

Structure of Laves phase in equiatomic CrFeNbNiV alloy

Multiprincipal elemental alloys commonly referred to as High Entropy Alloys (HEA) are a relatively new class of materials gaining large interest in recent times due to their new microstructures and excellent mechanical and corrosion properties. An equiatomic high entropy Cr-Fe-Ni-Nb-V alloy synthesized by multiple vacuum arc melting. Though the formation of BCC solid solution was predicted by considering the thermodynamic parameters like entropy and enthalpy of mixing, atomic size differences, valence electron concentration and electronegativity [1], analysis of XRD pattern of the as cast alloy, showed the presence of a major HCP Laves phase of CrNiNb type and minor tetragonal and BCC phases. The lattice parameters of these phases calculated by Rietveld refinement are as follows: (i) HCP Laves phase: a=0.485 ± 0.003 & c= 0.790 ± 0.009 nm (ii) BCC phase: a=0.327 ± 0.001 nm (iii) Tetragonal phase: a= 0.895 ± 0.002 & c= 0.462 ± 0.002 nm. Microstructural and microchemical analysis through Transmission Electron Microscopy confirmed that the HCP Laves phase is Nb rich, while the tetragonal and BCC phases are lean w.r.t Nb and enriched with Cr and V. The microstructure of the alloy was found to be stable upto 1100°C. Bright field TEM image of the Laves phase of this alloy aged at 1100°C along with the Selected Area Diffraction Pattern along [-11.0] zone axis from the Laves phase is shown in the figure below. The formation of HCP Laves phase in contrast to the theoretical predictions was understood based on the calculation of average d orbital energy level, which decides the formation of topological close packed phases in superalloys containing transition elements [2]. Since HEAs also contains transition elements, this concept is extended to these novel alloys. For the current alloy, the average dorbital energy level was calculated to be 1.46, which is well above the threshold value of 1.09 for formation of intermetallic phases. This dictates the formation of intermetallic Laves phases in this system. Detailed analysis regarding the crystallographic aspects of this Laves phase using Rietveld refinement and Precession Electron Diffraction (PED) technique is under progress, results of which will be presented in the paper.

Estimation of Enthalpy of Formation of Liquid Transition Metal Alloys: A Modified Prescription Based on Macroscopic Atom Model of Cohesion

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.

Modelling the role of nucleation on recrystallization kinetics: A cellular automata approach

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.