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This study presents an extensive experimental characterization of high temperature phase stability and phase transformation kinetics of intermetallic sigma and alpha phases of Fe 55 Cr 45 alloy carried out by dynamic calorimetry. Sigma phase was synthesized from its parent alpha phase of Fe 55 Cr 45 alloy by annealing at 973 K for 360 h in vacuum. A comparative thermal study of sigma and alpha phase alloy was performed from 373 to 1400 K by Differential Scanning Calorimeter. Single inflection of ferro to para magnetic transition (T c ) was observed in DSC upto melting at 831 K for alpha phase. For sigma phase, the sigma to alpha decomposition temperature was measured as 1122 K. It was observed that sigma phase decomposition temperature is nearly 30 K higher than the presently reported 1093 K. Kinetics study of sigma to alpha phase transformation has been carried out by employing various heating rates from 1 to 50 K min −1 in DSC. The non isothermal KJMA model has been adopted to model the fraction transformed to determine the order and kinetics parameters. The activation energy for sigma to alpha phase transformation was obtained 240–260 kJ mol −1 by employing KJMA model.

In the present study, the feasibility of employing the indigenously developed ferroboron alloy (Fe-15 wt.%B) as an alternate neutron shield material in combination with 9Cr-based ferritic steel (P91) clad in future Indian fast breeder reactors (FBR), has been investigated from a metallurgical perspective. Towards this goal, a series of diffusion couple experiments have been conducted at three different temperatures namely, 600, 700 and 800 °C for time durations up to 5000 h. The thickness of interaction layer has been monitored using standard metallographic procedures. The experiments revealed that ferroboron/P91 combination exhibited a tendency to form complex intermetallic borides at the interface. The structural and microstructural characterization of the interface confirmed that the reaction layer consists predominantly of borides of Fe and Cr of type FeB, Fe 2 B, (Fe,Cr) 2 B and (Fe,Cr)B. The measured variation of interaction layer thickness as a function of time and temperature have been modelled in terms of diffusion mediated interaction. The growth kinetics of borided layer has followed the parabolic law at each temperature, and the apparent activation energy for boride layer formation is found to be of the order of 115 kJ mol −1 . This indicates that the kinetics of boriding could be governed by diffusion of B into the P91 matrix. Based on the findings of present study, an extrapolative estimate of the clad attack thickness at 550 °C for 60 years of operating time has been made and it turns out to be 210 ± 15 μm, which is less than the clad thickness of FBR shielding subassembly (4 mm) [1]. Thus, this study confirms that at testing temperatures from 550 to 600 °C, the ferroboron/P91 steel combination can be safely employed for shielding subassembly applications in fast reactors.

This article presents the results of an experimental study on the evolution of phases during annealing of ball milled Fe – ZrO2 nanocomposite powder using Differential Scanning Calorimetry, X-ray Diffraction and Electron Microscopy techniques. The formation of FeO phase from Fe and Fe3O4 is identified through Differential Scanning Calorimetry and confirmed using Synchrotron Diffraction and Transmission Electron Microscopy. Qualitative X-ray phase analysis showed the stoichiometry of the FeO phase as Fe0.87O. This phase is expected to be beneficial as it offers strengthening in the Fe matrix and is thermally stable. The activation energy of formation of the Fe0.87O phase was estimated to be ~ 195 kJ mol− 1. This phase is also found to be partially stabilizing the cubic phase of ZrO2, at high temperatures. A cubic ⇆ tetragonal transformation of ZrO2 was identified at 1140 K. A rather broad peak was observed for Fe α → γ transformation in the Calorimetry thermograms as the Fe grains were nanocrystalline which became sharper as the crystallites grew in size.

In this study thermophysical properties of alloy 617 have been measured as a function of temperature using various thermal analysis techniques and complemented by structural and microstructural characterization. The enthalpy increment (HT−H298) has been measured using static calorimetry in the temperature range of 300–1523 K. It is found that (HT−H298) increases monotonically with temperature up to about 1000 K. However, above this temperature two distinct inflections are detected at about 1030 K and 1205 K respectively. A critical comparison of these inflections with Thermo-Calc simulations and dynamic calorimetry measurements indicates that they are associated with dissolution of M23C6, and γ′-Ni3Al phases respectively. The enthalpy data has been numerically fitted using non-linear regression analysis to obtain the temperature dependence of heat capacity. Further, the thermal diffusivity and thermal expansivity of alloy 617 have been measured using the laser flash method and thermo-mechanical analyzer in the temperature range of 300–1523 K. Finally, with the knowledge of heat capacity, density and thermal diffusivity data, thermal conductivity has also been estimated in the temperature range of 300–1523 K. The measured properties for alloy 617 have been compared with Thermo-Calc, JMatPro simulation and other literature data, which clearly establishes the role of precipitates dissolution. The various properties measured in the present study are original and new addition to the database for alloy 617, which can serve as new input for Thermo-calc based optimization. This paper also provides the insight into the temperature and composition dependence of thermal conductivity of alloy 617.

Present study demonstrates by experiments and computation that a fully martensitic microstructure does not form in 9 Cr steels containing 1.9 wt% W (P92). The presence of a magnetic transition prior to the martensitic transformation during fast cooling supports the co-existence of ferrite and martensite. The formation of diffusional ferrite at high cooling rates is precluded by a systematic variation of three parameters namely cooling rate, austenitising temperature and hold times. However, complete martensite formation has not been experimentally feasible in P92 steel at all for all cooling rates (1–90 K min −1 in DSC and water quenching) employed from austenitisation temperatures (950–1150 °C), unlike in P91 steel where complete martensite forms during cooling at the rate of 30 K min −1 or higher. The experimental results have been substantiated by Thermo Calc simulation for varying W contents in P91 and P92 steels. Thermo Calc simulation revealed that P91 steel remains in the single phase γ domain during austenitisation at 950–1150 °C (1223–1423 K). It is observed that γ phase field gets shifted towards the right with addition of W in P91. P92 steel remains in the three phase domain of α + γ +MX region at 950–1150 °C (1223–1423 K), which explains the presence of ferrite above Ac 3 temperatures.

Despite the availability of extensive information on Fe–Cr alloys, there have not been many recent studies toward generating high temperature calorimetry data and possible integration of these in terms of simple and physically based modeling protocols, to yield a wholesome picture of phase stability in Fe–Cr system. This study aims at partially filling this gap. In specific terms, accurate measurements of high temperature enthalpy increment (HT − H298.15) have been made on Fe–15 wt.% Cr alloy using inverse drop calorimetry in the temperature range, 298–1323 K. The Curie temperature and the transformation enthalpy associated with the magnetic transformation have been experimentally determined. These data are combined with relevant information available in literature for other compositions, to develop a physically based modeling approach that takes into account explicitly various contributions to the thermodynamics of αferro → αpara phase change as a function of Cr-content.

The lattice and bulk thermal expansion behavior of an Indian version of reduced activation ferritic–martensitic (INRAFM) steel has been quantified using high temperature X-ray diffraction and dilatometry. The lattice parameter of tempered α-ferrite phase exhibited a smooth quadratic increase with temperature, while that of γ-austenite remained fairly linear up to 1273 K. The results suggest that α-ferrite + Carbides → γ-austenite transformation occurs upon continuous heating in the temperature range, 1146 ⩽ T ⩽ 1173 K. Further, this transformation is found to be accompanied by a reduction in average atomic volume. The mean linear thermal expansion coefficients of tempered α-ferrite and γ-austenite phases are estimated to be about 1.48 × 10 −5 and 2.4 × 10 −5 K −1 respectively. The magnetic contribution to relative thermal dilatation (Δ l / l 298 ) mag is found to be small and negative, as compared to phonon contribution.

This paper presents the results of an experimental and modelling study to determine the thermophysical properties of α and σ phases in Fe55Cr45 (wt%) alloy. A comprehensive analytical model was used to predict the temperature dependent variation of enthalpy and specific heat, which are compared with experimental results. σ phase was synthesized from homogenized α alloy through heat treatment at 973 K for 300 h. Analysis of high temperature phase stability showed a magnetic transformation for α phase at 831 K and σ→α structural transformation for sigma phase at 1120 K with an enthalpy of 2.6 ± 0.2 kJ mol−1. Room temperature linear thermal expansion coefficient (CTE) for α and σ phases were obtained as 7.39 × 10−6 and 15.03 × 10−6 K-1 respectively. While the α phase exhibited an increase of 118% in CTE up to 1000 K, only 1.7% increase was observed for σ phase. Room temperature thermal conductivity for σ was much lower (3.1 W m−1 K−1) than that of the α phase (16.22 W m−1 K−1). The systematic experimental investigations carried out in the present study, established that formation of σ has an adverse effect on the heat transport properties of structural steels and their welds.