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This paper presents the results of an experimental and computational study carried out to elucidate the effect of Al on the microstructure and phase stability of P91 F/M steel in as-cast, homogenized and normalized conditions. Al-added steels followed ‘Ferritic-Austenitic’ mode of solidification and the as-cast microstructures consisted of δ-ferrite + α′-martensite, the volume fraction of ferrite and hardness of martensite increased with Al concentration. Heat treatments and DSC experiments confirmed increased stability for δ-ferrite with Al addition. Systematic change in the phase transformations temperatures and volume fraction of equilibrium phases due to Al addition was estimated with the help of Thermo-Calc®. Al addition promoted the formation of AlN which was confirmed through electron microscopy-based investigations. AlN dissolution temperature was always above γ-loop which made it impossible to dissolve during austenization. With the help of Scheil and equilibrium simulations using Thermo-Calc®, elemental partitioning between δ-ferrite and α′ phases was found to be the reason for higher hardness of martensite. Based on experimental evidences, it is concluded that except in the case of 0.48 wt pct Al-added steel it is impossible to obtain single phase γ-field (without ferrite) at high temperature thereby a fully martensite structure on cooling.

The current study examines the evolution of nanocrystallites of ZrO2 with time and temperature in a Fe-ZrO2 composite. The crystallite sizes were determined through X-ray peak broadening analysis by the Williamson–Hall method together with dark field transmission electron microscopy. The ZrO2 crystallites were found to be stable and retained their sizes at 973 K and 1073 K for hold durations up to 600 minutes. On the other hand, the crystallites were seen to grow at 1173 K and reached up to ~ 200 nm for a hold time of 600 minutes. The Ostwald ripening model was adopted to understand crystallite growth while a dislocation-driven pipe diffusion was adopted for understanding the kinetics of grain growth. The activation energy of grain growth was calculated as ~ 379 kJ mol−1. The modeled and experimentally calculated size evolutions with time and temperature were shown to be in good agreement with each other. A detailed discussion on the kinetics and activation energy of grain growth of ZrO2 crystallites in a Fe matrix is presented in this manuscript.

This paper presents the results of a study on grain boundary characteristics in cold deformed and annealed 304HCu grade austenitic stainless steel (SS 304HCu) using electron backscatter diffraction. The microstructure exhibited an increasing fraction of Σ1 to 29 coincidence site lattice boundaries with annealing temperature, resulting up to ~60 pct at 1573 K with 92 pct contribution from Σ3-type twin boundary. However, the twin boundary interaction at the triple points with a network of Σ3–Σ3–Σ9 was found to decrease from 4 to 0.5 pct with annealing temperature. To understand the resultant boundary advancement of the Σ3 n (n = 1, 2, 3) boundaries, their migration was traced in the annealed specimen. However, in the specimen with extended annealing Σ3 boundary fraction was found to be higher with a concomitant decrease in the boundary fraction generated by the Σ3 interactions. In this study, a procedure to analyze the coherency of Σ3 boundaries and its interfaces that form due to Σ3 interactions has been evolved based on single-section analysis using the pole concentration across the grains. Further, a crystallographic description of the two planes meeting at the interface of Σ3-type boundary has been provided by adopting serial sectioning methods, which help to understand the morphological changes. The quantitative deviation from exact coherent Σ3 has been estimated to be within ~6 deg in this study.

A novel ODS ferritic steel with ZrO2 dispersoids is developed as a candidate structural material for future fast reactor applications. Encouraged by our earlier results demonstrating the retention of crystallinity by ZrO2 dispersoids during milling, the 9Cr ODS steel powder has been synthesized under optimized ball milling conditions and subsequently characterized. The analysis of milled powder showed fine dispersions of ZrO2 with a size range of 2–10 nm. Upon annealing at 1223 K, precipitation of non-stoichiometric FeO was observed along with fine ZrO2 dispersoids. All the constituent phases are identified using Synchrotron XRD, Conventional and HRTEM. An excellent thermal stability has been manifested by the dispersoids as it retained its fine structure during the annealing treatment with ~ 95% of dispersoids measuring

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 paper presents the results of a systematic study on the role of alloying elements W and Ta on the microstructural evolution in 9Cr-W-Ta-V-C Reduced Activation Ferritic/Martensitic steels during long-term thermal exposure in the temperature range of 773 K to 923 K (500 °C to 650 °C). The kinetics of evolution of secondary phases like M23C6, MX, and Laves phase crucially depend upon the W and Ta content of the steel in addition to temperature and time, which has been studied in detail using analytical transmission electron microscopy as well as predictive methods. The steel with 1 wt pct W and 0.06 wt pct Ta showed slow recovery below 873 K (600 °C) and no evidence for Laves phase at any temperature. Significant change in microstructure was observed after 10,000 hours of exposure at 923 K (650 °C), while recovery at short durations was retarded by nucleation of MX precipitates. Increase in both W and Ta content of the steel enhanced the tendency for the formation of Laves phase.

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.

The present study describes the changes in the creep properties associated with microstructural evolution during thermal exposures to near service temperatures in indigenously developed reduced activation ferritic-martensitic steels with varying tungsten (1 and 1.4 wt pct W) contents. The creep behavior has been studied employing impression creep (IC) test, and the changes in impression creep behavior with tungsten content have been correlated with the observed microstructures. The results of IC test showed that an increase in 0.4 pct W decreases the creep rate to nearly half the value. Creep strength of 1.4 pct W steel showed an increase in steels aged for short durations which decreased as aging time increased. The microstructural changes include coarsening of precipitates, reduction in dislocation density, changes in microchemistry, and formation of new phases. The formation of various phases and their volume fractions have been predicted using the JMatPro software for the two steels and validated by experimental methods. Detailed transmission electron microscopy analysis shows coarsening of precipitates and formation of a discontinuous network of Laves phase in 1.4 W steel aged for 10,000 hours at 823 K (550 °C) which is in agreement with the JMatPro simulation results.

Oxide dispersion strengthened (ODS) steels have very high thermal stability and creep resistance due to reinforcement of hard and stable nano-sized ceramic dispersoids in metallic matrix which act as barriers to dislocation motion. This study established the role of Ti in the structural evolution of yttria during mechanical milling and subsequent annealing in a Fe–15 wt% Y2O3–5 wt% Ti model ODS alloy, using electron microscopy and XRD techniques. The alloy was synthesized in a high energy planetary ball mill in Ar atmosphere by varying the milling durations in the range of 0 (un-milled) to 60 h. The XRD result revealed amorphisation of Y2O3/Ti during milling and evolution of YTiO3 complex oxide upon annealing at 1273 K for 1 h. The electron microscopy studies revealed the refinement of alloy powders from ~50 μm to few nanometers during milling. Electron diffraction analysis and high resolution transmission electron microscopy of 60 h milled as well as and annealed powder showed formation of different types of Y–Ti–O complex oxides such as Y2Ti2O7, Y2TiO5 and YTiO3.

This paper presents the results of an electron microscopy study on the formation of metastable fcc Ti phase under shock loading conditions in an explosive clad of α + β Ti–5Ta–2Nb alloy and the mechanism and kinetics of its reversion during annealing. The fcc Ti phase with a volume fraction of over 0.6 formed in hcp Ti matrix with a characteristic needle shaped morphology. The biphasic fcc + α structure when heat treated in the temperature range of 100–1100 °C showed the gradual transformation of fcc to parent α phase and the maximum reversion was observed at temperatures > 600 °C. The α phase was nucleated within the fcc as well as on the fcc/α interfaces. Orientation relationship between fcc and hcp phases has been derived using standard 1 − 12 fcc and [0001]α stereographic projections. The kinetics of the reversion process followed diffusion controlled nucleation and growth process with an effective activation energy corresponding to the self diffusion of Ti in hcp matrix.

An α + β Ti–5Ta–2Nb alloy has been developed for applications under severe oxidizing conditions of boiling concentrated nitric acid in spent fuel reprocessing plant. The formation of nanocrystalline grains has been observed in the cryo-rolled sample accompanied by a significant improvement in yield strength whereas the grain refinement is not significant in case of room temperature rolled sample. Suppression of dynamic recovery plays a major role in the formation of nanostructured material. The evolution of micro-texture during room temperature rolling and cryo-rolling has been compared in this study. The rolling texture of room temperature and cryo-rolled samples are similar to the rolling texture predicted for hexagonal materials with c/a

Explosive clads of mild steel (MS) and Ti Grade 2 have been fabricated using different explosive load ratios (R) of 1.07, 2.01 and 3. Interface microstructure, microhardness variation and bond strength of the MS/Ti clad joints were studied as a function of R. A sinusoidal wavy interface of uniform wavelength and amplitude was observed in clads fabricated with R = 1.07, which degraded as the value of R exceeded 2, and was used to select low R values for further fabrication. The presence of martensite and metastable fcc Ti phases in MS and Ti plates respectively has been confirmed by transmission electron microscopy (TEM). Microchemical characterization by electron probe micro analyzer (EPMA) and JMatPro® simulations showed coexistence of intermetallic Fe2Ti and TiC phases at the vortex regions of the wavy interface. The formation of Fe2Ti phase was also confirmed by TEM investigations, although isolated at the vortices of the wavy interface, and hence exert only a minor influence on the bond strength of the ‘as clads’. Based on the detailed analysis of interface microstructure and interface bond strength predicted by JMatPro® simulations, the post clad heat treatments for MS/Ti explosive joints have been optimized. Keywords: Mild steel, Titanium, Explosive cladding, Electron microscopy, JMatPro® simulations, Post-clad heat treatments

Dissimilar joints of 304L austenitic stainless steel (SS) and Ti–5Ta–2Nb alloy were fabricated using explosive cladding process with an aim to avoid the formation of brittle intermetallic phases at the interface. Subsequently, diffusion annealing heat treatments were carried out at temperatures in the range of 550–800 °C for various durations. In the present study, concentration and temperature dependence of the distinct diffusion zones, formed at the clad interface, due to interdiffusion of the alloying elements, have been established using Electron Probe Micro Analysis and imaging of the interface. Molar volume showed a close match with the ideal Vegard’s law at low temperatures while a non-linear negative deviation has been observed at high temperatures (800 °C) due to the formation of secondary phases such as FeTi, Fe2Ti and βTi(Fe) phases. Interdiffusion parameters evaluated by Wagner’s method showed sluggish diffusion kinetics of Fe and Ti at concentration corresponding to a two phase mixture of FeTi and Fe2Ti. Further, an attempt has been made to understand multicomponent diffusion using Dayananda’s approach and estimation of effective interdiffusion coefficients revealed ~22 µm shift of the interface from Matano plane at 800 °C.

Explosive clad joints of 304L SS and Ti-5Ta-2Nb alloy, fabricated for an important application in the spent nuclear fuel reprocessing industry showed formation of deformation induced metastable α′ martensite and fcc Ti phase in SS and TiTaNb alloy respectively. A biphasic structure consisting of metastable phases is not preferred for industrial applications due to degradation of corrosion and mechanical properties of the structural materials during service. Hence, it is essential to carry out post cladding heat treatments. The results reported in this paper provide evidence for the presence of α′ phase in 304L SS in ‘as clad’ joints and its reversion process during thermal exposure. The temperature window in the range of 400-700 °C and time was optimized based on complete transformation of the metastable phases to parent phases, and avoiding the formation of brittle Fe-Ti intermetallics at the interface. A systematic increase in the fraction of austenite phase associated with the reversion phenomena has been studied using electron back scattered diffraction and transmission electron microscopy. Orientation relationship between product fcc and parent bcc phases was found to obey the K-S relationship. The reverted γ phase was found to nucleate within the martensite laths. A temperature of 550 °C for duration of about 10 h was found to be optimum for the post cladding treatments of the explosive clad joints.

This paper discuss the microstructural variations across a dissimilar weld joint between SS316 and 9Cr-RAFM steel and its modifications on post weld heat treatments (PWHT). Detailed characterization showed a mixed microstructure of austenite and martensite in the weld which is in agreement with the phases predicted using Schaeffler diagram based on composition measurements. The presence of very low volume fraction of δ-ferrite in SS316L has been identified employing state of the art electron back-scattered diffraction technique. PWHT of the ferritic steel did not reduce the hardness in the weld metal. Thermal exposure at 973 K (700 °C) showed a progressive reduction in hardness of weld joint with duration of treatment except in austenitic base metal. However, diffusion annealing at 1073 K (800 °C) for 100 hours resulted in an unexpected increase in hardness of weld metal, which is a manifestation of the dilution effects and enrichment of Ni on the transformation characteristics of the weld zone. Migration of carbon from ferritic steel aided the precipitation of fine carbides in the austenitic base metal on annealing at 973 K (700 °C); but enhanced diffusion at 1073 K (880 °C) resulted in coarsening of carbides and thereby reduction of hardness.

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.

Ti-5Ta-2Nb alloy was joined to 304L austenitic stainless steel by explosive cladding technique. Explosive cladding resulted in the formation of deformation-induced martensite in 304L SS and fcc phase of Ti in the Ti-5Ta-2Nb side of the joint. The stability of these metastable phases was systematically studied using high-temperature X-ray diffraction technique and transmission electron microscopy, which enabled the optimization of the temperature window for post-cladding heat treatments.

9Cr-Reduced Activation Ferritic-Martensitic steels with 1 and 1.4 wt pct tungsten are materials of choice for the test blanket module in fusion reactors. The steels possess a tempered martensite microstructure with a decoration of inter- and intra-lath carbides, which undergoes extensive modification on application of heat. The change in substructure and precipitation behavior on welding and subsequent thermal exposure has been studied using both experimental and computational techniques. Changes i.e., formation of various phases, their volume fraction, size, and morphology in different regions of the weldment due to prolonged thermal exposure was influenced not only by the time and temperature of exposure but also the prior microstructure. Laves phase of type Fe2W was formed in the high tungsten steel, on aging the weldment at 823 K (550 °C). It formed in the fine-grained heat-affected zone (HAZ) at much shorter durations than in the base metal. The accelerated kinetics has been understood in terms of enhanced precipitation of carbides at lath/grain boundaries during aging and the concomitant depletion of carbon and chromium and enrichment of tungsten in the vicinity of the carbides. Therefore, the fine-grained HAZ in the weldment was identified as a region susceptible for failure during service.

Explosive cladding, a solid-state welding technique was used to fabricate dissimilar joints between Ti-5Ta-2Nb alloy and 304L austenitic stainless steel. To remove the residual stresses and improve the ductility of the ‘as clad’ joints, further heat treatments were carried out in the temperature range of 873 K to 1073 K (600 °C to 800 °C) for varying durations. Systematic change in the interface microstructure and microchemistry due to thermally activated interdiffusion of alloying elements was studied. This information together with that of the thermal stability of non-equilibrium phases in base materials was used to evaluate the probability of formation of various phases across the explosive clad interface using JMatPro®, a materials modeling software. In the clad heat treated at 1073 K (800 °C) for 20 hours, evidence was obtained for the presence of different phases (predicted to form) in the diffusion zone by transmission electron microscopy on site-specific specimens prepared by focused ion beam milling. An attempt to understand the specific influence of the type of phase, their relative mole fraction, and microchemistry on the mechanical property of the joint has also been made based on both experimental and computational study.

This paper presents a detailed electron microscopy study on the microstructure of various regions of weldment fabricated by three welding methods namely tungsten inert gas welding, electron beam welding and laser beam welding in an indigenously developed 9Cr reduced activation ferritic/martensitic steel. Electron back scatter diffraction studies showed a random micro-texture in all the three welds. Microstructural changes during thermal exposures were studied and corroborated with hardness and optimized conditions for the post weld heat treatment have been identified for this steel. Hollomon–Jaffe parameter has been used to estimate the extent of tempering. The activation energy for the tempering process has been evaluated and found to be corresponding to interstitial diffusion of carbon in ferrite matrix. The type and microchemistry of secondary phases in different regions of the weldment have been identified by analytical transmission electron microscopy.

This paper presents the results of an experimental study on the microstructural evolution in 9Cr reduced activation ferritic/martensitic steels during short term thermal exposures. Since the microstructure is strongly influenced by the alloying additions, mainly W, Ta and C contents, the effect of varying W and Ta contents on the martensite structure that forms during normalizing treatment and the subsequent changes during tempering of the martensite in the temperature regime of 923–1033 K have been studied. Microstructural changes like subgrain formation and nature of precipitates have been evaluated and correlated to hardness variations. The systematic change in size distribution and microchemistry of M23C6 carbide is studied with variation in W content at different temperatures.

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.

Ferritic steel powder was mechanically milled in a dual drive planetary ball mill, under different milling conditions to optimize the milling parameters. The resulted powder was characterized, using particle size analyzer, X-rays and electron microscope. X-ray peak broadenings were investigated to estimate crystallite size, lattice strain and deformation stress. Better Pearson's coefficient was observed for uniform stress deformation model (USDM) (0.988) in comparison to uniform deformation model (UDM) (0.64) which shows better estimation of lattice parameter. An increase in fineness was observed with an increase in ball to powder ratio as well as for an increase in rotational speed. At the optimized condition, ferritic steel powder, together with Y 2 O 3 , was milled in the dual drive mill to produce oxide dispersion strengthened ferritic steel powder, suitable to be used in nuclear applications. Convoluted morphology, desired for better alloying, was confirmed using an electron microscope. A significant increase in per unit surface area was noticed due to milling using BET surface area analysis. Negligible contamination was observed due to milling atmosphere and mill container. The steel powder produced, was sintered using spark plasma sintering and its density and hardness were measured. High hardness and lower crystallite size were recorded using spark plasma sintering. Addition of Y 2 O 3 shows decreases in the thermal expansion coefficient. Effect of added titanium was studied and an adverse effect on oxide dispersion strengthened steel was noticed.

Mod. 9Cr-1Mo is used as the structural material in the steam generator circuit of liquid metal-cooled fast breeder reactors. Microstructural modifications on the surface of this steel are investigated after exposing to flowing sodium at a temperature of 798 K (525 °C) for 16000 hours. Sodium exposure results in the carburization of the ferritic steel up to a depth of ~218 µm from the surface. Electron microprobe analysis revealed the existence of two separate zones with appreciable difference in microchemistry within the carburized layer. Differences in the type, morphology, volume fraction, and microchemistry of the carbides present in the two zones are investigated using analytical transmission electron microscopy. Formation of separate zones within the carburized layer is understood as a combined effect of leaching, diffusion of the alloying elements, and thermal aging. Chromium concentration on the surface in the α-phase suggested possible degradation in the corrosion resistance of the steel. Further, concentration-dependent diffusivities for carbon are determined in the base material and carburized zones using Hall’s and den Broeder’s methods, respectively. These are given as inputs for simulating the concentration profiles for carbon using numerical computation technique based on finite difference method. Predicted thickness of the carburized zone agrees reasonably well with that of experiment.

Inconel 600, a Ni-based alloy was considered as a possible replacement for austenitic stainless steels for service at high temperatures in nitrogen containing atmosphere of the neutron detector. To obtain a fundamental understanding of the differences in the diffusivity of nitrogen in pure Fe and Ni lattices, molecular dynamics simulations were carried out. Based on the simulation of nitrogen trajectories in the temperature range of 1200–1400 K, pre-exponential factor, activation energy and jumping frequency were calculated and compared for fcc Ni and Fe. MD simulations confirmed that rate of diffusion of nitrogen is lower in Ni when compared to Fe, suggesting the replacement of austenitic steel with Inconel 600 for better performance of the detectors.

Boron carbide containing boron enriched in 10B isotope (10B ∼ 67 at. %) will be used as the control rod material in India's fast breeder reactors. The enriched boron carbide will be produced by high temperature reaction between enriched elemental boron and carbon. The control rod will consist of high-density boron carbide pellets enclosed in D9 clad tubes. The boron carbide pellets will be surrounded by liquid sodium since the tube is of vented type. Out-of-pile chemical compatibility of boron carbide with D9 alloy in the presence of liquid sodium has been studied by carrying out isothermal annealing of closed capsules containing sodium and boron carbide at 973 K for a period ranging from 1000 – 5000 h. The extent of reaction was evaluated by visual examination, metallographic analysis, chemical analysis and electron probe microanalysis (EPMA). The results of these measurements are discussed in detail in this paper.

The Fe-15 wt% Y2O3 model Oxide Disperson Strengthened (ODS) alloy powder was prepared by mechanical milling in an inert atmosphere using high energy planetary ball mill. The present paper deals with the study on structural changes of nano-crystalline yttria dispersoids in Fe matrix during mechanical milling for various durations and subsequent annealing. The crystallinity of Y2O3 in the milled powder was studied by X-ray diffraction (XRD) technique which showed that the intensity of diffraction peaks of Y2O3 gradually disappeared with increase in milling time. Dark field TEM imaging of the milled powder showed that both Fe and Y2O3 particles were refined to few nanometers during milling. Electron diffraction analysis showed polycrystalline rings for Fe and broad rings with a few spots corresponding to the (4 1 1) plane of Y2O3 after 10 h of milling time. The yttria diffraction peaks became broad and diffuse with increasing milling time. Subsequent annealing of the 60 h milled powder at 1273 K for 1 h results in recrystallization of Y2O3 and growth of Fe crystallites.

This study aims at understanding the microstructural changes due to variation of W and Ta content and heat treatment in 9Cr–W–Ta–0.2V–C reduced activation ferritic/martensitic steels, with W and Ta concentrations varied from 1 to 2 wt.% and 0.06 to 0.14 wt.% respectively. An increase in concentration of W and Ta in the steels refined prior austenite grain size and lath width, though the change was not significant beyond 1.4 wt% W. Increase in W and Ta concentrations also resulted in a decrease in size and increase in number density of carbides on tempering. Microchemical analysis of M 23 C 6 precipitates in tempered steel showed enrichment of W with increase in W content of the steel, whereas addition of Ta showed no significant change in chemistry of M 23 C 6 or MX, but only increased the number density of fine MX precipitates.

Effects of tungsten and tantalum contents on impact, tensile, low cycle fatigue and creep properties of Reduced Activation Ferritic–Martensitic (RAFM) steel were studied to develop India-specific RAFM steel. Four heats of the steel have been melted with tungsten and tantalum contents in the ranges 1–2 wt.% and 0.06–0.14 wt.% respectively. Increase in tungsten content increased the ductile-to-brittle transition temperature (DBTT), low cycle fatigue and creep strength of the steel, whereas the tensile strength was not changed significantly. Increase in tantalum content increased the DBTT and low cycle fatigue strength of the steel whereas the tensile and creep strength decreased. Detailed TEM investigations revealed enhanced microstructural stability of the steel against creep exposure on tungsten addition. The RAFM steel having 1.4 wt.% tungsten with 0.06 wt.% tantalum was found to possess optimum combination of impact, tensile, low cycle fatigue and creep properties and is considered for Indian-specific RAFM steel.

Cold worked austenitic stainless steel type AISI 316 is used as the material for fuel cladding and wrapper of the Fast Breeder Test Reactor (FBTR), India. The evaluation of mechanical properties of these core structurals is very essential to assess its integrity and ensure safe and productive operation of FBTR to very high burn-ups. The changes in the mechanical properties of these core structurals are associated with microstructural changes caused by high fluence neutron irradiation and temperatures of 673–823 K. Remote tensile testing has been used for evaluating the tensile properties of irradiated clad tubes and shear punch test using small disk specimens for evaluating the properties of irradiated hexagonal wrapper. This paper will highlight the methods employed for evaluating the mechanical properties of the irradiated cladding and wrapper and discuss the trends in properties as a function of dpa (displacement per atom) and irradiation temperature.

Ni-based alloys are preferred as structural materials over austenitic stainless steels in nitrogen containing atmospheres like in neutron detectors due to severe problem of nitriding of stainless steels and subsequent loss of nitrogen. Extensive work has been carried out in literature on the nitriding behavior of austenitic stainless steels whereas similar information on Ni-based alloys is very few. Hence, to assess the nitriding kinetics of a Ni-based alloy (Inconel 600) nitriding experiments were carried out. Inconel 600 was plasma nitrided at temperatures in the range of 450–600 °C for time durations of 1 to 24 h. Evolution of the microstructure and kinetics of nitrided layer formation and its growth were investigated using various experimental techniques like electron probe micro-analysis and electron microscopy. Nitriding at a temperature of 600 °C for 24 h resulted in the formation of a nitrided zone of ~ 14 μm thickness. Inconel 600 was found to get internally nitrided resulting in the formation of nano-meter sized fcc CrN precipitates in Cr depleted Ni–Fe based matrix phase. Diffusion coefficient and activation energy values for the diffusion of nitrogen in fcc Ni-based matrix were evaluated using the studies on kinetics of growth of nitrided layer. These values were compared with the available information in literature on other Ni–Cr–Fe alloys.

A sodium leak was observed in a reducer in the carbon meter circuit of a dynamic sodium loop used for high temperature sodium testing of materials. The reducer in the carbon meter circuit had been exposed to a temperature of 623 K for 6600 h during testing. Detailed metallurgical characterization was undertaken to understand the cause of failure. Based on micro chemical and carbon analysis the structural material of the reducer was identified as 304 stainless steel containing 0.08 wt% of carbon. Cracks were observed at a distance of 4 mm from a weld interface where the microstructure appeared to be fully sensitized in nature. Electron microscopy investigations confirmed the presence of (Fe, Cr) 23 C 6 carbides in the grain boundary. Cause of the failure was concluded as low temperature sensitization followed by intergranular stress corrosion cracking. It is recommended that 304LN stainless steel may be used as the structural material for components operating in this temperature regime for prolonged duration.

Oxide dispersion strengthened ferritic steel powder was prepared by mechanical alloying of pre-alloyed ferritic steel powder together with nano Y 2 O 3 in a dual drive planetary ball mill. A detailed investigation was carried out using X-ray diffraction, field emission electron microscopy and transmission electron microscopy. Microstructural parameters such as, crystallite size, lattice strain, deformation stress and dislocation character were evaluated using different Williamson–Hall models; uniform deformation model, uniform stress deformation model and modified Williamson–Hall model and the results obtained were compared and discussed. Uniform stress deformation model and modified Williamson–Hall model were observed to give better estimation of crystallite size as they consider strain anisotropy. With milling, dislocation character was observed to be changing, from near edge to mixed type. Lattice parameters of the milled powders were also estimated. Uniform milling with convoluted particle shape and homogeneous distribution of Y 2 O 3 throughout the matrix was observed by using electron microscopy.

An α + β Ti–5Ta–1.8Nb alloy was solutionized in the β phase field and cooled at different rates from 20 to 99 K/min. Detailed microstructural characterization of the products of β → α transformation after continuous cooling from the β phase field has been carried out, to understand the effect of cooling rate on the transformation mechanisms. An increase in cooling rate causes a suppression of the β → α transformation temperature domain, resulting in composition variation of α and β phases. Such a variation in the composition of β phase was observed to induce different secondary transformations, which are sensitive to the cooling rate.

The methods to predict and prevent the formation of hard and soft zones in dissimilar weldments of 9Cr-1Mo and 2¼Cr-1Mo ferritic steels during high-temperature exposure are examined in this article. The computational studies have been carried out using multicomponent diffusion model incorporated in Dictra and validated by experimental methods using EPMA and TEM. Carbon concentration profiles across the interface of the weld joint between the two ferritic steels were simulated in the temperatures ranging from 823 K to 1023 K (from 550 °C to 750 °C) for various time durations using “diffusion in dispersed phase model” in Dictra. When precipitation and diffusion were incorporated into the calculations simultaneously, the agreement was better between the calculated and the experimentally measured values of carbon concentration profiles, type, and volume fractions of carbides in the hard zone and diffusion zone, width, and the activation energy. Calculation results of thermodynamic potentials of carbon in 2¼Cr-1Mo and 9Cr-1Mo steels suggested that the diffusion is driven by the activity gradient of carbon across the joint. The effectiveness of nickel-based diffusion barrier in suppressing the formation of hard and soft zones is demonstrated using calculations based on the cell model incorporated in Dictra.

Replacement of Mo and Nb by W and Ta in modified 9Cr-1Mo class of steels has been significantly useful for achieving reduced activity and improved mechanical properties. The addition of W and Ta strongly influences microstructure which comprises of the substructural changes and precipitation kinetics under both thermal and stress exposure. This study focusses on the effect of W and Ta on microstructural changes on thermal and creep exposure at 823K in 9Cr-W-Ta-0.2V- 0.1C steels, with W and Ta concentrations varying from 1 to 2% and 0.06 to 0.14% respectively. Increase in W from 1 to 1.4 or 2%, was beneficial to retain lath structure and retard the coarsening of M23C6 during long term thermal aging. Detailed microstructural investigation of the steesl after creep deformation showed extensive subgrain formation and coarsening of M23C6 carbides than just thermal exposure. Influence of W on subgrain size was more pronounced than Ta. Increase in W was found to be useful to control the coarsening rate of M23C6 carbides, even under stress, while MX precipitate size showed no significant change.

The failure of the stainless steel housing of Electro Chemical Hydrogen Metre (ECHM) in the Steam Generator Test Facility resulting in a minor sodium fire has been investigated through systematic metallographic and fractographic analyses. The root cause of the failure was identified as caustic stress corrosion of the stainless steel flange that forms a part of the ECHM housing.

The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite.

Ti-5Ta-2Nb alloy was clad on 304L austenitic stainless steel (SS) using the explosive cladding process. Both Ti-5Ta-2Nb and 304L austenitic steel were severely deformed due to high pressure (in the gigapascal range) and strain rate (105/s), which are characteristics of explosive loading conditions. Consequent changes produced in the microstructure and crystal structure of both the alloys are studied using electron microscopy techniques. The microstructure of both Ti-Ta-Nb alloy and 304L steel showed evidence for the passage of the shock waves in the form of a high number density of lattice defects such as dislocations and deformation twins. In addition, both the alloys showed signatures of phase transformation under nonequilibrium conditions resulting in metastable transformation products. 304L SS showed martensitic transformation to both α′(bcc) and e(hcp) phases. Microscopic shear bands, shear band intersections, and twin boundaries were identified as nucleation sites for the formation of strain-induced phases. Ti-Ta-Nb alloy underwent metastable phase transformation to an fcc phase, which could be associated with regions having a specific morphology.

It is well known that Ti and Zr based alloys are best suitable for oxidizing environments due to their superior corrosion resistance. This has been exploited in the development of the ternary Ti–Ta–Nb alloy, for the dissolver in a fast reactor fuel reprocessing plant, the efforts of which are summarized in the present overview. Closing of fuel cycle is a priority for the three stage nuclear power program of India, towards which, indigenous development of reprocessing technology and materials is an important milestone. Corrosion resistance of structural materials controls the operating life of the dissolver in a nuclear reprocessing plant, where the spent nuclear fuel is dissolved in highly oxidizing boiling nitric acid. A complete physical metallurgy database of the alloy has been generated to understand the various phase transformations and the resultant microstructures. Corrosion control in different phases of nitric acid is achieved through microstructural optimization and the mechanisms of corrosion are understood using a detailed study of passive oxide film. The dependence of transformation texture on the mode of deformation and the role of severe plastic deformation on mechanical properties have been elucidated. These extensive R&D efforts resulted in a complete understanding of the metallurgy the alloy, which is discussed in this overview.

This paper presents the creep deformation and rupture behaviour of indigenously produced 9Cr–1W–0.2V–0.06Ta Reduced Activation Ferritic–Martensitic (RAFM) steel for fusion reactor application. Creep studies were carried out at 773, 823 and 873 K over a stress range of 100–300 MPa. The creep deformation of the steel was found to proceed with relatively shorter primary regime followed by an extended tertiary regime with virtually no secondary regime. The variation of minimum creep rate of the material with applied stress followed a power law relation, έ m = Aσ n , with stress exponent value ‘ n ’ decreasing with increase in temperature. The product of minimum creep rate and creep rupture life was found to obey the modified Monkman–Grant relation. The time to onset of tertiary stage of deformation was directly proportional to rupture life. TEM studies revealed relatively large changes in martensitic sub-structure and coarsening of precipitates in the steel on creep exposure as compared to thermal exposure. Microstructural degradation was considered as the prime cause of extended tertiary stage of creep deformation, which was also reflected in the damage tolerance factor λ with a value more than 2.5. In view of the microstructural instability of the material on creep exposure, the variation of minimum creep rate with stress and temperature did not obey Dorn's equation modified by invoking Lagneborg and Bergman's concepts of back stress.

This paper presents the results of an experimental investigation where an enhancement in Charpy impact toughness and decrease in DBTT was obtained through grain refinement in 9Cr–1Mo steel. The steel in the normalized and tempered condition (1323 K/air cool + 1023 K/2 h/air cool) had an average prior-austenite grain size of 26 μm. By designing a two-stage normalizing (1323 K/2 h/water quench + 1223 K/2 h/air cool) and tempering treatment (1023 K/2 h/air cool), a homogeneous tempered martensite microstructure with a lesser prior-austenite grain size of 12 μm could be obtained. An improvement trend in impact properties of standard sized Charpy specimens was obtained in fine-grained steel: upper shelf energy increased from 175 J to 210 J, and DBTT reduced from 243 K to 228 K. This heat treatment is unique since an attempt to carry out a single-stage low temperature normalizing treatment (1223 K/2 h/air cool) did not give a complete martensite structure, due to the incomplete dissolution of carbides during austenitization.

The 9Cr-1Mo-0.1C steel was solutionzed (1323 K for 1hr) and step cooled (at 40 K/hr between Ms and Mf) to effect martensitic transformation at slower rates, and then subsequently tempered (1023 K for 2 hr). The microtexture of the tempered martensite structure was evaluated using SEM-Electron Back Scattered Diffraction (EBSD) technique, and the crystal orientation data of ferrite crystallites formed within a single prior-γ grain were analyzed in detail. The Kurdjumov-Sachs (K S) orientation relationship (111)γ // (011)α´ [-101]γ // [-1-11]α´ is commonly obeyed in low carbon steels, and a restitution methodology was used to back calculate the orientation of parent γ grain assuming the K-S relationship. The austenite was identified as the common solution of the data set, and its mean orientation has been calculated. The mean deviation from the K-S was calculated to be 3.75º.

This paper presents the results on the physical metallurgy studies in 9Cr Oxide Dispersion Strengthened (ODS) and Reduced Activation Ferritic/Martensitic (RAFM) steels. Yttria strengthened ODS alloy was synthesized through several stages, like mechanical milling of alloy powders and yttria, canning and consolidation by hot extrusion. During characterization of the ODS alloy, it was observed that yttria particles possessed an affinity for Ti, a small amount of which was also helpful in refining the dispersoid particles containing mixed Y and Ti oxides. The particle size and their distribution in the ferrite matrix, were studied using Analytical and High Resolution Electron Microscopy at various stages. The results showed a distribution of Y 2 O 3 particles predominantly in the size range of 5-20 nm. A Reduced Activation Ferritic/ Martensitic steel has also been developed with the replacement of Mo and Nb by W and Ta with strict control on the tramp and trace elements (Mo, Nb, B, Cu, Ni, Al, Co, Ti). The transformation temperatures (A c1 , A c3 and M s ) for this steel have been determined and the transformation behavior of the high temperature austenite phase has been studied. The complete phase domain diagram has been generated which is required for optimization of the processing and fabrication schedules for the steel.

This paper presents the microstructural modification in a dissimilar joint of Ti–5Ta–1·8Nb alloy with 304L austenitic stainless steel, fabricated using explosive cladding process. The interface had a wavy nature with occasional presence of shrinkage cavities and solidified melt zones. X‐ray Rietveld and electron microprobe based analysis did not reveal the presence of intermetallic phases at the weld interface within their detection limits. Evidences for the transformation of fcc to bct phases in 304L stainless steel and formation of metastable fcc phase in Ti–Ta–Nb alloy, not predicted in the phase diagram are provided. These phase transformations are understood in terms of severe plastic deformation during explosive cladding process.

Austenitic stainless steel SS316 and its variants are the common materials for the fast reactor structural components. Using the Fast Breeder Test Reactor (FBTR) as an irradiation test bed, a systematic analysis of the irradiation performance of the austenitic stainless steel has been undertaken. The performance of 20% cold worked SS316 has been assessed by examining the cladding and wrapper of FBTR at various displacement damages. The modified version of SS316, alloy D9, chosen for PFBR has been subjected to test irradiations in FBTR. Further modification of alloy D9 with respect to minor elements is also being studied The salient features of (i) mechanical and microstructural behaviour of SS316 at different fluence levels, (ii) the ongoing irradiation experiments on alloy D9 and (iii)microstructural studies on modified versions of alloy D9 are presented in this paper.

This paper presents the results of a study on the characteristics of the passive oxide film that forms on the surface of an {alpha} + {beta} Ti-5%Ta-1.8%Nb alloy, which possesses good corrosion resistance in severe oxidizing environment of boiling 11.5 M nitric acid. Through systematic structure-property studies, the microstructure with low corrosion rate (< 1 mpy) in liquid, vapor and condensate phases of nitric acid was identified. The characteristics of the passive film, which imparts corrosion resistance to the alloy, are influenced by its microstructure, temperature and concentration of the acid. The microstructure, thickness and composition of the oxide film were characterized using different techniques. TiO{sub 2}, Nb{sub 2}O{sub 5} and Ta{sub 2}O{sub 5} formed on exposure to vapor and condensate phases, while TiO{sub 2} was observed on exposure to the liquid phase. Detailed microstructural studies showed that the passive film consists of nano-crystalline phases of titanium and tantalum oxides, predominantly anatase in an amorphous matrix. Based on these studies, the mechanism of corrosion of the alloy is derived. - Research Highlights: {yields}Liquid phase corrosion results in a thicker and protective oxide film. {yields}Oxide film is a mixture of amorphous and nano-crystalline anatase. {yields}Higher amounts of Nb{sub 2}O{sub 5} andmore » Ta{sub 2}O{sub 5} form on vapor and condensate phase corrosion. {yields}High corrosion rate in condensate phase is due to selective dissolution of Ti in {alpha} phase.« less

When dissimilar weldments between 9Cr-1Mo and 2¼Cr-1Mo ferritic steels are exposed to high temperature, microstructural and microchemical modifications are observed near the weld interface. Diffusion of carbon driven by the activity gradient from low Cr to high Cr steel leads to the formation of carbon enriched ‘hard’ zone and carbon depleted ‘soft’ zone near the fusion joint. The present paper deals with the measurement of carbon diffusion profiles and the evaluation of concentration dependent diffusion coefficients of carbon across the interface at a temperature of 1023 K. Accurate carbon concentration profiles are generated using carbon calibration graph. The profiles are smoothened to reduce the experimental scatter and the concentration dependant diffusion coefficients are determined using Den Broeder’s method. In the base materials, where the concentration gradient is extremely small, D(c) values are determined using Hall’s method. Variation in D(c) across the weld interface is understood based on the microstructural and microchemical changes that take place during heat treatment.