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The development of robust and efficient methods for constructing and joining complex metal specimens with high bonding quality and durability is of paramount importance for various industries, e.g., aerospace, deep space, and automobiles. This study investigated the fabrication and characterization of two types of multilayered specimens prepared by tungsten inert gas (TIG) welding: Ti–6Al–4V/V/Cu/Monel400/17-4PH (Specimen 1) and Ti–6Al–4V/Nb/Ni–Ti/Ni–Cr/17-4PH (Specimen 2). The specimens were fabricated by depositing individual layers of each material onto a Ti–6Al–4V base plate, and subsequently welding them to the 17-4PH steel. The specimens exhibited an effective internal bonding without any cracks, accompanied by a high tensile strength, with Specimen 1 exhibiting a significantly higher tensile strength than Specimen 2. However, the substantial interlayer penetration of Fe and Ni in the Cu and Monel layers of Specimen 1 and the diffusion of Ti along the Nb and Ni–Ti layers in Specimen 2 resulted in a nonuniform elemental distribution, raising concerns about the lamination quality. This study successfully achieved elemental separation of Fe/Ti and V/Fe, which is vital for preventing the formation of detrimental intermetallic compounds, particularly in the fabrication of complex multilayered specimens, representing the prime novelty of this work. Our study highlights the potential of TIG welding for the fabrication of complex specimens with high bonding quality and durability.

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 polycrystalline elastic constants of an indigenous variant of 9Cr-1W-based reduced activation ferritic-martensitic (RAFM) steel have been determined as a function of temperature from 298 K to 1323 K (25 °C to 1000 °C), using impulse excitation technique (IET). The three elastic constants namely, Young’s modulus E, shear modulus G, and bulk modulus B, exhibited significant softening with increasing temperature, in a pronounced non-linear fashion. In addition, clearly marked discontinuities in their temperature variations are noticed in the region, where ferrite + carbides → austenite phase transformation occurred upon heating. Further, the incidence of austenite → martensite transformation upon cooling has also been marked by a step-like jump in both elastic E and shear moduli G. The martensite start Ms and Mf finish temperatures estimated from this study are, Ms = 652 K (379 °C) and Mf =580 K (307 °C). Similarly, the measured ferrite + carbide → austenite transformation onset (Ac1) and completion (Ac3) temperatures are found to be 1126 K and 1143 K (853 °C and 870 °C), respectively. The Poisson ratio μ exhibited distinct discontinuities at phase transformation temperatures; but however, is found to vary in the range 0.27 to 0.29. The room temperature estimates of E, G, and μ for normalized and tempered microstructure are found to be 219 GPa, 86.65 GPa, and 0.27, respectively. For the metastable austenite phase, the corresponding values are: 197 GPa, 76.5 GPa, and 0.29, respectively. The measured elastic properties as well as their temperature dependencies are found to be in good accord with reported estimates for other 9Cr-based ferritic-martensitic steel grades. Estimates of θ D , the elastic Debye temperature and γG, the thermal Gruneisen parameter obtained from measured bulk elastic properties are found to be θ D = 465 K (192 °C) and γG = 1.57.

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.

This paper presents the results of a study on high-temperature thermophysical properties of an indigenously developed 18Cr–9Ni–2.95Cu–0.58Nb–0.1C (mass%) austenitic stainless steel. The temperature-induced phase evolution under equilibrium conditions has been theoretically simulated using Thermocalc. Accurate estimates of enthalpy, bulk thermal expansion and thermal diffusivity have been obtained as a function of temperature, using drop calorimetry; dilatometry; and laser flash transient thermal diffusivity techniques, respectively. The experimental measurements were subject to robust analytical treatment to obtain specific heat, linear thermal expansion coefficient, density and thermal conductivity, in the temperature range of 298–1273 K. A comparative analysis of the experimental data obtained in this study with the reported values for other 300-series of austenitic stainless steels has also been carried out. In addition, a quasiharmonic Debye–Gruneisen modeling of thermal properties has also been attempted.

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.

The sequence of phase instabilities that take place in a Fe-17.7Cr-9.3Ni-0.58Nb-2.95Cu-0.12N (wt pct) austenitic stainless steel (304H Cu grade) as a function of temperature has been investigated using dynamic calorimetry. The results obtained from this investigation are supplemented by Thermocalc-based equilibrium and Scheil–Gulliver nonequilibrium solidification simulation. The following phase transformation sequence is found upon slow cooling from liquid: L → L + γ → L + γ + MX → γ + MX + δ → γ +MX + M23C6 → γ + MX + M23C6 + Cu. Under slow cooling, the solidification follows austenite + ferrite (AF) mode, which is in accordance with Thermocalc prediction and Scheil–Gulliver simulation. However, higher cooling rates result in skeletal δ-ferrite formation, due to increased segregation tendency of Nb and Cr to segregate to interdendritic liquid. The solidification mode is found to depend on combined Nb + Cu content. Experimental estimates of enthalpy change associated with melting and secondary phase precipitation are also obtained. In addition a semi-quantitative study on the dissolution kinetics of M23C6 type carbides has also been investigated. The standard solution treatment at 1413 K (1140 °C) is found to be adequate to dissolve both Cu and M23C6 into γ-austenite; but the complete dissolution of MX type carbonitrides occurs near the melting region.

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.

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.

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.

A comprehensive calorimetric study of high-temperature phase equilibria and phase transformation characteristics in U-xZr (x = 2, 5, 10 wt pct) alloys has been undertaken, as a function of heating and cooling rates. It is found that the following sequence of phase transformation takes place upon slow heating in annealed U-2 wt pct Zr alloy: α + α′ + δ-UZr2 → α + γ 2 → β + γ 2 → β + γ 1 → γ. For alloys of 5 and 10 wt pct Zr, the additional presence of a miscibility gap (γ 1 U-rich bcc + γ 2 Zr-rich bcc) in the high-temperature γ(bcc) phase region resulted in the following transformation sequence: α + α′ + δ-UZr2 → α + γ 2 → β + γ 2 → γ 1 + γ 2 → γ. Further, it has been demonstrated that depending on the nature of starting microstructure, namely whether it is α eq + δ-UZr2, or a mix of α′ + α eq + δ-UZr2 phases, the relative extents of two possible co-occurring modes of the first on-heating phase transformation step differ. In case of starting microstructure having mixture of three phases α′ + α eq + δ-UZr2, it is found that α′-martensite relaxation via α′ + α eq + δ-UZr2 → α eq + δ-UZr2 constitutes the first on-heating thermal response. The α′-martensitic relaxation is very closely followed by the dissolution of δ-UZr2. The co-occurrence of these two events gives rise to a composite thermal arrest in a normal dynamic calorimetry profile. However, if the starting microstructure is the one having the equilibrium mix of α eq and δ-UZr2, then only the peritectoidal dissolution of δ-UZr2 is found in the calorimetry profile. Unless, a very slow cooling rate of the order of 0.1 K min−1 is adopted from high-temperature γ(bcc) phase, it is not possible to obtain 100 pct of α eq phase along with equilibrium amount of δ-UZr2. At normal and high cooling rates, it is possible to suppress the diffusional decomposition of γ to varying extents. The direct γ → α′-martensite transformation has been observed at sufficiently higher cooling rates. It has been also noticed that even after γ → α′-martensite transformation the precipitation of δ-UZr2 phase is possible at lower temperature during non-isothermal cooling. Further, the critical cooling rate required for γ → α′ displacive transformation is found to decrease with increasing Zr content. For U-2, 5, and 10 wt pct Zr alloys, it is found to be of the order of, 60, 20, 10 K min−1, respectively. The cooling rate from high-temperature γ(bcc) is found have a strong influence on microstructure evolution as well. The kinetic aspects of α → β diffusional transformation that occurs on heating have been modeled in terms of Kolmogorov–Johnson–Mehl–Avrami formalism, and it is found that the transformation is effectively controlled by the diffusion of Zr in α′-orthorhombic phase. Continuous heating and cooling transformation diagrams have also been obtained for U-2 wt pct Zr alloy.

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.

The high temperature thermal stability, phase transformation characteristics and heat capacity of a Fe-0.28Mn-0.55Si-0.045C (mass%) magnetic iron base alloy have been investigated by dynamic and drop calorimetry techniques. Upon slow heating, the following phase changes are found to take place: αferro + (Fe,Mn)3C αpara + (Fe,Mn)3C α + γ γ γ + δ δ δ + L L; with distinct enthalpy effects. The α γ transformation taking place upon slow heating is appreciably influenced by the dissolution kinetics of (Fe,Mn)3C phase. Enthalpy increment values were measured as a function of temperature, up to 1523 K by drop calorimetry. It is found that magnetic effects play a significant role in influencing overall thermodynamic stability. The experimental enthalpy data have been modeled using Debye–Gruneisen and Hillert–Jarl formalisms for accounting respectively the vibrational and magnetic contributions to heat capacity.

There is considerable uncertainty among diverse assessments of phase equilibrium in Fe-Cr alloys, especially around (α′ + α″)/α miscibility gap region. This is largely due to the difficulty associated with the rigorous incorporation of the interplay between magnetic and chemical contribution to phase stability, in particular its composition and temperature dependencies through theory, in the absence of reliable experimental data. Toward this cause, accurate enthalpy measurements have been made on homogenized Fe-20 wt pct Cr alloy using inverse drop calorimetry, in the temperature range 298 K to 1473 K (25 °C to 1200 °C). The experiments revealed two distinct phase transformations: (i) at 720 ± 10 K (447 ± 10 °C), the Fe-20Cr alloy transformed from α′(Fe-rich) + α″(Cr-rich) two-phase microstructure to α single phase and (ii) at 925 ± 10 K (652 ± 10 °C), the ferromagnetic single-phase α transformed to paramagnetic state. Both these transformations are clearly attested by the measured enthalpy increment variation with temperature. The enthalpy data obtained in this study have been combined with available literature information to forge an integrated theoretical assessment of the energetic aspects of α′ + α″ → α, and α ferro → α para transformations. In addition, a comprehensive evaluation of enthalpy and heat capacity data for Fe-20Cr alloy in the temperature range 0 K to 1473 K (−273 °C to 1200 °C), with explicit incorporation of magnetic contribution has also been made.

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.

The influence of magnetic interactions on high temperature thermodynamic stability of Fe-Cr binary system has been analysed in the light of accurate isothermal calorimetry measurements (400-1473 K) on Fe-20wt.%Cr alloy. The onset of two successive principal transformations namely, (i) α(Fe-rich bcc)+α(Cr rich bcc)α(HT bcc) at 702±10 K; and (ii) αferroαpara at 925 ±10 K, with their associated enthalpy effects (ΔoHmag = 2 kJ mol -1; Cpmag = 20 J mol-1 K-1) have been clearly delineated by the measured enthalpy variation with temperature. A precise quantification of magnetic contribution to high temperature thermodynamic stability has been attempted using physically based modelling approach.

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) α or α′ (distorted orthorhombic martensite) + δ(UZr2) → α + γ2 (bcc phase enriched in Zr); (ii) α + γ2 → β (tetragonal) + γ2; (iii) β + γ2 → β + γ1 (bcc phase enriched in U); (iv) β + γ1 → γ; (v) γ (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.