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urate measurements of enthalpy increment ( H − H) values have been made on a Ti-5 mass% Ta-1.8 mass% Nb alloy using the inverse drop calorimetry technique in the temperature range from 463 K to 1457 K. The measured enthalpy increment values show a steady increase with temperature in both α- hcp and β- bcc solid solution regions. It is found that both the onset as well the completion of the α → β phase change are demonstrated by a marked deviation of the enthalpy increment behavior from the otherwise smooth variation encountered in the respective low-temperature α- and high-temperature β-phase domains. The transformation start ( T) and finish ( T) temperatures of the α → β phase change are found to be (1072±10) K and (1156±10) K, respectively. In the actual α → β phase transformation region, the variation of the enthalpy with the progress of transformation is found to follow a sigmoidal shape which is in line with the diffusive nature of the phase transformation. An estimation of the total enthalpy change associated with the α → β phase transformation (Δ° H) has been made by assuming a simple diffusion limited kinetic model for the phase change. The net enthalpy change for the α → β transformation is found to be 76 J · g. The measured temperature variation of the enthalpy increment in both α- and β-phase regimes are fitted to simple analytical functional forms to obtain temperature-dependent estimates of the specific heat, C. The total specific heat change associated with the α → β phase transformation $${\Delta^{\circ}{C_{P}^{\alpha}}^{\rightarrow{\beta}}}$$ is estimated to be 904 J · kg · K. [ABSTRACT FROM AUTHOR]

The temperature dependence of enthalpy increment ( H T − H298) of 9 mass% Cr–1 mass% W–0.23 mass% V–0.06 mass% Ta–0.09 mass% C reduced activation steel has been measured by inverse drop calorimetry in the temperature range 400 K to 1273 K. A critical comparison of present isothermal enthalpy measurements with the results of our previous dynamic calorimetry studies has been made to reveal clearly the occurrence of various diffusional phase transformations that occur at high temperature. These phase changes are marked by the presence of distinct inflections or cusps in an overall nonlinear variation of enthalpy values with temperature. The principal thermal relaxation step of the martensitic microstructure obtained through quenching from the high-temperature γ-austenite phase is observed around 793 K. The ferromagnetic-to-paramagnetic transition of the α-ferrite phase is found to occur at 1015 K. The equilibrium values of γ-austenite start (A e1) and finish (A e3) temperatures are found to be 1063 K and 1148 K, respectively. A value of 12 J · g−1 has been estimated for Δ° H α→ γ the latent heat associated with the α → γ transformation. The measured enthalpy increment variation of the α-ferrite phase with temperature has been fitted to a suitable empirical function to estimate the temperature-dependent values of the specific heat. A comparison of the drop calorimetry-based indirect estimate of the specific heat with the direct differential scanning calorimetry-based values revealed that the drop calorimetry estimates are systematically lower than its dynamic calorimetry counterpart. This difference is attributed to the fact that, under finite heating rate conditions that are typical of dynamic calorimetry, measurements are made under nonequilibrium conditions. Notwithstanding this limitation, there is a good overall agreement between the two C p values and also among the phase transformation temperatures so that a reliable assessment of thermal properties and phase transformation characteristics of reduced activation steel can be determined by a combined analysis of the results of drop and differential scanning calorimetry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The salient characteristics of martensite formation kinetics in 9Cr–1W–0.23V–0.063Ta–0.09C–0.02N (wt.%) reduced activation steel have been investigated using differential scanning calorimetry and supplemented by microscopy characterisation. Accurate determination of the martensite start (M S) and finish (M f) temperatures as a function of cooling rate (1–99Kmin−1) and holding time (1–120min) in the γ-austenite phase field have been made. The critical cooling rate to form nearly 100% martensite is found to be of the order of 5–6Kmin−1. The measured latent heat for the martensite transformation is found to be in the range 62–75Jg−1. In the case of austenitisation at 1323K, it is found that the grain size of the parent austenite exhibited significant growth and the observed M S temperature too exhibited a systematic increase with austenite grain size. However, for those samples that are solution treated at a slightly lower temperature of 1253K, the M S is found to be initially insensitive to the increase in holding time for up to about 30min. This trend is followed later on by the usual increasing character of M S for further rise in the holding periods, up to about 120min. This unusual initial behaviour is also reflected by the corresponding variation in the hardness values of the quenched in martensite. Moreover, the growth of parent austenite grains has also been restricted during the initial phase of solution annealing at 1253K. This is attributed to the presence of undissolved carbide particles, which exert a pinning influence on the movement of austenite grain boundaries. The observed athermal martensite formation kinetics is described well by the Koistinen–Marburger relation. [ABSTRACT FROM AUTHOR]

Abstract: The on-heating phase transformation temperatures up to the melting regime and the specific heat capacity of a reduced activation ferritic–martensitic steel (RAFM) with a nominal composition (wt%): 9Cr–0.09C–0.56Mn–0.23V–1W–0.063Ta–0.02N, have been measured using high temperature differential scanning calorimetry. The α-ferrite+carbides→γ-austenite transformation start and finish temperatures, namely Ac 1, and Ac 3, are found to be 1104 and 1144K, respectively for a typical normalized and tempered microstructure. It is also observed that the martensite start (M S) and finish (M f) temperatures are sensitive to the austenitising conditions. Typical M S and M f values for the 1273K normalized and 1033K tempered samples are of the order 714 and 614K, respectively. The heat capacity C P of the RAFM steel has been measured in the temperature range 473–1273K, for different normalized and tempered samples. In essence, it is found that the C P of the fully martensitic microstructure is found to be lower than that of its tempered counterpart, and this difference begins to increase in an appreciable manner from about 800K. The heat capacity of the normalized microstructure is found to vary from 480 to 500Jkg−1 K−1 at 500K, where as that of the tempered steel is found to be higher by about, 150Jkg−1 K−1. [Copyright &y& Elsevier]