25 research outputs found

    Development of a self-consistent thermodynamically optimized database along with phase transition experiments in Ni-Mn-Ga system for magnetocaloric applications

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    Magnetocaloric materials have received significant attention of research community as they can minimize the use of harmful gases (CFCs, HFCs) and render eco-friendly refrigeration. Heusler alloys (Ni2MnGa) are known for their magnetocaloric effects, which make them useful as energy efficient and eco-friendly refrigerating materials. Magnetocaloric properties significantly depend on the composition of these alloys. Ni-Mn-Ga is one of the interesting Heusler systems, which exhibits magnetocaloric properties. In the present study, we performed the thermodynamic optimization of two sub binaries of the Ni-Mn-Ga system: Mn-Ga and Ni-Ga, using CALPHAD approach. A Modified Quasichemical Model (MQM) was used to describe the thermodynamic properties of the liquid solutions in both the binaries. Both the binaries were combined with Mn-Ni to develop a self-consistent thermodynamic database for Ni-Mn-Ga. In order to resolve the existing experimental discrepancies in the Mn-Ga and Ni-Ga system, few alloy compositions were prepared and analyzed using differential thermal analysis. Finally, the developed thermodynamic database was used to calculate the ternary isothermal section of the Ni-Mn-Ga (Heusler alloy) system at 1073 K with a proposed phase region for magnetocaloric applications.Comment: 26 Pages, 10 Figure

    Microstructural development in Mg alloys during solidification: an experimental and modeling study

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    Magnesium alloys have been the focus of active research especially in automotive industry for designing light weight vehicles in order to achieve better fuel efficiency and lower greenhouse gas emissions. The control of as-cast microstructure of Mg alloys is critical because many automotive components are made of Mg alloys and used in their as-cast form. Microstructure development during any casting process is controlled by solidification phenomena. Thus it is imperative to understand the relationship between casting process parameters and microstructural features to control the quality of final product. In the present work, a solidification model is developed to predict the as-cast microstructural features of Mg alloys. In order to validate the modeling results, extensive solidification experiments were performed by employing various casting techniques. A comprehensive microstructural evolution of Mg-3, 6 and 9 wt. % Al alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (GV) and solute Al content were investigated. The results are presented in chapter 4. Different solidification techniques were employed to obtain the as-cast microstructure in the cooling rate range between 0.05 and 700 K/sec. The microstructural length scales of Mg-Al alloys such as secondary dendrite arm spacing (SDAS) and primary dendrite arm spacing (PDAS) were experimentally determined and compared with currently available models in the literature. In addition, the solidification parameters for morphological transitions like cellular to columnar dendrite and columnar to equiaxed dendrite were also determined. Based on all the experimental data and the solidification model, a solidification map was built in order to provide guidelines for the as-cast microstructural features of Mg-Al alloys. In chapter 5, a systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6 and 9 wt. % Al) has been carried out over a wide range of cooling rate (0.05 -700 K/sec) by employing various casting techniques. In order to explain the experimental results, a solidification model taking into account dendrite tip undercooling, eutectic undercooling, solute back diffusion and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not only determined by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model. Microstructural features of Mg-1.5, 4.0 and 5.5 wt. % Zn alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (GV) and solute (Zn) were comprehensively studied in chapter 6. Microstructural features were experimentally determined and compared with the simulation results from the solidification model. The microstructural solidification model, which can predict the microsegregation and microstructural features, has been extended to multicomponent alloy systems and a detailed formulation of the model is presented in chapter 7. The model is dynamically linked to thermodynamic library for accurate input of thermodynamic data. The modeling results are tested against the directional solidification experiments for Mg-Al-Zn alloys. The experiments were conducted in the cooling rate range of 0.1-3 K/sec and the microstructural features are compared with the model. Based on the model and the experimental data a solidification map was built in order to provide guide lines for as cast microstructural features of Mg-Al-Zn alloys in a wide range of solidification conditions.Les alliages de magnésium ont fait l'objet de recherches actives notamment dans l'industrie automobile pour concevoir des véhicules légers afin de parvenir à une meilleure efficacité énergétique et réduire les gaz à effet de serre. Le contrôle de la microstructure de coulée d'alliages de magnésium est essentielle, car de nombreux composants automobiles sont faits d'alliages de magnésium et utilisées dans leur forme brute. Même pour les alliages de magnésium forgé, la compréhension de la microstructure de coulée est très importante pour optimiser les processus en aval tels que l'homogénéisation, le laminage et le recuit. Le développement de la microstructure au cours d'une opération de coulée est contrôlé par des phénomènes de solidification. Ainsi, il est impératif de comprendre la relation entre les paramètres du procédé de coulée et les caractéristiques de microstructure pour contrôler la qualité du produit final. Dans le présent travail, un modèle de solidification est développé pour prédire les caractéristiques microstructurales des alliages de coulée de magnésium. Afin de valider les résultats de la modélisation, de nombreuses expériences de solidification ont été réalisées en employant diverses techniques de moulage. Une évolution de la microstructure complète de Mg-3, 6 et 9 % poids d'alliages d'Al par rapport aux paramètres de solidification comme le gradient thermique (G), la vitesse de solidification (V), la vitesse de refroidissement (GV) et la teneur en soluté d'Al ont été étudiés. Les résultats sont présentés dans le chapitre 4. Techniques de solidification différentes ont été utilisées pour obtenir le mictostructure de coulée dans la vitesse de refroidissement entre 0.05-700 K / sec. Les échelles de longueur des microstructures des alliages Mg-Al tels que l'espacement des bras de dendrites secondaires (SDAS) et l'espacement des bras de dendrites primaires (PDAS) ont été déterminées expérimentalement et comparés avec les modèles actuellement disponibles dans la littérature. De plus, les paramètres de solidification pour les transitions morphologiques des dendrites de cellulaire à colonnaire et colonnaire à équiaxe ont également été déterminés. Sur la base de toutes les données expérimentales et du modèle de solidification, un schéma de solidification a été construite afin de fournir des lignes directrices pour les caractéristiques microstructurales des alliages de coulée Mg-Al.Dans le chapitre 5, une étude expérimentale systématique sur la microségrégation et la fraction de la seconde phase d'alliages Mg-Al binaires (3, 6 et 9 % poids d'Al) a été réalisée sur une large gamme de taux de refroidissement (0,05-700 K/sec) en employant diverses techniques de moulage. Pour expliquer les résultats expérimentaux, un modèle de solidification qui tient compte de la surfusion des pointes des dendrites, de la surfusion eutectique, de la rétrodiffusion du soluté et du grossissement des bras de dendrites secondaires a également été développé en lien dynamique avec une base de données thermodynamiques précises. A partir des données expérimentales et du modèle de solidification, il a été constaté que la fraction de la seconde phase de la microstructure solidifiée n'est pas uniquement déterminée par la vitesse de refroidissement, mais varie indépendamment avec le gradient thermique et la vitesse de solidification. Enfin, les schémas de fraction de la seconde phase pour les alliages Mg-Al ont été calculés à partir du modèle de solidification.Une analyse similaire est effectuée pour Mg-1.5, 4.0 et 5.5 en poids. % D'alliages de Zn dans le chapitre 6. Le modèle de solidification binaire est étendu à des systèmes multi-composants dans le chapitre-7. Les résultats de la modélisation sont testés contre les données expérimentales pour Ternay alliages Mg-Al-Zn solidifié dans la plage de vitesse de refroidissement de 0.1-3 K / sec.

    Influence of solute elements (Sn and Al) on microstructure evolution of Mg alloys: an experimental and simulation study

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    Wedge casting technique was employed for binary Mg alloys to obtain the solidified microstructure with the cooling rates ranging from 5 to 150?K/sec. Microstructural features such as secondary dendrite arm spacing (SDAS) and secondary phase fractions were experimentally determined. The experimental results of Mg-3.0, 6.0 and 9.0?wt% Sn alloys were compared with Mg-Al alloys to understand the solute effect upon the evolution of SDAS and second phase fraction in Mg binary alloys. Solidification calculations that incorporate solute back diffusion, secondary arm coarsening, dendrite tip undercooling and is dynamically linked with accurate thermodynamic databases were performed for accurate analysis of the experimental results.by Arushi Dev and Manas Paliwa

    1D solidification model for the prediction of microstructural evolution in light alloy

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    As-cast microstructure is important for alloy development. Solidification is a complex process involving both phase diagram information and diffusion kinetics. The present 1D solidification model coupling the CALPHAD-type thermodynamic database and classical solidification theories can reasonably predict the microstructural evolution including solidification path, primary and secondary dendrite arm spacing, microsegregation, and secondary phase fraction. The solidification calculations even in multicomponent system can be calculated in less than a few minutes. The combination of the present solidification model and thermodynamic calculations can be useful for the optimization of casting parameters of existing alloys and the development of new alloys.by Manas Paliwal and In-Ho Jun

    Precipitation kinetic model and its applications to Mg alloys

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    Synthesis, Consolidation and Modelling Study of AA2014-TiB2 Composite Prepared by Powder Metallurgy (P/M) Method

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    This Aluminum alloy matrix composite reinforced with TiB2 particulates with different volume % of TiB2 (5, 10 and 15) has been successfully synthesized by P/M route. The composite powders were consolidated by cold uniaxial compaction pressure followed by sintering at 590 ?C in nitrogen atmosphere. The effect of reinforcement on the densification was studied and reported in terms of the relative density, densification parameter, tensile rupture strength and Vickers hardness of the composite. The above physical and mechanical properties increase with compaction pressure irrespective of TiB2 content. Scheil cooling and equilibrium calculations were performed using FactSage for qualitative understanding of the microstructural evolution during sintering. The experimental results showed that samples with 5 volume % TiB2 exhibits optimum densities after sintering and correspondingly highest hardness.by Rana Pratap Singh, Gaurav Kumar Gupta, and Manas Paliwa

    Influence of second phase precipitates on mechanical and in-vitro corrosion behaviour of Mg-4Zn-0.5Ca-0.8Mn alloy in optimum homogenized conditions

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    The present study investigates the mechanical and in-vitro corrosion behavior of Mg-4Zn-0.5Ca-0.8Mn alloy in optimum homogenized conditions. The optimization of the homogenization parameters has been carried out employing thermodynamic calculations and kinetic modeling. The model utilizes the inter-diffusivity of the solute elements and predicts that ∼6–24 h of homogenization at 633 K effectively redistributes the elements in the Mg matrix. Based on the insights obtained from the simulations, the as-cast Mg-4Zn-0.5Ca-0.8Mn alloy was subjected to homogenization heat treatment process for 6–24h. The microstructural study through optical microscopy and scanning electron microscopy (SEM) revealed that the interconnected network of second phase precipitates substantially dissolve within 24 h, implying adequate homogenization. Moreover, fine Mg-Zn based precipitates with varied morphology and phase fractions also evolved during homogenization treatment, as confirmed through SEM and transmission electron microscopy. In the 12 h homogenized specimen, the highest fraction of uniformly dispersed fine precipitates resulted in the highest strength (∼225 MPa). On the other hand, a substantial disruption in coarse precipitate network and lower aspect ratio of fine Mg-Zn precipitates led to the highest ductility (∼8%) in this specimen. In the 24 h homogenized specimen, the ductility reduced marginally owing to higher aspect ratio of fine precipitates. The immersion and electrochemical tests (viz., potentiodynamic polarization and electrochemical impedance spectroscopy) carried out in Hank's solution revealed that the 24 h homogenized specimen exhibits the best corrosion properties. The least fraction of Ca2Mg6Zn3 phase with maximum disruption in interconnectivity, in combination with a small fraction of fine equilibrium MgZn2 precipitates, resulted in suppression of localized corrosion in this specimen. This promotes the formation of the most stable and compact product layer over the specimen, resulting in the highest corrosion resistance

    Numerical modeling of diffusion-based peritectic solidification in iron carbon system and experimental validation

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    Continuous casting of high-strength steels is challenging owing to peritectic phase transformation during solidification. This transformation is reported to be either diffusion controlled or “massive” like. The experimental evidence suggests that constant thermal gradients lead to diffusion-controlled phenomena, whereas the concentric solidification technique induces massive transformation. Diffusion-controlled peritectic solidification is more desirable during continuous casting to ensure a suitable cast quality compared with massive transformation. Accordingly, the authors demonstrate a general one-dimensional numerical modeling of the solidification process in steel by incorporating a diffusion-controlled peritectic phase transformation. The model is dynamically linked with the FactSage thermodynamic database through ChemAppV 7.1.4 library for input of accurate thermodynamic data. The modeling details are presented for a binary Fe-C system, and the results are compared with the experimental data available in the literature. The growth and dissolution of phases are accurately predicted as a function of composition and cooling rate.by Ipsita Madhu Mita Das, Nishant Kumar and Manas Paliwa

    Microstructure and Mechanical Properties of Al10SiMg Fabricated by Pulsed Laser Powder Bed Fusion

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    A series of high-density Al10SiMg specimens were fabricated using a custom built pulsed laser powder bed fusion unit operating with a pulsed-laser source. The fabricated components were analyzed using optical microscopy, computerized tomography (CT), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) mapping, and X-ray diffraction (XRD). A significantly refined cellular microstructure was observed, where Al cell diameter refinement upto ~210 nm was obtained throughout the component. Age hardening T6 treatment was also performed to investigate the heat treatment response of this fine microstructure. The mechanical properties in the as-built condition were assessed by microhardness testing (136 HV) and compressive tests (true compressive yield strength of 380 MPa and true ultimate compressive strength of 485 MPa). On the other hand, the mechanical responses of T6 specimens displayed strength reduction while demonstrating enhanced ductility.by R. Choua, A. Ghosha, S. C. Choua, Manas Paliwal and M. Brochu