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As-cast Mg–Sn alloys (3, 6, and 9 wt% Sn) were solution treated at 653, 703 and 753 K (380, 430 and 480 °C) for 1, 4, 8, 12 and 24 h to determine the variation of secondary phase with respect to Sn content, temperature and time. Mg-3 wt% Sn exhibits Mg 2 Sn dissolution at all solution treatment temperatures whereas Mg-6 and 9 wt% Sn alloy displays Mg 2 Sn reprecipitation and dissolution depending on the heat treatment temperature. In addition, a combined mathematical model that predicts the secondary phase dissolution and solute redistribution as a function of temperature and time is presented in this work. This model is a significant improvement compared to the previous studies where the dissolution and homogenization processes are considered independently. The effect of grain size and solute mobility upon the dissolution and homogenization kinetics is discussed as well. [ABSTRACT FROM AUTHOR]

Abstract Precipitation calculations for Mg alloys have been performed using an in-house precipitation model. The size distribution, number density and volume fraction of precipitates are complex functions of thermodynamics, nucleation, and diffusion kinetics dependent on solute composition and heat treatment parameters. The mathematical description and the numerical approach for the precipitation model are given in details. The precipitation model is dynamically linked to the FTlite light alloy database of the FactSage system for accurate input of thermodynamic data. The simulation results are compared with the available experimental data for Mg alloys such as Mg-Al, Mg-Zn and Mg-Al-Zn alloys to ascertain the predictive ability of the precipitate model. The precipitation model is also applied to deduce the precipitation kinetics in AZ91 + Ca alloy in comparison to conventional AZ91 alloy. [ABSTRACT FROM AUTHOR]

Highlights • Secondary dendrite arm spacing (SDAS) decreases with increasing solute Sn content. • Second phase fraction increases with cooling rate in wedge casting experiments. • Mg-Sn alloys have lower secondary arm spacing (SDAS) when compared with the Mg-Al. • The secondary phase fraction in Mg-Sn alloys is lower compared to Mg-Al alloys. • Solidification simulations match with the experimental data for Mg-Sn alloys. Abstract 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. [ABSTRACT FROM AUTHOR]

Diffusion couple experiments for Mg–Nd system were performed to investigate the Nd diffusion in Mg solid solution and the interdiffusivities of intermediate phases. Experiments were conducted at 460, 480 and 500 °C for 3, 4 and 3 days respectively. Phase diagram data obtained from the diffusion couple experiments are consistent with previous experimental data and thermodynamic modeling. Impurity diffusion coefficient of Nd in Mg solid solution is almost three times of Gd and Y but lower than that of Al and Zn. [ABSTRACT FROM AUTHOR]

A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes 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 determined only 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. [ABSTRACT FROM AUTHOR]

A numerical formulation of solidification model which can predict the microsegregation and microstructural features for multicomponent alloys is presented. The model incorporates the kinetic features during solidification such as solute back diffusion, dendrite tip undercooling, and secondary arm coarsening. 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 0.13 to 2.33 K/s and microstructural features such as secondary arm spacing, primary dendrite arm spacing, second phase fraction, and microsegregation were compared with the modeling results. Based on the model and the experimental data, a solidification map was built in order to provide guidelines for as-cast microstructural features of Mg-Al-Zn alloys in a wide range of solidification conditions. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive microstructural evolution of Mg–1.5, 4.0 and 5.5wt% Zn alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (GV) and solute (Zn) content were investigated in the present study. Solidification techniques such as directional solidification and wedge casting were employed in order to obtain cooling rates between 0.05 and 250K/s. Microstructural features such as secondary dendrite arm spacing (SDAS), primary dendrite arm spacing (PDAS), microsegregration along the secondary dendrites and secondary phase fractions were experimentally determined. A solidification model that incorporates solute back diffusion, secondary arm coarsening, dendrite tip undercooling and dynamically linked with accurate thermodynamic databases is used to explain the experimental results. [Copyright &y& Elsevier]

Abstract: The comprehensive microstructural evolution of Mg–3, 6 and 9wt.% 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 in the present study. Various solidification techniques, including directional solidification, wedge casting, sand and graphite mould casting, gravity casting in a Cu mould and water quenching, were employed in order to obtain wide ranges of cooling rates between 0.05 and 1000Ks–1. The microstructural length scales of Mg–Al alloys, such as secondary dendrite arm spacing and primary dendrite arm spacing, were determined experimentally and compared with published models. In addition, the solidification parameters of morphological transitions such as 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. [Copyright &y& Elsevier]

Abstract: In order to investigate the short range ordering behavior of liquid Mg–Bi and Mg–Sb solutions, thermodynamic modeling of the Mg–Bi and Mg-Sb binary systems has been performed. All available thermodynamic and phase diagram data of the Mg–Bi and Mg–Sb binary systems have been critically evaluated and all reliable data have been simultaneously optimized to obtain one set of model parameters for the Gibbs energies of the liquid and all solid phases as functions of composition and temperature. In particular, the Modified Quasichemical Model, which accounts for short-range-ordering of nearest-neighbor atoms in the liquid, was used for the liquid solutions. A comparative evaluation of both systems was helpful to resolve inconsistencies of the experimental data. The thermodynamic modeling shows the strong ordering behavior in the liquid Mg–Bi and Mg–Sb solutions at Mg3Bi2 and Mg3Sb2 compositions, respectively, and predicts the metastable liquid miscibility gaps at sub-solidus temperatures. All calculations were performed using the FactSage thermochemical software. [Copyright &y& Elsevier]

Present work focuses on developing a mechanism based strengthening model to predict the mechanical properties of age hardened Mg binary alloys. The study attempts to unify several model parameters and successfully demonstrates the ability of the model to predict the hardness and effective stiffness properties of aged Mg based binary alloys. The model considers the contribution of several strengthening mechanisms to estimate the hardness of Mg–Al and Mg–Zn alloy systems that are precipitation hardened at different aging conditions. The proposed model uses input parameters that are obtained from a precipitation model proposed by Paliwal and Jung (2019). Vickers micro-hardness experiments are conducted on aged binary Mg–Al (6 wt% Al) and Mg–Zn (4.0 and 5.5 wt% Zn) alloys. Verification and validation of the proposed model is carried out by comparing: (a) the hardness measures from experiments with model predictions, (b) the effective stiffness measures obtained from experiments with model predictions based on mean-field approach (Mori–Tanaka method) and (c) the model predictions relative to those obtained from experimental and theoretical data for Mg binaries in literature. • Proposes a strength based model to predict mechanical response of aged magnesium alloys. • Early onset of precipitate coarsening is predicted using continuum models. • Back stress contributions are critical in Mg-binary alloys. • Mechanism based strength contributions are estimated using physics based models. [ABSTRACT FROM AUTHOR]