The Evolution of As-cast Microstructure of Ternary Mg-Al-Zn Alloys: An Experimental and Modeling Study

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.     

Phase equilibria of the constituent ternaries of the Mg-Al-Ca-Sr system

Thermodynamic modeling of the Al-Ca-Sr, Mg-Ca-Sr, Mg-Al-Ca and Mg-Al-Sr systems was conducted using the modified quasichemical model. A self-consistent database has been established for these systems. Mg-Al-Ca and Mg-Al-Sr ternary systems were studied experimentally through microstructure characterization, phase identification, and thermal analysis and thermodynamic modeling based on these experimental findings. It has been observed that the intermetallic compounds in the Mg-Ca, Mg-Sr, Al-Ca, and Al-Sr binary systems dissolve the third component in the respective ternary phase diagrams. In addition, two ternary compounds, Mg₅₆Al₄₀Sr₄ and Mg₂Al₄Ca₃, have been reported.     

Unraveling Recrystallization Mechanisms Governing Texture Development from Rare-Earth Element Additions to Magnesium

Metallurgical and Materials Transactions A - The origin of texture components often associated with rare-earth element (REE) additions in wrought magnesium alloys is a long-standing problem in...     

Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters

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.     

Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on plain C, Mo, and Mo−Nb−V microalloyed steels in order to study the effect of austenite deformation on the ferrite nucleation and growth rates. The nucleation rate increases with deformation and the degree of supersaturation, Ae₃−T; it appears to be reduced by the substitutional elements Mo, Nb, and V through reduction of the austenite grain boundary energy. The growth rate increases with the degree of supersaturation and is also reduced by these elements, apparently through the solute drag-like effect. Under static conditions, increasing the prestraining strain rate increases the nucleation rate, but this increase is small compared to the effect of concurrent deformation. The growth rate under static conditions decreases as the deformation or the strain rate is increased. E. E, formerly with the Department of Metallurgical Engineering, McGill University     

A texture and microstructure analysis of high speed rolling of AZ31 using split Hopkinson pressure bar results

This study used very high strain rate uniaxial compression testing to analyze the microstructure and texture evolution during high speed rolling of as-cast AZ31B alloy. A split Hopkinson pressure bar equipped with induction radiation furnace was used to attain a strain rate of 1200 s^?1 in the temperature range of 25–350 °C and the result was compared with low strain rate (0.01 s^?1) behavior. As well, high speed rolling at 500 m min^?1 was employed to successfully roll AZ31 alloy in one pass with 71 % reduction at 200 °C. During rolling, the mill was suddenly stopped and the sheet was withdrawn from rolling gap and the microstructure and texture evolution was observed. Grain boundary misorientation analysis shows that coincident site lattice boundaries related to contraction twins and secondary twins are more numerous in the samples deformed at high strain rate. With increasing strain for both rolling and compression at 200 °C, the splitting of basal poles was observed, indicating the activation of more contraction twins and secondary twins compared to low strain rate deformation. Also, the recrystallized volume fraction increased significantly with strain rate, probably due to increasing the twin-induced recrystallization fraction. On annealing of the samples compressed at 200 °C, secondary twins and their vicinity were observed to be the preferential sites for nucleation and it seems that rapid recrystallization on secondary twins contributes to the basal texture weakening. Therefore, an increasing number of such twins increase the texture weakening.     

The Development of a Processing Window for the Continuous Galvanizing of a Mn–Cr–Si Martensitic Steel

Martensitic or complex phase steels are leading candidates for automotive impact management applications. However, achieving high strengths while obtaining high quality coatings via continuous galvanizing is a challenge due to cooling rate limitations of the processing equipment and selective oxidation of alloying elements such as Cr, Mn, and Si adversely affecting reactive wetting. The galvanizability of a Cr? Mn? Si steel with a target tensile strength above 1250 MPa was investigated within the context of the continuous galvanizing line. The continuous cooling transformation behavior of the candidate alloy was determined, from which intercritical and austenitic annealing thermal cycles were developed. The evolution of substrate surface chemistry and oxide morphology during these treatments and their subsequent effect on reactive wetting during galvanizing were characterized. The target strength of 1250 MPa was achieved and high quality coatings produced using both intercritical (75% γ) and austenitic (100% γ) annealing using a conventional 95%N₂–5%H₂, ?30°C dew point process atmosphere and 0.20 wt% dissolved (effective) Al bath, despite the presence of significant Mn and Cr oxides on the substrate surfaces. It is proposed that complete reactive wetting by the Zn(Al, Fe) bath was promoted by in situ aluminothermic reduction of the Mn and Cr‐oxides by the dissolved bath Al.     

Characterization of Isothermally Heat-Treated High Carbon Nanobainitic Steels

Isothermal heat treatment close to the martensite-start temperature at various transformation times, followed by hardness and compression tests, has been performed for three new high carbon experimental nanobainitic steels. Microstructural characterization clearly revealed the formation of lower bainitic structures with plate thickness in the range of nanometers. Analysis of the volume fraction of the bainitic phase via x-ray diffraction indicates that the presence of Co and Al accelerates the transformation resulting in almost complete transformation within 24 h. The effect of transformation and the resulting microstructure on the mechanical properties are also presented. Finally, the data collected from the compression tests have been used to develop an enhanced correlation between the yield strength and hardness in steels. Comparison of the improved correlation with three other frequently used correlations from the literature reveals a good performance of the proposed correlation.     

Microstructural characterization of Mg–Al–Sr alloys

The microstructural details of fourteen Mg–Al–Sr alloys were investigated in the as-cast form by a combination of scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis and quantitative electron probe microanalysis (EPMA). The heat transfer method coupled with the DSC measurement has been utilized to determine the solidification curves of the alloys. The morphology and the chemical composition of the phases were characterized. The microstructure of the alloys is primarily dominated by (Mg) and (Al₄Sr). In the present investigation, ternary solid solubility of three binary compounds extended into the ternary system has been reported and denoted as: (Al₄Sr), (Mg₁₇Sr₂) and (Mg₃₈Sr₉). The (Al₄Sr) phase is a substitutional solid solution represented by Mg_xAl_4–xSr and has a plate-like structure. The maximum solubility of Al in Mg₁₇Sr₂ was found to be 21.3 at%. It was also observed that Mg₃₈Sr₉ dissolved 12.5 at% Al.