Keyna O'Reilly

Intermetallic phase selection in 1XXX Al alloys

The wide range of both equilibrium and metastable binary Al-Fe and ternary Al-Fe-Si are examined

The effect of vanadium and grain refiner additions on the nucleation of secondary phases in 1xxx Al alloys

High purity Al-0.3 wt% Fe-0.1 wt% Si alloys with different Si, V and grain refiner contents were melt spun to produce microstructures of submicron secondary phases entrained in a higher melting point Al matrix. On reheating, a dispersion of eutectic liquid droplets forms that represents an exaggerated version of the liquid puddles that solidify punched-off between Al dendrite arms during conventional casting. The subsequent resolidification of the droplets, analyzed using differential scanning calorimetry (DSC), allows the nucleation-controlled aspects of secondary phase selection to be studied. The droplets solidify as the metastable FeAl{sub m} phase in ribbons containing {approx{underscore}equal}500 ppm V or {approx{underscore}equal}100 ppm V plus Al-Ti-B, Al-Ti-C or Al-B grain refiner. This phase contributes to the fir-tree surface defect in commercial sheet products. this work suggests that the combination of V and Al-Ti-B promotes FeAl{sub m} in commercial ingots, and confirms that solidification rate and bulk Si content also influence phase content.

Effect of semisolid microstructure on solidified phase content in 1xxx Al alloys

Abstract Melt-spun high purity Al–0.3 wt% Fe–0.1 wt% Si alloys, containing V and Al–5Ti–1B grain refiner, were melted at 2 K/min from 635°C to different temperatures above the solidus in a differential scanning calorimeter (DSC). Quenched and slow cooled (at 2 K/min) microstructures were examined using scanning electron microscopy (SEM). Melting was observed in a hot-stage reflected light microscope. Al–Fe4Al13 melted first, forming cell boundary liquid films. Rapid coarsening occurred at

Heterogeneous nucleation of solidification of equilibrium and metastable phases in melt-spun Al-Fe-Si alloys

Abstract 1xxx series Al-0.3 wt% Fe-0.1 wt% Si alloys with different impurity contents have been melt spun to produce microstructures of fine scale ~ 0.1–0.2 μm diameter secondary phase particles entrained in a higher melting point Al matrix. The melting and resolidification behaviour of the secondary phase particles while entrained in the solid matrix was monitored using differential scanning calorimetry, to simulate the nucleation of secondary phases in liquid interdendritic regions during the final stages of solidification in casting. The effect of impurities on the nucleation of solidification behaviour, and in particular on secondary phase selection, has been investigated by comparison of commercial and higher purity alloys.

The nucleation of solidification in liquid droplets embedded in a solid matrix

Abstract Experimental progress in the study of the nucleation of solidification has significantly increased our understanding of heterogeneous nucleation. A wide range of metallic systems has been studied to investigate, firstly, fundamental theories of heterogeneous nucleation; secondly, the roles of chemical and structural factors in determining catalytic efficiency; and thirdly, the effects of ppm levels of impurities on nucleatin behaviour in commercially relevant systems.

Characterization of Fe-Rich Intermetallic Phases in a 6xxx Series Al Alloy

Intermetallic phases formed during directional Bridgman solidification of a 6xxx series Al alloy in the growth velocity range 5-120 mm/min have been characterised using conventional and high-resolution transmission electron microscopy. Cubic αc-AlFeSi and β-AlFeSi were always present in the alloy, but Al13Fe4 was only observed at 5mm/min and monoclinic αT-AlFeSi at 80 mm/min. Cubic αc-AlFeSi was observed to have a variety of morphologies resulting from flexibility in its growth mechanisms. β-AlFeSi with twins and faults showed strongly anisotropic growth by steps, resulting in its platelet morphology. Occasionally blocky β-AlFeSi particles were observed particularly at 30-60 mm/min, suggesting that other mechanisms could influence their morphology. Two types of composite particles, β-AlFeSi/Al13Fe4 and β-AlFeSi/cubic αc-AlFeSi, were observed in this alloy, and likely formed by two quasi-peritectic reactions.

Melt Conditioned Direct Chill Casting (MC-DC) Process for Production of High Quality Aluminium Alloy Billets

A novel direct chill (DC) casting process, melt conditioned direct chill (MC-DC) casting process, has been developed for production of high quality aluminium alloy billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleating particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. Experimental results have demonstrated that, the MC-DC casting process can produce aluminium alloy billets with significantly refined microstructure and reduced cast defects. In this paper, we give an overview of the MC-DC casting process and report on results obtained from an industrial scale trial.

Microstructural transitions in Al–Cu ribbons manufactured by planar flow casting

Abstract Morphological transitions in Al–4.2wt%Cu and Al–14.3wt%Cu ribbons manufactured by planar flow casting were analyzed by a combination of optical, scanning and transmission electron microscopy as a function of the solidification velocity. There is a {200} preferred orientation of the α -Al phase at the wheel surface of the Al–4.2wt%Cu ribbon, but no distinguishable preferred orientation in the Al–14.3wt%Cu ribbon. The absolute stability velocities of the Al–4.2wt%Cu and Al–14.3wt%Cu ribbons were determined to be 2.28 and 8.40 ms −1 , respectively. For the Al–4.2wt%Cu ribbon, a segregation-free columnar layer was developed near the bottom of the ribbon, where the solidification velocity was higher than the critical value of 2.28 ms −1 . At lower solidification velocities transitions occurred to cellular columnar, and then equiaxed dendritic layers. In Al–14.3wt%Cu ribbons, however, no segregation-free layer was found and only cellular columnar and equiaxed dendritic layers were observed, due to the solidification velocity being lower than the critical velocity for absolute interface stability for an Al–14.3wt%Cu ribbon of 8.40 ms −1 . A solidification sequence has been proposed for the formation of Al–4.2wt%Cu ribbons.

A calorimetric evaluation of the role of impurities in the nucleation of secondary phases in 1xxx Al alloys

Superpurity (> 99.995wt% pure) lxxx series Al-0.3wt%Fe-0.lwt% Si alloys with different impurity additions have been melt spun to produce ribbons with microstructures of fine scale ∼0.1–0.2μm diameter secondary phase particles entrained in a higher melting point Al matrix. The melting and resolidification behaviour at 2Kmin −1 of the secondary phase particles while entrained in the solid matrix was monitored using differential scanning calorimetry, to simulate the nucleation of secondary phases in liquid interdendritic regions during the final stages of solidification in casting. The melting and resolidification behaviour of 500ppm V and Zr doped superpurity ribbons is the same as that of a commercial purity ribbon. This indicates that V is responsible for the promotion of the metastable FeAl m phase in the commercial purity ribbon. The presence of FeAl m in direct chill (DC) cast commercial purity alloys indicates that V also influences phase selection in the faster cooled (∼5-10Ks −1 ) regions of these alloys.

In Situ Al3Nb Formation in Liquid Al by Nb Particle Addition

Master alloys are used in the metals industry to control chemical composition and to help to achieve a particular microstructure or promote growth of desired phases. This study reports on making a Al3Nb containing aluminium (Al) - niobium (Nb) master alloy by solid-liquid reaction processing, where solid Nb particles are added to the liquid Al. Nb react with Al to form in-situ Al3Nb. The in-situ formed Al3Nb particles were facet and polygonal in shape. The three dimensional analysis revealed that the outer surface of the partially reacted Nb was covered with faceted Al3Nb particles. The different nature and morphologies of the in-situ phases that were produced were determined using SEM, EDX, XRD and extraction techniques. A mechanism for the observed microstructural difference is discussed.