L.M. Hogan

The crystal growth of silicon in Al-Si alloys

Some crystallographic observations of primary silicon and unmodified eutectic silicon in aluminium-silicon alloys are summarized and used to support the theory that sodium modification is due to the poisoning of active growth centres. The implications of endogenous growth in unmodified aluminium-silicon sand castings are explored and mechanisms are proposed for the flake-fibre transition in chill-modified and sodium-modified alloys.

The modification of aluminium-silicon alloys with strontium

High purity Al-Si alloys in the range 9–17% Si, and containing strontium additions of 0–0.1%, were directionally frozen at rates from 11 to 7500 μm/sec with imposed temperature gradients from 90 to 410°C/cm. A region of coupled growth, in which modified eutectic alone grows, has been delineated and the effect of strontium level on the shape of this region determined. Observations are recorded of overmodification, the interparticle spacing/growth rate relationship for strontium modified alloys, twinning behaviour and crystal growth directions of the silicon phase, and grain structure of the aluminium phase. Explanations are offered for features of the unmodified/modified eutectic transition, suppression of primary silicon crystals and the form of the coupled region boundaries.

A simple prediction of the rate of the peritectic transformation

The rate of a peritectic transformation is controlled by diffusion through the peritectic envelope. The rate constant is determined by the diffusion coefficient in the peritectic product phase and by the form of the phase diagram features. Peritectic systems can be classified on the basis of phase diagram form to permit easy prediction of relative rate constants.

The peritectic transformation

Some experimental observations on the Al-Al3Ti and Cd-Cd3Ag peritectic systems are discussed in relation to transformation rates. In steady-stage unidirectional solidification equilibrium between the solid and liquid phases is closely approached at all points of the solid-liquid interface. An approximate diffusion analysis permits prediction of the rate of growth of the peritectic envelope by diffusion through the envelope. In the Al-Al3Ti system the peritectic product is formed principally by direct transformation from the liquid phase, and attack on the properitectic phase is minimal. In the Cd-Cd3Ag system the peritectic transformation goes almost to completion, and direct transfor- mation from the liquid plays a minor part.

Crystal morphology of unmodified aluminium-silicon eutectic microstructures

The crystallography of eutectic silicon in an unmodified A1-12.7 wt% Si alloy has been studied by transmission electron microscopy and diffraction of thin films cut from bulk samples. Twinning mechanisms are described by which the silicon changes direction or adjusts interflake spacing at the solid-liquid interface. The branching mechanism observed by previous workers is confirmed but found to be infrequent at moderate freezing rates. A side-branching mechanism is described which is considered to be the primary mechanism for branching in the wheatsheaf configuration. Orientation relationships between the eutectic silicon and aluminium phases are described. These differ from those previously observed in thin films drawn from the melt. It is proposed that orientation relationships vary with freezing rate. Evidence is presented that the eutectic aluminium re-nucleates repeatedly during growth of a single eutectic grain.

Growth morphology of primary silicon in cast Al-Si alloys and the mechanism of concentric growth

Abstract Faceted growth of primary silicon crystals in cast hypereutectic aluminium–silicon alloys is studied by measurement of spacings between successive growth traces observed in microsections. A general equation, derived to specify conditions for stable growth of silicon crystal, is supported by spacing measurements. Some examples of departures from stable silicon growth are studied. Three stages in the development of faceted crystal growth are recognized, changing from spheroidal to faceted to unstable with increasing crystal diameter.

Some crystallographic observations of growth-twinned dendrites in aluminium

Growth-twinned dendrites in semi-continuously cast aluminium billets were studied with the object of elucidating some of their crystallographic details. It is shown that there is no unique growth direction for twinned dendrites in aluminium and that under the conditions of D.C. semi-continuous casting, twinned dendrites can occur in competition with columnar dendrites when the twin plane deviates by as much as 30° from the normal to the solid-liquid interface. The growth advantage experienced by twins in competition with normal columnar dendrites is explained in terms of a growth mechanism which depends upon twin boundary energy, solute content, freezing rate and temperature gradient.

Thermal analysis of peritectic alloys

Predictions that increasing cooling rate will result in increasing elevation of the apparent peritectic arrest temperature have been confirmed for Cd-Ag and Zn-Cu alloys in line with observations of Al-Ti alloys. The shape of the peritectic cooling curve is related to the competition between the peritectic transformation and direct crystallization from the melt in the formation of the secondary α phase. The effect on solidification of the relative depressions of the α and β liquidus during cooling is considered.

Crystallography of the Al-Al3 Fe eutectic

Crystallographic characteristics and limits of stability of the Al-Al2Fe abnormal eutectic microstructure have been observed. When the G R ratio is low, Al3Fe grows as frequently branched, narrow platelets in a dendritic aluminium matrix. When G R > 1000°C sec/mm2 branching is largely eliminated and [010]Al3Fe ∥ [100]Al ∥ growth direction. The Al3Fe platelets grow in the [010] direction by a twin plane re-entrant edge mechanism in which grooves between faceting {111} planes are formed between multiple (001) twins. (001) twins were observed at 50-200 A intervals for all freezing conditions, and (100) twins were also observed at higher values of G R.

Metallography and growth crystallography of Al3Ti in Al-Ti alloys up to 5 wt% Ti

The morphologies of Al3Ti primary dendrites in Al-Ti alloys are studied for a range of compositions and growth conditions, and related to growth kinetics. The characteristics growth pattern is a flat dendrite with effectively inert (001) surfaces and orthogonal [110] branches. This faceted morphology is ascribed to growth rate anisotropy related to the large density difference between (001) and all other crystallographic planes. No evidence was found of the TPRE growth mechanism. A foliated growth pattern in which dendrites form parallel layers is described as antiskeletal growth. High freezing rates and low Ti contents promote fine twinned dendritic particles which are effective nuclei for the grain refinement of aluminium. At high Ti contents the growth anisotropy is reduced and 〈hk0〉 growth directions other than [110] become prominent.