Directional solidification of eutectic and off-eutectic Au-Co composites with and without magnetic field

Abstract

A fibrous eutectic with Co fibres embedded in a Au-matrix can be obtained at G/v ≥ 2×10⁵ deg/cm² whereG = temperature gradient andv = growth velocity. Interfibre spacing 2R can be described byR ² v=1.2×10⁻¹¹ cm³/sec. Below G/v=2×10⁵ deg/cm² cellular growth is observed. Fibre to blade transitions have been found over the entire range of growth velocities employed, i.e. between 10⁻⁵ and 10⁻²cm/sec and are thought to be due to local growth perturbations. Composite growth under the influence of a magnetic field produces two effects: an orienting effect and a shape and size modification. Orienting occurs at slow growth rates, preferably, in hypereutectic alloys and is explained as a magnetic aligning of the ferromagnetic primary Co-precipitates. The shape and size modification which results in thick and segmented, caterpillar-like Co fibres, appears to be independent of growth rate (within the limits indicated above), occurs mainly in cell boundaries at all compositions, including hypoeutectic ones, and is thought to be due to a field induced enhancement of local temperature gradients.