Five-fold branched Si particles in laser clad AlSi functionally graded materials

Abstract

Abstract-Many five-fold branched Si particles (Si p ) were observed in Al-40 wt% Si functionally graded materials produced by a single-step laser cladding process on cast Al-alloy substrate. In this paper the fivefold twinning and growth features of Si p are scrutinized with orientation imaging microscopy and electron microscopic examination. It is a more in depth study of formation of the Si particles in functionally graded materials as published in our previous paper [Pei, Y. T. and De Hosson, J. Th. M., Acta mater., 2000, 48, 2617]. These Si particles have grown from twinned decahedron nuclei consisting of five tetrahedrons that share a common 110 axis. The twin plane re-entrant edge (TPRE) mechanism explains both the branch growth in the radial direction and the elongation of Si p along their common 110 axis. Subsequent twinning within the twinned tetrahedrons provides additional re-entrant grooves on their top faces, which are important for the rapid elongation and consequently for the continuous growth of the branched particle. The 7.5°m ismatch that arises by putting together five tetrahedrons around a common 110 axis is accommodated by small-angle grain boundaries (SAGBs). The SAGBs may disturb the progress of growth steps, which causes the particles to branch. The most remarkable facts of the study are that the five-fold branched silicon particles are much bigger (25ෂ40 µm) than the nanometer sizes previously reported in the literature and the 7.5°m ismatch is accommodated mainly by multiple SAGBs. The examples of a single SAGB reported before are just a special case of the SAGB mechanism.

Re-formation of AlP compound in Al–Si melt

“…Recently, there has been a growing interest in hypereutectic Al-Si alloys as the candidate material for highquality engines instead of conventional near eutectic Al-Si alloys by virtue of a higher volume fraction of primary silicon which provides excellent mechanical properties [1][2][3]. The brittleness of Si crystals, however, decreases the properties of hypereutectic Al-Si alloys.…”

Growth of octahedral primary silicon in cast hypereutectic Al–Si alloys

“…A series of morphologies of primary silicon has been observed in hypereutectic Al-Si alloys: plate-shaped crystal of hexagonal form, octahedral equi-axed crystals, star-like crystals containing two to five radiating twin planes, and more or less spherical shapes, etc. [6][7][8][9][10][11].…”

Computer Simulations of the Aluminium-Silicon Anomalous Eutectic Growth Based on Multiphase Field Method

“…The symmetry of the interface between Si and liquid is not known, but experimental observations show five-fold branched silicon particles. 17) Therefore, a value of k "20 ¼ 5 has been assumed in the present simulations. The initial melt contains 12.2 mol% silicon, which is the eutectic composition.…”