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

Many five-fold branched Si particles (Si-p) were observed in A1-40 wt% Si f unctionally graded materials produced by a single-step laser cladding proce ss on cast Al-alloy substrate. In this paper the fivefold twinning and grow th 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 p revious paper [Pei, Y. T. and De Hosson, J. Th. M.: Acta mater., 2000, 48, 2617]. These Si particles have grown from twinned decahedron nuclei consist ing of five tetrahedrons that share a common 110 axis. The twin plane re-en trant edge (TPRE) mechanism explains both the branch growth in the radial d irection and the elongation of Si, along their common 110 axis. Subsequent twinning within the twinned tetrahedrons provides additional re-entrant gro oves on their top faces, which are important for the rapid elongation and c onsequently for the continuous growth of the branched particle. The 7.5 deg rees mismatch that arises by putting together five tetrahedrons around a co mmon 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 br anched silicon particles are much bigger (25 similar to 40 mum) than the na nometer sizes previously reported in the literature and the 7.5 degrees mis match is accommodated mainly by multiple SAGBs. The examples of a single SA GE reported before are just a special case of the SAGE mechanism. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved .