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

Many five-fold branched Si particles (Sip) 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 five-fold twinning and growth features of Sip 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 earlier. 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 Sip 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° mismatch 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 (25~40 µm) than the nanometer sizes previously reported in the literature and the 7.5° mismatch is accommodated mainly by multiple SAGBs. The examples of a single SAGB reported before are just a special case of the SAGB mechanism.