High doses of 350 keV Al+ ions were implanted into 6H-SiC single crystals at 500 degreesC. The Al atoms occupy preferentially Si sites in the SiC lattice. The replaced Si atoms seem to be mobile under the given implantation conditions and diffuse out. At higher Al concentrations the SiC matrix is decomposed and Si and Al4C3 precipitates are formed. It is found that the Al4C3 precipitates have a perfect epitaxial orientation to the SiC matrix. The phase transformation is accompanied by atomic redistribution and strong volume swelling.