We have seen that the main effect of chemical modification is on the microstructure, and it is these microstructural changes which influence directly the mechanical properties. The coarse silicon plates of the unmodified acicular silicon structure act as internal stress raisers in the microstructure and provide easy paths for fracture. With modification, the structure becomes finer and the silicon more rounded, both of which contribute to somewhat higher values of ultimate tensile strength and greatly increased values of ductility.