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Pure silicon can withstand large stresses before permanently deforming.  However, this high strength makes silicon very brittle since a small amount of deformation would dissipate local concentrations of high stress and, as a result, slow down crack propagation.  This technology alloys silicon with other elements to reduce brittleness.  When silicon is melted with another element at the eutectic composition and cast into a mold, cubic silicon and silicide phases can exist simultaneous in the cooled structure.  The silicide phases serve as obstacles to crack propagation and helps to bridge cracks by slightly delaminating when the structure is under high stress.  These mechanisms hinder the spread of microscopic cracks and allow the alloy to be several times tougher than pure silicon.