FORMATION OF AN ICOSAHEDRAL PHASE BY CRYSTALLIZATION OF AMORPHOUS AL-CU-V ALLOYS

Amorphous ribbons of Al-(Si, Mo)-(Cu,V) alloys transform upon heating into a mixture of crystalline compounds and an icosahedral phase. The composition of this latter phase is roughly Al//7//5 (Si,Mo)//5 (Cu,V)//2//0 as measured by STEM microanalyses. Its formation is always preceded or accompanied by the growth of Al//2Cu tetragonal crystals and, in vanadium-rich alloys, fcc Al, Al//1//0 V and Al//3V compounds. It is therefore not a polymorphous crystallization but rather a eutectic process. This implies either that long range diffusion of copper is a prerequisite to the transformation or that the amorphous structure is a non-homogeneous medium made of copper-rich and copper-depleted regions at a nanometer length scale. As a consequence, glass formation cannot be uniquely related to the occurrence of icosahedral order, as suggested, but rather to defects introduced by (atomic scale) concentration fluctuations in the connection between structural units.