Structural Evolution Of Highly Undercooled Al₇₂Ni₁₂Co₁₆ Alloy And Preparation Of Decagonal Single-phase Quasicrystal

In this paper, the fundamental technique and engineering controlling condition of obtaining high undercooling in Al72Ni12Co16 alloy was systematically investigated by the method of the nucleation-inhibitive coating mold and recycle superheating under Ar atmosphere. Moreover, the formation and microstructure evolution of quasicrystal phase under the condition of high undercooling was also studied. Based on the classical nucleation theory and the time-dependent nucleation theory, the thermodynamics and the kinetics for the competitive nucleation between the quasicrystal and crystal phase have been calculated, and hence a critical condition of forming the quasicrystal phase was proposed. Finally, the single quasicrystal sample was identified by SEM, TEM and X-ray powder diffraction. The main results are as follows:(1) By the method of nucleation-inhibitive coating mold and recycled superheating under Ar atmosphere, a highly undercooling of 200K was obtained in(2) While the undercooling of Al72Ni12Co16 alloy is more than 60K, quasicrystal phase will precipitate from the undercooled melt as initial phase.(3) With the increase of undercooling, the microstructure of quasicrystal will undergo the transformation of 'dendrite-columnar crystal-equiaxed crystal', and the two critical undercooling for the transformation are 60K and 120K respectively.(4) The effect of the cooling rates on the nucleation of competing phases is negligible in the undercooled alloy melts. The initial undercooling of Al72Ni12Co16 alloy is a chief factor in determining the type of primary phase on the basis of the classical nucleation theory and the time-dependent nucleation theory.(5) All the results of SEM, TEM and X-ray powder diffraction indicate that the sample undercooled more than 60K is the decagonal single-phase quasicrystal.