Formation Criteria and Growth Morphology of Quasicrystals in Al–Pd–TM (TM=Transition Metal) Alloys

Rapidly solidified and annealed structures were examined in a series of Al70Pd20TM10 (TM=transition metal) and Al70Pd20TM15TM25 alloys, based on the fact that an Al70Pd20Mn10 alloy consists of a stable icosahedral (I) phase. As a result, new stable I alloys were found in Al70Pd20Cr5Fe5, Al70Pd20V5Co5, Al70Pd20Mo5Ru5 and Al70Pd20W5Os5. In addition, a metastable decagonal (D) phase was found in rapidly solidified Al70Pd20Nb5Rh5 and Al70Pd20Ta5Ir5 alloys. We found that the formation tendency of the quasicrystalline structure decreases with the deviation of the periodic group number and periodicity of the TM1 and TM2 elements from those of Mn. The decrease of the formation tendency corresponds to the increase in the difference in the heat of mixing between Pd–TM1 and Pd–TM2 pairs and hence the decrease of selectivity in the occupation of the TM1 and TM2 elements at the Mn sites leads to the disappearance of the I and D phases. These stable I alloys have a dendritic solidification morphology with preferential growth along the three-fold direction.