Tsai-Type Quasicrystal and Its Approximant in Au-Al-Tm Alloys

A P-type icosahedral quasicrystal with a six-dimensional lattice parameter a(6D) = 7.411 angstrom is formed as an equilibrium phase in Au-Al-Tm alloy, of which composition was analyzed to be Au(46)AL(38)Tm(16) by electron probe microanalysis. This quasicrystal is observed as a predominant phase in both as-cast and Au49Al34Tm17 alloys annealed at 910 degrees C, and as one of main phases in the alloy slowly cooled from 1020 degrees C. A 1/1 approximant, Au48Al38Tm14, is also formed near the composition of the quasicrystal. This is a body-centered cubic structure (space group: /m(3)) with a lattice parameter a = 14.458 angstrom that is an isostructure to the recently reported 1/1 Tsai-type approximant in Au-Al-Yb. This approximant is characterized by disorderly arranged four Au-Al atoms centered at the Tsai-type cluster, presence of atoms at 8c site, and chemical ordering of Au and Al at sites forming a partial triacontahedron.