FORMATION OF F-TYPE AND P-TYPE ICOSAHEDRAL QUASI-CRYSTALS IN THE ZN-MG-HO ALLOY SYSTEM

Two types of icosahedral quasicrystals, F- and P-types, formed at 563 degrees C in the Zn60Mg40-xHox alloy depending on Ho content. In the c omposition range of 6 less than or equal to x less than or equal to 10 , F-type icosahedral quasicrystal formed as a major phase in these all oys. The intensity of superlattice reflections which occurred due to F -type ordering became weaker in the alloys with lower Ho content. At x = 5, P-type icosahedral quasicrystal formed, coexisting with MgZn2-ty pe Laves phase at 563 degrees C and 503 degrees C. Some electron diffr action patterns of the P-type icosahedral quasicrystal exhibited diffu se scattering intensity at positions corresponding to F-type superlatt ice reflections. By annealing at 403 degrees C, a mixture of the P-typ e icosahedral quasicrystal and the MgZn2-type phase changed to a mixtu re of the F-type quasicrystal and a decagonal quasicrystal. The former state was reversibly obtained by additional annealing at 503 degrees C. These experimental results support the interpretation that the thre e types of quasicrystals form as a thermodynamically stable state in t he Zn-Mg-Ho alloy system.