Formation ability, phase decomposition and mechanical properties of Ti39Zr39Ni20Ag2 and Ti39Zr39Ni20Cu2 bulk icosahedral quasicrystalline alloys

An icosahedral quasicrystalline (IQ) phase forms in the as-spun ribbons as well as as-cast bulk rods with a diameter of 2 mm for the Ti39Zr39Ni20Ag2 and Ti39Zr39Ni20Cu2 alloys. The Ti39Zr39Ni20Ag2 alloy ribbon consists of a mostly amorphous phase containing a small amount of IQ phases and exhibits good bending plasticity. The decomposition of their metastable phases occurs accompanying exothermic peaks at 694 K and 890 K, followed by an endothermic peak at 991 K. The former two peaks are due to the transitions from amorphous to IQ + cF96-(Ti, Zr)(2)Ni phases, and to C14-TiZrNi Laves phase, respectively. The latter endothermic peak originates from the change of IQ to C14-TiZrNi Laves and beta-(Ti, Zr) phases. Compared with the complex phase decomposition behavior of the quaternary alloys, the decomposition of the metastable phase of the Ti40Zr40Ni20 alloy ribbon occurs accompanying only an endothermic peak at a peak temperature of 952 K, followed by a melting peak. For the quaternary alloys, the as-cast rods consist of an IQ single phase, in contrast to three phases of IQ, C14-TiZrNi Laves and beta-(Ti, Zr) phases for the Ti40Zr40Ni20 rod. The ternary alloy rod fractures in the quasi-cleavage fracture mode, while the fracture mode of the quaternary alloy rod is cleavage fracture. In the Ti39Zr39Ni20Ag2 rod, IQ phase displays a spherical morphology with an average diameter of approximately 0.27 mu m. The formation of the bulk IQ rod as well as the enhancement of amorphous phase formability for the Ag/Cu-containing IQ alloys are presumably due to the stabilization of supercooled liquid caused by the development of more dense-packed atomic configurations resulting from the sequent atomic size change of Zr > Ti > Ag/Cu > Ni as well as by the necessity of the separation to Ti-Zr-Ni and Ti-Zr-Ag/Cu atomic pairs resulting from the positive heats of mixing for Ni-Ag and Ni-Cu pairs.