High strength and good ductility of bulk quasicrystalline base alloys in Zr₆₅Al_7.5 Ni₁₀ Cu_17.5-xPd_x system

An icosahedral quasicrystalline phase was found to precipitate as a primary phase from the supercooled liquid in the Zr₆₅Al_7.5Ni₁₀Cu_17.5-xPd_x (x=5 and 10 at%) amorphous alloys. No icosahedral phase is formed for the present Zr₆₅Al_7.5Ni₁₀Cu_17.5 alloy and hence the addition of Pd promotes the precipitation of the icosahedral phase. The volume fraction of the icosahedral phase increases with increasing Pd content and exceeds 90% for the 10%Pd alloy. The icosahedral phase changes to Zr₂Cu+Zr₂Ni+Zr₂Al₃ phases with further increasing annealing temperature. The icosahedral phase consists of fine grains with a size of about 35 nm and has a disordered (P-type) structure. Bulk amorphous alloys were produced in the diameter range up to 4 mm at 5%Pd and 2 mm at 10%Pd by copper mold casting. The icosahedral base alloys obtained by annealing the bulk amorphous phase exhibit improved mechanical properties. The Young's modulus, compressive yield strength, compressive fracture strength and fracture elongation including elastic elongation of the bulk 10%Pd alloy are 85 GPa, 1510 MPa, 1640 MPa and 2.2%, respectively, in the as-cast amorphous state and increase to 88 GPa, 1780 MPa, 1830 MPa and 3.1%, respectively, for the icosahedral base state obtained by annealing for 60 s at 705 K. The success of synthesizing the high-strength bulk alloys consisting mainly of icosahedral phase is promising for the future development of a new class of high-strength quasicrystalline material.