Synthesis of high strength bulk amorphous Zr-Al-Ni-Cu-Ag alloys with a nanoscale secondary phase

Abstract

Bulk amorphous alloys in a cylindrical form were prepared in the diameter range up to about 5 mm for Zr65Al7.5Ni10Cu17.5-xAgx (x=0 to 5 at%) by copper mold casting. These bulk amorphous alloys exhibit a wide supercooled liquid region before crystallization and the temperature interval defined by the difference between glass transition temperature (Tg) and crystallization temperature (Tx), ΔTx(=Tx-Tg) is 111 K for the 0%Ag alloy and 63 K for the 5%Ag alloy. The replacement of Cu by 5 at%Ag causes the change in the crystallization process from a single stage of Am→Zr2(Cu, Ni)+Zr2(Al, Ni) to two stages of Am→Am′+Zr3(Al, Ag)2→Zr3(Al, Ag)2+Zr2(Cu, Ni). The Zr3(Al, Ag)2 phase has a spherical shape with a small size of about 20 nm and is dispersed homogeneously at an interparticle spacing of about 30 nm. The change in the crystallization process by the addition of Ag is presumably due to the generation of the distinction of bonding nature among the constituent elements because of the positive heats of mixing for Ag-Ni and Ag-Cu pairs. The dispersion of the nanoscale Zr3(Al, Ag)2 phase causes a significant increase in tensile fracture strength (σf) from 1150 MPa for the amorphous single phase to 1520 MPa for the amorphous phase containing about 14% volume fraction of Zr3(Al, Ag)2 phase. The shear-type fracture mode along the maximum shear plane remains unchanged. The effectiveness of the Zr3(Al, Ag)2 phase for the increase in σf is presumably because the size of the phase is too small to contain dislocations. The success of synthesizing the high-strength bulk amorphous alloy consisting of nanoscale Zr3(Al, Ag)2 phase embedded in an amorphous matrix is encouraging for future development of bulk amorphous alloys as a high-strength material.