Phase formation of Cu50−xCoxZr50 (x = 0–20 at.%) alloys: Influence of cooling rate

• Pseudo-binary (Cu,Co)Zr phase diagram.
• Stable and metastable phases of Cu–Co–Zr alloys.
• Martensitic transformation.

The dependence of phase formation on quenching rate and the thermodynamical stability of Cu50−xCoxZr50 (x = 0–20) was investigated. It was found that cobalt decreases the glass forming ability of the alloys and changes the crystalline products of the system from Cu10Zr7 + CuZr2 to a (Cu,Co)Zr phase with a B2 structure. The results indicate that for the melt-spun ribbons with at least 5 at.% Co, the glass crystallizes directly into B2 (Cu,Co)Zr, while in the case of bulk specimens, compositions with 0 ⩽ x < 5 of Co contain the monoclinic (Cu,Co)Zr phase as well as Cu10Zr7 and CuZr2, whereas for x ⩾ 10, the B2 (Cu,Co)Zr phase is the equilibrium phase at room temperature. Complete solubility of cobalt in B2 CuZr is indicated by the linear change of the lattice constant, which can be readily understood by Vegard’s law. Furthermore, increasing the cobalt content decreases the martensitic transformation temperatures. The phase formation in the ternary system is summarized in a pseudo-binary (Cu,Co)Zr phase diagram. The results are useful for designing new shape memory alloys, as well as bulk metallic glass composites with the addition of glass former elements.