Effect of Nb on Transformation Kinetics and Mechanical Properties in Zr-Al-Ni-Cu Metallic Glasses

The effect of Nb addition on the nucleation and grain growth behaviors has been investigated in the monolithic Zr₆₅Al_7.5Ni ₁₀Cu_17.5Nb₅ metallic glass. Nb has been well-known as an element for icosahedral quasicrystalline (QC) phase formation in the primary crystallization stage as well as for improving mechanical properties and corrosion resistance. The grain growth rate of the QC phase at near the crystallization temperature is 1.2×l0^-9 m/s, which is approximately 10 times smaller than that of the primary fcc Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass. In contrast, nucleation rate increases drastically by Nb addition. It can be calculated as 1.2×10²¹/m³ s, which is approximately 10⁵ times larger than that of the Zr ₆₅Al_7.5Ni₁₀Cu_17.5 alloy. The results indicate that Nb is a very effective element for controlling grain growth and nucleation rates. Good plasticity is exhibited in the as-cast alloy, however, it is lost drastically even after the structural relaxation as well as the QC precipitation. Such significant change may be attributed to the local ordering and/or phase separating tendencies by a positive chemical affinity of the Zr-Nb pair.