Observation and kinetic analysis of a metastable b.c.c. phase in rapidly solidified Ni9at.%Zr and Ni8at.%Zr1at.%X alloys

The microstructures of rapidly solidified Ni9at.%Zr and Ni8at.%Zr1at.%X (≡ Si, Ti or Mo) alloys are found to consist predominantly of micron-sized grains having a b.c.c. (A2) structure with lattice parameter a = 0.280 ± 0.005 nm. This b.c.c. phase is metastable and undergoes decomposition into equilibrium phases α-Ni + Ni₅Zr upon heating. Using relevant thermodynamic data and classical nucleation and crystal growth theory, the formation of the b.c.c. phase is rationalized in terms of kinetic hierarchy of alternative crystalline phases. We find that the calculated critical cooling rate to obtain an amorphous phase in Ni9at.%Zr alloy is about 9 × 10⁷ K s^-1 which is too high for the melt-spinning process.