Decomposition of a metastable bcc phase in rapidly solidified Ni-9 at.% Zr and Ni-8 at.% Zr-1 at.% X alloys

A metastable body centred cubic (bcc) phase with lattice parameter a = 0-280 ± 0-005 nm is obtained upon rapidly solidifying Ni-9 at.% Zr and Ni-8 at.% Zr-1 at.% X (= Si, Ti, Mo) alloys. This bcc (A2) phase and its decomposition behaviour are studied by X-ray diffraction, differential scanning calorimetry (DSC), conventional transmission electron microscopy, transmission electron diffraction and high resolution electron microscopy. Both continuous heating experiments in DSC and ex-situ isothermal annealing studies suggest that the bcc phase is relatively more stable in the ternary alloys. Upon aging, the bcc phase decomposes into equilibrium phases (αNi) and Ni₅Zr. While (αNi) forms directly from the bcc phase, formation of layered structures precedes the formation of the other equilibrium phase Ni₅Zr. Using hexagonal representation, the lattice parameters of the layered structure in the ordered state are a≈ 0-471 nm and c≈l-612nm and those in the disordered state are a≈0-471 nm and c≈0-403 nm. The morphology of the decomposition products at a very early stage is found to be dominated by strain energy. The microstructural evolution as a function of temperature is also reported and discussed.