Nucleation in crystallization of Zr–Cu–Ni–Al metallic glasses

Abstract

Zr–Cu–Ni–Al alloys belong to the best glass forming systems known. Zr 68.5Cu 13Ni 11Al 7.5 metallic glasses [1] are known to transform by primary crystallization into a quasicrystalline structure. Since small Hf-addition were found to stabilize the formation of quasicrystals, i.e. to reduce the influence of oxygen on the crystallization, a glass with 5 at.% Hf as a replacement of Zr was chosen to compare with the transformation kinetic of Zr 69.5Cu 12Ni 11Al 7.5. From crystallization statistics nucleation was found to be homogeneous in the temperature regime between 653 and 713 K with transient rates. In order to understand the observed crystallization process in more detail a new virtual space algorithm is presented. It is capable of calculating the real projected distribution of crystallite sizes in a thin TEM transparent layer including hard impingement up to a crystalline volume fraction of 100%. Another improvement as compared to the standard Kolmogorov–Johnson–Mehl–Avrami (KJMA) procedure [J. Chem. Phys. 7 (1939) 1103; J. Chem. Phys. 8 (1940) 212; J. Chem. Phys. 9 (1941) 177] is the parallel fit of several experimental distributions with large ensembles of crystallites for one temperature and different annealing times. A least sum of error squares optimization method is implemented, yielding the set of relevant parameters, i.e. the stationary nucleation rate, the incubation time and the effective diffusion coefficient for the primary crystallization. By fitting the nucleation rates with a diffusion controlled classical model, the interface energies and the metastable melting temperature of both alloys are estimated. Based on these data is the evaluation of a TTT diagram for the crystallization of the icosahedral phase in Zr 64.5Cu 12Ni 11Al 7.5Hf 5 and Zr 69.5Cu 12Ni 11Al 7.5.