Crystal nucleation in metallic glass forming systems is explored by means of a non-classical model assuming a diffuse solid/liquid interface. The model predictions are demonstrated initially for an example system with general hypothetical properties. The predicted nucleation rate matches that obtained from the classical nucleation theory for shallow undercoolings, and deviates from it with increasing undercooling. For certain system properties, a physical spinodal is predicted, i.e. the calculated energy barrier to crystal nucleation vanishes at a finite critical undercooling. The numerical results for the example system are then fitted into a simple formula for nucleation frequency, in which the critical undercooling is taken as an adjustable parameter. This formula fits well into the nucleation data of the bulk metallic glass forming system Zr_(41)Ti_(14)Cu_(12)Ni_(10)Be_(23), when a critical undercooling of 440 K is taken. The existence of a physical spinodal in this system is shown to be consistent with various other experimental observations. It is also shown that the nucleation behaviour in such systems cannot be explained by classical nucleation theory, even if phase Separation in the liquid is taken into account.
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