PHASE EVOLUTION IN QUATERNARY Zr-Cu-Ni-Al AND QUINARY Zr-Cu-Ni-Al-Pd BULK METALLIC GLASS FORMING ALLOYS

摘要

Inhomogeneous deformation tendency and the unhindered flow of shear bands in bulk metallic glasses, leading to catastrophic failure, can be mitigated by partial crystallisation or reinforcement by second phase particles in the amorphous matrix. Quaternary Zr_(55)Al_(10)Ni_5Cu_(30) and quinary Zr_(60)Al_(10)Ni_5Cu_(15)Pd_(15) alloys have been cast in a copper mould in the form of 3 mm diameter rods. Though the alloys are multicomponent in nature with widely different atomic size of the constituent elements and with high negative heat of mixing, a change in the processing conditions results in partial crystallisation of the alloys while casting. cF96 NiZr_2 and 0C68 (Cu, Ni)_(10)Zr_7 phases are observed after casting and crystallisation. The nature of the crystallising phases indicates that polytetrahedral units exist in the amorphous alloys, which can be explained in terms of Bernal deltahedra and Frank-Kasper polyhedra. Pseudo-ternary composition plot of the alloys indicates that the alloy compositions are very close to the anti-Laves composition in the relevant system. The phase evolution in the alloys has been explained in terms of enthalpy of mixing, atomic radii mismatch and the structure of the crystalline phases.