Microstructure and Phase-Transformation Studies of Cu-Ni-Sn Alloys

Abstract

Six compositions of Cu-Ni-Sn alloys in the α and (α+θ) phase containing 2.7 to 13 wt% Sn and 5 to 22 wt% Ni have been prepared for structure-property correlation by X-ray diffraction, optical and scanning electron microscopy, microhardness and ageing studies. X-ray diffraction line profile analysis characterizes the plastically deformed state of the homogenised α-matrix through the evaluation of different defect parameters. The results reveal that the α-matrix is more prone to faulting and the solute Sn predominantly controls the defect state. With increasing solute concentration the matrix becomes slightly depleted of Sn and Ni resulting in some lowering in the lattice parameter values. The decomposition process has been investigated by ageing study on prior cold-worked (hand-filed) samples and the precipitating phases identified as 2H, DO3 and DO19 along with α-phase with varying Sn and Ni concentrations. However, no evidence of spinodal decomposition has been noticed. From optical microscopy and energy dispersive X-ray (EDX) study grain boundary precipitation having composition (Cux Ni1-x )3Sn has been detected in higher Sn content alloys and the composite matrix reveals a dendritic structure in the micrographs. Microhardness study of homogenised alloy shows considerable increase in the microhardness values for high Sn content alloys undergoing precipitation. Unlike deformed alloys in the form of filings, deformed (compressed) bulk samples are found to be less prone to decomposition on ageing as revealed from X-ray and microhardness studies.