MICROSTRUCTURE AND PHASE-TRANSFORMATION STUDIES OF CU-NI-SN ALLOYS

Six compositions of Cu-Ni-Sn alloys in the alpha and (alpha + theta) p hase 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 deform ed state of the homogenised alpha-matrix through the evaluation of dif ferent defect parameters. The results reveal that the alpha-matrix. is more prone to faulting and the solute Sn predominantly controls the d efect state. With increasing solute concentration the matrix becomes s lightly depleted of Sn and Ni resulting in some lowering in the lattic e parameter values. The decomposition process has been investigated by ageing study on prior cold-worked (hand-filed) samples and the precip itating phases identified as 2H, DO3 and DO19 along with alpha-phase w ith Sn and Ni concentrations. However, no evidence of spinodal decompo sition has been noticed. From optical microscopy and energy dispersive X-ray (EDX) study grain boundary precipitation having composition (Cu xNi1-x)(3)Sn has been detected in higher Sn content alloys and the com posite matrix reveals a dendritic structure in the micrographs. Microh ardness study of homogenised alloy shows considerable increase in the microhardness values for high Sn content alloys undergoing precipitati on. Unlike deformed alloys in the form of filings, deformed (compresse d) bulk samples are found to be less prone to decomposition on ageing as revealed from X-ray and microhardness studies.