Grain size effect on the phase growth in CoNi/Sn sandwich diffusion couples

Abstract

Grain size effect during interdiffusion is investigated by designing a sandwich couple of CG-CoNi/Sn/UFG-CoNi; (CG coarse grained (>200 µm), UFG: ultrafine grained (∼240 nm)). Phase growth is examined at temperatures 175 & 200 ℃ for various times. CG CoNi and UFG CoNi have been fabricated by Vacuum arc melting and spark plasma sintering of mechanically alloyed CoNi powders, respectively. At 175 ℃, (CoNi)Sn3 intermetallic phase is detected at CG CoNi/Sn interface and (CoNi)Sn2 phase at UFG CoNi/Sn interface. At 200℃, formation of (CoNi)Sn2 and (CoNi)Sn3 phases is observed at CG interface and only (CoNi)Sn2 at the UFG interface. Phase compositions have been confirmed by electron probe microanalysis while the corresponding crystal structure with micro-x-ray diffraction. Phase selection in the interdiffusion zone (IDZ) of CG CoNi/Sn is governed primarily by kinetic factors, while thermodynamic driving forces, assessed using CALPHAD approach, also contribute to phase formation in the IDZ of UFG CoNi/Sn. Time-dependent measurements at 175 & 200℃ for UFG CoNi/Sn reveal an accelerated growth of phases when compared with the CG CoNi/Sn. The enhanced kinetics and preferential formation of the (CoNi)Sn2 phase at the UFG CoNi/Sn interface can be ascribed to the higher proportion of grain boundaries (GBs), leads to an increase in overall diffusion flux. The sandwich diffusion couple approach ensured that all the comparisons between CG and UFG interfaces are conducted under similar conditions yielding dependable correlations. The formation of intermetallic phases and role of GBs are studied at a finer detail using correlative microscopy approach.