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

Grain size effect during interdiffusion is investigated by designing a sandwich couple of CG-CoNi/Sn/UFG-CoNi; (CG coarse grained (>200 mu m), UFG: ultrafine grained (similar to 240 nm)). Phase growth is examined at temperatures 175 & 200 degree celsius 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 degree celsius, (CoNi)Sn-3 intermetallic phase is detected at CG CoNi/Sn interface and (CoNi)Sn-2 phase at UFG CoNi/Sn interface. At 200degree celsius, formation of (CoNi)Sn-2 and (CoNi)Sn-3 phases is observed at CG interface and only (CoNi)Sn-2 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 & 200degree celsius 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)Sn-2 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.