Grain boundary interdiffusion in CoFeNi/Sn system: Analysis of phase growth and compositional complexity

Grain boundary interdiffusion in the CoFeNi/Sn system is explored by investigating diffusion couples of coarse-grained (CG > 200 μm) and ultrafine-grained (UFG ∼800 nm) CoFeNi alloys with Sn in the temperature range 175–215 °C. The CG and UFG CoFeNi samples were prepared through vacuum arc melting (VAM) and spark plasma sintering (SPS) of mechanically alloyed CoFeNi powders, respectively. At 175 °C, formation of (Co, Ni)Sn₃ and FeSn₂ intermetallic phases is observed at both the interfaces. At 200 °C, (Co, Ni)Sn₃ and FeSn₂ persisted across annealing durations of 120, 180, and 240 h, with a gradual increase in their thickness. However, at 300 h, a phase mixture of (Co, Ni)Sn₃ and FeSn₂ emerged between the two single phase layers. At 215 °C, the presence of FeSn₂, (Co, Ni)Sn₃, and their phase mixture was observed consistently, with progressive thickening over time. Phase formation in the interdiffusion zone has been confirmed using correlative microscopy methods. Time dependent studies reveal the pronounced phase growth in UFG CoFeNi/Sn compared to CG CoFeNi/Sn. The accelerated diffusion kinetics at the UFG interface can be attributed to the higher fraction of grain boundaries (GBs). The comparison of CoFeNi/Sn diffusion couples with reported studies of Ni/Sn and CoNi/Sn reveal that the effect of enhanced GB fraction on kinetics is partially nullified by the compositional complexity of end members in the interdiffusion zone. The experimental data across the 175–215 °C range provide valuable insights that can aid in refining the phase diagram and expanding the available database for this multicomponent alloy system.