Accelerated phase growth kinetics during interdiffusion of ultrafine-grained Ni and Sn

Abstract

The phase growth is investigated during the interdiffusion of an ultra-fine grained Ni (UFG) (grain size, d ∼ 320 nm) and coarse-grained Sn diffusion couple in the temperature range of 175–215 °C. The UFG Ni was produced through spark plasma sintering (SPS) of ball-milled Ni powder. Formation of only Ni3Sn4 intermetallic phase is observed, as confirmed by electron probe microanalysis (EPMA), micro-x-ray diffraction (XRD), and Transmission Kikuchi Diffraction (TKD) measurements. Using the CALPHAD approach, the driving forces for the formation of possible intermetallic phases in the Ni-Sn system were estimated and the highest value was found for Ni3Sn4, which matches the experimental observations. Time-dependent measurements at 200 °C revealed an acceleration in the growth (∼2 orders of magnitude) of Ni3Sn4 compared to the scenario prevalent in the coarse-grained (CG) Ni/Sn diffusion couple. This is reflected in the calculated integrated diffusivities for the UFG-Ni/Sn diffusion couple. The accelerated phase growth kinetics are attributed to the increased fraction of grain boundaries (GBs) in the UFG-Ni, which leads to an increased contribution of GB diffusion to the overall diffusion flux. The formation of Ni3Sn4 at GB was further ascertained using correlative microscopy strategy including transmission electron microscopy, transmission Kikuchi diffraction and scanning transmission electron microscopy.