Figure 4: TiC0.7N0.3 nanoparticles (NPs) coating on the Bi phase and effective control of diffusional growth.
(a) SEM image of a Bi droplet with nanoparticle coating. To see nanoparticle coating clearly, ion milling was used to preferentially remove Bi away to reveal the nanoparticle shell on the droplets of Bi. (b) Magnified view of the area in the box in a. The SEM images show that the nanoparticles form a dense coating on the surface of Bi droplets. (c) Bright field (BF) TEM image of nanoparticle coating on the surface of Bi phase. The nanoparticles are randomly and closely packed on the surface of Bi phase. (d) High-angle annular dark-field (HAADF) TEM image of two closely packed nanoparticles. A gap of ~1 nm is observed between two closely packed nanoparticles. (e) Bi size as a function of γ=Cnp/S under a cooling rate of 1 K s−1. Droplet sizes are plotted from experimental results and theoretical predictions with various diffusion blocking efficiencies f. (f) Bi size as a function of the cooling rate for Al-20Bi-2vTiCN. Without diffusional growth control (diffusion blocking efficiency f=0), the size of the Bi phase is orders of magnitude larger than the experimental value. The small droplet size can only be obtained with high diffusion blocking efficiency (f=0.99987). Scale bars, 5 μm in a, 500 nm in b, 200 nm in c and 5 nm in d.
