CBE30361 Lecture 21: Materials lecture 21
21 Mar 2016
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Materials page 1 (cid:1855)(cid:3080)=0. 022% (cid:1829) a. (cid:1855)(cid:3007)(cid:3032)(cid:3119)(cid:3004)=6. 8% (cid:1829) b. c. Phase transformations can be rather slow and the final structures often depend on the rate of cooling/heating. Nucleation - formation of stable small particles (nuclei) of the new phases. Nuclei are often formed at grain boundaries and other defects. Growth of the new phase(s) at the expense of the original or parent phase(s). Nuclei (seeds) act as templates on which crystals grow. For nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss. Once nucleated, growth proceeds until equilibrium is attained. Driving force to nucleate increases as we increase (cid:1846) Small supercooling --> slow nucleation rate - few nuclei - large crystals - coarse. Large supercooling --> rapid nucleation rate - many nuclei - small crystals - fine. Nuclei form in the bulk of liquid metal. Nuclei form preferentially at structural in-homogeneities, such as container surfaces, insoluble impurities, grain boundaries, dislocations, etc.
