ABSTRACT
In Chapter 4, while discussing nondendritic crystallization mechanisms we noted that a melt solidified into ingots, pellets, platelets (flakes) or fibers may have a dendritic or nondendritic structure depending on the solidification rate, the purity of the melt, nucleation, and dynamic treatments.The nondendritic (subdendritic, predendritic or microcrystalline) structure implies an ultimate grain refinement achieved at a given freezing rate with polyhedral grains having large-angle boundaries and no dendritic branches. The size of such nondendritic grains is approximately equal to the average cross section of dendritic branches in a similar casting with dendritic morphology.Most commercial light alloys solidified at freezing rates of 0.1-100 K/s can be crystallized into a nondendritic structure in a cavitation ultrasound environment with trace additions of transition metals (Eskin, 1988; Dobatkin and Eskin, 1991b). The cavitation treatment assists in wetting the uncontrolled fine impurities and activates heterogeneous nucleation. 255
At increased freezing rates (up to 105 K/s), a nondendritic structure may also form if the melt was supercooled prior to solidification, thus intensifying its homogeneous nucleation.In the phase of developed cavitation prior to rapid solidification, an ultrasonic treatment leads more reliably to a nondendritic structure in rapidly solidified castings.
