An introduction to crystal chemistry by Robert Crispin Evans PDF

By Robert Crispin Evans

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A more in-depth discussion on dendrite fragmentation is provided in Sect. 5. Fragmentation can also be induced through increased convection, ultrasonic vibrations, or a pressure pulse. In the last two cases, nucleation follows because of the change in equilibrium temperature caused by the pressure changes during the collapse of cavitation bubbles. An example of the effect of increased convection through shearing under mechanical mixing for the Mg alloy is presented in Fig. 14. ” It is therefore not surprising that estimation of the volumetric density of nucleation sites before and during solidification of casting 48 3 Nucleation and Growth Kinetics—Nanoscale Solidification Fig.

At equilibrium, the flux of atoms toward and away from the interface must be equal, that is, (dn/dt)M = (dn/dt)S . Thus, the two curves must intersect at Te (Fig. 12). For solidification to occur, more atoms must jump from L to S than from S to L. Consequently, the solidifying interface must be at lower temperature than Te by an amount that is called as kinetic undercooling , Tk . , Biloni and Boettinger 1996) would be to consider that the overall solidification velocity is simply: V = Rate of solidification − Rate of melting = Vc − Vc exp (− G/RTi ) where G is expressed in J/mole.

However, experimental data on the maximum undercooling are less reliable than for pure metals because the entrained droplet (“mush quenching”) technique used is susceptible of allowing some weak heterogeneous nucleation on the solid solution. The undercooling must be calculated from the liquidus temperature. 2. In commercial casting alloys, homogeneous nucleation is virtually inexistent. It is, however, relevant to the new processes involving rapid solidification. 4 , eV/atom Fig. 3 Molecular dynamics calculated interface free energies for metals as a function of latent heat of melting.

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An introduction to crystal chemistry by Robert Crispin Evans

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