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Ebook The new metallurgy of cast metals casting (2nd edition): Part 2

(BQ) Part 2 book "The new metallurgy of cast metals casting" has contents: Gas porosity, solidification shrinkage, linear contraction; structure, defects and properties of the finished casting, processing, environmental interactions. | Chapter 6 Gas porosity 6.1 Nucleation of gas porosity Although the problem of the nucleation of cavities and bubbles is in principle similar to that of the nucleation of dense phases such as inclusions, there are differences that make it worthwhile to look at non-condensed phases separately in some detail. In particular, we shall find that there are special difficulties with the nucleation of void and gas phases, forcing us to adopt new concepts. 6.1.1 Homogeneous nucleation Following the beautifully elegant approach by Fisher (1948), we can quantify the conditions required for the formation of porosity in liquid metals. A quantity of work is associated with the reversible formation of a bubble in a liquid. If the local pressure in the liquid is P,, we need to carry out an amount of work P,V to push back the liquid far enough to create a bubble of volume V. The formation and stretching out of the new liquid/gas interface of area A requires work yA, where y is the interfacial energy per unit area. The work required to fill the bubble with vapour or gas at pressure Pi is negative and equal to - PiV (The negative sign arises because the pressure inside the bubble clearly helps the formation of the bubble, as opposed to the other work requirements, which tend to oppose bubble formation.) Thus the total work is: AG = yA =4 + P, - P i ~ + (4/3>1~r?(P, 2 -Pi) where clearly ( P , - Pi>is the pressure difference between the exterior and the interior of the bubble, which we may write as AP for convenience. Similarly to dense phase nucleation, a plot of AG versus bubble radius r shows a maximum that constitutes an energy barrier to nucleation, as in Figure 5.15. The critical radius r* in this case is: r* = 2y/AP* (6.1) Since bubbles growing from the bulk liquid will grow an atom at a time as the result of statistical thermal fluctuations, it is evident that small bubbles with radii less than r* will tend to disappear. Only exceptionally will a long chain of favourable .