Cavity Nucleation Assisted by Internal Gas Pressure
Gases or solid phases which are precipitated in alloys may exert high, pressures on the surrounding matrix. The driving force for precipitation against that pressure comes from the high chemical potential which the constituents of the precipitate may have in solid solution. If one assumes for the moment that the equilibrium gas pressure, p, is established within the subcritical vacancy clusters, the effect of the pressure on nucleation is exactly the same as that of a tensile stress (Russell, 1978, Raj, 1982). This means that in the formulae for cavity nucleation (Section 6.2) the normal stress, σ must be replaced by σ + p to account for internal pressure. The magnitude of the equilibrium pressure is calculated in Sections 9.1 to 9.3 below for a number of representative examples. It is, however, a very difficult question whether the equilibrium gas pressure is actually built up fast enough compared to the lifetime of the subcritical vacancy clusters. This kinetic aspect is addressed in Section 9.4. The effect of a gas pressure on the growth of cavities, as distinct from nucleation, will be described in Section 11.1.6.
KeywordsNickel Methane Carbide Chromium Enthalpy
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