diffractor;
Based on your question, it would appear that you are looking to compare gas permeability at forging temperatures between various materials. If this is the case, you can compare rates of gas permeability by rough calculation using the equations below;
1. Fick's first law of diffusion;
J (flux) = D* [delta C (change in concentration)/thickness]
2. In dealing with gas permeability, the C for concentration in equation 1) needs to be calculated based on the solubility of the gas species in the metal at temperature. This requires the use of Sievert's Law;
C gas in metal = K eq * partial pressure of gas ^1/2
You can obtain relative diffusivity constants (D) at temperature for the gas in a metal, and knowing the solubility of the gas at say 1 atm pressure, and temperature you can calculate the equilibrium constant of the gas. After solving for the equilibrium constant (K) for the particular gas species, you can determine the solubility of the same gas in the metal at any partial pressure.
I don't know of any actual tabulated data for the above, which is the reason I suggested this approach with a little research effort.
Instead of gas permeability, you might be interested in gas solubility when the ingots were poured for these materials. Sievert's Law can also be applied to determining gas solubility at a partial pressure of gas over a melt, also at a specific temperature. You could compare the solubility of gases like hydrogen, oxygen or nitrogen at a particular partial pressure in Ti versus iron-base alloys. Then go back to equation 1) and use Fick's first law of diffusion to calculate differences in flux between the alloys at a specific temperature.
I believe most of the diffusivity and gas solubility data could be obtained from a TMS metallurgical text book like "Transport Phenomena in Materials Processes" by Poiter and Geiger .
