Computer Simulation of Internal Oxidation and its Application to Heating-Up Experiments for Determination of Oxygen Diffusivities Metals—Palladium as an Example

摘要

Internal oxidation of metal alloys is of great applicational significance as an effective dispersion-hardening technique, e.g., for Ag, Cu, or Ni alloys, as well as having undesirable side effects to many heat treatment technologies of steels. Wagner's kinetics theory comprises three practically important restrictions: (1) homogeneous initial concentration distribution of the alloying metal M, (2) isobaric, and (3) isothermal processes. The presented new computer model, which is based on an iterative solution of Fick's 2nd law for the inward diffusion of oxygen with internal reaction, overcomes these limitations. Illustrative processes are simulated. Considering the restrictions of Wagner's theory, incomplete isothermal isobaric internal oxidation of dilute homogeneous binary alloys represents an established experimental method for determining the diffusion coefficient of oxygen in the matrix metal: the sought Arrhenius equation is derived by carrying out annealing series at several constant temperatures. The modified simulation-aided measuring technique of incomplete nonisothermal isobaric internal oxidation, introduced in the present work, reduces the necessary experimental time considerably. Also, as a test of the new computer model, the mathematical evaluation of the data obtained by microscopic analysis of polished cross-sections is performed by means of this tool. To demonstrate the applicability of the novel experimental technique of temperature-programmed internal oxidation, the diffusivity of oxygen in palladium is exemplarily measured this way in a Pd-9.19 at.% Fe alloy and, expressed in cm~2/s, found to be D_o = 5.32·10~(-1) · exp(-1 8728.27/T). Here, the temperature T ranges from 1073 to 1473 K. This result is compared with literature sources and agrees well with previously published reliable data.