Papers by Author: Franz Dieter Fischer

Abstract: The interaction between interstitially diffusing atoms and substitutional solute atoms, acting as trapping sites, causes a non-negligible influence on the diffusion process itself and, consequently, on many aspects of alloys, such as phase transformations, solubility, precipitation of carbides and nitrides etc. The most important quantity in this treatment is the so-called trapping enthalpy (depth of trap), which has been used in several approaches in literature over the last century. However, the determination of the trapping enthalpy so far relies on approximations or assumptions on the one hand (statistical approaches, quasi chemical approach) or is significantly limited due to high complexity (ab initio approaches) on the other hand. The model introduced in this paper illustrates a rigorous and efficient thermodynamically-based concept utilizing only the dependence of the chemical potential of the interstitial component on the chemical composition of the alloy. Such a dependency is available in a very precise form from CALPHAD thermodynamic databases. Using the most recent databases available, the trapping enthalpies of carbon and nitrogen at various solute atoms (trapping sites) are evaluated for austenitic and ferritic steels. Good agreement with previous literature results is observed. The flexibility of the concept allows also for the treatment of trapping in a multi-component system, where different types of solute atoms are responsible for different depths of traps.

Abstract: The stoichiometric M_pX_q hollow nanospheres are produced by reaction of metallic M nanospheres with the gaseous X phase. In the first stage a sufficiently thick M_pX_q nanoshell on the metallic core of phase M is formed. During this stage high supersaturation of vacancies in the M core or very high hydrostatic stress in the M core, due to the misfit between the core and the nanoshell, are developed and provide favourable conditions for the hollow nucleation. The misfit is caused by the Kirkendall effect. Based on the application of the thermodynamic extremal principle a kinetic model of M_pX_q nanoshell formation is derived. The kinetics is driven by the change of the chemical energy due to reaction of M and X components, of the interface and surface energies, and of the elastic strain energy due to misfit strain of the whole system. The model is used for simulation of the Cu₂O shell growth kinetics due to oxidation of a Cu nanosphere, and the results of simulations are discussed.

Abstract: This paper deals with the determination of the crack driving force in elastic-plastic materials and its correlation with the J-Integral approach. In a real elastic-plastic material, the conventional J-integral cannot describe the crack driving force. This problem has been solved in Simha et al. [1], where the configurational force approach was used to evaluate in a new way the J-integral under incremental plasticity conditions. The crack driving force in a homogeneous elastic-plastic material, Jtip, is given by the sum of the nominally applied far-field crack driving force, Jfar, and the plasticity influence term, Cp, which accounts for the shielding or anti-shielding effect of plasticity. In this study, the incremental plasticity J-integral and the crack driving force are considered for a stationary and a growing crack.

Abstract: A new concept of generation and annihilation of vacancies at uniform sinks and sources for vacancies is incorporated into the standard Monte Carlo model for vacancy mediated diffusion. This model enables to treat the vacancy wind as well as the deformation of the specimen and the shift of the Kirkendall plane. The Monte Carlo model is used for the testing of the recent phenomenological theories of diffusion by Darken, Manning and Moleko. The agreement with the self-consistent Moleko theory is excellent. On the other hand the agreement with the classical Darken theory used very often for the explanation of the Kirkendall effect is rather poor.