Rene Radis

Kinetics of AlN precipitation in microalloyed steel

In this work, the thermodynamic information on AlN formation in steel and experimental data on AlN precipitation kinetics are reviewed. A revised expression for the Gibbs energy of AlN is presented with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation are performed and compared with independent experimental results from the literature. A new model for grain boundary precipitation is employed, which takes into account fast short-circuit diffusion along grain boundaries as well as slower bulk diffusion inside the grain, together with the classical treatment for randomly distributed precipitates with spherical diffusion fields. It is demonstrated that the precipitation of AlN can be modelled in a consistent way if precipitation at grain boundaries and dislocations is taken into account, dependent on chemical composition, grain size and annealing temperature. It is also demonstrated that, for consistent simulations, the influence of volumetric misfit stress must be taken into account for homogeneous precipitation of AlN in the bulk and heterogeneous precipitation at dislocations.

Mean-field model for the growth and coarsening of stoichiometric precipitates at grain boundaries

In this paper, a model for growth and coarsening of precipitates at grain boundaries is developed. The concept takes into account that the evolution of grain boundary precipitates involves fast short-circuit diffusion along grain boundaries as well as slow bulk diffusion of atoms from the grain interior to the grain boundaries. The mathematical formalism is based on a meanfield approximation, utilizing the thermodynamic extremal principle. The model is applied to the precipitation of aluminum nitrides in microalloyed steel in austenite, where precipitation occurs predominately at the austenite grain boundaries. It is shown that the kinetics of precipitation predicted by the proposed model differs significantly from that calculated for randomly distributed precipitates with spherical diffusion fields. Good agreement of the numerical solution is found with experimental observations as well as theoretical treatment of precipitate coarsening.

Numerical simulation of NbC precipitation in microalloyed steel

This work describes with the investigation of the precipitation kinetics of NbC in microalloyed steel. Using the thermo-kinetic software MatCalc, computer simulations of NbC precipitation are carried out and compared with several independent experimental results, measured in austenite and ferrite. The selected experiments involve a variety of different dislocation densities originating from distinct thermo-mechanical treatments. Two separate populations of NbC precipitates are accounted for in the simulations, representing precipitates on grain boundaries as well as dislocations. Furthermore, three different diffusion mechanisms are taken into account, which are bulk diffusion in the undisturbed crystal, accelerated diffusion along the dislocation core and fast diffusion along grain boundaries. It is demonstrated that deformation-induced dislocation densities higher than 1013 m−2 lead to prominent diffusion along dislocation networks. Therefore, in such cases, the precipitation kinetics of NbC is dominated by the pipe diffusion mechanism, and the precipitation process is several orders of magnitude faster compared with NbC precipitation in unstrained, well-annealed microstructures.

Interaction of the Precipitation Kinetics of δ And γ’ Phases in Nickel-Base Superalloy ATI Allvac® 718PlusTM

In this paper, the precipitation behaviour of  (Ni3(Nb,Al)) and ’ (Ni3(Al,Ti,Nb)) phases in the nickel-base superalloy ATI Allvac® 718PlusTM, as well as their kinetic interactions are discussed. Important parameters such as volume fraction, mean radius and number density of precipitates are experimentally determined and numerically simulated as a function of the heat treatment parameters time and temperature. To match the experimentally observed kinetics, the predicted interfacial energy of the precipitates, as calculated for a sharp, planar phase boundary, is adjusted to take into account the interfacial curvature and entropic effects of a diffuse interface. Correction functions for the interfacial energies of  as well as ’ precipitates are presented. Using these modified interfacial energies, the calculated results show excellent agreement with the experimental measurements.

Loss of Ductility Caused by AlN Precipitation in Hadfield Steel

Two modified X120Mn12 Hadfield steels, differing in the amount of the alloying elements Al and N, are analyzed with respect to AlN precipitation and its effects on ductility. Charpy impact tests are performed, demonstrating the loss of ductility in the one grade containing a high density of AlN precipitates. The characterization of the precipitates is carried out by high-resolution scanning electron microscopy (HRSEM). Depending on chemical composition, primary and secondary AlN precipitates are detected on prior austenite grain boundaries and within the bulk volume. The experimental observations are confirmed by thermokinetic simulations, using the software package MatCalc (Vienna University of Technology, Vienna, Austria).

Precipitation Kinetics of Aluminium Nitride in Austenite in Microalloyed HSLA Steels

In this work, the thermodynamic information on aluminium nitride formation and experimental precipitation kinetics data are reviewed. A revised expression for the Gibbs energy of AlN is developed with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation kinetics are performed and compared to several independent experimental results from literature. To mimic the geometrical arrangement of AlN precipitates along austenite grain boundaries, a new model for precipitation at grain boundaries is used, which takes into account fast short-circuit diffusion along grain boundaries as well as the slower bulk diffusion of atoms from inside the grain to the grain boundaries. This is essential for the calculation of AlN precipitation in austenite where nucleation occurs predominantly on grain boundaries. By studying the AlN precipitation at grain boundaries numerically, and by comparison with experimental data, it is demonstrated that the precipitation kinetics of AlN differs significantly from the simulated precipitation kinetics of randomly distributed precipitates assuming spherical diffusion fields.

Microstructure control in processing nickel, titanium and other special alloys

Abstract: This chapter gives an overview of materials modelling and microstructure control for some special alloys, namely nickel-based superalloys, titanium alloys and intermetallics. After a description of the applications and production processes of these alloys, the resulting microstructures and related mechanical properties are presented. Also, the possibility of process and materials optimization is shown in two case studies. The chapter closes with future trends and some sources of additional information.