Scientific Events

Host: Prof. Dr. Gerhard Dehm / Dr. Christian Liebscher

Close Packed Phases in Nickel-Based Superalloys - Investigation by Diffusion Multiples

Close Packed Phases in Nickel-Based Superalloys - Investigation by Diffusion Multiples
Precipitation of close-packed phases is a common problem of modern nickel-based superalloys, containing refractory or higher melting point elements such as Re, Ru, Cr, Mo and W. Thus, a fundamental understanding of phase stabilities of close-packed phases governed by these elements is of high relevance regarding the improvement of databases for nickel-based superalloys and the development of next generation superalloys. Diffusion multiples have been used to investigate the ternary systems Ni-Mo-Cr, Ni-Mo-Re and Ni-Mo-Ru at 1100°C and 1250°C. A novel manufacturing technique for diffusion multiples based on a two-step casting process will be presented. EDS and EBSD measurements lead to isothermal sections of phase diagrams. Additionally investigations of certain quaternary systems will be shown. Solubility limits of sigma-, P-, delta- and hcp-phase were determined. Adaptation of the MatCalc database to the experimental results by project partners in Vienna lead to significant improvements in predictions for multicomponent alloys. [more]

Predicting phase behavior of grain boundaries with evolutionary algorithms and machine learning

Diffusion and segregation of solutes in grain boundaries: from pure metals to high-entropy alloys

Diffusion and segregation of solutes in grain boundaries: from pure metals to high-entropy alloys

Many faces of interfaces

Properties of materials are sensitively influenced by the microstructure inherited from their synthesis and processing. In response to high stress, temperature and composition gradients microstructures evolve in a complex way that involves nucleation of new phases, interface migration and mass redistribution that lead to complex morphological evolution on the mesoscale. Understanding this evolution and the ways it influences properties can be key to optimizing materials for targeted applications. Due to their importance related to grain structure evolution and properties of polycrystals, significant effort has been devoted to calculation of free energies and mobilities of isolated grain boundaries using atomistic simulations. It is commonly assumed that interface properties are continuous functions of temperature, pressure and chemical composition. On the other hand there is accumulating evidence suggesting that interfaces are capable of first order structural transitions, in which case the properties like segregation, excess volume, mobility, cohesive strength and sliding resistance may change discontinuously. This talk will review the results of recent atomistic computer simulations, investigating the nature of structural phase transitions in metallic grain boundaries, induced by changes in temperature and composition. We start by reviewing changes in the structure of elemental boundaries that are observed when these interfaces are exposed to very high homologous temperatures, and the nature of the qualitatively different types of structural disordering that can arise. The transitions involve changes in atomic density in the grain-boundary plane, which was discovered only when new simulation methodologies were developed that permit such variations. We show that interfaces can absorb large number of point defects through a first order phase transition, which may play important role in recovery of materials from radiation damage. Our simulations demonstrated strong effect of the transitions on self and impurity diffusion as well as grain boundary migration and shear strength. [more]
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