WEB Towards a New Generation of Superalloy Single Crystals – Creep Strength, the Importance of Early Nanoscale Dislocation Processes and the Role of Dislocations in Microstructural EvolutionAudimax
The German Research Association DFG funds a collaborative research center (SFB/TR-103) which explores the scientific basis for on a new generation of single crystal superalloys, which are used to make critical high temperature components, which have to withstand mechanical loads and chemical attack at very high temperatures. In SFB/TR-103, researchers from the Universities of Bochum and Erlangen, from the German Aerospace Research Center (DLR), the Max Planck Institut für Eisenforschung (MPIE), the Research Center Jülich (FZ Jülich) and from industry join forces. The structure of SFB/TR 103 will be briefly presented and research highlights from different areas (processing, mechanical and microstructural characterization, scale bridging modelling) will be presented. Then the lecture focusses on a newly discovered elementary creep mechanism, which operates in the medium temperature (below 800°C) high stress (resolved shear stress: above 300 MPa) creep regime. Creep governs the service lives of critical high temperature components. It shows a strong stress and temperature dependence and one must therefore understand the elementary processes which govern creep in order to safely design and operate high temperature systems. Creep is generally subdivided into periods of primary, secondary, and tertiary creep, where creep rates decrease, reach a creep rate minimum and then increase towards final rupture. However, in the medium temperature and high stress creep regime of SX, two creep rate minima (at about 0.5 and 5% strain) can be clearly distinguished. High resolution miniature tensile and double shear specimen creep testing, analytical scanning transmission electron microscopy and 2D discrete dislocation modelling are combined to identify the elementary processes which govern this phenomenon. Emphasis will finally be placed on the role of dislocations in the evolution of local alloy chemistry and local phase equilibria in gamma / gamma prime microstructures.