Characterizing defect phases at surfaces and interfaces

The design of new metallic materials is essential in fulfilling the promise of emerging and improving key technologies from efficient energy conversion over lightweight transport to safe medical devices. Over the last decades, two approaches in materials physics have proven immensely successful in the design of new metallic materials: Firstly, thermodynamic descriptions of crystalline phases have enabled materials scientists and engineers to tailor and process alloys to obtain a desired internal structure at the microscale. Secondly, better understanding and manipulation of the crystal defects, which govern the material’s strength, formability and corrosion resistance, has led to the development of new alloying and processing concepts that provide some of the most advanced high-performance alloys in operation today.

In the Collaborative Research Centre SFB1394 “Structural and Chemical Atomic Complexity: From Defect Phase Diagrams to Material Properties”, defects and their thermodynamic stability are brought together in the framework of defect phase diagrams. Within the subproject B01, we study surfaces and interfaces in model Mg-Al-Ca alloys. Aberration-corrected STEM and EELS are applied to resolve the structural complexity (local crystalline structure) of defects and chemical complexity (atomic distribution among different types of phases and defects), respectively. As an example, an extension twin boundary in Mg (the middle of Fig. 1) is found to be decorated by dislocations and heavy elements. Within SFB1394, we develop methods with mathematicians (Benjamin Berkels group) to automatically detect such interfaces and the associated strain field to compare with atomistic simulation (Tilmann Hickel group). Moreover, approaches to correlate STEM imaging with atom probe tomography are under development together with Marcus Hans group to have an accurate description of the defect phases both from crystallography and their chemical composition in 3D.