Marlene Kapp

Investigation of reversible plasticity in a micron-sized, single crystalline copper bending beam by X-ray μLaue diffraction

The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ µLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ µLaue experiments are useable. A critical size is evidenced below which, depending on Youngs modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample.

Characteristics of an optimized active metal cast joint between copper and C/C

In this paper, an optimized active metal cast copper to carbon composite (C/C) joint is examined. Mechanical characterization by means of shear and tensile tests as well as fracture toughness measurements at ambient temperature are employed, which show that the obtained values are above those of the reference joint that has been used so far. The phases occurring at the interface region and the fracture surface of the optimized joint are investigated in detail to explain the improved mechanical behaviour. The composition and the morphology at the interface region are analysed by means of scanning electron microscopy. In addition, the crack path is analysed by stereomicroscopy. The improved properties can be explained by the circumstance that the fracture of the optimized joint is not restricted to the interface between the occurring carbide and the C/C but changes between C/C, the carbide to C/C interface and the copper, depending on the local structure in the C/C composite.

Ultra-strong and damage tolerant metallic bulk materials: A lesson from nanostructured pearlitic steel wires

Structural materials used for safety critical applications require high strength and simultaneously high resistance against crack growth, referred to as damage tolerance. However, the two properties typically exclude each other and research efforts towards ever stronger materials are hampered by drastic loss of fracture resistance. Therefore, future development of novel ultra-strong bulk materials requires a fundamental understanding of the toughness determining mechanisms. As model material we use today’s strongest metallic bulk material, namely, a nanostructured pearlitic steel wire, and measured the fracture toughness on micron-sized specimens in different crack growth directions and found an unexpected strong anisotropy in the fracture resistance. Along the wire axis the material reveals ultra-high strength combined with so far unprecedented damage tolerance. We attribute this excellent property combination to the anisotropy in the fracture toughness inducing a high propensity for micro-crack formation parallel to the wire axis. This effect causes a local crack tip stress relaxation and enables the high fracture toughness without being detrimental to the material’s strength.

Strength and ductility of heavily deformed pearlitic microstructures

The fracture toughness and deformation behavior of heavily deformed pearlitic steels have been investigated. A strong anisotropy of the fracture toughness and the plastic deformation behavior with respect to the lamellar orientation is observed. The consequences of this anisotropy both for processing and application, as well as for the limits in strengthening, are discussed.

Synthesis and Properties of Bulk Metallic Glass Composites

The aim of this study is to show that new types of bulk metallic glass composites (BMGCs) can be produced via severe plastic deformation (SPD). The initial materials are mixtures of a Zr-metallic glass (MG) and crystalline Cu powders that were mixed and then consolidated, cold welded together and refined by high pressure torsion (HPT). Four different compositions (Zr-MG Xwt% Cu, X = 20, 40, 60, 80) were produced as well as single phase Zr-MG samples as reference. To investigate the influence of the degree of deformation and the ratio of the two phases on the evolution of the microstructure and mechanical properties, scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness and microcompression measurements were used.