O. Renk

Hardening by annealing: insights from different alloys

In contrast to the general notion about the annealing behavior of coarse grained materials, hardening phenomena in nanocrystalline materials can occur. Although the phenomena have already been recognized several years ago, the mechanisms behind are still controversially discussed. For example, the influence of solutes segregated to grain boundaries on the strengthening mechanism is unclear. We present a combination of atom probe tomography and mechanical data to reveal the role of segregations to the strengthening. The results show that despite large modifications of the boundary chemistry the mechanical behavior remains widely unaffected. Additionally, it will be shown that hardening upon annealing can only occur below a material-specific grain size threshold value.

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.

Microstructural and texture evolution of copper-(chromium, molybdenum, tungsten) composites deformed by high-pressure-torsion

Abstract Cu-refractory metal composites containing Cr, Mo or W were subjected to severe plastic deformation using room temperature high-pressure torsion (HPT). A lamellar microstructure developed in each of the composites at equivalent strains of ∼75. The refractory metals developed {hkl}⟨111⟩ fibre textures with a slight tilt to the tangential direction. This texture was stronger and more clearly defined in Mo and W than in Cr. By applying additional HPT deformation to these samples, perpendicular to the original shear strain, it was found that the lamellar structure of Cu30Mo70 and Cu20W80 (wt%) composites could be retained at high equivalent strains and the refractory layer thickness could be reduced to 20–50 nm in Cu20W80 and 10–20 nm in Cu30Mo70. Although neighbouring regions of the microstructure were aligned and there was evidence of local texture in both composites, the bulk texture of Cu30Mo70 became weaker during this second step of HPT deformation. This was attributed to the refractory metal lamellae being discontinuous and imperfectly aligned. This work shows that it is possible to form ultrafine composites of Cu-group VI refractory metals via high-pressure torsion, with namolamellar structures being possible where there is a sufficient volume fraction of Mo or W.