Karoline Kormout

Supersaturation in Ag–Ni alloy by two-step high-pressure torsion processing

Bulk Ag–Ni composites with an ultrafine grained structure were obtained by the use of a new two-step high-pressure torsion consolidation and deformation process after the first processing step. The evolving microstructure in the fully dense composites of 6 and 19 at.% of Ag was investigated by scanning and transmission electron microscopy and X-ray diffraction after applying the first and second deformation step. A deformation-induced formation of single-phase supersaturated solid solution of Ag in nanocrystalline Ni was observed after the second deformation step at the highest degree of deformation independent of the initial composition.

Transmission electron microscope investigations on Cu-Ag alloys produced by high-pressure torsion

Cu-Ag alloys in three different compositions (Cu – 25/50/75wt% Ag) were produced by powder consolidation followed by high-pressure torsion. Deformation was performed till a saturation regime was reached. The generated microstructures were investigated by transmission electron microscopy and vary from ultra-fine grained to nanocrystalline to even partially amorphous structures. Vickers hardness measurements show a strong increase in hardness compared with the pure metals, annealing at 130°C leads to an additional increase in hardness.

Influence of Annealing on Microstructure and Mechanical Properties of a Nanocrystalline CrCoNi Medium-Entropy Alloy

An equiatomic CrCoNi medium-entropy alloy was subjected to high-pressure torsion. This process led to a refinement of the microstructure to a grain size of about 50 nm, combined with a strong increase in the materials hardness. Subsequently, the thermodynamic stability of the medium entropy alloy was evaluated by isothermal and isochronal heat treatments. Annealed samples were investigated by scanning and transmission electron microscopy as well as X-ray diffraction, and were subjected to tensile tests to establish microstructure-property relationships. Furthermore, a comparison of mechanical properties with a grade 316L stainless steel was performed in order to evaluate if the CrCoNi alloy is competitive with commercially available structural materials in the nanocrystalline state. A minority phase embedded in the face-centered cubic matrix of the CrCoNi alloy could be observed in multiple annealed states, as well as the as-received and high-pressure torsion processed material. For 200 h of annealing at 500 °C, it was determined that the minority phase has a hexagonal-closed-packed crystal structure. A possible explanation for the formation of the phase is a preferential segregation of Co to stacking faults.

An investigation on shear banding and crystallographic texture of Ag–Cu alloys deformed by high-pressure torsion

Ag–Cu alloys of two different initial microstructures—a cast eutectic alloy (AgCu-E) and an equivolume Ag–Cu powder mixture (AgCu-P)—were deformed by high-pressure torsion. The codeformation of Ag and Cu grains led to uniform refinement and a nanolamellar microstructure for both alloys. However, the lamellar structure in AgCu-P alloys was broken at intermediate shear strains (γ > 150) by extensive shear banding. On the other hand, no shear banding was observed for AgCu-E alloy at similar microstructural refinement. At higher strains deformation induced intermixing of Ag and Cu atoms was observed. Further, three-dimensional diffraction analysis of AgCu-E alloy showed that in contrast to conventional single phase alloys, the Ag and Cu phases develop similar crystallographic texture.

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.

Influence of Processing Parameters on the Mechanical Properties of HPT-Deformed Nickel/Carbon Nanotube Composites

Nickel carbon nanotube composites with varying amounts of carbon nanotubes are deformed by high pressure torsion at different deformation temperatures to high strains, where no further refinement of the Nickel matrix microstructure is observed. Mean Nickel grain sizes increase with increasing HPT deformation temperature, while the size of the carbon nanotube agglomerates is significantly reduced. Additionally, the distribution of the agglomerates in the metal matrix becomes more homogenous. To investigate the mechanical performance of the HPT deformed composites, uniaxial tensile and compression tests are conducted. Depending on the HPT deformation temperature and the resulting microstructure, either brittle or ductile fracture occurs. Increased HPT deformation temperatures induce a decrease in the anisotropy of the mechanical properties, mainly caused by a shrinking of the carbon nanotube agglomerates. It is shown, that tuning the HPT deformation temperature is the key for optimizing both the microstructure and the mechanical performance.

Role of interfaces on microstructure refinement and mechanical properties of severe plastically deformed copper and copper-silver eutectic

The process of microstructural refinement during deformation by high pressure torsion (HPT) of commercially pure copper (Cu-B), copper powders (Cu-P) and cast copper silver eutectic alloy (CuAg-E) were investigated. The presence of a surface oxide layer or of immiscible silver lamellae leads to much finer grain size. A saturation in microstructural refinement was attained in both Cu-B and Cu-P samples, while the CuAg-E sample showed a linear hardening propensity down to a 30 nm interlamellae spacing. The possible mechanisms for strengthening in the CuAg-E alloy are further discussed in view of the evolution of interlamellae spacing with applied shear strain.