Andrea Bachmaier

Technical parameters affecting grain refinement by high pressure torsion

Abstract High pressure torsion is a well known and widespread processing technique for severe plastic deformation. The aim of high pressure torsion and other comparable techniques is to obtain ultrafine-grained or even nanocrystalline materials with enhanced mechanical and physical properties compared with their coarse-grained counterparts. Generally this refinement process is strongly influenced by processing parameters such as temperature or accumulated strain, but can also simply be affected by the entire experimental setup. Therefore, the benefits and limitations of the process with regard to grain refinement, homogeneity and specimen size, underlined with experimental results using different tools, will be discussed.

Generation of metallic nanocomposites by severe plastic deformation

Abstract In this article, the potential to fabricate composites with ultrafine grained structures, as well as composites with a nanostructure, by different severe plastic deformation methods, is reviewed. A broad spectrum of diverse composites produced by severe plastic deformation methods exist which include metal–metal composites, metal matrix composites and amorphous metal matrix composites. Furthermore, the influence of the strain path and initial structure on the final composite material is outlined.

New insights on the formation of supersaturated solid solutions in the Cu–Cr system deformed by high-pressure torsion

Abstract In the Cu–Cr system, the formation of supersaturated solid solutions can be obtained by severe plastic deformation. Energy-dispersive synchrotron diffraction measurements on as-deformed Cu–Cr samples as a function of the applied strain during deformation confirm the formation of supersaturated solid solutions in this usually immiscible system. Due to evaluation of the diffraction data by a newly developed energy-dispersive Rietveld program, lattice parameter and microstructural parameters such as domain size and microstrain are determined for as-deformed as well as annealed samples. The obtained information is used to deepen the understanding of the microstructural evolution and the formation of supersaturated solid solutions during severe plastic deformation. Complimentary transmission electron microscopy investigations are furthermore performed to characterize the evolving microstructure in detail. After annealing at elevated temperatures, the formed solid solutions decompose. Compared to the as-deformed state, an enhanced hardness combined with a high thermal stability is observed. Possible mechanisms for the enhanced hardness are discussed.

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.

Development of a New Testing Procedure for Performing Tensile Tests on Specimens with Sub-Millimetre Dimensions

A new method to conduct tensile tests with specimens ranging from a few millimetres down to 100 μm is presented. The tensile specimens are fabricated using a newly developed water-cooled circular grinding process that guarantees a high-accuracy geometry by keeping the fabrication-related material change to a minimum. Furthermore, the grinding tool is equipped with a mechanical polishing unit to make well-polished surfaces. Besides the specimen fabrication, the test setup is modified to allow a specimen related displacement measurement. This image-based system does not require any additional specimen markings and enables the evaluation of true stress, true strain, and the reduction in area. The whole method is successfully demonstrated for different types of metals ranging from ductile nickel to very brittle tungsten. In addition to the quality of the tensile test in this specimen size range, another advantage is the enormous reduced time for specimen fabrication and testing.

Temperature dependent structural evolution in nickel/carbon nanotube composites processed by high-pressure torsion

Nickel/Carbon nanotube (CNT) composites with varying amounts of CNTs were processed at different temperatures by high-pressure torsion (HPT) with the aim to optimize the process parameters to obtain a homogenous dispersion of CNTs in the metallic matrix. As the CNT distribution has an enormous influence on the composite properties, the structural evolution with increasing strain and the final microstucture of the composites are investigated by scanning and transmission electron microscopy. Microhardness measurements were additionally performed. Microhardness increases up to 800 Vickers (HV) and the mean grain size decreases to an equivalent radius smaller than 40 nm for HPT at room temperature (RT), while the CNTs form rather large agglomerates. HPT deformation at 200 °C shows no significant change in hardness, grain size and CNT agglomerate size. For HPT deformation at 300°C and 400°C grain sizes increase to 60 nm respectively 90 nm, microhardness decreases to 500 HV respectively 400 HV and the size of the CNT agglomerates decreases from more than 5 times the grain size at RT to smaller than the grain size. It could be shown that the optimal HPT processing route to improve the CNT distribution is a combination of deformation at 400°C with subsequent deformation at RT.

Microstructural evolution in immiscible alloys processed by High-Pressure Torsion

The application of High-Pressure Torsion (HPT) to two different Co-Cu alloys permits the generation of materials with nanocrystalline grain sizes. The evolution of the microstructure with increasing strain and the resulting microstructure are investigated by scanning electron microscopy. The final attainable grain sizes are significantly smaller than in corresponding pure metals deformed by HPT. Special attention is given on microstructural evolution and deformation behavior of the Cu and Co phases revealing the importance of nature, morphology and deformation behavior of the single phases on the formation of nanocrystalline structures during HPT processing.

Microstructure and Properties of a Fe-Cu Composite Processed by HPT Powder Consolidation

A method to produce nanocrystalline Fe-Cu composites by means of high-pressure torsion (HPT) deformation is presented. Mixtures of micrometer sized powders of Fe and Cu with different ratios of the two components were precompacted and subsequently deformed by HPT at room temperature to a certain amount of strain. Afterwards, new samples were cut out of these previously deformed samples and further HPT deformation was conducted. The evolution of the microstructure during the different steps of the HPT process and the resulting microstructure of the composites were investigated by scanning electron microscopy. In summary it could be shown that the final attainable grain sizes in the composite materials in the two step process are much smaller than in the simply HPT deformed composites. The reduction of the grain size is also reflected in an enhancement of the hardness.

Study of the structural defects on carbon nanotubes in metal matrix composites processed by severe plastic deformation

Carbon nanotubes (CNT) have been recently proposed as stabilizers against grain growth that can happen even at low temperature inputs in nano-crystalline and ultrafine-grained materials obtained by severe plastic deformation. In this study, we analyzed the evolution of the structural defects on the nanotubes in CNT-reinforced nickel matrix composites with different reinforcement weight fractions. The composites were processed by high pressure torsion, and we used Raman spectroscopy as the main characterization technique. The results indicate that for CNT subjected to highly energetic processing, it is not sufficient to analyze only the ID/IG ratio (as proposed in the available literature), but it is also necessary to evaluate the shifting of the G band, which traces the amorphization trajectory undergone by the CNT. Furthermore, we observed that the deformation suffered by the CNT is related to the accumulated strain and varies with the partial CNT fractions of these composites. This is related to their capacity to withstand the plastic strain that occurs during deformation. In addition, the defective state reaches a saturation before achieving the saturation in the microstructural refinement. These results will help to efficiently optimize the processing of this type of engineering composites.

Thermal and Mechanical Stability of Nano-Crystalline and Nano-Structured Metals

The aim of the current work is to study the microstructure stability under thermal and mechanical loads of a nano-structured Cu/Co alloy and nc-Nickel with different content of solute elements and nano-particles. For this annealing at different temperatures as well as fatigue loading are performed and the microstructure evolution is characterized. The effectiveness of mechanisms that impedes grain growth (solute drag, Zener pinning) are evaluated and studied for both, thermal and mechanical loads.