A. V. Ganeev

Grain Boundary Segregation in UFG Alloys Processed by Severe Plastic Deformation

Grain boundary (GB) segregations were investigated by atom probe tomography in an Al–Mg alloy, a carbon steel and Armco® Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminum alloy are Mg depleted, but after SPD some local enrichment up to 20 at% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as “non-equilibrum” in ultra fine grained materials processed by SPD. The GB segregation mechanisms are discussed and compared in the case of substitutional (Mg in fcc Al) and interstitial (C in bcc Fe) solute atoms.

Refinement of tungsten microstructure upon severe plastic deformation

The effect of equal-channel angular pressing and high-pressure torsion on the formation of an ultrafine-grained structure in commercially pure tungsten has been studied. The structure was tested by the methods of transmission and scanning electron microscopy using electron backscatter diffraction analysis. The material microhardness depending on the stored deformation has been evaluated. In the samples subjected to severe plastic deformation, the quantitative structure characteristics (the average size of grains/subgrains and misorientation-angle distribution) have been determined.

Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.

Influence of HPT Deformation Temperature on Microstructures and Thermal Stability of Ultrafine-Grained Tungsten

In the present work high pressure torsion (HPT) was imposed on commercial purity (CP) tungsten at different temperatures of 450 °С and 490 °С to achieve different microstructures and grain boundary misorientation spectra (GBMS). After HPT at 450°С, ultrafine grained microstructure with an average grain size of ~150 nm was developed in the metal. HPT at 490 °С results in an elongated structures with average width of ~500nm. EBSD investigations showed that over 92% are HAGB type in microstructure HPT-processed at 450°С, and in contrast, over 50% of grain boundaries are LAGB in sample processed at 490°С. Annealing at 900°С for 1h, of the sample with homogeneous UFG structure resulted from HPT at 450°C, leads to only limited decrease (~20%) in microhardness.

On the nature of high-strength state of carbon steel produced by severe plastic deformation

Contributions of different strengthening mechanisms to yield strength of carbon steels C 10 (0.1 % C) and C 45 (0.45 % C) with ultrafine-grained microstructures have been analyzed in this work based on the precision investigation of the microstructure by electron microscopy, 3D atom probe tomography, and mechanical properties. Estimated values of yield stress showed satisfactory agreement with the experimental results. It is shown that significant contribution to strengthening is made by carbides. In particular, as the carbon content increases in carbon steels, the volume fraction of cementite increases and, accordingly, there is a bigger deviation of the yield strength value from the classical Hall-Petch relationship. The relative contribution of grain boundary strengthening into the yield stress is reduced due to an increased proportion of precipitate strengthening.

Strengthening and Toughening Effect on Tungsten Subjected to Multiple ECAP

This paper presents the research results of the grain refining effect on the ductile-to- brittle transition temperature (DBTT) of commercial purity tungsten. The as-received tungsten was subjected to eight passes of equal-channel angular pressing (ECAP) at decreasing temperatures from 1300 to 1150 °C. According to optical and TEM microscopy the average grain size was refined considerably from ~80 μm to ~1 μm. The mechanical tensile tests, carried out at various temperatures for the tungsten samples, showed that DBTT decreased approximately 80 °C as a result of microstructure refinement by ECAP, at the same time the strength also increased 50-100 % by grain refinement. SEM observation of the fractures confirmed the mechanical testing results.

Microstructure, properties, and failure characteristics of medium-carbon steel subjected to severe plastic deformation

The paper deals with two-stage processing of medium-carbon steel 45 (0.45 % C; 0.27 % Si; 0.65 % Mn) via quenching and high pressure torsion. Such processing combination allowed producing a nanocomposite microstructure with a ferrite matrix and high-dispersed carbides. The ultimate tensile strength of the nanostructured steel is over 2500 MPa. The processing effect on the structure, mechanical properties and failure mechanisms of steel 45 samples is studied. The peculiarities of static fractures in the samples after HPT are demonstrated in comparison with those after quenching.

Structural strength and corrosion resistance of nanostructured steel 10

The ultrafine-grain steel 10 obtained by equal-channel angular pressing at 200°C is investigated. The structural strength of the steel samples is analyzed, with tensile tests and assessment of the impact strength and crack resistance. The strength of steel increases by a factor of 2.5 after equal-channel angular pressing. In the specified conditions, such treatment does not change the cold-brittleness threshold of steel 10. However, the interval of ductile-brittle transition becomes narrower. Calculations show that the crack resistance K1C of steel 10 with an ultrafine grain structure is somewhat greater than that of the regular steel. The corrosion rate of uncoated steel 10 samples after equal-channel angular pressing is somewhat greater than in the initial state. However, the corrosion rate of coated steel 10 samples after equal-channel angular pressing is half that in the initial state. The significance of these findings is discussed.