Materials Science Forum Vols. 584-586

Abstract: High-pressure torsion (HPT) is an important processing technique in which a disk is subjected to a high pressure with concurrent torsional straining. In principle at least, the imposed strain is zero at the center of the disk and a maximum at the outer edge. This difference suggests, therefore, that materials processed by HPT will exhibit considerable inhomogeneity. This paper describes the results obtained in a series of experiments which were designed to evaluate the evolution of homogeneity during the processing of two materials by HPT. It is demonstrated that it is possible to achieve a reasonable level of homogeneity in both materials but there are important differences which reflect the dependence of the microstructure on the occurrence of dynamic recovery.

Abstract: This report presents main achievements of international R&D activities of the Institute of Physics of Advanced Materials of Ufa State Aviation Technical University (Ufa, Russia) with a special attention to the innovative potential of nanostructured metals and alloys produced by severe plastic deformation techniques. Several examples of the first promising applications of bulk nanostructured materials as well as potential competing technologies are considered and discussed.

Abstract: The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.

Abstract: The concept of grain boundary (GB) engineering of ultrafine-grained (UFG) metals and alloys is proposed for enhancement of properties by tailoring different GBs (low angle and high angle ones, special and random, equilibrium and non-equilibrium) and formation of GB segregations and precipitations using SPD processing. By variations of regimes and routes of SPD processing we show for several light alloys (Al and Ti) the ability to produce UFG materials with different grain boundaries, and this can have a dramatical effect on mechanical behavior of the processed materials. This paper demonstrates also several new examples of this approach for attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various UFG metals and alloys.

Abstract: Deformation methods of nanostructuring (DMNs) of materials are proposed to classify into severe plastic deformation (SPD) and mild plastic deformation (MPD) methods according to fundamentally different low- and high-temperature grain refinement mechanisms they exploit. A general analysis of the fundamentals and nanostructuring efficiency of three most developed DMNs, high pressure torsion (HPT), equal-channel angular pressing (ECAP), and multiple isothermal forging (MIF) is done with a particular attention to ECAP and MIF. It is demonstrated that MIF is the most efficient method of DMNs allowing one to obtain the bulkiest nanostructured samples with enhanced mechanical properties.

Abstract: The present work is devoted to the investigation of the influence of the grain size on the main mechanical characteristics of nanopolycrystals of different metals. The Hall-Petch parameter behaviour for Al, Cu, Ni, Ti and Fe was examined in the wide grain size interval. The stages of plastic deformation and the parameters of work hardening for nanocrystalline copper were analysed in detail. The deformation mechanisms and critical grain sizes accounting for the transition from the dislocation slip to the grain boundary sliding were described.

Abstract: Recently, a new class of materials - called nanoglasses - with a glassy structure was synthesized. The novel feature of these materials is that the atomic structure in the entire volume of the material as well as the density of the material can be tuned. Nanoglasses are generated by introducing interfaces into metallic glasses on a nanometer scale. Interfaces in these nanoglasses delocalize upon annealing, so that the free volume associated with these interfaces spreads throughout the volume of the glass. This delocalization changes the atomic structure and the density of the glass throughout the volume. In fact, by controlling the spacing between the interfaces introduced into the glass as well as the degree of the delocalization (by modifying the annealing time and/or annealing temperature), the atomic structures as well as the density (and hence all structure/density dependent properties) of nanoglasses may be controlled. A comparable tuning of the atomic structure/density of crystalline materials is not conceivable, because defects in crystals do not delocalize upon annealing.

Abstract: Nanostructured titanium (nTi) with essential enhanced strength and fatigue characteristics is an advanced material for dental implant applications. Nano Ti is commercially pure titanium, that was nanostructured by a special technique of severe plastic deformation. It is bio inert, does not contain even potentially toxic or allergenetic additives and has significantly higher specific strength properties than any other titanium applied in dental implants. Cylindrical threaded screw implants Nanoimplant® sized 2.4 mm in diameter and 12 mm in length were made from nTi. It is the first application of nTi dental implant in the world reported. Recently more than 250 successful clinical applications dealing with surgery on the front teeth were carried out. No complications were noticed during the early postoperative period and early loading. Laboratory cytocompatibility tests undertaken so far on mice fibroblast cells have indicated that nanocrystalline Ti surface has a significantly better property for cell colonisation and healing of tissue consequently.

Abstract: Grain refinement is well known to influence the mechanical properties of materials, especially the strength characteristics. The promising method for grain refinement is a SPD process and it produces the homogenized nano grain material which exhibits very high strength and limited ductility. Recently the grain refinement technique by the SPD in powder metallurgy (PM) field has received much attention. The SPD-PM process is one of new processes combining mechanical milling (MM) or alloying (MA), heat treatment and sintering processes. Microstructure of the SPD-PM materials is easily controlled by the MM condition, and hence we can intentionally make a heterogeneous microstructure. In the present study, commercially pure titanium, Ti-6Al-4V alloy and SUS316L stainless steel powders are applied to the SPD-PM process. These MM powders are sintered by Hot Roll Sintering (HRS) process. These SPD-PM materials demonstrate a heterogeneous microstructure and high strength and advanced plastic strain. The microstructure of materials consists of a shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. All of the materials fabricated by these processes demonstrate not only superior strength but also enough elongation. The mechanical properties are strongly influenced by the shell / core microstructure. The nano / meso hybrid microstructure by these processes has been proved to be very effective to improve mechanical properties.