Grain Boundaries and Mechanical Properties of Ultrafine-Grained Metals
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TRODUCTION
ULTRAFINE-GRAINED (UFG) materials with a grain size in the submicrometer (100 to 1000 nm) or nanometer (less than 100 nm) range possess a much higher area of grain boundaries (GBs) than the usual coarse-grained (CG) counterparts; that is why the properties of UFG materials depend considerably on the structure and behavior of internal interfaces–GBs. Therefore, the UFG materials are commonly defined as interface-controlled materials.[1] At the same time, in the UFG materials produced by severe plastic deformation (SPD) techniques, GBs might vary significantly depending on the regimes and routes of processing and they can belong to high- and low-angle GBs, special and random, equilibrium and nonequilibrium ones, as well as contain GB segregations or precipitations.[2–6] In this context, there appears a possibility to control and enhance the properties of UFG materials by variation of the structure of GBs using SPD processing. This approach can be considered as GB engineering of UFG metals and alloys.[5,6] The concept of GB engineering or GB design was introduced by Watanabe in Reference 7, where it has been proposed that the properties of polycrystalline materials may be effectively changed by deliberate and RUSLAN Z. VALIEV, Professor and Director, MAXIM YU. MURASHKIN, Researcher, and IRINA P. SEMENOVA, Leading Researcher, are with the Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450000, Russia. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Mechanical Behavior of Nanostructured Materials,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Electronic, Magnetic, and Photonic Materials Division, the TMS Materials Processing and Manufacturing Division, the TMS Structural Materials Division, the TMS Nanomechanical Materials Behavior Committee, the TMS Chemistry and Physics of Materials Committee, and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online December 19, 2009 816—VOLUME 41A, APRIL 2010
careful tailoring of the distributions of boundary misorientation angles. This approach has been used successfully for several studies, including, for example, improving the susceptibility to intergranular stress corrosion cracking.[8] However, there are generally difficulties in achieving different boundary distributions in conventional CG materials. In this connection, UFG materials in which GB structure features are associated with SPD processing regimes allow many more possibilities for GB engineering. During recent years, in our laboratory, a number of investigations have been performed dealing with nanostructuring of metals and alloys[2,9] by using the two most popular SPD techniques, high-pressure torsion (HPT) and equal-channel angular pressing (ECAP), and with the focus on GB evolution. This article reports several new examples of GB engineering for attaining enhanced strength and ductility, fatigue, and sup
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