The deformation mechanisms in crystalline materials, covering metals, alloys, ceramics, and geologic rocks, have to date been well established in the deformation map, which include the dislocation motion in a wide temperature range, grain boundary (GB) sliding at high temperature, and/or twinning at low temperature (LT). Here we report a reverse twinning mechanism-detwinning, which operates at LTs during the tensile deformation of an electrodeposited Cu with a high density of nanosized growth twins. In situ synchrotron X-ray diffraction (XRD) methods provide the direct experimental evidences for detwinning of nanoscale twins at LTs, which is distinct from the twinning activities previously observed in metals deformed at LTs or during thermal annealing. This finding indicates that the highly moveable nanoscale twin boundaries (TBs) contribute greatly to the accommodation of plastic strains and promote the homogeneous deformation of crystals. We believe that the detwinning behavior should be closely related to mechanical properties and functional behaviors of many technical materials.The detwinning as a reverse deformation mode of twinning was rarely observed in traditional metals or alloys while the deformation twinning can be evidenced in many technical materials at LT. The motion of dislocations is hindered during the deformation of the nanocrystalline metals due to the extremely high population of GBs, suggesting that new deformation mechanisms, related to the GB-mediated processes, absent in their coarse-grained counterparts, could be activated in the nanostructured metals or alloys. [1] The deformation behavior of nanocrystalline metals was studied by molecular-dynamics (MD) simulations of threedimensional (3D) grains [2][3][4][5] and in situ characterized by the transmission electron microscopy (TEM) on the thin speci-The origin of the plasticity in bulk nanocrystalline metals have, to date, been attributed to the grain-boundary-mediated process, stress-induced grain coalescence, dislocation plasticity, and/or twinning. Here we report a different mechanism-detwinning, which operates at low temperatures during the tensile deformation of an electrodeposited Cu with a high density of nanosized growth twins. Both three-dimensional XRD microscopy using the Laue method with a submicron-sized polychromatic beam and high-energy XRD technique with a monochromatic beam provide the direct experimental evidences for low temperature detwinning of nanoscale twins. 906 wileyonlinelibrary.com ß
This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic (RAFM) steels. High-angle grain boundaries, subgrain boundaries, nano-sized M 23 C 6 , and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength. M 23 C 6 carbides are easily coarsened under high temperatures, thereby weakening their ability to block dislocations. Creep properties are improved through the reduction of M 23 C 6 carbides. Thus, the loss of strength must be compensated by other strengthening mechanisms. This review also outlines the recent progress in the development of RAFM steels. Oxide dispersion-strengthened steels prevent M 23 C 6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength. Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel. The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries. This procedure increases the creep life of TMT(thermo-mechanical treatment) 9Cr-1W-0.06Ta steel by ~20 times compared with those of F82H and Eurofer 97 steels under 550°C/260 MPa.
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