Grain refinement of pure Ti during plastic deformation

Abstract

a b s t r a c tThe mechanism of grain refinement was studied in titanium subjected to high-rate surface mechanical attrition treatment. By cross-sectional transmission electron microscopy, the deformation structures were systematically characterized in a nanocrystalline (NC) layer of the treated surface which exhibits a grain size gradient. The microstructural evolution with increasing strain involves (1) the formation of slip bands with dislocation-free cells in their interiors, (2) nucleation and migration of high-angle grain boundaries at cell boundaries consisting of high-density tangled dislocations, leading to the formation of submicrosized grains inside the slip bands, followed by (3) the formation of subgrains and dislocation cells inside the submicrosized grains with increasing misorientations, which finally become NC grains. The mechanism of grain refinement was interpreted in terms of the classical migration dynamic recrystallization (DRX) and continuous rotation DRX, respectively, leading to submicrosized and NC grains with increasing strains at high strain rates. The cooperative grain boundary sliding is active at high strain rate, by the presence of arrays of coplanar grain boundaries.

Dislocation Density-Based Grain Refinement Modeling of Orthogonal Cutting of Titanium

“…The cell structure can be considered as a 'precursor' of the eventually developing grain structure, as have been experimentally observed in the UFG microstructures pro duced by the SPD processes [5,7,[31][32][33][34][35][37][38][39]50]. The cell structure can be considered as a 'precursor' of the eventually developing grain structure, as have been experimentally observed in the UFG microstructures pro duced by the SPD processes [5,7,[31][32][33][34][35][37][38][39]50].…”

“…Figure 1 shows the transmission electron micro scope (TEM) images of CP Ti workpieces produced by (a) and (b) orthogonal cutting [5], (c) and (d) multipass cold rolling [31], (e) ten-pass ECAP plus cold rolling with 77% thickness reduction [32], (/) two-pass ECAP [33], (g) multipass hydrostatic extrusion [34] and (h) surface mechanical attrition treatment (SMAT) proc esses [35], Figure 1(a) shows a TEM micrograph of a chip machined with the +20 deg rake angle tool, which microstructure consists of sub-lOOnm dislocation cells/grains interspersed with elongated, less developed subgrain structures. A dislocation cell structure with high dislocation den sity is often observed in CP Ti workpieces treated by a variety of SPD processes.…”

Temperature and strain rate dependence of microstructural evolution and dynamic mechanical behavior in nanocrystalline Ti

“…The mechanism of DRX may be classified into migrational and rotational types [39] where it was reported that primarily migrational DRX operates in metals subjected to static deformation or low strain rate dynamic deformation [27,40,41]. Nevertheless, in deformation at very high strain rates, there is insufficient time for long-range migration of the boundaries so that the DRX operates via the rotation of subgrain boundaries [40,42].…”

Effect of grain size on compressive behaviour of titanium at different strain rates

“…It has been demonstrated that dislocation motion and the value of the activation volume both vary with grain size during plastic deformation [46] and for CP Ti the magnitudes of V* were estimated as ~276b 3 , ~57b 3 and ~5b 3 for CG, UFG and NC Ti, respectively [47]. Although a reduction in grain size gives rise to an enhancement in stress, nevertheless this rise becomes less important when considering the drastic increase in the dislocation density in the coarser grains so that an increase in m with decrease in grain size is observed in the CP Ti.…”