Papers by Keyword: Dislocation Density

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Authors: A. Ma, Franz Roters, Dierk Raabe
Abstract: Crystallographic slip, i.e. movement of dislocations on distinct slip planes, is the main source of plastic deformation of most metals. Therefore, it was an obvious idea to build a constitutive model based on dislocation densities as internal state variables in the crystal plasticity. In this paper the dislocation model recently proposed by Ma and Roters (Ma A. and Roters F., Acta Materialia, 52, 3603-3612, 2004) has been extended to a nonlocal model through separating the statistically stored dislocation and geometrically necessary dislocation densities. A nonlocal integration algorithm is proposed, which can be more easily used in conjunction with commercial software such as MARC and ABAQUS than the model proposed in the work of Evers(Evers L.P., Brekelmans W.A.M., Geers M.G.D., Journal of the Mechanics and Physics of Solids, 52, 2379-2401, 2004).
Authors: Miao Quan Li, Jiao Luo
Abstract: Isothermal compression of near alpha Ti-5.6Al-4.8Sn-2.0Zr alloy is conducted on a Thermecmaster-Z simulator at the deformation temperatures ranging from 1173 K to 1333 K, the strain rates ranging from 0.001 s-1 to 10.0 s-1 at an interval of an order magnitude and the height reductions ranging from 50% to 70%. The primary grain size is measured at an OLYMPUS PMG3 microscope with the quantitative metallography SISC IAS V8.0 image analysis software. A multi-scale constitutive model coupling the grain size, volume fraction and dislocation density is established to represent the deformation behavior of near alpha Ti-5.6Al-4.8Sn-2.0Zr alloy in high temperature deformation, in which the flow stress is decomposed a thermal stress and an athermal stress. A Kock-Mecking model is adopted to describe the thermally activated stress, and an athermal stress model accounts for the working hardening and Hall-Petch effect. A genetic algorithm (GA)-based objective optimization technique is used for determining material constants in this study. The mean relative difference between the predicted and experimental flow stress is 5.98%, thus it can be concluded that the multi-scale constitutive model with high prediction precision can efficiently predict the deformation behavior of near alpha Ti-5.6Al-4.8Sn-2.0Zr alloy in high temperature deformation.
Authors: Seo Young Ha, William M. Vetter, Michael Dudley, Marek Skowronski
Authors: Yi Chu Wu, M.K. Teng, Yuan Fu Hsia, X.R. Chang, Z.Z. Tian, C.M. Hsiao
Authors: Jalil P. Vafa, Shahriar J. Fariborz
Abstract: Based on the modified couple stress theory the solution to a screw dislocation is obtained, in an isotropic elastic plane, via Fourier integral transform method. The asymptotic analysis of displacement field at the tip of a stationary crack reveals that stress field is not singular. A crack under anti-plane deformation is modeled by the distribution of screw dislocations. The ensuing integral equations are solved numerically to determine the density of dislocation on a crack surface. The dislocation solution is used to study the interaction between two parallel non-collinear micro-cracks.
Authors: Alex Penlington, Bradley Diak, Hai Ou Jin
Abstract: An experimental 6XXX series aluminum alloy, Al-0.4Mg-1.2Si-0.49Cu-0.14Mn-0.2Fe(wt.%), was cold rolled 73% and the kinetics of its static recovery studied isochronally between 80 to 350°C, and isothermally at 175 and 205°C. Typical recovery is described by an extrinsic property such as yield stress, however, this study utilized the intrinsic dislocation density extracted from x-ray line profile analysis using a modified Williamson-Hall analysis. The static recovery of dislocation density was fit to the models of Nes [Acta Metall. Mater. 43 (1995) 2189–2207], suggesting that recovery is controlled by the migration of jogged screw dislocations assuming no lateral drift during annealing. The model fit of isothermal annealing at 175°C and 205°C yields activation energies of 0.99 and 1.7 eV/at., respectively. The change in energies can be correlated to an observed change in lattice strain with recovery.
Authors: U. Gilabert, A.B. Trigubó, G.E. Lascalea, N.E. Walsöe de Reca
Authors: Zhi Fu Yang, Qing Yuan Meng, Yu Hang Jing
Abstract: During the metal hot working process, the dislocation density will vary with strain and strain rate, and the variation of the dislocation density will affect the grain evolution subsequently. The cellular automaton (CA) method is an effective technique used to simulate the grain evolution of materials. In this work, a dynamic recrystallization (DRX) model of titanium alloy TC11 under varied strain rates was established by the use of cellular automaton method and verified by experimental observation. Two types of loading processes called “begin fast and then slowly” and “begin slowly and then fast” were simulated to investigate the titanium alloy TC11 grain evolution processes during hot working. The simulation results are in good coincidence with experimental data. Both cellular automaton simulation and experimental results show that the flow stresses and DRX transformation percentage during hot working process of the TC11 alloy are closely related not only to the strain rate but also to the loading sequence. Compared to the “begin slowly and then fast” loading sequence, the flow stress with the “begin fast and then slowly” loading sequence is relatively smaller under the same strain rates, and the DRX transformation percentage is relatively larger.
Authors: Shigeo Saimoto, Kaan Inal, Hai Ou Jin
Abstract: A new rationale to assess the work-hardening locus for pre-rolled sheets is described based on the realization that since the internal stresses necessarily sum to zero, the mean dislocation density remains the same upon re-pull in the rolling direction. Thus the 0.2 % yield stress as function of thickness strains results in an estimate of the stress-strain relation during rolling. Under plane strain, the thickness strain is negative to that of extension and hence the deduced rolling locus is compared to that of extrapolated tensile one of the start sheet. This comparison indicates that the onset of Stage IV occurs when volume fraction of point defects produced attains about 2 %.
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