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Register a new userTowards a theory of flow stress in multimodal polycrystalline aggregates. Effects of dispersion hardening
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Added by
Alexander Reshetnyak
Publication date
2019
fields
Physics
and research's language is
English
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We elaborate the recently introduced theory of flow stress, including yield strength, in polycrystalline materials under quasi-static plastic deformations, thereby extending the case of single-mode aggregates to multimodal ones in the framework of a two-phase model which is characterized by the presence of crystalline and grain-boundary phases. Both analytic and graphic forms of the generalized Hall-Petch relations are obtained for multimodal samples with BCC ($alpha$-phase Fe), FCC (Cu, Al, Ni) and HCP (Cu, Al, Ni) and HCP ($alpha$-Ti, Zr) crystalline lattices at $T=300K$ with different values of the grain-boundary (second) phase. The case of dispersion hardening due to a natural incorporation into the model of a third phase including additional particles of doping materials is considered. The maximum of yield strength and the respective extremal grain size of samples are shifted by changing both the input from different grain modes and the values at the second and third phases. We study the influence of multimodality and dispersion hardening on the temperature-dimensional effect for yield strength within the range of $150-350K$.
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In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with a recently proposed physically-based crystal plasticity constitutive equations validated for neutron-irradiated austenitic stainless steel. Intergranular normal stress probability density functions are found with respect to plastic strain and irradiation level, for uniaxial loading conditions. In addition, plastic slip activity jumps at grain boundaries are also presented. Intergranular normal stress distributions describe, from a statistical point of view, the potential increase of intergranular stress with respect to the macroscopic stress due to grain-grain interactions. The distributions are shown to be well described by a master curve once rescaled by the macroscopic stress, in the range of irradiation level and strain considered in this study. The upper tail of this master curve is shown to be insensitive to free surface effect, which is relevant for IGSCC
A theory of flow stress (FS), reviewing and developing our research,e.g. arxiv:1803.08247;1908.09338, is proposed,including yield strength (YS) of PC materials for quasi-static plastic loading for grain of average size d in range 10^{-8}-10^{-2}m. Its based on statistical model of energy spectrum distribution in each grain of 1-mode PC sample under plastic loading,with highest level equal to maximal dislocation energy. Found distribution of scalar dislocation density leads to FS due to Taylor strain hardening containing usual and anomalous HP laws for coarse and NC grains, respectively, and reaches maximum for extreme grain size d_0 of order 10^{-8}m. Maximum undergoes shift to region of larger grains for decreasing T and increasing strains. Coincidence is established among theoretical and experimental data on YS for BCC(alpha-Fe), FCC(Cu,Al,Ni),HCP(alpha-Ti,Zr) PC materials at T=300K.The T-dependence of strength quantities is studied. It is shown using Al that YS grows with decrease in T for all grains with d>3d_0,and then YS decreases in NC region,thus determining a temperature-dimension effect (TDE).1-phase model of PC sample is extended by including softening GB phase into 2-phase model,and then by dispersion (un)hardening. A quasi-particle interpretation of crystallite energy quantization is suggested.Analytic and graphic forms of HP laws are obtained in above samples with different values of small-,large-angle GB and constant pores.The maximum of YS and respective extremal grain size of the samples are shifted by change of 2-nd phase.The T-dependence of YS in range of 150-350K for Al demonstrates the validity of TDE. An enlargement of 2-nd phase neutralizes TDE.Deformation curves for 1- and 2-mode 2-phase alpha-Fe PC model are constructed with Backofen-Considere fracture criterion,as compared to experimental,1-phase model data, and strongly depend on multimodality and GB
In this paper, we study the fully developed gravity-driven flow of granular materials between two inclined planes. We assume that the granular materials can be represented by a modified form of the second-grade fluid where the viscosity depends on the shear rate and volume fraction and the normal stress coefficients depend on the volume fraction. We also propose a new isotropic (spherical) part of the stress tensor which can be related to the compactness of the (rigid) particles. This new term ensures that the rigid solid particles cannot be compacted beyond a point, namely when the volume fraction has reached the critical/maximum packing value. The numerical results indicate that the newly proposed stress tensor has an obvious and physically meaningful effects on both the velocity and the volume fraction fields.
Charged pattern formation on the surfaces of self--assembled cylindrical micelles formed from oppositely charged heterogeneous molecules such as cationic and anionic peptide amphiphiles is investigated. The net incompatibility $chi$ among different components results in the formation of segregated domains, whose growth is inhibited by electrostatics. The transition to striped phases proceeds through an intermediate structure governed by fluctuations, followed by states with various lamellar orientations, which depend on cylinder radius $R_c$ and $chi$. We analyze the specific heat, susceptibility $S(q^*)$, domain size $Lambda=2pi/q^*$ and morphology as a function of $R_c$ and $chi$.
Structural aspects of crystal nucleation in undercooled liquids are explored using a nonlinear hydrodynamic theory of crystallization proposed recently [G. I. Toth et al., J. Phys.: Condens. Matter 26, 055001 (2014)], which is based on combining fluctuating hydrodynamics with the phase-field crystal theory. We show that in this hydrodynamic approach not only homogeneous and heterogeneous nucleation processes are accessible, but also growth front nucleation, which leads to the formation of new (differently oriented) grains at the solid-liquid front in highly undercooled systems. Formation of dislocations at the solid-liquid interface and interference of density waves ahead of the crystallization front are responsible for the appearance of the new orientations at the growth front that lead to spherulite-like nanostructures.
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