Abstract
Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain ...boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production.
Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric ...rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.
•An active learning method combining Kriging and Subset Simulation (AK–SS) is proposed.•AK–SS takes advantages of Subset Simulation and the Kriging metamodel.•The proposed method is applied to ...several benchmark functions and a tunnel lining structure.•AK–SS is shown to be more efficient than the other methods in the literature.•AK–SS can deal with small probability problems with time-consuming function evaluations.
With complex performance functions and time-demanding computation of structural responses, the estimation of small failure probabilities is a challenging problem in engineering. Although Subset Simulation (SS) is a powerful tool for small probabilities, the computation amount is still large for time-consuming numerical procedures. Metamodelling is an important approach to increase the computational efficiency for engineering problems, however, a larger set of sample points is required for higher accuracy. This is a time-consuming task when the performance function needs to be numerically evaluated. To address this issue, AK–SS: an active learning method combining Kriging model and SS is proposed in this paper. The efficiency of this new method relies upon the advantages of SS in evaluating small failure probabilities and the Kriging model with active learning and updating characteristic for approximating the true performance function. The proposed method is applied to several benchmark functions in the literature, and to the reliability analysis of a shield tunnel, which requires finite element analysis. The results demonstrated that as compared to the other approaches in literature, AK–SS can provide accurate solutions more efficiently, making it a promising approach for structural reliability analyses involving small failure probabilities, high-dimensional performance functions, and time-consuming simulation codes in practical engineering.
The tensile properties and the deformation microstructure of pearlitic steel (0.8 wt % C) have been quantified in wires drawn to strains in the range from 3.7 to 5.4, having a flow stress in the ...range from 3.5 to 4.5 GPa. With increasing strain the interlamellar spacing (ILS) decreases from about 20 to 10 nm and the thickness of the cementite lamellae decreases from about 2 nm to about 0.7 nm, representing a structure, which breaks up at large strains, decomposes and releases carbon to the ferrite lamellae. The dislocation density increases continuously with strain and reaches about 5 × 1016 m−2 at a strain of 5.4; the dislocations are stored as threading dislocations, as dislocation tangles and as cell boundaries with low to medium misorientation angles. An analysis of the evolution of microstructure and strength with increasing strain suggests that dislocation-based plasticity is a dominating mechanism in the wire and three strengthening mechanisms are applied: boundary strengthening, dislocation strengthening and solid solution hardening with their relative contributions to the total flow stress which change as the strain is increased. Based on linear additivity good correspondence between the calculated and the measured flow stress is observed over the strain range 0–5.4. However at large strains beyond 3.7 deviations are observed which are discussed in terms of the applied strength-structure relationships.
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A Hall-Petch slope (k) that is highly changeable with texture, as extensively reported in Mg alloys, is ultimately related to the variation of deformation modes. In this paper, the influence of ...different (0002) distributions on k for twinning and slip was systematically studied using an AZ31 rolled plate (0002//ND) and extruded rod (0002⊥ED together with a random distribution around the ED). The ND and ED refer to the normal direction of the plate and extrusion direction of the rod, respectively. A high dependency of k on the (0002) distribution is found, namely, a much lower k for {101¯2} twinning in the plate (219 MPa μm1/2) than that in the rod (435 MPa μm1/2), but a much higher k for slip in the plate (437 MPa μm1/2) than that in the rod (235 MPa μm1/2). Compound use of the difference in Schmid factor (ΔSF) and geometric compatibility factor (m′) quantitatively explains this orientation effect on k. ΔSF relates to the extra stress needed for the activation of slip/twinning in a neighboring grain, and m′ reflects the efficiency of the stress concentration at the onset of slip/twinning in an adjacent grain. The lower m′ for twinning in the rod versus the plate primarily accounts for the higher k for twinning in the rod. A much larger inclination of basal poles away from the ideal texture exists in the plate than in the rod, which induces a higher activity of basal slip during tension. The resultant high fraction of slip transfer from basal slip in one grain to prismatic slip in the neighboring grain largely amplifies ΔSF and reduces m′, both of which yield a higher k for slip in the plate than in the rod. The relationship between the crystallographic orientation and m′ was also calculated for different types of deformation transfer, and the main factor that determines m′ was revealed.
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An ultrafine grained Al-0.3 wt %Cu has been produced by cold rolling to a thickness reduction of 98% (εvM = 4.5). The deformed structure is a typical lamellar structure with a boundary spacing of ...200 nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100–300 °C. Good thermal stability has been observed up to 175 °C with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198 MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that the constants in this equation are in very good agreement with previous observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance.
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Samples of Mg-3Gd (wt.%) were prepared by accumulative roll-bonding followed by annealing at different temperatures to produce samples with average grain sizes ranging from 3.3 µm to 114 µm. The ...samples were tensile tested at room temperature to characterize their strength and ductility, both of which were found to be significantly affected by transitions in mechanical behavior and deformation mechanisms. These transitions occurred with decreasing grain size and are described by: (i) a transition in the mechanical behavior from continuous flow to discontinuous flow associated with a yield point phenomenon, and (ii) a transition in the deformation mechanisms from 〈a〉 slip and twinning to 〈a〉 and 〈c + a〉 slips. The dislocation structures and deformation twins in the tensile samples have been characterized by transmission electron microscopy and electron backscatter diffraction, respectively. Dislocations of 〈a〉 and 〈c + a〉 type were identified based on two-beam diffraction contrast experiments. The results reveal that 〈a〉 dislocations and tension twins dominate in the samples with grain sizes larger than 10 µm, while 〈a〉 and 〈c + a〉 dislocations dominate in the samples with grain sizes smaller than 5 µm. In parallel, a consistent trend for both the strength and ductility to increase with decreasing grain size is observed. The appearance of a yield point phenomenon at small grain sizes has a significant effect on both strength and ductility, illustrated by an increase in boundary (Hall–Petch) strengthening and an increase in the total elongation to 36.6%. These results demonstrated a positive effect of a superposition of the transitions on both the strength and ductility of Mg-3Gd.
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Understanding the interaction between void and grain boundary (GB) is important to the design of radiation resistant materials by GB engineering and to achieve high quality metallurgical diffusion ...joining. In this study, the interaction between nano-voids and GBs has been systematically investigated by molecular dynamics simulations. The bicrystal Cu sample was used throughout the work, and the dynamic GB-void interaction was achieved by GB migration under shear deformation. Both high-angle GBs (Σ5 (310) GB, Σ5 (210) GB) and low-angle GBs (Σ37 (750) GB, Σ61 (650) GB) were investigated, and the effect of void size and temperature on the simulation result was examined. The transition of the deformation mechanism from GB migration to dislocation propagation was observed during the interaction between voids and high-angle GBs at low temperature (T = 10 K). At higher temperature (T = 300 and 600 K), the migrating GB can be pinned to voids, freely traversed voids, or dissolved voids in the process of their interaction. The void-drag effect on GB motion was analyzed based on the Zener-like equation, which indicates that the retarding pressure applied to the migrating GB by a void is closely related to the surface area of the void, the degree of contact between GB and void, and GB energy. By investigating the thermal stability of a void at the stationary GB, it was found that the dissolution of voids at a moving GB cannot be attributed solely to the thermal diffusion mechanism. The dynamic migration of high-angle GBs can significantly accelerate the dissolution time of the void. Atomistic analysis indicated that the migrating GB rearranged the atoms on the void surface by the collective motion of structural units, and the GB structural phase transformation provided an efficient diffusion channel for transporting the vacancies. The low-angle GBs show a reduced ability to dissolve the voids than the high-angle GBs, which can be ascribed to their low GB energy and diffusion coefficient, the fast GB migration velocity, and the discrete GB structure.
Dynamic interaction of shear-coupled grain boundary motion and nano-voids. Display omitted
The presence of a dislocation structure associated with low-angle dislocation boundaries and interior dislocations is a common and characteristic feature in nanostructured metals produced by plastic ...deformation, and plays an important role in determining both the strength and ductility of the nanostructured metals. The dislocation structure can be modified by post-process annealing and deformation which points to new ways of optimizing the mechanical properties. Such ways are demonstrated and discussed.
The microstructure and chemical composition of a 316L stainless steel prepared by selective laser melting have been characterized using electron backscatter diffraction, transmission electron ...microscopy and atom probe tomography (APT). A multi-scale microstructure in the 316L stainless steel was observed in the as-built samples, consisting of equiaxed and columnar grains, dislocation cell blocks, dislocation cells, individual dislocations and nano-sized particles. The misorientations across dislocation cells were determined based on local crystallographic orientation measurements using a Kikuchi pattern method. The dislocation cells have very small misorientation angles with an average angle of 0.9°, and are arranged to form dislocation cell-blocks with cell-block boundary misorientation angles generally larger than 2°. APT data reveal that alloying elements are evenly distributed in the matrix as well as a high nitrogen content in the as-built material. Based on quantification of the microstructural parameters, good agreement is achieved between the yield strength as calculated from a linear sum of different strengthening contributions, and the experimentally measured value, with significant contributions from dislocation strengthening and solid solution strengthening effects.
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