Strength Variations during Mechanical Alloying Through the Nanostructural Range
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Nanocrystalline material
Nanometre
Length scale
Grain boundary strengthening
Strengthening by grain refinement via the Hall-Petch mechanism and softening by nanograin formation via the inverse Hall-Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall-Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, and iron produced by severe plastic deformation in the literature. Magnesium, aluminum, copper, and their alloys follow the Hall-Petch relationship with a low slope, but an up-break appears when the grain sizes are reduced below 500-1000 nm. This extra strengthening, which is mainly due to the enhanced contribution of dislocations, is followed by a down-break for grain sizes smaller than 70-150 nm due to the diminution of the dislocation contribution and an enhancement of thermally-activated phenomena. For pure iron with a lower dislocation mobility, the Hall-Petch breaks are not evident, but the strength at the nanometer grain size range is lower than the expected Hall-Petch trend in the submicrometer range. The strength of nanograined iron can be increased to the expected trend by stabilizing grain boundaries via impurity atoms. Detailed analyses of the data confirm that grain refinement to the nanometer level is not necessarily a solution to achieve extra strengthening, but other strategies such as microstructural stabilization by segregation or precipitation are required.
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Orowan strengthening,thermal mismatch strengthening and Hall-Petch strengthening are known as the main strengthening mechanisms of nano particle reinforced magnesium matrix composites.However,the distribution of the particles in the matrix has an important influence on the enhancement effect and determines the dominant mechanism.In this paper the existed strengthening models were modified.The influence of three types of nanoparticle distribution,intragranular,grain boundary and intragranular-boundary distribution,on the yield strength of the nano SiC particle reinforced AZ91D composites was analyzed based on the modified models.The calculated results were compared with the experimental results.The results show that the composite has the best strengthening effect when the particles are completely distributed within the grain and the dominant mechanism will be Orowan strengthening;the composite shows the least strengthening effect while the particles are fully distributed along the grain boundary and the main mechanism will be Hall-Petch strengthening;the multi strengthening mechanisms will work when the particles are distributed both in the grain and on the grain boundary,in which case the strengthening effects will be weakened as the proportion of the fraction of the particle inside the grain to that on the grain boundary decreases.
Grain boundary strengthening
Particle (ecology)
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Grain boundary strengthening
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Solid solution effects on the strength of the finest nanocrystalline grain sizes are studied with molecular dynamics simulations of different Cu-based alloys. We find evidence of both solid solution strengthening and softening, with trends in strength controlled by how alloying affects the elastic modulus of the material. This behavior is consistent with a shift to collective grain boundary deformation physics, and provides a link between the mechanical behavior of very fine-grained nanocrystalline metals and metallic glasses.
Nanocrystalline material
Solid solution strengthening
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Nanocrystalline material
Grain boundary strengthening
Severe Plastic Deformation
Deformation mechanism
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Grain boundary strengthening
Volume fraction
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An improved combination of strength and ductility is generally a trade-off relationship, and it remains a major research topic in the field of structural materials. A bimodal grained microstructure, consisting of a coarse-grained region "core" and a surrounding fine-grained region "shell", exhibits a good balance between strength and ductility. Therefore, the exact reason for the strengthening mechanism needs to be investigated. In the present study, we conducted nanomechanical characterization to evaluate the individual strengthening factors, including matrix strength (σ0), and grain boundary effect (k), in the Hall–Petch model for each region of the core and shell to clarify the strengthening mechanism in the bimodal grained microstructure. The nanoindentation technique was applied locally in the "grain interior" to evaluate σ0, and "on grain boundary" and "near grain boundary" to assess the grain boundary effect associated with the k value. The grain interior nanohardness was found to be higher in the core region than that in the shell, which is explained by the higher pre-existing dislocation density in the core region. The nanomechanical characterization of the "on grain boundary" and the "near grain boundary" regions show a higher barrier effect due to the grain boundary in the shell than that in the core, which is presumably dominated by the higher internal strain at the shell grain boundary. Furthermore, a Hall–Petch plot was constructed using nanohardness, Vickers hardness, and grain size to estimate the k value. The plot showed a higher k value in the shell, which is consistent with the higher strengthening effect of the shell grain boundary that is evaluated independently in the local region. Therefore, the macroscopic strength of the shell region is significantly affected by both grain boundary effect as well as fine grain size.
Grain boundary strengthening
Ductility (Earth science)
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Grain boundary strengthening
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Nanocrystalline material
Solid solution strengthening
Lattice (music)
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DD simulations are performed to unravel the coupled effects between grain boundary strengthening and Orowan strengthening. With the simulations of a plastically-deformed grain embedded in elastic matrix, the internal stresses associated with inter- and intragranular obstacles are quantified. First, plastic yielding is controlled by the length of dislocation sources. Then, in plastic deformation, the stress contribution of bypassing mechanism increases significantly with the size and number of precipitates, while the stress contribution of geometrically necessary dislocations at grain boundaries slightly decreases. Compared with regular spatial distribution of precipitates, random distribution induces a higher strengthening effect by reducing the dislocation mobility. Simulations of four-grain aggregates involving grain refinement and precipitation are systematically investigated. The volume fraction of precipitates is the key factor controlling the Orowan strengthening in addition to the grain size effect. A generalized Hall-Petch equation based on the mean free path of dislocations is proposed. At low strain, a relatively uniform internal stress field is found inside the aggregates with weak stress concentrations around grain boundaries and precipitates. The underlying mechanisms identified in this work are essential for understanding the plastic behavior of precipitate-strengthened polycrystals.
Grain boundary strengthening
Volume fraction
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