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ACTA

In sensitized (700°C for 10 h) AISI 316L stainless steel samples attacked by an HF solution, coincidence site lattice (CSL) ∑7 grain boundaries (GBs) always show corrosion while ∑5 GBs never do. This distinct corrosion behaviour can be related to the distinct precipitation behaviour of the different GBs, which itself depends on the GB characters. It is found that ∑5 GBs, although being curved both macro- and microscopically, neither show facets nor precipitates. The amount of segregation is enhanced in sensitized ∑5 GBs compared to solution-annealed ones. In contrast, ∑7 GBs show clear faceting in both the solution-annealed and sensitized state, with one facet orientation showing strong segregation while the other not. GB energy anisotropy is applied to explain the faceting phenomenon. The formation of two kinds of precipitates, i.e., C14 Laves phase and M₂₃C₆ carbide, holding responsible for two distinct morphologies of corrosion, are believed to be both linked to intensive faceting and the anisotropic segregation of ∑7 GBs. For the carbide, a preferred {111} orientation out of all ∑7 facets measured by trace analyses is presumed to facilitate its nucleation.

ACTA

Nucleation in an aluminium tricrystal cold rolled along its columnar direction, to 40% thickness reduction, is studied. This experimental configuration was used to obtain the same deformation microstructure through the sample length, which made it possible to cut it into several slices of similar microstructures and use these slices differently. Some slices served to analyse the deformation microstructure and others to analyse the annealing microstructure. Nucleation developed only in one of the three crystals and not at grain boundaries. The relationship between the crystallographic orientations and the local density of nuclei and different attributes of the parent, deformation microstructure was then analysed. As generally presumed, the nuclei were observed to inherit orientations from the parent matrix. Much more surprisingly, the stored energy alone, which is often considered as the driving force for recrystallisation nucleation, was found not to provide a reliable criterion for recrystallisation nucleation in the investigated sample. Instead, the density of nuclei was the highest where the substructure is composed of sharp bands, which correspond to regions of highly anisotropic orientation distributions. A new energy criterion for recrystallisation nucleation is proposed, which is called “primary stored energy” and depends on the stored energy and the anisotropy of the orientation distribution.

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A CrCoNi based medium entropy alloy with small additions of Ti, Al and Nb (denoted as (CrCoNi)₉₃Al₄Ti₂Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Dark field transmission electron microscopy (TEM), atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with atom probe tomography (APT) show that spatially-localized long range ordering (LRO) L1₂ domains exist in this alloy. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and STEM based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2<110>dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)₉₃Al₄Ti₂Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of LRO domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of LRO domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

ACTA

向上滑動閱覽英文摘要

Phase evolution in FCC metals with strongly interacting alloy components during severe plastic deformation is investigated using molecular dynamics simulations. Specifically, we study the alloy microstructure in steady state, nucleation and growth of precipitates in supersaturated alloys, and the dissolution of precipitates in undersaturated alloys. The results are compared to a modified effective temperature model, providing a physical understanding for the atomic processes underlying the model and a perspective on its strengths and weaknesses. Key observations in this work are nucleation and growth of precipitates during SPD at a temperature of 100 K; Gibbs-Thomson-like behavior relating steady-state solubility to precipitate size under sustained shearing; a direct relationship between the effective temperature and the shear modulus; and the importance of cluster agglomeration during precipitate growth. The study also reveals that the mechanism of forced chemical mixing depends on precipitate size, adding complications for effective temperature models describing inhomogeneous systems. The simulations are shown to provide good semi-quantitative agreement with experimental findings reported in the literature.

ACTA

向上滑動閱覽英文摘要

Anisotropic tensile behaviors of a heterogeneous nanostructured Cu composed of isotropic nanograin matrix and anisotropic nanotwin bundles were investigated under loading directions parallel, normal, and 45° inclined to the twin boundaries (TBs). Distinct from the anisotropic strengthening effect in the homogeneous nanotwinned (NT) counterpart, the heterostructure exhibits the highest strength under parallel tension, and moderate strength under normal tension. High-resolution digital image correlation analysis indicated an orientation-dependent deformation compatibility between the nanotwin bundles and nanograin matrix, i.e., compatible deformation in the parallel orientation; but significant incompatibility in orientations both normal and 45° inclined to TBs. The results reveal that the strengthening effect of nanotwins in heterogeneous nanostructure is not only dependent on the strength of themselves, but also influenced by the deformation compatibility with surrounding components. The orientation-dependent deformation compatibility was attributed to the interactions between isotropic shear bands in nanograin matrix and anisotropic deformation in nanotwin bundles.

Vol. 210，15 May. 2021, 116844