Constrained minimal-interface structures in polycrystalline copper with extremely fine grains

Abstract

Metals usually exist in the form of polycrystalline solids, which are thermodynamically unstable because of the presence of disordered grain boundaries. Grain boundaries tend to be eliminated through coarsening when heated or by transforming into metastable amorphous states when the grains are small enough. Through experiments and molecular dynamics simulations, we discovered a different type of metastable state for extremely fine-grained polycrystalline pure copper. After we reduced grain sizes to a few nanometers with straining, the grain boundaries in the polycrystals evolved into three-dimensional minimal-interface structures constrained by twin boundary networks. This polycrystalline structure that underlies what we call a Schwarz crystal is stable against grain coarsening, even when close to the equilibrium melting point. The polycrystalline samples also exhibit a strength in the vicinity of the theoretical value.

Lattice Phase Field Model for Nanomaterials

“…For the last case, consider a nano-scaled grain growth process under tensile stress. The nano-grained metals have attracted great attention due to their excellent properties for the structure designed [29,30]. Since the grain growth process is widely simulated Another advantage of exchange coupled composites is that the structure is easy to be designed for better performance.…”

Formation of Nanolaminated Structure with Enhanced Thermal Stability in Copper

“…As a result, stability of pure Cu increases with the decreasing grain size, as plotted in Figure 4 . The 10-nm Schwarz crystal in Cu is even stable against grain coarsening when close to the equilibrium melting point [ 24 ]. Although intensive boundary relaxation can be triggered by rapid heating [ 30 ], deformed grains of about 70 nm in size show the worst stability and grain coarsening occurs when annealed at temperature lower than 0.3 T m .…”

“…Further refining lamellae below the saturation size is challenging due to the increasing tendency of GB annihilation via dislocation annihilation and migration of GBs during straining [ 20 ]. By increasing strain rates and/or decreasing deformation temperature, grains of pure Cu can be refined to 40 nm [ 21 , 22 , 23 ] and even down to 10 nm [ 24 ]. However, the nanostructured Cu tends to show morphology of roughly equiaxed grains with plenty of twins and stacking faults rather than laminated structure as that in Ni [ 1 , 11 ] and Al [ 5 ].…”

‘Colorful’ Polyline Grain Boundaries in Two-dimensional Transition Metal Dichalcogenides

“…Fig. 3 | Structural evolution of the p-GBs in 2D MoSe2 . a, Proposed migrations of polygons along the p-GBs.…”

“…The average grain size and microstructures of the GBs are critical to material properties. Elemental and alloyed metals can exhibit greatly enhanced yield strength by optimizing grain sizes and boundaries [1][2][3] , while strong covalent crystals can be continuously hardened by decreasing grain sizes down to a few nanometers [4][5][6] . The GB effects become more significant in two-dimensional (2D) materials [7][8][9] , where the crystalline order is highly vulnerable to various types of disorder and can be directly disrupted by a line defect.…”