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In the present study, repetitive upsetting-extrusion (RUE) process was employed to induce severe plastic deformation (SPD) for an AZ91D magnesium alloy at temperatures ranging from 285 °C to 380 °C. Finite element method (FEM) and technical experiment were performed to reveal the non-uniform behavior during the RUE process. The results from the finite element analysis showed that effective strain was unevenly distributed in one-cycle strained sample and the non-uniformity would exist permanently with increasing deformation cycles. The microstructure evolution of the RUE processed alloys was characterized by optical microscopy, which clarified the microstructural development from heterogeneous and coarse as-cast structure, heterogeneous and refined structure to final homogeneous and fine-grained structure of the 340 °C and eight-cycle processed sample. In addition, a critical strain was suggested from the metallographic observation results. It was proposed that α -Mg grain structure was sufficiently refined by imposing accumulated strain beyond the critical value and the invariability of grain sizes was illustrated. A homogeneous structure with mean grain size of ∼3.04 μm was obtained after 285 °C and three-cycle RUE process, corresponding to an accumulated true strain of about 3.75. Furthermore, the fragmentation and dispersed distribution of the β -Mg 17 Al 12 phase induced by shearing was confirmed by referring to scanning electron microscope (SEM) examination. Moreover, refining mechanisms responsible for grain structure were discussed in detail with reference to the transmission electron microscope (TEM) analysis results.

LiNi0.8Co0.1Mn0.1O2 (NCM811) often suffers from the poor cycle stability as the cathode material for the lithium-ion battery (LIB), while Al2O3 or MgO coating is the most common method to resolve such problem. However, the reported techniques often produced Al2O3 or MgO particles with the large size or zero dimensional (0 D) spheres, which resulted in ununiform or thick layer coating on the surface of NCM811. Herein, Mg-Al layered double oxide (Mg-Al-LDO) nanosheets derived from Mg-Al layered double hydroxide (Mg-Al-LDH) were firstly studied as a potential coating material because of their two dimensions (2D) and few-layer structures (less than 10 nm). The Mg-Al-LDO nanosheets were coated on NCM811 microspheres via the intercalation-exfoliation-calcination strategy. The Mg-Al-LDO nanosheets modified NCM811 samples exhibited enhanced cycle performances. The capacity retention improvements of this work (11–18%) were higher than those of the reports (0.7–15.6%). Typical sample showed the capacity retention of 95.5% after 50 cycles, and it increased by 18% compared with that of pristine NCM811 after 100 cycles within 2.75–4.3 V at 0.5 C rate. This improved cycle performance can be attributed to the unique thin and continuous Mg-Al-LDO nanosheets coated on NCM811, which not only inhibits HF corrosion and the residual alkali salt formation, but also reduces Ni2+/Li+ cation mixing. This method provides a new strategy for the surface modification of nickel-rich layered cathode materials.