ABSTRACT
In order to obtain the good match between yield strength and low-temperature toughness, the short-time partial austenitization (SPA) process was employed for V/Mo-bearing 0.22C-5.24Mn steel. The initial microstructure after intercritical tempering was dual-phase ferrite and reversed austenite (RA), while the final microstructure consisted of ferrite, RA, and secondary martensite (SM) after being subjected to the SPA process. (V, Mo)C with disclike morphology mainly precipitated during intercritical tempering, and the aspect ratio of particles decreased, leading to the appearance of near-spherical morphology. After being subjected to SPA process, the resultant multiphase hierarchical microstructure (three layers: outer layer of ferrite, interlayer of SM, and inner layer of RA) enabled a high yield strength of 1097 MPa, a total elongation of 14%, and an impressive impact energy of 33.3 J at -20 °C. The strengthening contribution of (V, Mo)C precipitation was estimated to be about 108 MPa.
ABSTRACT
P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 â O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na+. Moreover, the as-prepared MgO-coated Na0.67Ni0.17Co0.17Mn0.66Mg0.1O2 exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g-1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g-1 within 2-4.5 V, which has been obviously enhanced than that of Na0.67Ni0.17Co0.17Mn0.66O2. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.
ABSTRACT
A cyclic wet/dry accelerated corrosion test was used to compare the corrosion behavior of medium-Mn steel and Q345 steel. In terms of scanning electron microscope (SEM), using X-ray diffraction (XRD), electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS), and analysis of the corrosion process, the results showed that the medium-Mn steel did not exhibit higher corrosion resistance than Q345 steel due to the greater content of Mn-rich compounds in the rust layer. Moreover, the effect of a small amount of anti-corrosion elements in medium-manganese steel can regulate the corrosion rate. The conceptual model of the corrosion process of the medium-Mn steel in a 3.5 wt % NaCl solution is proposed.
