Dislocation Structures in Low-Angle Grain Boundaries of α-Al2O3

Time-Dependent Evolution of Al-Al4C3 Composite Microstructure and Hardness during the Sintering Process

In this study, Al-Al4C3 compounds were manufactured by mechanical milling followed by heat treatment. To analyze the microstructural evolution, the composites were sintered at 550 °C at different sintering times of 2, 4 and 6 h. The mechanical results suggest that dislocation density and crystallite size primarily contribute to hardening before the sintering process, with a minimal contribution from particle dispersion in this condition. The compound exhibited a significant 75% increase in hardness after 2 h of sintering, primarily attributed to the nucleation and growth of Al4C3 nanorods. The HRTEM analysis, combined with geometric phase analysis (GPA) at and near the Al-Al4C3 interface of the nanorods, revealed strain field distributions primarily associated with partial screw dislocations and the presence of closely spaced dislocation dipoles. These findings are consistent with the microstructural parameters determined from X-ray diffraction pattern analysis using the convolutional multiple whole profile (CMWP) method. This analysis showed that the predominant dislocation character is primarily of the screw type, with the dislocation dipoles being closely correlated. Based on these results, it is suggested that samples with a lower weight percentage of reinforcement and longer sintering times may experience reduced brittleness in Al/Al4C3 composites. Strengthening contributions were calculated using the Langford-Cohen and Taylor equations.

Effect of Deep Cryogenic Treatment on Corrosion Behavior of AISI H13 Die Steel

AISI H13 die steel specimens were subjected to heating at 1020 °C followed by oil quenching and double tempering at 520 °C. Subsequently, these specimens were subjected to deep cryogenic treatment at −185 °C in liquid nitrogen environment for 16 h and then subjected to soft tempering at 100 °C once the specimens attained room temperature. Thereafter, the specimens were subjected to scanning electron microscopy (SEM) analysis and electron backscatter diffraction (EBSD) analysis. The electrochemical corrosion activity was investigated in 3.5% NaCl at 23 ± 0.5 °C by evaluating the evolution of open circuit potential over time and potentiodynamic curves, and electrochemical impedance spectroscopy study was also carried out. The heat-treated specimens exhibited better resistance to corrosion through more electropositive values of open circuit potential. This could be attributed to lower grain boundary area in heat-treated specimens as compared to 16 h cryogenically treated specimen as higher grain boundary areas behave as an anode in an electrochemical cell, thereby enhancing the rate of corrosion. According to electrochemical tests, the cryogenically treated surface is more resistant to corrosion, followed by heated alloy. However, both surface modification treatments improved the corrosion behavior of the untreated alloy.

Atomic structures of twin boundaries in CoO

The twinning plane of crystals with a face-centered-cubic (FCC) structure is usually the (111) plane, as found in FCC metals and oxides with FCC sublattices of oxygen, like rock-salt-type NiO and spinel-type Fe₃O₄. Surprisingly, we found in this work that the twinning plane of rock-salt-type CoO is the (112) plane, although Co is adjacent to Ni in the periodic table. The atomic and electronic structures of the CoO(112) twin boundary with in-plane shift vector 1/2[111] have been studied combining aberration-corrected scanning transmission electron microscopy (STEM), electron-energy-loss spectroscopy (EELS), and density functional theory (DFT) calculations. It was found that the atoms at the twin boundary have nominal oxidation states, and the twin boundary remains insulating and antiferromagnetically coupled. Importantly, through the electronic structures and the crystal orbital Hamilton population (COHP) analyses, the (112) twin boundary is found to be more stable than the (111) twin boundary.