Papers by Keyword: Lattice Defect

Abstract: Hydrogen embrittlement (HE) is increasingly becoming a critical issue for using high-strength steels in the automotive and infrastructure industries. To overcome the risk posed by HE of structural components under a hydrogen uptake environment in long-term service, it is necessary to clarify the mechanism of HE. In the present study, the presence of hydrogen-enhanced strain-induced vacancies (HESIVs)—one type of defect associated with proposed HE mechanisms—was validated by low-strain-rate tensile tests with in-situ electrochemical hydrogen charging for tempered martensitic steel showing quasi-cleavage fracture with a tensile strength. The effect HESIVs on the mechanical properties of tempered martensitic steel was also studied. The combined use of low-temperature thermal desorption spectroscopy and tensile tests led to the following observations: (i) hydrogen enhanced the accumulation of vacancy-type defects under plastic strain, (ii) accumulated vacancy-type defects adversely affected the ductility of the tempered martensitic steel after hydrogen release, and (iii) aging at 150 °C after applying a given plastic strain with hydrogen charging decreased the amount of newly formed vacancy-type defects and resulted in recovery of ductility.

Abstract: Work hardening is one of the most widely used methods in strengthening metals by increasing dislocation density, which can be achieved by raising plastic strain and/or suppressing dynamic recovery of the dislocations upon plastic deformation. Based on the analyses on the data reported in our previous work in cold-rolled Pd-H system (Scripta Materialia, Vol. 68 (2013), p. 743), we propose a new strategy in hardening Pd using hydrogen as a catalytic element. It is shown that since the introduction of hydrogen facilitates dislocation formation and increases the dislocation density in Pd upon plastic deformation, subjected to a same deformation level and subsequent removal of hydrogen, Pd can obtain a higher hardness compared to that without hydrogenation before deformation. It is further pointed out that the proposed strategy may, in addition, be applied to other metals, which can dissolve a relatively large amount of hydrogen, e.g. magnesium, nickel and niobium.

Abstract: Surface roughening regions running like scratches are often observed locally after epitaxy film grown on a very flat 4H-SiC wafer surfaces. We investigated generation mechanism of such roughening surface by using X-ray topography and confocal optical microscopy. We found that lattice defects were often introduced during CMP at local regions, and those local regions cannot be recognized by optical microscopy, since very flat surface can be observed. By H₂ etching which is preprocess of epitaxy film growth, those lattice defects are almost etched off, but local rough surface consists of pits and step bunching regions appear like scratches, and those local pits and surface roughening regions grew up to step bunching during epitaxy film growth.

Abstract: Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.

Abstract: The M1.99MgSi2O7: Ce3+0.01 (M: Ba, Sr, Ca) phosphors were prepared by the solid-state reaction method. All the samples emit the violet-ultraviolet light with a broad emission band from about 330 nm to 500 nm. All the phosphors samples show a long afterglow. The strongest afterglow intensity and the longest decay duration of the afterglow come from the Ca1.99MgSi2O7: Ce3+0.01 sample. It is attributed to the suitable trap depth and the high trap concentration of this sample. The traps of these phosphors are induced by the lattice defects. A suitable model involving the cation vacancies and the oxygen vacancies, which act as the hole traps and the electron traps respectively, is proposed.

Abstract: Effects of lattice defects on cathode properties of LiMn2O4 synthesized at low temperatures were investigated. LiMn2O4 powders were synthesized by a sol-gel method. The specific capacities of LiMn2O4 decreased from 134 to 81 mAh g-1 with decreasing heating temperature from 750 to 200°C. X-ray absorption spectroscopy showed that a large amount of lattice defects such as cation vacancies existed and cation mixing occurred in LiMn2O4 calcined at low temperatures. It was found that the low specific capacities of LiMn2O4 calcined at low temperatures were attributed to these lattice defects.

Abstract: The recent evolution of powder diffraction line profile analysis toward full pattern methods is discussed. Specific reference is made to the Whole Powder Pattern Modelling (WPPM), as applied to metals and ceramics subjected to strong plastic deformation. Examples concerning three different materials science studies are shown to illustrate features and potentialities of the WPPM approach.

Abstract: A magnetic technique was applied to examine cyclic deformation behavior and deformation damage in ordered FeAl and Ni3(Al,Ti) single crystals. The fatigue lifetime of FeAl was evaluated by an abrupt increase in spontaneous magnetization. The morphology of γ precipitates in Ni3(Al,Ti) during annealing and deformation was examined by changes in spontaneous magnetization and coercive force.