Papers by Author: X.Y. Ma

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: Nanocrystalline Fe-1.77at.%C and Fe-3.27at.%C alloys prepared by ball milling iron powders and graphite powders are annealed below 573K where the precipitation of Fe₃C does not occur. Upon annealing, a significant grain coarsening is observed in Fe-1.77at.%C alloy, whereas the grain coarsening is inhibited in Fe-3.27at.%C alloy. Within the framework of thermodynamic theories, the inhibition of grain coarsening in nanocrystalline Fe-C alloys is discussed. It is demonstrated that the inhibition of grain coarsening in the nanocrystalline Fe-C alloys can be ascribed to a vanished driving force for grain growth which is caused by the interaction between carbon and the grain boundaries of nanograins.

Abstract: In this study, we performed a statistical analysis of 500 Ni Schottky diodes distributed across a 2-inch, n-type 4H-SiC wafer with an epilayer grown by chemical vapor deposition. A majority of the diodes displayed ideal thermionic emission when under forward bias, whereas some diodes showed ‘double-barrier’ characteristics with a ‘knee’ in the low-voltage log I vs. V plot. X-ray topography (XRT) and polarized light microscopy (PLM) revealed no correlations between screw dislocations and micropipes and the presence of double-barrier diodes. Depth resolved cathodoluminescence (DRCLS) indicated that certain deep-level states are associated with the observed electrical variations.

Abstract: We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) to correlate subsurface deep level emissions and double barrier current-voltage (I-V) characteristics across an array of Ni/4H-SiC diodes on the same epitaxial wafer. These results demonstrate not only a correspondence between these optical features and measured barrier heights, but they also suggest that such states may limit the range of SB heights in general. DRCLS of near-ideal diodes show a broad 2.45 eV emission at common to all diode areas and associated with either impurities or inclusions. Strongly non-ideal diodes exhibit additional defect emissions at 2.2 and 2.65 eV. On the other hand, there is no correlation between the appearance of morphological defects observed by polarized light microscopy or X-ray topography and the presence of double barrier characteristics. The DRCLS observations of defect level transitions that correlate with non-ideal Schottky barriers suggest that these sub-surface defect features can be used to predict Schottky barrier behavior.