RESUMO
The mechanical behavior of titanium alloys has been mostly studied in quasi-static conditions when the strain rate does not exceed 10 s-1, while the studies performed in dynamic settings specifically for Ti-based composites are limited. Such data are critical to prevent the "strength margin" approach, which is used to assure the part performance under dynamic conditions in the absence of relevant data. The purpose of this study was to obtain data on the mechanical behavior of Ti-based composites under dynamic condition. The Metal Matrix Composites (MMC) on the base of the alloy Ti-6Al-4V (wt.%) were made using Blended Elemental Powder Metallurgy with different amounts of reinforcing particles: 5, 10, and 20% of TiC or 5, 10% (vol.) of TiB. Composites were studied at high strain rate compression ~1-3 × 103·s-1 using the split Hopkinson pressure bar. Mechanical behavior was analyzed considering strain rate, phase composition, microstructure, and strain energy (SE). It is shown that for the strain rates up to 1920 s-1, the strength and SE of MMC with 5% TiC are substantially higher compared to particles free alloy. The particles TiC localize the plastic deformation at the micro level, and fracturing occurs mainly by crushing particles and their aggregates. TiB MMCs have a finer grain structure and different mechanical behavior. MMC with 5 and 10% TiB do not break down at strain rates up to almost 3000 s-1; and 10% MMC surpasses other materials in the SE at strain rates exceeding 2200 s-1. The deformation mechanism of MMCs was evaluated.
RESUMO
Solar cells containing a polycrystalline Cu(In,Ga)Se2 absorber outperform the ones containing a monocrystalline absorber, showing a record efficiency of 22.9%. However, the grain boundaries (GBs) are very often considered to be partly responsible for the enhanced recombination activity in the cell and thus cannot explain the registered record efficiency. Therefore, in the present work, we resolve this conundrum by performing correlative electron beam-induced current-electron backscatter diffraction investigations on more than 700 grain boundaries and demonstrating that 58% of the grain boundaries exhibit an enhanced carrier collection compared to the grain interior. Enhanced carrier collection thus indicates that GBs are beneficial for the device performance. Moreover, 27% of the grain boundaries are neutral and 15% are recombination-active. Correlation with microstructure shows that most of the ∑3 GBs are neutral, whereas the random high-angle grain boundaries are either beneficial or detrimental. Enhanced carrier collection observed for a big fraction of high-angle grain boundaries supports the "type-inversion" model and hence the downward band bending at GBs. The decrease in current collection observed at one of the high-angle grain boundaries is explained by Cu being enriched at this GB and hence by the upward shift of the valence band maximum.
