A New Method for Research of Grown-In Microdefects in Dislocation-Free Silicon Single Crystals

As a virtual experimental device for analysis and calculation of grown-in microdefects formation in undoped silicon dislocation-free single crystals the software is proposed. The software is built on the basis on diffusion model of formation, growth and coalescence of grown-in microdefects. Diffusion model describes kinetics of defect structure changes during cooling after growth on crystallization temperature to room temperature. The software allows the use of personal computer to investigate the defect structure of dislocation-free silicon single crystals with a diameter on 30 mm to 400 mm grown by floating-zone and Czochralski methods.

Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

The microdefects and free electron densities in B2,R and B19’ phases of Ni50.78Ti49.22 alloy were studied by positron lifetime measurements.Comparing the lifetime parameters of the Ni50.78Ti49.22 alloy measured at 295 K and 225 K,it is found that the free electron density of the R phase is lower than that of the B2 phase;the open volume of the defects of the R phase is larger,while the concentration of these defects is lower than that of the B2 phase.The Ni50.78Ti49.22 alloy exhibits B19’ phase at 115 K.In comparison with the R phase,the free electron density of the B19’ phase increases,the open volume of the defects of the B19’ phase reduces,and the concentration of these defects increases.The microdefects and the free electron density play an important role during the multi-step transformations(B2→R→B19’ phase transformations)in Ni50.78Ti49.22 alloy with the decrease of temperature.

Effects of B, Zr and Si on Microdefects in FeAl Alloys

The long-range ordered intermetallic compounds FeAl alloys with B2 crystalstructure are characterized by superior high-temperature strength, excellent oxidation re-sistance, low density and low cost, so that they are available for the futurehigh-emperature structure materials. But the room temperature brittleness of FeAl al-loys has been the major obstacle for its engineering applications. The binary FeAl withmore than 40 at.% Al content showed mainly intergranular fracture. The cohesions

Influence of Fe on electronic density of defects in NiAl alloys

NiAl alloy is considered to be a good candidate material for high performance, high-tempera-ture structural applications since this material offers a wide range of attractive mechanical andphysical properties, such as low density (5. 86 g/cm^3), high melting point (1 640℃), goodthermal conductivity (4--8 times compared with Ni-based superalloys) and oxidation resis-tance. However, the use of NiAl is limited by its room-temperature brittleness. Georgeand Liu have investigated the effects of microalloying elements B, C and Be on the fractureproperties and grain boundaries of NiAl alloy. Their results showed that the improvement

Effects of Nb and Si on densities of valence electrons in bulk and defects of Fe_3Al alloys

Positron lifetime measurements have been performed in binary Fe3Al and Fe3Al doping with Nb or Si alloys. The densities of valence electrons of the bulk and microdefects in all tested samples have been calculated by using the positron lifetime parameters. Density of valence electron is low in the bulk of Fe3Al alloy. It indicates that, the 3d electrons in a Fe atom have strong-localized properties and tend to form covalent bonds with Al atoms, and the bonding nature in Fe3Al is a mixture of metallic and covalent bonds. The density of valence electron is very low in the defects of Fe3Al grain boundary, which makes the bonding cohesion in grain boundary quite weak. The addition of Si to Fe3Al gives rise to the decrease of the densities of valence electrons in the bulk and the grain boundary thus the metallic bonding cohesion. This makes the alloy more brittle. The addition of Nb to Fe3Al results in the decrease of the ordering energy of the alloy and increases the density of valence electron and the bonding cohesion of the grain boundary. However, since the radius of Nb atom is larger than that of Fe atom, when Nb atoms substitute for Fe atoms, they will distort the lattice and enlarge the volume of the lattice, which decreases the density of valence electron and the cohesion of metallic bond in the bulk of the alloy.