Papers by Keyword: Nucleation Sites

Abstract: The influence of grain refiner on the microstructure and mechanical properties of pure aluminum was investigated by separate and combined addition trace ScZr elements. The results show that the grain refinement effect of 0.2%Zr added alone is better than that of 0.2%Sc, each of them separate addition can make the strength and hardness of alloys increased obviously, and the decrease of the ductility is not remarkable, but the hardness increase of the alloy with 0.2%Sc is more significant than 0.2%Zr because of the strongly solid solution strengthening of Sc. While, the combined addition of 0.2%Sc and 0.2%Zr possesses the most excellent grain refining effect and change the growth morphology from columnar grains to equiaxed ones, the average grain size of the alloy is dropped to only about 100μm, and the strength, plastic and hardness of the alloy meet the reasonable matching, which is largely attributed to the effectively reduce of the Al-Sc eutectic point and the formation of Al₃(Sc, Zr) composite particles, These Al₃(Sc, Zr) particles can not only keep all the beneficial effect of A1₃Sc but can act as extremely effective nucleation sites of α (Al) and cause strongly fine-grain strengthening effect.

Abstract: The Fe-V-Nb-RE(Ce) alloy was rapidly solidified with melt-spin method, and the nano-sized ribbon was gained with the rolling wheel speed of 45m/s.The microstructure and the grain size of the Fe-V-Nb-RE(Ce) inoculants were performed by SEM and XRD respectively. Subsequently the microstructure of the steel modified by the Fe-V-Nb-Re(Ce) inoculants was also studied. The results show that the average grain size of the nano-scale Fe-V-Nb-RE(Ce) inoculants obtained is 26.1nm and a large mounts of nucleation sites in the steel exist in the form of (V, Nb)C and FeCeSi which effectively increases the nucleation rate and bring about the refinement of austenite grains.

Abstract: We investigated the expansion of single Shockley stacking faults (SSFs) in a 4H-SiC epitaxial layer under high-intensity scanning laser beam during room temperature photoluminescence mapping, which is similar to the degradation of bipolar pin diodes during forward current injection. In an epitaxial layer on an 8 off-axis (0001) substrate, the SSF-related intensity patterns induced by scanning high-intensity laser beam were classified into two types. The first one was a triangular pattern and the second a pattern which expanded in accordance with the motion of the scanning laser beam. The origins of the SSFs responsible for both patterns are presumably due to the preexisting basal plane dislocations and the dislocation-loops on the basal plane in the epitaxial layer, respectively. On the other hand, most of the SSF-expansion in on-axis (11 2 0) epitaxial layers were similar to the second type in the (0001) epitaxial layer. We, therefore, suggest that the dislocation-loops, which were located close to the surface, were dominant nucleation-sites of the SSFs in the (11 2 0) epitaxial layers.

Abstract: In extra-low carbon steels, the generation of specific nucleation sites in the deformed microstructure determines the formation of an optimum recrystallization texture. In particular, during the cold rolling of the steel sheets, transition bands are generated in order to accommodate the different deformation paths followed by the deformation bands (DB-s) within the grains. -fibre grains (ND-fibre grains) are, in general terms, more fragmented than -fibre grains (RD-fibre grains). Consequently, the higher orientation gradients and stored energy levels of the -fibre grains determine the ND-fibre annealing texture. Nevertheless, during recrystallization of the ELC steel, nucleation in different type of transition bands (TB-s) between α-fibre components has also been observed in the present work. From a previous crystallographic classification of transition bands done by the authors, the effect of the lattice curvature, either by gradual or sharp orientation gradients, is studied during early recrystallisation stages. Specifically, the crystallographic characteristics of recrystallized nuclei formed at transition bands generated between α-fibre components are analyzed using electron back-scatter diffraction (EBSD). This technique enables the orientation of deformation bands, the misorientation across them, the orientation of the new recrystallized grains and the misorientation of those grains with the adjacent matrix grains to be determined. Recrystallization of components different to α at the expense of α components has also been found.

Abstract: The nucleation sites of stacking faults (SFs) during forward current stress operation of 4H-SiC PiN diodes were investigated by the electron beam induced current (EBIC) mode of scanning electron microscopy (SEM), and the primary SF nucleation sites were found to be basal plane dislocations (BPDs). Damage created on the diode surface also acts as SF nucleation sites. By using a novel BPD-free SiC epilayer, and avoiding surface damage, PiN diodes were fabricated which did not exhibit SF formation under current stressing at 200A/cm2 for 3 hours.

Abstract: During annealing of cold rolled steel sheets, the evolution of the texture, to produce the characteristic texture of a recrystallised material, depends on a series of factors, including the starting deformation texture, composition and processing conditions. In the present work, electron backscattered diffraction (EBSD) techniques have been used to investigate the deformation substructures being developed by cold rolling into grains belonging to different texture components. The strain localisation, the deformation banding, the spread in orientation into grains with different orientations are some of the aspects that have been considered. Annealing cycles at different temperatures have also been carried out in order to promote recovery and initial stages of recrystallisation . The obtained substructures have been compared to the as-cold rolled ones. It has been observed that recovery induces the dislocation structures to arrange into subgrains. Associated to the ND fibre, a network of relatively high angle boundaries develop within the subgrain structure. The recrystallisation nuclei have been observed to evolve from such a network.