One-dimensional ledges and migration mechanism of incoherent interphase boundaries

Since the edge-to-edge matching relationship of close-packed planes on an incoherent interphase boundary was found, the one-dimensional ledge migration mechanism has been put forward. However, owing to the lack of direct experimental evidence, the existence of the one-dimensional ledge is still questioned and it is thus usually treated as just an assumption. In this study, focusing on the existence of one-dimensional ledges and the migration mechanism of incoherent interphase boundaries, an atomic scale investigation on the migration of incoherent interphase boundaries in a body- to face-centered cubic transformation has been carried out using the phase-field crystal model. Simulation results demonstrated the presence of one-dimensional ledges on incoherent interphase boundaries, but only on those boundaries with high atomic densities. The simulation results further showed that the interphase boundaries with one-dimensional ledges migrate as a result of the nucleation and extension of the one-dimensional ledge, similar to the mechanism for two-dimensional ledges; meanwhile the interphase boundaries without one-dimensional ledges migrate according to a continuous mechanism by random atomic jumping. Because it is difficult for one-dimensional ledges to nucleate under low driving forces, interphase boundary migration based on the one-dimensional ledge mechanism is slower than that based on the continuous mechanism. This study reveals the structures and mechanisms of complex transitions of incoherent interphase boundaries and can aid a deeper understanding of solid phase transformations.

Investigation of long-term aging of high-temperature Nb-matrix composite material reinforced by α-Al2O3 fibers

Solving the problems when developing high-temperature composite materials (HTCM) requires non-standard approaches, as for example, using the long-term high-heat treatment (HHT), which has a significant effect on mechanical properties of the HTCM at high temperatures. To obtain a niobium-based HTCM reinforced with α-Al2O3 single crystal fibers (“Nb – SCF α-Al2O3”) hot pressing technique was used in the research. The HHT effect at 1350 °C of the HTCM on its high-temperature (1300 °C) bending strength, hardness and density at 22 °C after 100 hours HHT with a step in 25 hours was investigated. The samples structure and elements distribution at the interfacial boundaries of HTCMs were studied. It was established that the elements interdiffusion width at the interphase boundary of the continuous composition “Nb – SCF α-Al2O3” doesn’t exceed 2 μm for the whole HHT term; in outside the interphase boundary, only niobium oxides and carbides were detected. It was found that the bending strength after 25 hours HHT slightly exceeded the strength of the initial sample (before HHT); with further high-temperature HHT, the strength increased by 1.7 – 2 times in comparison with the initial sample. The hardness (HV 0.5) after 25 hours HHT remained actually unchanged (70), and subsequently sharply increased and the average hardness in three aging stages (50, 75 and 100 hours) was 330. The density of HTCMs increased with HHT, and after 100 hours HHT increased in comparison with the original sample by 1.3 times.