In situ atomic scale mechanisms of strain-induced twin boundary shear to high angle grain boundary in nanocrystalline Pt

Twin boundary can both strengthen and soften nanocrystalline metals and has been an important path for improving the strength and ductility of nano materials. Here, using in-lab developed double-tilt tensile stage in the transmission electron microscope, the atomic scale twin boundary shearing process was in situ observed in a twin-structured nanocrystalline Pt. It was revealed that the twin boundary shear was resulted from partial dislocation emissions on the intersected {111} planes, which accommodate as large as 47% shear strain. It is uncovered that the partial dislocations nucleated and glided on the two intersecting {111} slip planes lead to a transition of the original symmetric tilt ∑3/(111) coherent twin boundary into a symmetric tilt ∑9/(114) high angle grain boundary. These results provide insight of twin boundary strengthening mechanisms for accommodating plasticity strains in nanocrystalline metals

Defects and Their Elemental Distributions in a Crept Co-Al-W-Ti-Ta Single Crystal Superalloy

The structure of defects and their elemental distributions is the decisive factor to metal’s mechanical properties. The γ/γ′ structure Co-Al-W-based superalloy is a potential replicable alloy to Ni-based superalloys that have been used for a long time. Revealing the microstructure of a defect induced by high-temperature crept provides direct clues for the alloy development. Our work revealed the high-resolution, high-angle annular dark field images and corresponding elemental distributions on crept defects. Defects in the γ phase mainly contain dislocation networks, while the γ′ phase contains lots of stacking faults and anti-phase boundaries. The results indicate that dislocation networks and stacking faults contain more weight elements than the base γ′ phase, while anti-phase boundaries contain more light elements. Disclosing atomic structure and its elemental distributions gives direct evidence for alloy design and further high-temperature mechanical improvement

Control Optimization of Small-Scale Thrust-Vectoring Vertical/Short Take-Off and Landing Vehicles in Transition Phase

The core of the short takeoff and landing problem in thrust-vectoring V/STOL vehicles is the tilt angle control of the thrust vector nozzles. This work resolves it by figuring out the optimal tilt angle time history with optimization methods. Since the optimization process is constrained by the transition corridor of the vehicle and the mission requirements, the transition corridor is firstly established by the AES theory with the longitudinal model of the V/STOL protype, where the jet-induced effect of the 3BSD nozzle and the lift fan are especially considered. In addition, the control redundancy caused by the multiple physical control actuators is addressed by a suitable control allocation and flight-mode-based control strategy, which ensures a smooth conversion. By establishing appropriate mission references and optimization constraints, the optimal control strategy and the corresponding transition process are obtained, based on the direct inverse and SQP algorithms

Control Optimization of Small-Scale Thrust-Vectoring Vertical/Short Take-Off and Landing Vehicles in Transition Phase

The core of the short takeoff and landing problem in thrust-vectoring V/STOL vehicles is the tilt angle control of the thrust vector nozzles. This work resolves it by figuring out the optimal tilt angle time history with optimization methods. Since the optimization process is constrained by the transition corridor of the vehicle and the mission requirements, the transition corridor is firstly established by the AES theory with the longitudinal model of the V/STOL protype, where the jet-induced effect of the 3BSD nozzle and the lift fan are especially considered. In addition, the control redundancy caused by the multiple physical control actuators is addressed by a suitable control allocation and flight-mode-based control strategy, which ensures a smooth conversion. By establishing appropriate mission references and optimization constraints, the optimal control strategy and the corresponding transition process are obtained, based on the direct inverse and SQP algorithms

Effects of TCP and creep cavity on creep life in the rafting regime for Ru-containing Nickel-based single crystal superalloys

Ru is crucial in improving creep properties in Nickel-based single-crystal superalloys. The effect of Ru content on the creep behavior of nickel-based single-crystal superalloy was studied. Ru exhibits unprecedented mechanical properties, increasing the creep life at 1120 °C but decreasing it at 1180 °C. This is due to the competition in the stress concentration formed between the topologically close-packed phases and the creep cavity. They are both related to the dissociation of the γ′ phase. The effect of oxidation caused by the addition of Ru is lower than these two factors. This work helps to recognize the comprehensive effect of Ru and can provide guidance for alloy design in the future

Effect of directional solidification methods on solid solution window in Ni-based single-crystal superalloy

In this work, the effect of directional solidification methods on solid solution windows in two Ni-based single-crystal superalloys is investigated. The sample prepared using the liquid metal cooling (LMC) method was found to contain smaller dendrite spacing, lower dendrite segregation and eutectic, and a smaller size of γ′ phase than that of the high-speed rapid solidification (HRS) method. The alloy-2 contained higher Cr, Co and lowered Re exhibited similar dendrite spacing and volume fraction of eutectic, and the lower dendrite segregation and smaller size of γ′ phase than that of the other alloy-1. The solution treatment window is broadened by adopting the LMC method or changing the composition to alloy-2. This is mainly realized by decreasing the lower limit temperature, and the upper limit temperature remains unchanged. According to the eutectic only contains the melt temperature, a similar eutectic is responsible for the unchanged upper limit temperature. At this state, the lower bound of the temperature is the dissolution of the γʹ phase in the dendrite core. The smaller size of γ′ phase and the elements' partition ratio are responsible for the change in the solid solution window

Reveal the size effect on the plasticity of ultra-small sized Ag nanowires with in situ atomic-scale microscopy

Revealing the atomic-scale deformation mechanisms of metallic nanowires (NWs) is important for their practical application. However, there are few reports providing direct atomic-scale experimental elucidation on those metallic NWs. Here, we conduct serial in situ deformation tests on silver (Ag) nanowires with diameters of 3-11 nm. The in situ atomic-scale observations reveal a transition in the deformation mechanism with a decrease in the diameter of Ag NWs. For the [5 (5) over bar4] and [001] oriented NWs with diameters of similar to 11 nm, the plastic deformation is dominated by full dislocation that involves leading and trailing partial dislocations, whereas the full or extended dislocations are rarely observed in the NWs with diameters in the range of similar to 5-8 nm, and their plastic deformation is governed by SF generation and annihilation. Moreover, for the [(1) over bar 11] oriented NWs, 60 degrees mixed and pure edge dislocations are frequently observed when the diameter is approximately 5 nm and the plastic deformation is accommodated by relative slip between two adjacent {111} planes for NWs with diameters below similar to 3 nm. These results indicate that the plastic deformation not only depends on the size of NWs but also can be significantly impacted by the loading orientation. (C) 2016 Elsevier B.V. All rights reserved