声学四极子拓扑绝缘体中的位错态

四极子拓扑绝缘体是人们提出的第一类高阶拓扑绝缘体,它具有量子化的四极矩而偶极矩为零.四极子拓扑绝缘体拓宽了传统的体-边对应关系,从而观察到了更低维度的拓扑边界态.最近,由局域在位错附近的拓扑缺陷态主导的体-位错对应关系引起了许多研究者的关注,其将晶格倒易空间的拓扑结构与位错态的出现联系起来.本文研究了声学四极子拓扑绝缘体中的位错态.在具有非平庸相的声学四极子拓扑绝缘体中嵌入部分具有平庸相的晶格,此时在由两种具有不同拓扑相晶格形成边界的角落处就会产生可以用1/2量化分数电荷表征的位错态.通过在系统内部引入缺陷,验证了此拓扑位错态的鲁棒性.此外,还证明了通过运用不同嵌入晶格的方式可以随意设计位错态的位置.本工作中研究的拓扑位错态拓宽了人工结构中高阶拓扑物态的种类,并为高阶拓扑绝缘体在声学中的应用(如声传感和高性能能量收集)提供了新的思路.

Microstructure evolution and dislocation configurations in nanostructured Al-Mg alloys processed by high pressure torsion

Microstructure evolution and dislocation configurations in nanostructured Al–Mg alloys processed by high pressure torsion （HPT） were analyzed by transmission electron microscopy （TEM） and high-resolution TEM （HRTEM）. The results show that the grains less than 100 nm have sharp grain boundaries （GBs） and are completely free of dislocations. In contrast, a high density of dislocation as high as 1017 m^-2 exists within the grains larger than 200 nm and these larger grains are usually separated into subgrains and dislocation cells. The dislocations are 60° full dislocations with Burgers vectors of 1/2〈110〉and most of them appear as dipoles and loops. The microtwins and stacking faults （SFs） formed by the Shockley partials from the dissociation of both the 60° mixed dislocation and 0° screw dislocation in ultrafine grains were simultaneously observed by HRTEM in the HPT Al–Mg alloys. These results suggest that partial dislocation emissions, as well as the activation of partial dislocations could also become a deformation mechanism in ultrafine-grained aluminum during severe plastic deformation. The grain refinement mechanism associated with the very high local dislocation density, the dislocation cells and the non-equilibrium GBs, as well as the SFs and microtwins in the HPT Al-Mg alloys were proposed.

Dislocation mechanism for dynamic recrystallization in twin-roll casting Mg–5.51Zn–0.49Zr magnesium alloy during hot compression at different strain rates

Dislocation mechanism operating in dynamic recrystallization （DRX） during hot compression of Mg-5.51Zn-0.49Zr alloy was investigated by X-ray diffraction, optical microscopy and transmission electron microscopy. The results showed that the continuous DRX occurred at a low strain rate of 1×10^-3s^-1, which was associated with the operation of the single gliding dislocation climbing. At the intermediate strain rate of 1×10^-2s^-1, the continuous DRX was associated with the climbing of the gliding dislocation array as deformed at an elevated temperature of 350 ℃, and in contrast, the discontinuous DRX was observed and associated with the bulging of subgrain boundaries as the deformation temperature was raised to 400 ℃. The continuous DRX was associated with the climbing of the leading dislocation ahead of pile-ups, and resultant rearrangement of misorientated flat dislocation pile-ups as the strain rate was increased to 1×100s^-1. It is suggested that the mechanism predominating the dislocation climbing was changed from the vacancy migration to the stress acting on the leading dislocation ahead of the pile-up as the strain rate was gradually increased.

Hot deformation behavior of high Ti 6061 Al alloy

The hot deformation behavior of a high Ti 6061 aluminum alloy in the temperature range from 350 to 510 ℃ and strain rate range from 0.001 to 10 s^-1 was investigated using stress-strain curve analysis, processing map, transmission electron microscopy and electron backscatter diffraction analysis. The results show that the peak stress decreases with increasing deformation temperatures and decreasing strain rate. The average deformation activation energy is 185 kJ/mol in the parameter range investigated. The flow stress model was constructed. The main softening mechanism is dynamic recovery. The processing map was obtained using dynamic material model, and the suggested processing window is 400-440℃ and 0.001-0.1 s^-1.

Dislocation density model and microstructure of 7A85 aluminum alloy during thermal deformation

The microstructure evolution of 7A85 aluminum alloy at the conditions of strain rate(0.001−1 s^(−1))and deformation temperature(250−450°C)was studied by optical microscopy(OM)and electron back scattering diffraction(EBSD).Based on the K-M dislocation density model,a two-stage K-M dislocation density model of 7A85 aluminum alloy was established.The results reveal that dynamic recovery(DRV)and dynamic recrystallization(DRX)are the main mechanisms of microstructure evolution during thermal deformation of 7A85 aluminum alloy.350−400°C is the transformation zone from dynamic recovery to dynamic recrystallization.At low temperature(≤350°C),DRV is the main mechanism,while DRX mostly occurs at high temperature(≥400°C).At this point,the sensitivity of microstructure evolution to temperature is relatively high.As the temperature increased,the average misorientation angle(θˉ_(c))increased significantly,ranging from 0.93°to 7.13°.Meanwhile,the f_(LAGBs) decreased with the highest decrease of 24%.

Microstructure homogeneity regulation of 7050 aluminum forgings by surface cumulative plastic deformation

To regulate the microstructure homogeneity of large aluminum structural forgings for aircraft,the surface cumulative plastic deformation was proposed.The microstructure of 7050 aluminum forgings after the surface cumulative plastic deformation was investigated by electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and X-ray diffraction(XRD).The results showed that the microstructure evolution of 7050 aluminum forgings was more sensitive to the deformation temperature than the strain rate.The dislocation density continued to increase with the decrease of the deformation temperature and the increase of the strain rate.Dislocation density and stored energy were accumulated by the surface cumulative plastic deformation.Besides,a static recrystallization(SRX)model of 7050 aluminum forgings was established.The SRX volume fraction calculated by this model was in good agreement with the experimental results,which indicated that the model could accurately describe the SRX behavior of 7050 aluminum forgings during the surface cumulative plastic deformation.

Effect of Mg and semi solid processing on microstructure and impression creep properties of A356 alloy

The effects of Mg and semi solid processing on the creep properties ofA356 A1 alloy were investigated. The results show that the dislocation climb controlled creep is the dominant creep mechanism and it is not affected by the semi solid processing and further addition of Mg. Mg improves the alloy creep properties probably by forming large Chinese script Mg2Si compounds at the interdendritic regions. The semi solid processed specimens exhibit better creep properties in comparison with the as cast ones. It is attributed to the reduction in the stacking fault energy resulting from the significant dissolution of Mg in the a（A1） phase.

Research on dynamic model of rotors with bearing misalignment

Based on the structural and mechanics analysis of aero-engines rotor system, the dynamic model of the flexible rotor system with multi-supports are presented in order to solve the bearing misalignment problem of rotor system in aero-engines. The motion equations are derived through Lagrange method. The relationship between structural and mechanics characteristics parameters are built up. Finally, the dynamic influence of bearing misalignment on rotor system are divided into three kinds： additional rotor bending rigidity, additional bearing misalignment excitation force and additional imbalance. The equations suggest that additional imbalance excitation force activates the nonlinearity on rotor system and an additional 2x excitation force might appear.

Influence of structure transition on plastic behaviors of iron based ordered alloys

The influence of ordered structure on the dislocation configuration,structure of anti-phase domain boundary,partial dislocation slips,etc.are analyzed in the background of promoting the plasticity of iron based ordered solid solutions with second-order phase transformation.The principles of deformation softening and annealing hardening in ordered solid solutions are discussed because of deformation induced structure disordering.It is concluded that the independent slip ability of the partial dislocations and the corresponding low temperature plasticity of ordered solid solutions could be promoted obviously by proper alloying effects,which reduces the anti-phase domain boundary energy,or by maintaining the disordering state into the low temperature range.The similar principles could be also used to modify the low temperature plasticity of other metal based ordered solid solutions.