高强β钛合金热变形过程中组织演变及热加工图研究进展（英文）

高强度β钛合金广泛应用于航空领域的大型承重部件。目前,大型零件一般采用模锻成型。不同的初始显微组织和变形工艺参数会显著影响流动行为。为了实现显微组织的精确控制,研究人员进行了大量的研究工作去分析热压缩过程中的组织演变和变形机制。本文重点综述高强度β钛合金在热变形过程中的组织演变,包括单相区的动态再结晶和动态回复,以及两相区中α相的动态演变。此外,总结热加工图中的最佳热加工区域、不稳定区域以及功率耗散效率与变形机制之间的关系。最后,强调利用热加工图优化工艺参数存在的问题和发展方向。     

基于点状β智能体的飞行器集群避障控制方法

针对传统的飞行器集群避障方法不能有效规避非规则障碍物的问题,提出一种基于点状β智能体的集群避障控制方法。在加速度输入的基础上,基于β智能体概念定义点状β智能体,将α智能体对其在障碍物表面投影的规避变为对点状β智能体的规避,调整个体加速度输入中(α, β)协议量的β智能体的坐标和相关向量,使多智能体面向非规则障碍物形成集群。仿真结果表明,该方法能实现集群对非规则障碍物的规避。     

Dependence of mechanical properties on the microstructure characteristics of a near β titanium alloy Ti-7333

In this work, the microstructure and the corresponding tensile properties of the rolled Ti-7Mo-3Nb-3Cr-3Al(Ti-7333) alloy before and after the thermal treatments were investigated. The results show that a strong α-fiber texture is developed in the rolled Ti-7333 alloy. The deformed matrix and the texture significantly induce the variant selection of β phase. The high strength of the rolled Ti-7333 alloy is attributed to the 110 texture parallel to the tensile direction and the dispersed α phase within the matrix. After the solution treatment followed by the aging treatment, the texture decreases and the microstructure consists of the equiaxed β grains, the spheroidal α_p phase and various needle-like α variants. Eventually, the alloy could achieve an optimal combination with the strength of about 1450 MPa,the ductility of about 10.5% and a considerable shear strength of about 775 MPa. This balance can be ascribed to the performance of the spheroidal α_p phase and various needle-like α_s variants. The results indicate that the Ti-7333 alloy could be a promising candidate material for the high-strength fastener.     

The effect of notch plasticity on the behaviour of fatigue cracks emanating from edge-notches in high-strength β-titanium alloys

The present paper is aimed at investigating the behaviour of fatigue cracks emanating from edge-notches for two different microstructures of the Ti-6246 alloy, produced by two specific thermo-mechanical treatments and defined as β-annealed and β-processed, respectively. Pulsating four point bending tests were performed on double-edge-notched specimens. The initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material’s yield stress. Plastic deformation at the notch tip initially produced a local stress redistribution followed by elastic shake down due to the high cyclic strain hardening rates exhibited by both microstructures, as confirmed by finite element modelling. Crack closure effects, measured by an extensometric technique, and variations in crack aspect ratio were considered in the ΔK calculation. The obtained crack growth rate data were compared with those of long cracks measured on standard C(T) specimens as well as of microcracks measured on round, unnotched S-N type of specimens to evaluate the intrinsic fatigue crack propagation resistance of the two microstructures. The contribution of notch plasticization to crack closure was estimated by finite element modelling.     

Time-dependent deformation and craze initiation in PMMA: Volume effects

As part of a study of long-term deformation and failure in glassy polymers, the time-dependent tensile compliance and lateral contraction ratio have been simultaneously determined for PMMA as a function of applied stress at room temperature. In the time range 10² to 10⁶s, measurements of longitudinal and lateral strain were made using extensometers of novel design and a new optical interference technique was developed to serve as a check on lateral displacement data. At low stresses, the time-dependence of the tensile, shear and bulk compliance was derived over an extensive timescale range (10^?8 to 10⁶s) by combining the long-time data with transformed complex modulus and Poisson's ratio results. These data illustrated the large influence of the broad β retardation region on the room temperature creep behaviour and the onset of the glass-rubber or α region at long-times. With increasing stress a relatively large decrease in retardation times associated with the α process was principally responsible for the onset of non-linearity and resulted in an increased merging of the α and β regions. Stress-induced structural changes associated with this effect were indicated by a small initial positive contribution to the volumetric strain attributed to the partial erasing of previous physical ageing in the material. A subsequent decrease in volume might involve the mechanical enhancement of the original ageing. The substantial reduction in α retardation times with increasing stress was paralleled by a decrease in craze incubation times and this result is discussed in relation to craze initiation criteria based on considerations on non-linear viscoelastic response.     

Influence of β nucleation on the mechanical properties of isotactic polypropylene and rubber modified isotactic polypropylene

The tensile and fracture behaviour of neat α and β nucleated isotactic polypropylene and rubber-modified α and β nucleated isotactic polypropylene has been investigated at test speeds of 0.0001-10 ms⁻¹ in the temperature range −30 to +60 °C. The presence of the β phase had little effect at low temperature. However, at +25 and +60 °C, it increased the speeds corresponding to the ductile-brittle transition in the neat polymer by more than three decades. This behaviour has been linked to changes in microdeformation mechanisms observed at the lamellar and spherulitic level, an increase in cavitational deformation in tensile tests and an increase in the strength of the β relaxation in dynamic mechanical spectra. In the blends, the presence of the β phase led to somewhat higher energy dissipation in regimes of ductile fracture. However, the ductile-brittle transitions were not significantly affected. The modifier phase was therefore inferred to control the initiation and propagation of the plastic zone ahead of the crack tip during fracture.     

镝对AZ31合金β相形状、均质化处理时间及单道次最大轧制率的影响

研究了AZ31合金均质化处理时间对β相形状和单道次最大轧制率的影响,随均质化处理时间的增加,β相形状由网状变为断续网状,单道次的最大轧制率增加。随着Dy元素添加量的增加,合金中β相形状由网状变为断续网状、细小颗粒状再到断续网状。轧制性能也呈同样规律先提高后下降,其中AZ31-1.5%Dy合金不经过均质化处理,单道次最大轧制率仍高于AZ31合金12 h均质化处理的单道次最大轧制率。     

Specific Detection of Extended-Spectrum β-Lactamase Activities with a Ratiometric Fluorescent Probe

The dissemination of antimicrobial resistance around the world is one of the biggest threats to global public health. The acquisition and expression of extended-spectrum β-lactamases (ESBLs) in pathogenic bacterial are mainly responsible for bacterial resistance to third-generation cephalosporins. Reported herein is a ratiometric fluorescent probe for the detection of the activity of ESBLs. This imaging reagent adopts the core structure of cefotaxime as an enzymatic recognition moiety, and exhibits excellent selectivity to ESBLs over other β-lactamases. The specific activation of this sensor by ESBLs can lead to over 2500-fold changes in the fluorescent ratio, which is independent of the concentration of the probe and environmental conditions. Further experiments have demonstrated that this ratiometric fluorescent probe can distinguish bacteria with extended-spectrum antibiotic resistance from a group of clinically important pathogens within a short period of time.