TC2钛合金的高温热变形行为

对TC2钛合金的高温变形行为进行了有限元模拟和热压缩实验研究,使用有限元自洽模型模拟提高流动应力曲线修正精度,分析材料的应力应变曲线特征,得到其高温流动本构方程和激活能,并进行了光学显微镜观察研究其微观组织演变规律,发现在高温低应变速率下α相的球化程度较高。绘制出TC2钛合金的功率耗散图和热加工图,结合应变速率敏感系数m研究了受m值控制的不同变形机制,最终确定了TC2钛合金的最佳加工窗口：(I)760~825℃、0.007~0.024 s^-1;(II)850~900℃、0.018~0.37 s^-1;(III)900~950℃、1~10 s^-1,在此区间功率耗散因子较大,在材料变形过程中发生充分动态再结晶,试样的微观组织呈细小等轴状。     

Effects of Thermally Induced Cyclic γ ↔ ε Transformation on Shape Memory Effect of a Quenched FeMnSiCrNi Alloy

The effects of thermally induced cyclic γ ? ε transformation on microstructures and shape memory effect (SME) are investigated in a quenched Fe14Mn5.5Si8.0Cr5.0Ni alloy. The results show that the annealing at 773 K remarkably improves the SME in the quenched alloy. One thermal cycling between 290 and 773 K remarkably increases the SME, but the further thermal cycling hardly improves the SME. The reason is that the amount of thermal ε martensite remarkably reduces after annealing at 773 K, but it hardly changes with the further increase of thermal cycling between 290 and 773 K. The pre‐existing thermal ε martensite not only prevents the occurrence of stress‐induced ε martensitic transformation but also promotes the formation of α′ martensite.     

High temperature creep of modified α + β brasses

Good machinability and formability of technical brasses α + β have to be preserved when alternative modifications are developed instead of alloying with toxic lead. Several model brasses of this type, which exhibit satisfactory machinability, are creep tested at 780 °C to compare their high temperature formability. The observed stationary creep rate and local ductility may highly differ for various alloy types. To characterize the impact of individual additions, superposition laws for the respective quantities are proposed and applied to the measured values. The strong effect of even small amounts of some additions on the creep rate and the boundary component of ductility is suggested to be due to segregation phenomenon. Rigid particles, such as phosphides, enhance significantly the creep resistance of the material, though restrict the macroscopic stability of plastic flow. On the other hand, the contribution of liquid particles to the creep damage may be effectively obstructed by phosphides.     

Comparative determination of the α/β phase fraction in α+β-titanium alloys using X-ray diffraction and electron microscopy

A comparison is made between the measured α/β phase fractions in Ti-6246 using X-ray diffraction (XRD) and electron microscopy. Image analysis of SEM and TEM images was compared to the phase fraction estimate obtained using electron backscattered diffraction, lab and high-energy synchrotron XRD. There was a good agreement between the electron microscopic and diffraction techniques, provided that the microstructural parameters of grain size and texture are estimated correctly when using quantitative Rietveld refinement.     

Probabilistic prediction of minimum fatigue life behaviour in α + β titanium alloys

The objective of this work was to develop and demonstrate a probabilistic life prediction method for the prediction of minimum fatigue lives that are typically used in the design of fracture critical rotating turbine engine components. A Monte Carlo analysis was used to predict the variability in fatigue lives based on the distribution of microstructural features that lead to early crack initiation as well as the variability in small fatigue crack growth rates. Two titanium alloys, both with bimodal microstructures, were tested and analysed in this study. The distribution of critical microstructural features was calibrated based on test results and understanding of microstructure neighbourhood effects. Testing was conducted on both alloys and included both smooth and notched specimens. The predictions are presented and compared with the data for smooth and notch geometries for the various loading conditions. A parametric study was performed to identify the importance of several model inputs and to identify areas for future improvement.     

加工图在钛合金中的应用

简要介绍了加工图在钛合金中的应用,主要介绍了加工图的基本原理,分别以几种典型的α钛、近α钛合金、α β钛合金及β钛合金为例介绍了钛合金加工图的特点和加工图在研究合金元素、微观组织对钛合金热加工特性影响方面的应用,总结了优化这几种合金加工工艺的条件,给出了钛合金加工图的一般规律.     

TC21钛合金的热加工行为研究进展

综述了高强高韧损伤容限型钛合金TC21的热加工行为研究进展.重点介绍了热加工及热处理工艺参数对TC21钛合金的相组成、显微组织与力学性能、损伤容限性能等方面的影响.指出TC21钛合金在国内某些领域已经得到了应用,但有待进一步研究与开发.     

Notch stress analyses of high‐frequency mechanical impact‐improved welds by using ρf = 1 mm and ρf = ρ + 1 mm approaches

This paper presents further assessments of the previously reported round‐robin fatigue data obtained from high‐frequency mechanical impact (HFMI)‐improved longitudinal welds. A detailed statistical analyses of geometry measurements of HFMI‐treated weld toe profiles are presented. The fatigue analyses based on notch stress as defined by the International Institute of Welding are performed using the finite element method. Notch stresses are assessed based on both the fictitious weld toe radius and the addition of measured actual notch radius to the fictitious radius. While no large differences are observed between the results of methods, the former one is found to be more practical and faster to implement from the end‐user point of view.     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.