Titanium alloys are key materials for applications in major engineering areas, such as aerospace and marine equipment. Studies on structural titanium alloys focus on strengthening and toughening the alloys, especially the latter. The mainstream structural titanium alloys comprise both α and β phases. The optimization of the strength and toughness balance relies on the control of the compositions, volume fractions, and morphologies of both phases. In this study, some recent advances along the above line are reviewed, focusing on studies on the composition design, plastic-deformation mechanism, and microstructure tuning. Rational design of the compositions of both phases improved the deformation coordination within the α phase and across the α/β interface, suppressed the precipitation of brittle ω and α2 phases, and resulted in improved plasticity and toughness through the α-deformation twin and β-deformation-triggered phase transformation. The multiscale microstructure enhanced the strength and toughness of the titanium alloy. Using the abovementioned approaches, a series of titanium alloys with an improved strength-toughness combination were developed and fabricated. Finally, an attempt was made to predict the prospect of technology development in the field of high-strength and high-toughness titanium alloys for various applications.

研究了两相区固溶温度及固溶后冷速对Ti-6Al-4V (TC4)合金元素再分配行为的影响，利用EPMA技术表征了初生α相(α_p)以及β转变区域(β_t)的元素浓度，考察了β_t显微组织尺寸随固溶温度及元素浓度的变化。结果表明：随着固溶温度升高，β_t区域元素浓度变化显著，表现为Al含量升高、V含量降低，而α_p晶粒中元素浓度变化较小，导致两区域元素浓度差异减小；同一固溶温度下，以不同冷却方式(水冷、空冷及炉冷)冷却的显微组织及元素分布显示，冷却速率越低，α_p比例越高，α_p与β_t之间元素浓度差异越明显。合金经固溶水冷、空冷后，β_t分别为淬火马氏体、次生α相(α_s)+残余β相，2种冷速下β_t的显微组织尺寸均与高温β相内的元素浓度水平有关，即β_t内部显微组织尺寸受固溶温度的显著影响。利用纳米压痕技术表征了不同固溶温度下微区域(α_p、β_t)的力学特征，结果表明，密排六方(hcp)晶格α_p本身呈现的力学行为的各向异性对其纳米压痕性能起决定性作用，而β_t的弹性模量及硬度主要受α_s片层尺寸的影响。最后讨论了“固溶温度-微区元素浓度-微区显微组织-微区力学性能”之间的关系。

<p>TB9 titanium alloy has been widely used for aerospace due to it's superior low stiffness, corrosion resistance and workability. It has been reported that cold deformation can improve the comprehensive mechanical properties of titanium alloys. At the same time, the cold rotary-swaging deformation facilitates the production of small batches and the acquisition of special shape and size bars. However, current studies on the microstructure and properties of cold rotary-swaged titanium alloys are not systematic. So, the effects of cold deformation rate on the microstructure, texture evolution and mechanical property of TB9 alloy during cold rotary-swaging were investigated using OM, EBSD, XRD, TEM and tensile test. The results showed that the grain size of TB9 titanium was refined with the increase in diameter reduction. Meanwhile, with the deformation increases, the grains rotation along the swaging axis occurs, forming a preferred orientation, the textures change from initial {001}<110> and {001}<100> to <i>α</i>-fiber and <i>γ</i>-fiber {001}<110>, {112}<110> and {111}<110>. All of grains refinement, texture components and substructures contributed to the enhancement of strength after cold rotary-swaging. And the ductile kept on a high level after 70% cold working, which means the TB9 titanium has a great cold deformation ability.</p>

采用冷旋锻对TB9钛合金棒材进行多道次冷变形，利用OM、EBSD、XRD、TEM以及拉伸等实验研究了不同冷变形量TB9钛合金棒材的显微组织、织构和拉伸性能及其规律。结果表明，TB9钛合金棒材的晶粒尺寸随冷旋锻变形量的增大而减小，部分晶粒尺寸达到纳米级。同时，晶粒随变形量的增加沿旋锻轴向转动，形成择优取向，由初始{001}和{001}织构转变为取向的α-fiber和γ-fiber {001}、{112}和{111}织构。在亚结构、小尺寸晶粒以及织构的共同作用下，TB9钛合金的强度随变形量的增大而增加，延伸率和面缩率在70%冷变形后仍保持在一个较高的水平，具有优异的冷变形能力。

This paper concentrates on the research progress of titanium alloys and their diffusion bonding fatigue characteristics, and summarizes the laws of fatigue crack initiation and growth of titanium alloys with/without welding. The chemical composition, classification, and common welding method of titanium alloys are stated, with emphasis on the features and advantages of diffusion bonding. The phenomena of slip band formation and dislocation movement under cyclic loading are described, and the mechanism of fatigue crack initiation is clarified. The selection of microstructures is a common method to optimize mechanical properties of titanium alloys. Previous studies suggested that the laminated structure is an important mode to realize the low fatigue crack growth rate of titanium alloys. Improper parameters of the welding process can cause joint defects, and further heat treatment can reduce joint defects while improving the fatigue life and strength. Finally, the multilayer and heterogeneous laminates of titanium alloys produced by diffusion bonding are briefly described to realize the possibility of high damage tolerance.

本文主要对钛合金及其扩散焊疲劳特性研究进展进行了综述，总结了钛合金及其焊接后疲劳裂纹萌生和扩展的规律。概述了钛合金的化学成分和分类，以及常用焊接方法，重点介绍扩散焊的特点和优势。描述了循环载荷作用下滑移带形成和位错的运动现象，阐明疲劳裂纹萌生的机制。钛合金微观组织的选择是优化力学性能的常见方法，现有研究表明，制备层合结构是降低钛合金疲劳裂纹扩展速率的一种重要调控手段。不合适的焊接工艺参数会导致接头缺陷的形成，后续热处理能够降低接头缺陷，并提高焊接构件的疲劳寿命和疲劳强度。最后，简述了扩散焊制备多层和异种钛合金层合结构来实现构件高损伤容限的可行性。

Diffusion bonding is a process by which two flat, usually metallic, surfaces are welded together at a high temperature and moderate pressure. Bonding occurs due to a combination of diffusion and power law creep that close the voids formed by microscopic differences between the mating surfaces. While the different process parameters are well understood the effects of surface condition and void shapes during bonding has not been thoroughly researched. In this paper we use measured surface profiles, discretize them, and apply the diffusion and creep equations numerically to the profiles in order to provide insight into the effects of surface geometry on bonding. Using this method the voids can interact with each other and the effects of nearby voids can be computed. Experimental tests are performed to confirm the model and theoretical tests were created to determine what the effects of different surface geometries are on bonding performance. While in most cases the bonding was dominated by power law creep the most optimal void shape was one where the voids had completed the creep stage and were controlled by diffusive processes. It was also found that concentrating the overlap area also increases bonding performance.

Due to the excellent high temperature comprehensive performance and cost effective, the second-generation nickel-based single crystal superalloy has been widely used in the high-pressure turbine blades of advanced aero-engines. Microdefects such as micropores and interdendritic eutectic are seriously harmful to the high temperature mechanical properties of nickel-based single crystal superalloys. Hot isostatic pressure (HIP) technology, which has been widely used in powder and casting superalloys, can effectively reduce the micropores, interdendritic eutectic and other structural defects formed in the turbine blades during manufacturing, and improve the service reliability of turbine blades. However, the effect of HIP process on the high temperature stress rupture life of nickel-based single crystal superalloys is still controversial, especially with regard to the initial microstructure state of the nickel-based single crystal superalloys, i.e. the as-cast microstructure state or the as-solid-solution state. In this work, a kind of second-generation nickel-based single crystal superalloy with as-cast state or as-solid-solution state was selected as the research object. Through two-stage heat/booster type heat treatment process, in combination with microdefects quantitative analysis, quantitative characterization of alloying element segregation and high temperature stress rupture tests at 980 ℃ and 250 MPa, the effects of HIP process on the microdefects and high temperature stress rupture life of the used superalloy with different initial microstructures were studied. The results indicated that the solid-solution treatment can significantly promote the diffusion of alloying elements, such as Re, W, Al, and Ta, reduce the area fraction of interdendritic eutectic, but significantly increase the average area fraction and size of micropores in the used alloy with as-cast state. While, HIP process can effectively reduce the average area fraction and size of microspores in the used alloy with as-cast state or as-solid-solution state, but cannot eliminate the interdendritic eutectic as remarkable as the solid-solution treatment. By HIP process of the used alloy with as-solid-solution state, the area fraction of micropores is reduced to 0.005%, the eutectic structure is basically eliminated, and the dendrite segregation of Re, W, Al, Ta and other elements is significantly alleviated, resulting in the higher stress ruputure life of the used alloy, about 40% over that of the used alloy with the standard heat treatment state. Performing HIP process on nickel-based single crystal superalloy alloy with as-solid-solution state is of benefit to the high temperature stress rupture life due to the reduction of microdefects and the homogenization of alloying elements, in comparison with performing HIP process directly on the alloy with as-cast sate.

以初始组织分别为铸态组织和固溶态组织的第二代镍基单晶高温合金为研究对象，通过进行1300 ℃、30 MPa、2 h+1300 ℃、100 MPa、3 h两阶段的热等静压处理，对比热等静压前后显微缺陷及微观组织的变化，并在980 ℃、250 MPa条件下进行高温持久性能实验，明确了热等静压处理对不同初始组织状态下镍基单晶合金组织状态及持久性能改善的影响机制。结果表明：固溶处理显著促进Re、W、Al、Ta等合金元素的扩散，降低铸态组织共晶面积分数但显著提高显微孔洞平均面积分数及平均尺寸。热等静压处理可以显著降低显微孔洞平均面积分数及平均尺寸且对固溶态组织的作用更为显著，但热等静压对共晶组织的消除作用不如固溶处理明显。固溶态组织经热等静压处理后，显微孔洞面积分数降低至0.005%；共晶组织基本消除；Re、W、Al、Ta等元素枝晶偏析程度显著缓解；其980 ℃、250 MPa高温持久寿命相比未经热等静压处理的标准热处理态合金提高了40%左右。对固溶态组织进行热等静压处理的工序安排有利于提高显微孔洞闭合作用，促进成分均匀化并显著提高合金高温持久寿命。

The present work reports the effect of thermal induced porosity (TIP) on the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behaviors of hot-isostatic-pressed (HIPed) Ti-6Al-4V alloy from gas-atomized powder. The results show that the residual pores in the as-HIPed powder compacts present no obvious effect on the HCF life. The regrowth of the residual pores can be observed after solution heat treatment. The pore location ranks the most harmful for the fatigue life compared with the other initiating defects. The maximum stress intensity factors were calculated. The plastic zone size of fine granular area (FGA) is much less than the characteristic size of the microstructure, and the crucial size of the internal pores in this study is about 40 μm. The failure types of fatigue specimens in the VHCF regime were classified, and the competition of different failure types was described based on the modified Poisson distribution.

Grain boundary (GB) migration plays an important role in modifying the microstructures and the related properties of polycrystalline materials, and is governed by the atomistic mechanism by which the atoms are displaced from one grain to another. Although such an atomistic mechanism has been intensively investigated, it is still experimentally unclear as to how the GB migration proceeds at the atomic scale. With the aid of high-energy electron-beam irradiation in atomic-resolution scanning transmission electron microscopy, we controllably triggered the GB migration in α-AlO and directly visualized the atomistic GB migration as a stop motion movie. It was revealed that the GB migration proceeds by the cooperative shuffling of atoms on GB ledges along specific routes, passing through several different stable and metastable GB structures with low energies. We demonstrated that GB migration could be facilitated by the GB structural transformations between these low-energy structures.

Plastic deformation bonding (PDB) has emerged as a promising solid state bonding technique with limited risk of phase transformations and residual thermal stresses in the joint. In this study, the PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions. It was revealed that, with increasing PDB strain rate at 1000 °C, different extents of dynamic recrystallization (DRX) occur in the bonding area of IN718 joints. The extent of DRX, average size of DRXed grains, and a newly proposed “interfacial bonding ratio (ΨBonding)” parameter (to quantify the bond quality) were initially reduced with increase in the strain rate up to 0.1 s-1 and later increased at further higher strain rates. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) based interfacial microstructure analyses indicated that the quality of the bonded joints is closely related with the development of fine DRXed grains at the bonding interface with the increasing strain, which promotes adiabatic temperature rise. It was revealed that the initial bulging and subsequent migration of the original interfacial grain boundary (IGB) were the main mechanisms promoting DRX in the well bonded IN718 superalloy joints. Moreover, the mechanical properties of the bonded joints were not only controlled by the recrystallized microstructure but also depended upon the Bonding parameter of the joints.