两种组织TC11钛合金薄板弯曲疲劳性能的研究

本研究选取了具有（α＋β）等轴组织和α／β粗片层组织的TCll钛合金作为研究对象,采用研究了TCll合金薄板样品在恒总应变幅控制下的弯曲疲劳性能及其损伤行为,通过对疲劳开裂路径和断裂的观察与表征,探讨了疲劳损伤与组织结构间的关系。研究发现,恒总应变幅控制下的等轴组织TCll钛合金薄板的弯曲疲劳性能明显的高于粗片层组织合金。等轴组织合金的疲劳裂纹沿α相中的滑移带萌生并扩展,片层组织样品疲劳裂纹沿着α相或与片层垂直的方向扩展。     

TC4-DT钛合金疲劳裂纹扩展的微观机制

通过断口分析讨论了TC4-DT钛合金裂纹扩展的微观机制。从微裂纹的形成,疲劳裂纹初期、近门槛区以及稳态区的扩展,分析了不同组织形态的TC4-DT钛合金对应的裂纹扩展微观机制。分析结果表明,对于片层组织,循环载荷的作用导致断裂表面粗糙及塑性变形过程中相界面产生位错塞积而诱使裂纹萌生。等轴组织变形过程中晶粒产生的断裂表面成为裂纹的萌生源。在近门槛区,等轴组织变形过程中位错累积将导致沿晶界的开裂,从而加速裂纹扩展;双片层组织由于次生α相的尺寸效应会加速裂纹扩展。在稳态扩展区,断口表面由第Ⅰ阶段的锯齿状断裂模式过渡到辉纹断裂模式,表现为塑性条带断裂机制。     

显微组织对Ti-6Al-4V ELI合金疲劳裂纹扩展速率的影响

研究了Ti-6Al-4V ELI合金板材的显微组织对疲劳裂纹扩展速率的影响.用金相显微镜对不同热处理制度下该合金α和β转变组织的变化进行了观察分析.采用MTS-810低周疲劳试验机测试合金的裂纹扩展速率.通过Origin 6.0软件对数据进行处理并绘制裂纹扩展速率(△a/△N)与应力强度因子幅△K的关系曲线.结果表明:在Pairs区范围内,疲劳裂纹扩展速率对双态组织中初生α相含量的多少不敏感,而在近门槛区和快速扩展区,裂纹扩展速率对组织比较敏感;在本实验研究的条件下,细针编织状魏氏组织的da/dN<平直状片层组织的da/dN<双态组织的da/dN.     

片层厚度对损伤容限型TC21合金裂纹扩展速率的影响

分别采用α β锻造和β锻造方式制备了TC21合金φ90mm棒材,研究了两种加工方式下同规格棒材的裂纹扩展速率,分析了粗细两种片层结构对裂纹扩展速率的影响.结果表明:β锻造获得了细片层组织结构,具有较低的疲劳裂纹扩展速率;α β锻造得到粗片层组织结构,裂纹扩展速率较快.疲劳裂纹扩展速率随着组织片层厚度的增加而加快.在疲劳裂纹稳态扩展区,裂纹以条带循环机制向前扩展,同时能看到许多疲劳条纹.     

钛合金熔模精密铸件表面微裂纹形成过程研究

目的 表征钛合金熔模精密铸件表面组织的特性,研究其如何形成表面微裂纹的过程。方法 通过对钛合金精铸件表面微裂纹形貌进行观察,对表面α层的化学成分、显微硬度进行表征,并通过三点弯曲实验模拟α层在拉应力状态下开裂倾向。结果 微裂纹为冷裂,深度在α层范围内。α层的片层组织较铸件内部组织粗大,O含量和硬度高于铸件内部,在拉应力状态下较铸件内部容易导致裂纹萌生和扩展。结论 钛合金精铸件在凝固过程形成α层,在后续的冷却过程中形成的拉应力或翘曲变形导致相对脆弱的α层形成了表面微裂纹。     

裂纹扩展片技术在弧形面上半椭圆表面裂纹长度测试中的应用

简述了裂纹扩展片技术的研究现状和测试原理,标定出应用在单臂悬梁试样弧形面上的单片和串联双片10mm裂纹扩展片的裂纹长度增量与电压增量关系方程。通过扩展片与试样裂纹扩展同步性研究,证明了裂纹扩展片技术应用于弧形面上半椭圆表面裂纹长度测试的可行性。     

TiAl基合金层状组织中裂纹扩展区的塑性变形

本文通过观察ＴｉＡｌ合金层片组织四点弯曲试样侧面裂纹扩展与滑移迹线之间的关系，分析讨论了层状组织中塑性变形对裂纹扩展的影响。发现不论层片相对于裂纹处于何种取向，沿层片方向都有一定程度的塑性变形，这种变形对于平行、倾斜、垂直于片层扩展的主裂纹都可引起裂尖钝化，从而引发构成各种韧化机制     

TiAl基合金层状组织中裂纹扩展区的塑性变形

本文通过观察ＴｉＡｌ合金层片组织四点弯曲试样侧面裂纹扩展与滑移迹线之间的关系，分析讨论了层状组织中塑性变形对裂纹扩展的影响。发现不论层片相对裂纹处于何种取向，沿层片方向都有一定程度的塑性变形，这种变形对于平行，倾斜，垂直于片层扩展的主裂纹都可引起裂尖钝化，从而引发构成各种韧化机制。     

全片层与双态组织高铌TiAl合金高温原位拉伸研究EI北大核心CSCD

利用原位观察的方法研究了全片层(Fully Lamellar,FL)组织与双态(Duplex,DP)组织高铌TiAl合金在750℃下的拉伸力学行为。结果表明:DP组织具有较好的抗拉强度和塑性,其抗拉强度为958MPa,断面收缩率为45.1%,FL组织抗拉强度较低且几乎没有塑性;随着拉伸应力的增加,DP组织合金试样表面存在一个大量裂纹萌生和扩展的过程,这些裂纹均匀分布在试样表面,主要以萌生为主,几乎难以扩展,而在FL组织中只能观察到少量的裂纹萌生现象;拉伸断裂后的DP组织合金的裂纹是由多个孔洞合并而成,呈蠕虫状且尺寸很小,FL组织中与拉伸方向垂直或呈一较大夹角的片层团界及片层界较易萌生裂纹,在临近断口区域,裂纹主要以穿层扩展为主。     

TA15钛合金的高周疲劳性能和断裂特征

研究了双态组织的TA15钛合金的高周疲劳性能和疲劳断裂特征,结果表明,β转变组织中次生α相的数量和形态对疲劳性能有显著影响,次生相α相的球化显著降低了合金抗裂纹扩展的能力,而大量的片状次生α相则通过造成疲劳裂纹的分枝有效地降低了疲劳裂纹的扩展速度,提高了疲劳极限.     

铸造、锻造和粉末冶金TC4钛合金损伤容限行为对比研究

目的 研究影响铸造、锻造和粉末冶金TC4钛合金的损伤容限行为差异的主要因素。方法 分别从裂纹尖端塑性变形行为、二次裂纹及断口表面粗糙度3个方面对比,分析造成3种成形方法制备的TC4钛合金的断裂韧性和疲劳裂纹扩展速率差异的原因。结果 铸造TC4钛合金断裂韧性优于锻造和粉末冶金TC4钛合金,主要是因为新产生的裂纹面积大,消耗更多断裂能量。铸造TC4钛合金疲劳裂纹扩展速率低于锻造、粉末冶金TC4钛合金,其主要原因为曲折的裂纹路径和断面粗糙度诱发裂纹闭合效应以及长而深的二次裂纹。结论 3种成形方法制备TC4钛合金损伤容限行为差异的主要原因是断裂形成了不同裂纹路径形貌。     

多重热处理对TC4合金的组织和力学性能的影响

研究了多重热处理对TC4钛合金的显微组织和力学性能的影响.多重热处理温度中第一重处理位于单相区,第二重位于两相区,前两重热处理均采用不同的冷却速率,最后进行时效处理.结果表明:采用不同多重热处理制度可以调节材料的显微组织参数,包括片状α的长度、宽度、长宽比以及次生α片层的数量等,从而可以优化材料力学性能的搭配,特别是显著提高片层组织的塑性.采用适当的多重热处理不仅提高了材料的强度和塑性,而且使材料的断裂韧性有一定程度的提高.     

LIFE PREDICTION BY SIMULATION OF CRACK GROWTH IN NOTCHED COMPONENTS WITH DIFFERENT MICROSTRUCTURES AND UNDER MULTIAXIAL FATIGUE

A modelling procedure was developed which is applicable to crack growth in notched components subjected to multiaxial fatigue for materials with different microstructures. An algorithm for crack growth, in a microstructure that was modelled as hexagons, was established as a competition between growth by crack linkages during the crack initiation and propagation stages and the propagation of a dominant crack as a single crack. Analytical results simulated by using the developed model were compared with experimental results from fatigue tests which had been conducted using notched specimens of pure copper, carbon steel and two kinds of titanium alloy. Cracking morphology, which was experimentally observed to depend on the microstructure and the loading mode, was well simulated using the present model. The fatigue failure life of a notched specimen was statistically estimated by a Monte Carlo procedure based on the model. The simulated life with a statistical scatter-band almost coincided with the experimental data.     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

Fatigue crack tip plastic zone of α + β titanium alloy with Widmanstatten microstructure

The recent studies had focused on the fatigue crack propagation behaviors of α?+?β titanium alloys with Widmanstatten microstructure. The fascinated interest of this type of microstructure is due to the superior fatigue crack propagation resistance and fracture toughness as compared to other microstructures, which was believed to be related to the fatigue crack tip plastic zone (CTPZ). In this study, the plastic deformation in fatigue CTPZ of Ti-6Al-4V titanium alloy with Widmanstatten microstructure was characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The results showed that large-scale slipping and deformation twinning were generated in fatigue CTPZ due to the crystallographic feature of the Widmanstatten microstructure. The activation of twinning was related to the rank of Schmid factor (SF) and the diversity of twin variants developing behaviors reflected the influence of SF rank. The sizes of CTPZ under different stress intensity factors (K) were examined by the white-light coherence method, and the results revealed that the range of the plastic zone is enlarged with the increasing K (or crack length), while the plastic strain decreased rapidly with the increasing distance from the crack surface. The large-scale slipping and deformation twinning in Widmannstatten microstructure remarkably expanded the range of fatigue CTPZ, which would lead to the obvious larger size of the observed CTPZ than that of the theoretically calculated size.     

Effect of microstructure on the plain and notched fatigue properties of IMI 550 Ti alloy

The fatigue crack initiation and propagation in plain and notched specimens of titanium IMI 550 alloy with different percentages of transformed β-phase has been investigated. The S/N curves for plain specimens having a two-phase α/β microstructure were superior to those having a 100% transformed β microstructure. It is thought that this difference is due to easier crack paths in the β-stabilized samples, which contain very large grains. The notched S/N curves, however, were not dependent on microstructure. The overall crack propagation rates were similar for the three microstructures studied.     

DEVELOPMENT OF FATIGUE CRACKING IN AIRCRAFT ENGINE COMPRESSOR DISKS OF TITANIUM ALLOY Ti–6Al–3Mo–2Cr

Fractographic features related to fatigue growth in a titanium alloy Ti–6Al–3Mo–2Cr were examined in compressor disks that had failed in service. In-flight block cyclic loading that included hold-time loading and variable amplitudes of vibrational loading produced fracture characteristics which mainly reflected the two-phase (α + β) lamellar structure of the titanium alloy and contained localized areas of fatigue striations.
Blocks of cyclic loads which included cycles of hold-time loading during the testing of disks resulted mainly in the formation of fatigue striations with no evidence of the lamellar microstructure of the titanium alloy. An analytical equation Vi = [(1 − ν2 )/(16 π E σ_{0.2 )][Ke ]2i} is suggested to describe the relationship between the crack propagation rate, V, and the equivalent stress intensity factor, K_e , as applied to the semi- and then to the quarter-elliptical-shaped cracks. Fractographic features are discussed and the disks' fatigue crack growth behaviour over 50 flights is demonstrated.     

THE SIZE OF PLASTIC ZONES AND FATIGUE CRACK GROWTH BEHAVIOUR OF THREE FORMS OF A Ti–6Al–2.5Mo–1.5Cr ALLOY

The effect of microstructure on the fatigue properties of Ti–6Al–2.5Mo–1.5Cr alloy was investigated. The experimental results for both the fatigue crack initiation and propagation behaviour, as well as the dynamic fracture toughness ( K _Id ) showed clearly that a lamellar microstructure is superior to two other structures. It was found that, as in the case of steels, the initiation and subsequent growth of cracks in the titanium specimens with a sharp notch may also occur on loading levels below the threshold values of the K factor (Δ K _th ) determined for long fatigue cracks. In addition, measurements by interferential-contrast of the plastic zone size on the surface of specimens revealed that the different rate of crack growth at identical values of Δ K in individual structural states can roughly be correlated with the size of the plastic zone. A general relationship between the fatigue crack growth rate and plastic zone size, the modulus of elasticity and the role of crack tip shielding is discussed.     

Effects of alloy additions on the fatigue properties of cast Co-Cr-Mo alloy used for surgical implants

As the second part of the present study, the effects of modifying the composition of the Co-Cr-Mo alloy with additions of nickel and some trace elements aluminium, titanium and boron, have been investigated. A great improvement in the fatigue crack growth resistance of the cast alloy is obtained by nickel additions to the base alloy, mainly because of a significant increase in the alloy's stacking fault energy. In addition, the fatigue fracture ductility is observed to be improved strikingly with the nickel additions. Much smaller facets and better ductility with a mixed fatigue crack propagation mode, as compared to the base alloy, are observed in the alloys with low nickel content level, and in the alloys with high nickel content level, localized ductile fatigue striations are observed. It is also indicated that minor additions of such elements as aluminium, titanium and boron can be used to improve further the fatigue crack growth resistance resulting from the elimination of some microstructural casting defects.     

Titanium-silicon coatings and their influence on the efficiency of machine parts

The production of titanium-silicon coatings on VT8 titanium alloy and the effect of these coatings on the resistance to contact fatigue and fatigue crack propagation are investigated. It is established that such coatings increase the resistance of VT8 alloy to fatigue crack propagation and contact fatigue.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 79–81, September–October, 1993.