High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy

This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstatten structure was obtained by double-annealing treatment,. High-cycle fatigue （HCF） tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor Kt = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of a lamella and the tearing of β matrix. The soft a precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of a lamella or vertical to a lamella.

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.

Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing

In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing(LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α(αp) phase surrounded by a secondary α colony(αs) in the α phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/d N values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low K values(<13.8). The differences that are observed between the crack propagation behaviors(in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the αp lamellae and αscolony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the αpand α clusters have a large size.However, in the columnar grain region, small and randomly distributed αplamellae generate a zigzagshaped propagation path with a reduction in the da/d N value. Additionally, the change in the size of the αp lamellae in the equiaxed grains(heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.

EFFECT OF Na,K AND Ce ON CRACK PROPAGATION RESISTANCE FOR ALLOY 8090

The crack propagation resistance for alloy 8090 sheet decreases with the increase of content of Na and K impurities.An improvement over the fracture toughness and crack initiation resist- ance for the alloy containing Na and K within certain limit may be made by adding 0.05% Ce.This beneficial modification would not be manifested if the alloy contained more Na and K.One of the reasons why Na and K injure the toughness of the alloy might be due to that they cause the precipitation of T_1 phase along grain and subgrain boundaries and accelerate the abnormal growth of recystallized grains.

Effect of sodium tartrate concentrations on morphology and characteristics of anodic oxide film on titanium alloy Ti–10V–2Fe–3Al

The effect of sodium tartrate concentrations on morphology and characteristics of anodic oxide film on titanium alloy was investigated. The alloy substrates were anodized in different concentration solutions of sodium tartrate with the addition of PTFE emulsion and their morphology and characteristics were analyzed. The anodic oxide film presented a uniform petaloid drums and micro-cracks morphology. Additionally, micro-cracks dramatically swelled with the increase of the tartrate concentrations. The thickness of the anodic oxide film increased with the concentrations until the concentration reached 15 g/L. The results of Raman analysis illustrate that all samples have similarity in the crystal structure, consisting of mainly amorphous TiO2, some anatase TiO2and a small amount of rutile TiO2. And the ratios of anatase TiO2and rutile TiO2increase with the concentrations until it reaches 15 g/L. Furthermore, the intensity of the peaks increases with enhanced concentrations until the concentration reaches 15 g/L. The corrosion resistance of the anodic oxide film is increased by the sodium tartrate with higher concentrations before 15 g/L. The coefficient of friction of the anodic oxide film reduces with the concentrations until the concentration reaches 15 g/L, then the coefficient of friction of the anodic oxide film increases with the concentrations. © 2015 The Authors

CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

The superalloy GH2132 is equivalent to A286. The tests were carried out at 550°C under various cyclic frequencies (hold time) and load levels, and the fracture surfaces were examined by using a scanning electron microscope. It was shown that the fracture mode of creep-fatigue and the effect of cyclic loading on crack growth change with the growth of crack and the increase of net-section stress, and both are reversed when the net-section stress is up to the yield stress of material. When σn0.2, cracking is predominantly cyclic-dependent transgranular and cyclic loading accelerates creep crack growth, whereas when σn>σ0.2, the case is reversed.

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy （SEM） in-situ observation. The results indicated that the fatigue crack growth threshold △Kth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750℃ was determined as 12.89 MPa.m^1/2 and 8.69 MPa.m^1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

Tensile and creep properties of Ti-600 alloy

Ti-600 is one of the high performance titanium alloys used at 600 ℃,which was developed in Northwest Institute for Nonferrous Metal Research(NIN) in China. The tensile and creep properties of Ti-600 alloy with different thermal treatment conditions were investigated. The results indicate that Ti-600 alloy possesses favorite comprehensive properties solution-treated at 1 020 ℃ for 1 h,then air-cool,and aged at 650 ℃ for 8 h,finally air-cooling,especially possesses quite good creep resistance. The residual deformation is less than 0.1% for the alloy exposed at 600 ℃ for 100 h with the stress of 150 MPa,and the bimodal microstructures of the alloy are almost the same as that of the alloy treated by duplex thermal treatment,only needle primary α phases became relatively thicker and coarsened. The ultimate strength and the elongation of the alloy tested at ambient temperature are 1 080 MPa and 12%,respectively;while at 600 ℃,they are 690 MPa and 16%,respectively. The ductility of the alloy tested at room temperature is no less than 5% after thermal exposing at 600 ℃ for 100 h.

Creep Rupture Behavior of K40S Alloy at Elevated Temperatures

Creep testing was conducted on K40S alloy. The detailed creep deformation and fracture mechanisms under constant load were studied. The results show that the stress exponent ranges between 7 and 14.4 at elevated temperature 973~1173 K, and that the activation energy is approximately 449.1 kJ/mol. During creep, the grain boundary sliding cut off primary carbides at the boundary, generating the 'O' model cracks. The creep failure mode of K40S alloy is transgranular ductile and cracks originate at the primary carbides. A long carbide and matrix interface is often a preferential path for crack propagation. The creep mechanism is discussed in light of the creep microstructure, the stress exponent and the activation energy.

激光增材制造体育器材用TC4钛合金疲劳裂纹扩展行为研究

目的揭示应力比对增材制造TC4钛合金疲劳裂纹扩展行为的影响规律。方法采用紧凑型拉伸试样,在恒载荷幅条件下对激光增材制造TC4钛合金进行了应力比为0.1、0.3和0.5的疲劳裂纹扩展实验,定量评价了不同应力比下合金的疲劳裂纹扩展速率和变化规律。基于Paris公式对裂纹扩展速率进行了拟合,分析了应力比对各参数的影响规律。最后通过扫描电镜对断口表面形貌进行了观察,分析了应力比对断裂模型的影响。结果在相同的∆K条件下,疲劳裂纹扩展速率随着应力比的增大而增大。在Paris公式中,参数C随应力比的增大而减小,参数m随应力比的增大而增大,并且m和lgC呈现线性关系。随应力比的增大,断口表面的河流花样增多、疲劳辉纹变浅、二次裂纹数量增加。结论应力比引起的裂纹尖端闭合效应和平面应力比变化是导致裂纹扩展速率发生改变的主要原因。

多级冷压合与高温蠕变应力松弛TC4钛合金微观组织演变机理研究

采用多级冷压合与高温蠕变应力松弛成形工艺制备TC4钛合金薄壁构件,研究了不同蠕变工艺参数对微观组织的影响规律。通过光学显微镜和电子背散射衍射技术分析了晶粒尺寸及取向、位错密度和裂纹萌生扩展。结果表明,在低塑性变形条件下,较高的蠕变温度可以促进α相晶粒长大。然而在大塑性变形条件下,微观组织受到蠕变温度和塑性变形共同影响。较高的温度加快位错回复,引起位错密度降低,从而减小了晶粒生长的驱动力,最终导致在较高的蠕变温度下α相晶粒反而更小。与600℃蠕变工艺相比,750℃蠕变后的TC4板材出现更多的裂纹,可能是β相粗化所致。

深潜器钛合金室温蠕变-疲劳损伤模型及数值研究

针对钛合金深潜器耐压壳在实际作业过程中疲劳与蠕变损伤相互促进的现象,本文提出了适用于室温环境下钛合金材料的蠕变-疲劳损伤模型和数值模拟方法。基于连续损伤力学理论建立了蠕变-疲劳损伤模型,并通过单轴蠕变-疲劳试验标定了参数;将该模型用子程序进行定义,通过有限元软件二次开发模拟了紧凑拉伸试件蠕变-疲劳损伤演化过程,并将数值模拟结果与试验结果对比。试验结果表明:超过90%的数据点预测结果的相对误差包络在±50%范围内,紧凑拉伸试件蠕变-疲劳寿命的模型预测结果与试验值误差为2.9%。本文所提出的模型和数值模拟方法可以很好地描述钛合金结构的室温蠕变-疲劳失效过程。

残余应力与拘束对GH4169合金疲劳及蠕变-疲劳裂纹扩展速率影响研究

为深入理解残余应力与拘束在疲劳载荷下的交互作用,以镍基高温合金GH4169为研究对象,选用紧凑拉伸(CT)试样,对不同拘束CT试样的上方施加不同大小的预加载荷从而在裂尖产生不同应力。将该应力作为残余应力,系统研究残余应力和拘束交互作用下GH4169合金的疲劳和蠕变–疲劳裂纹扩展速率。结果表明:随着裂尖残余压应力的增加,不同拘束下GH4169合金的疲劳和蠕变–疲劳裂纹扩展速率均降低。与低拘束试样相比,高拘束试样的疲劳和蠕变–疲劳裂纹扩展速率对残余应力的变化更加敏感,这主要与裂尖Mises应力和垂直于裂纹扩展方向的正应力有关。与疲劳裂纹扩展速率相比,蠕变-疲劳裂纹扩展速率对残余应力的变化更加敏感。

大型钛合金构件激光熔化沉积制造过程残余应力诱发开裂机理研究

残余应力诱发开裂问题仍是限制大型构件激光熔化沉积工业化应用的一大瓶颈。因此,探究激光熔化沉积大型复杂构件残余应力的演化规律,以及残余应力诱发裂纹萌生和扩展的微观组织相关性具有重要意义。通过对大型钛合金构件开裂断口形貌分析及宏观热-力耦合有限元计算,首次发现激光熔化沉积过程独特的热应力三阶段非对称循环加载模式,即稳定循环-突增加载阶段、非线性循环加载阶段及线性循环加载阶段。以热-力耦合计算得到的残余应力为输入,采用耦合损伤的晶体塑性计算研究了3种热应力加载模式对激光熔化沉积特有的网篮组织的破坏程度,发现线性循环加载模式对网篮组织破坏性最大,稳定循环-突增加载模式破坏性次之,非线性循环加载模式破坏性最小。这种热应力加载模式、断口形貌及微观组织分析进一步表明,残余应力诱发开裂现象是由过大的热应力累积、零件的几何特征、热应力加载模式及成形缺陷等多因素协同控制的,而非单一因素影响,这也为从适时消除应力、优化零件结构及工艺参数、减少和抑制缺陷产生等方面系统化控制开裂问题提供了方向。

焊接残余应力重分布与裂纹扩展的耦合分析

焊接残余应力对于焊接结构疲劳寿命影响明显,同时焊接残余应力并不是一成不变的,随着裂纹扩展会发生重分布。因此,开展焊接残余应力重分布和裂纹扩展的耦合研究对于准确预测焊接结构的疲劳寿命至关重要。本文基于热弹塑性有限元法与扩展有限元法(XFEM),提出考虑焊接残余应力重分布和裂纹扩展耦合的疲劳寿命分析方法。以TC4钛合金拉伸疲劳试样为例,结合扩展有限元法,研究焊接残余应力随裂纹扩展的重分布情况,循环迭代计算得到裂纹扩展过程中裂尖前方焊接残余应力分布,以及焊接残余应力重分布下的疲劳裂纹扩展a-N曲线。计算结果表明,裂尖处焊接残余应力会随裂纹扩展先增大后减小,相比于将焊接残余应力视为恒定值,考虑焊接残余应力重分布的扩展有限元疲劳寿命分析方法能更准确地预报焊接结构疲劳寿命。

热处理对激光沉积修复TA15疲劳性能的影响

应用激光沉积修复(LDR)技术对TA15钛合金进行修复试验,对试样进行多种热处理,对比研究了修复件的疲劳裂纹萌生和扩展特性。采用扫描电子显微镜分析了试样疲劳断口。结果表明,退火态试样中值疲劳寿命能达到锻造态母材的40%;固溶时效态试样中值寿命能达到退火态的1.7倍。修复后试样的疲劳行为具有很强的组织敏感性,疲劳源区有与α片层集束尺寸和形貌相同的解理断裂面;固溶时效热处理后,马氏体的产生改变了部分片层取向,使裂纹扩展速率变慢。

微观组织对钛合金扩散连接层合板疲劳裂纹扩展行为的影响

采用扩散连接技术制备了TA2/TA2/TA2层合板。通过引入Ti55异质层制备了TA2/Ti55/TA2异质层合板。通过疲劳裂纹扩展试验,比较了两种层合板的疲劳寿命,研究了异质层对层合板疲劳裂纹扩展行为的影响。通过SEM和EBSD微观分析方法,揭示了异质层对层合板疲劳裂纹扩展行为的影响机理。异质层合板的疲劳裂纹扩展寿命是同质层合板的1.65倍。裂纹尖端从TA2层进入TA2/Ti55界面时,疲劳裂纹扩展速率降低了53.1%。界面壁垒增加、疲劳裂纹尖端塑性区尺寸减小和晶粒择优取向弱化均延缓了疲劳裂纹扩展速率的增速。

Oxidation damage zone formed in creep fatigue crack growth of GH4169 alloy at 650℃

In the creep fatigue crack growth of GH4169 alloy,oxidation is a prominent damage source,which is mainly manifested as the oxidation damage zone in front of crack tip.In order to investigate the property of the oxidation damage zone formed in the creep fatigue crack growth,crack growth tests of directly aged GH4169 alloy were conducted at 650℃ in air under various load conditions.Interrupted tests were performed to observe the damage characteristics at crack tip.Block tests were systematically executed to quantify the dependency of oxidation damage zone size on load and holding time.The crack propagation of the GH4169 alloy has a close relationship with grain boundary oxidation at 650℃.An oxidation damage zone in front of crack tip includes intergranular microcracks and oxidised but uncracked grain boundaries.Its size has been calculated from transient crack growth rate and described as a function of maximum stress intensity factor and holding time.Based on oxidation damage zone size,a novel model has been developed to predict the creep fatigue crack growth rate of the GH4169 alloy at 650℃.

TEM Morphology and FCP Rate of Ti-5Al-2Mo-3Zr Alloy

The transmission electron microscopic morphology of the phases and fatigue crack propagation (FCP) rate of Ti-5Al-2Mo-3Zr alloy were investigated in this study. Microstructure of the alloy consists of α and β phases after furnace- and air-cooling, and interfacial phase appears at the boundaries between these two phases. After water quenching, the microstructure consists primary of α and h.c.p. martensite α′ which assumes acicular. There are many twins within the α′ plates. No retained β phase exists after quenching from any temperature. During aging, β particles precipitated along the boundaries and inside the martensite plates with Burgers orientation relationship. The fatigue crack propagation rate (low frequency) is not sensitive to the microstructure, tensile strength and rolling direction. Analysis of the fractography shows that main cracks propagated serpentinely and secondary cracks existed everywhere. In high stress intensity range, the resistance of FCP is better than that of Ti-6Al-4V.

TC4ELI钛合金在3.5%NaCl溶液中的应力腐蚀行为研究

应力腐蚀是海洋装备用钛合金面临的主要威胁之一。为了给海洋装备的安全服役提供技术支撑,以海洋环境用TC4ELI钛合金为研究对象,根据恒位移WOL(楔形张开加载)试样测定KISCC的原理,系统开展了不同组织状态下TC4ELI钛合金在3.5%NaCl溶液中的应力腐蚀行为研究。用光学显微镜和扫描电镜观察了应力腐蚀试样断口形貌和裂纹扩展路径,用二次离子质谱仪分析了裂纹部位元素分布情况。结果表明:TC4ELI双态组织(α相与β相)板材的应力腐蚀门槛值KISCC为56.01 MPa·m^(1/2),魏氏组织板材的应力腐蚀门槛值KISCC大于67.48 MPa·m^(1/2),应力腐蚀试样断口主要由基体、疲劳预制区和撕裂区组成,双态组织试样疲劳预制区与撕裂区界面存在应力腐蚀区,裂纹尖端除主裂纹外还有部分次生裂纹出现;魏氏组织试样未发现明显应力腐蚀开裂特征,魏氏组织的抗应力腐蚀能力优于双态组织的。TC4ELI合金双态组织试样的应力腐蚀开裂以穿晶断裂为主,氢元素易于在裂纹尖端富集。