TC18合金大型锻棒冲击韧性的横纵向差异研究

针对TC18合金大型锻棒冲击韧性的横纵向差异及其内在原因进行了系统研究。夏比冲击实验显示,锻棒头部、中部和尾部3处C-L试样的冲击韧性均高于C-R试样。示波冲击实验发现,抵抗裂纹萌生的能力是决定合金冲击韧性的关键因素,C-L试样的裂纹萌生功明显大于C-R试样。断裂形貌分析表明,裂纹以微孔聚集方式萌生,主要起源于试样缺口附近的强化相界面处(如晶界α相)。C-L样品中微观组织的拉长方向和开裂方向平行,但和微孔萌生后聚集生长的方向垂直,裂纹不易生长至临界尺寸进行纵深扩展,因而消耗的裂纹萌生功较高,表现出较好的冲击韧性;相反,C-R样品的初生α相(包括晶界α相)和裂纹萌生的方向相同,裂纹容易顺着强化相界面生长至临界尺寸而失稳扩展,从而导致较低的冲击韧性。     

大规格Ti80合金棒材冲击韧性各向异性研究

针对大规格Ti80合金棒材冲击韧性各向异性现象及其内在原因进行了系统研究。夏比冲击试验显示,棒材R/2处弦向的C-R试样（缺口所在面与棒材轴向垂直）的冲击韧性远高于C-L试样（缺口所在面与棒材轴向平行）。金相组织结果表明,热处理态Ti80合金棒材横截面显微组织主要由均匀的等轴状初生α相组成,而纵截面组织中还存在与棒材轴向平行的长条状初生α相;冲击断口扫描电子显微镜（SEM）结果表明,C-R试样断面起伏更为剧烈,断面处存在大量横向裂纹,裂纹扩展方向垂直于长条状初生α相,长条状初生α相起到阻碍裂纹扩展的关键作用,从而冲击韧性明显高于C-L试样;断口侧面（与缺口所在面垂直的侧面）背散射电子衍射（EBSD）结果显示,C-R试样在裂纹萌生及扩展的过程中发生了更为严重的塑性变形,这个过程需要消耗更多能量,从而具有更高的冲击韧性。     

Ti-55531合金变形及断裂行为的原位SEM分析

采用原位SEM拉伸方法对比分析了Ti-55531合金片层和双态组织静载下的变形及断裂行为。结果表明,静载下α相的特征参数对该合金的形变、裂纹萌生及扩展有强烈影响。片层组织中粗大次生αs片较软,αs片最先变形促进位错滑移,位错运动至次生αs和残留βr的界面处堆积,塑性变形导致局部应力集中促进裂纹萌生,并沿αs/βr相界面扩展。双态组织中初生等轴αp是相对最软相且尺寸较大,位错滑移自由程较大,易启动多系滑移,αp内不同位向的滑移线交割促进应力集中,部分位错在αP/βtrans界面处堆积产生应力集中,两者导致微裂纹萌生于αp内及αp/βtrans界面,并沿αp/βtrans界面和αp聚集处扩展。     

FH550级海洋平台用钢冲击断裂行为实验研究

通过热模拟、拉伸和低温冲击实验,采用SEM,TEM,EDS和EBSD等手段研究了FH550海洋平台用钢的低温断裂行为.结果表明,轧态实验钢为下贝氏体和粒状贝氏体的混合组织,回火后为回火贝氏体组织;冲击断口多为韧窝断口,部分等轴韧窝底部有含Ca和Al的氧化物夹杂,个别试样呈现准解理断裂,存在尺寸大于10μm的含Fe,Mn的碳化物夹杂,恶化韧性,导致冲击吸收功波动;断裂过程中裂纹扩展的主要方式为微孔聚合长大并与裂纹颈缩连结生长,同时存在的剪切扩展裂纹易受到由于塑性变形而聚集成团的富C硬相的阻碍,从而增加了裂纹扩展功.79.3%的大角度晶界比例以及7.61μm的细小晶粒尺寸是获得优良低温冲击韧性的关键因素.     

激光冲击对GH742镍基合金疲劳短裂纹扩展的影响

对GH742镍基高温合金紧凑拉伸件预制裂纹,并分别进行1次及3次搭接激光冲击处理,利用复型技术和光学显微镜研究了镍基合金的裂纹起源、裂纹分布及尺寸演化全过程.结果表明,未作激光冲击强化试样形成的是沿晶裂纹,晶界处大量位错塞积群造成很高的应力集中;而经激光冲击强化后试样在远离冲击强化区域以裂纹群的形式萌生,且大约集中在同一时段萌生,疲劳后期萌生的新裂纹条数不多,且激光冲击作用的次数越多,裂纹扩展速率越小,激光冲击处理产生的强化效应能大大降低裂纹扩展速率,在某些强化区还能明显提高应力强度因子门槛值.     

退火工艺对Ti700钛合金冲击韧性的影响

对Ti700钛合金在不同热处理制度下的冲击韧性进行了研究.结果表明,Ti700钛合金冲击韧性随退火温度升高而发生明显变化,650～850℃处理后试样的冲击韧度为低值区,915℃处理可获得最高值.相变点以下热处理试样的冲击断口为韧性断裂,相变点以上热处理的为准解理的脆性断裂.915℃试样热处理的初生α相体积分数为12%,随着退火温度的降低,初生α相体积分数升高,尺寸增大,Al元素富集于α相中.初生α相界有利于微裂纹的形核,随着初生α相体积分数减少,避免了在初生α相界处孔洞萌生,可有效改进材料的冲击韧性.     

ZTC4合金高周疲劳断裂机理

对ZTC4合金室温高周疲劳断裂机理进行研究.结果表明:疲劳裂纹主要沿垂直和平行于α/β片层的方向扩展:裂纹萌生于试样表面、内部晶界以及α/β相界面;源区为类解理断裂,疲劳裂纹扩展区除有大量疲劳条带外,还出现扩展台阶和二次裂纹;断口形貌受晶体学取向和显微组织影响.     

钢结硬质合金GJW50热疲劳开裂的探讨

对钢结硬质合金GJW50试样进行了冷热循环实验,观察了试样在热应力作用下热疲劳裂纹的萌生、扩展以及试样开裂的全过程。结果表明:热疲劳裂纹出现前,在试样的光滑缺口边缘上产生了明显的塑性变形,呈现出凹凸不平。在试样缺口顶端的凹坑内萌生首条裂纹,萌生地是小颗粒的WC粒子集聚区或大颗粒的WC"自裂纹"。萌生的首条裂纹沿着与热循环方向平行的方向扩展,最终成为主裂纹。其扩展途径主要为沿WC聚集区和钢基体相的界面扩展以及在大面积的WC粒子聚集区内扩展。主裂纹遇到钢基体相后受阻,裂纹尖端钝化、转向,寻找耗能少的区域扩展。主裂纹在扩展时形成二次裂纹,但未形成明显上的龟裂;最终,仍然是主裂纹导致试样断裂。     

β锻TC17钛合金的高周疲劳研究

对β锻的TC17合金的高周疲劳性能进行了研究,获得了光滑试样的应力疲劳寿命曲线(S-N曲线)。结果表明,β锻的网篮组织具有较高的高周疲劳性能,疲劳极限达到544 MPa。对疲劳断口和裂纹扩展路径进行了分析,发现网篮组织的高周疲劳断口具有裂纹扩展路径曲折、二次裂纹较多的特征。原始β晶界的粗大α相是显微组织中的"弱区",疲劳裂纹易沿粗大α相萌生和扩展。     

高铌TiAl合金在疲劳蠕变作用下的裂纹萌生及扩展

利用SEM原位观察技术研究了近片层Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金在750℃疲劳蠕变交互作用下的裂纹萌生及扩展行为,循环实验采用在最大拉应力保载的梯形波.结果表明,裂纹主要在片层团界面萌生,裂纹萌生方式包括蠕变空洞和疲劳微裂纹.片层团界面处的微裂纹先通过吞并蠕变空洞或在裂纹尖端应力集中作用下沿片层团界面进行扩展,然后相互连接长大;当裂纹扩展受到不同取向的片层团界面阻碍时,受阻的裂纹开始沿试样厚度方向扩展,且附近伴随出现垂直于载荷方向的微裂纹;最终受阻的裂纹相互连接直至合金断裂.将实验结果与该合金在相同条件下疲劳变形和蠕变变形的原位观察结果进行了比较.结合实验结果建立了高铌TiAl合金在疲劳蠕变交互作用下裂纹萌生及扩展示意模型.     

Effects of Nb Addition on Charpy Impact Properties of TiVTa Refractory High-Entropy Alloy

In the present study, the effects of Nb addition on Charpy impact properties of TiVTa refractory high-entropy alloy with high strength-ductility trade-off were systematically studied by using the instrumented Charpy impact testing machine. The experimental results showed that the impact toughness was remarkably improved by Nb addition in TiVTa to form TiVTaNb alloy. The crack initiation energy and propagation energy of TiVTaNb were 67.3% and 24.9% higher than that of TiVTa, indicating that Nb addition simultaneously reinforced the resistance to crack initiation and propagation. The impact fracture of TiVTaNb exhibited larger bending degree of shear lips, deeper dimples and more secondary cracks which effectively dissipated more impact energy. The deformation mechanism of TiVTa alloy was dominated by dislocation activities. While in TiVTaNb, the deformation mechanism was synergized by dislocation activities and deformation twinning, which were the main contributors for the improved impact properties and the stronger crack resistance of TiVTaNb alloy under impact loading.     

Dynamic fracture resilience of elk antler: Biomimetic inspiration for improved crashworthiness

The antler of the North American elk has been shown to have impressive fracture resistance under quasi-static loads, but given its viscoelastic behavior and impact nature of loading, questions remain as to its mechanical, and in particular, fracture behavior under dynamic loading. Samples were tested using a unique split-pressure Hopkin-son bar (SPHB) for four-point bending experiments in order to measure the fracture toughness of this material Interestingly, the hierarchical structure of antler had a strong influence on crack propagation characteristics, and cracks tended to propagate along the osteonal growth direction, whether loaded parallel or perpendicular to the osteonal growth direction. This occurred to such a degree so as to stop all crack propagation through the sample on transverse specimens, thus inhibiting the ability to measure a valid crack initiation toughness and demonstrating the extreme resilience of antler to resist dynamic fracture. The high resilience of antler to impact loading may serve as biomimetic inspiration to future material development for crashworthiness and defense applications.     

2197(Al-Li)-T851合金的疲劳裂纹萌生与扩展行为研究

研究了2197-T851合金的疲劳裂纹萌生与扩展特性。结果表明,2197铝锂合金的疲劳裂纹萌生与应力水平有关,在较低应力下,主要萌生在第二相粒子、第二相粒子/基体界面以及表面缺陷处,而在较高应力水平下,还出现沿滑移带和晶界处萌生。裂纹的扩展优先沿有利滑移面,在晶粒间的扩展以沿{111}面的穿晶扩展为主,也有沿小角度晶界扩展的情况,合金具有很好的平面滑移性。裂纹在晶粒内的扩展并非沿一个滑移系的直线扩展而是频繁发生偏折,却始终沿{111}面,可剪切共格相δ’粒子造成晶内扩展路径曲折。     

铸造γ+α双相不锈钢的裂纹生长与扩展速率

采用阶梯能量示波冲击试验法研究了铸造双相不锈钢冲击弯曲破断时,裂纹的萌芽、生长、扩展3个阶段与载荷-位移曲线及断口的对应关系,研究了裂纹的生长与扩展速率。结果表明,裂纹萌芽于屈服后的第2个载荷-位移曲线高峰。裂纹生长的临界尺寸对应于载荷-位移曲线平台后的第2个下跌槛。裂纹生长对应于断口上的启裂区。裂纹扩展对应于断口上的扩展区。在启裂前区裂纹以较低速率波动生长,启裂后区裂纹的生长速率更趋减缓。裂纹扩展的前期扩展速率很高,后期则迅速下降。裂纹从萌芽到生长至临界尺寸耗费了总破断功的76.2%。     

GH536高温合金焊接接头疲劳裂纹萌生与扩展原位试验研究

采用原位疲劳试验方法,实时观察了GH536焊接接头疲劳裂纹的萌生和扩展行为,从而揭示了GH536焊接接头疲劳裂纹的萌生和扩展机制:疲劳加载过程中,位错沿滑移带在晶界前沿塞积,晶界阻碍位错运动,裂纹沿滑移带开裂,萌生疲劳裂纹;疲劳裂纹扩展初期,受单滑移的交替作用,裂纹呈“Z”字型向前扩展,随后裂纹的扩展逐渐以主应力控制为主,垂直于加载方向、平直向前扩展;GH536合金焊接接头组织中的晶界和碳化物会阻碍疲劳裂纹的扩展。     

In-situ tensile deformation behavior of powder metallurgy Ti6Al4V alloys

In this study, in-situ tensile deformation behavior of powder metallurgy (PM) Ti6Al4V alloys was investigated to analyze the crack initiation and propagation. Accordingly, the fracture mechanisms of the as-sintered and forged PM alloys were summarized. At the initial stage of plastic deformation, cracks appeared in the stress concentration area of pores in the as-sintered Ti6Al4V alloy, and the crack propagation direction was along the phase boundary. Due to the existence of pores, early fracture was obtained, resulting in low elongation of 6.3%. After forging, the crack initiation occurred between α lamellar structure, and the propagation direction was along the lamellar direction. The fine lamellar structure in different directions in the forged PM Ti6Al4V alloy can hinder the crack propagation, thus improving the plasticity. As a result, better comprehensive mechanical performance was obtained in the forged sample, with UTS of 960 MPa, YS of 850 MPa, and EL of 16%.     

不同温度下X100管线钢的冲击韧性

通过对X100管线钢试件冲击示波曲线、冲击断口形貌的观察,对比分析了在温度20℃、-60℃及-80℃条件下试件的冲击性能,研究了冲击过程中裂纹的形核和扩展规律。结果表明,裂纹启裂及扩展功所表现出的韧脆特性与总冲击功不同。随试验温度降低,断口分层现象明显,断口分层小平台可提高试验钢的断裂韧性,片状夹杂物则促进了脆性断口的产生;随温度降低,裂纹的扩展路径呈直线化,且尖角状裂纹较多。     

BEHAVIOUR OF INITIATION AND GROWTH OF SHORT FATIGUE CRACKS

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Effects of Tempering Temperature on Microstructure and Mechanical Properties of Drill Pipe Steel 26CrMoEI北大核心CSCD

The effects of tempering temperature on microstructure and mechanical properties of steel 26CrMo were studied based on mechanical property tests and microstructure observation. The results show that a phase matrix gradually occurs recovery and recrystallization with increasing temperature during 540 similar to 690 degrees C temper process, martensite morphology fades away gradually, flake or rocklike carbides separate out along the martensite boundaries, and then change into granulated dispersed distribution, at 690 degrees C tempering carbides happen aggregation and growth on grain boundaries. With tempering temperature increasing, the strength of 26CrMo steel is gradually reducing, plasticity and toughness are gradually increasing. The tensile property and impact energy can meet all different grade drill pipe requirements in API 5DP standard with different tempering conditions. The total impact energy, crack initiation energy and crack propagation energy of 26CrMo steel are gradually increasing with the tempering temperature rising, the crack propagation energy is three times of crack initiation energy which shows great anti-crack propagation capability, but their ratio has no obvious change. The change of impact property is closely related to the strength and plasticity change, impact toughness stand or fall depends on high or low plasticity.     

SLM成形17-4PH不锈钢微观组织与动态断裂性能

通过激光选区熔化(selective laser melting, SLM)技术制备了17-4PH不锈钢,采用电子背散射衍射(electron backscattered diffraction, EBSD)和透射电子显微镜(transmission electron microscope, TEM)等方法对沉积态和固溶态试样微观组织结构进行了分析.通过示波冲击试验确定了裂纹萌生扩展的特征阶段和动态裂纹扩展阻力曲线(J-R曲线),研究了微观组织与动态断裂性能之间的关系.结果表明,沉积态试样主要由<100>择优且沿增材方向拉长的δ铁素体柱状晶、取向随机的细小马氏体,以及少量奥氏体组成,不同截面具有显著的组织各向异性;大尺寸δ铁素体柱状晶与细小晶粒的结合面作为薄弱环节,使其脆性增加,J-R曲线的撕裂模量较低,以准解理方式断裂.固溶热处理明显弱化组织各向异性,微观组织由尺寸细小、均匀的马氏体组成,其冲击吸收能量提升1倍,动态断裂韧性优良,属于韧性断裂.大尺寸δ铁素体柱状晶与周围细小马氏体晶粒界面结合较弱是沉积态17-4PH不锈钢动态断裂性能较差的主要原因.