Ce对Al－Li合金物理短裂纹扩展特性的影响

本工作研究了２０９０和８０９０合金名义门槛值、本征门槛值和闭合效应对裂纹尾迹的响应关系。Ａｌ－Ｌｉ合金具有优良的疲劳长裂纹门槛值和强的短裂纹效应，其物理本质是高的裂纹闭合效应和裂纹闭合效应的尾迹依赖性．２０９０合金微小裂纹的扩展抗力劣于７０７５合金，系因△Ｋｉ低；８０９０合金的△Ｋｉ与２０２４合金相当，其短裂纹扩展抗力相当或优于２０２４合金，微量稀土元素Ｃｅ可显著改善２０９０合金的本征门槛值，有益于裂纹闭合效应，较大幅度提高其疲劳长、短裂纹扩展抗力。Ｃｅ降低８０９０合金的本征门槛值，有损于其疲劳长、短裂纹扩展抗力。     

Ce对Al—Li合金物理短裂纹扩展特性的影响

本工作研究了２０９０和８０９０合金名义门槛值、本征门槛值和闭合效应对裂纹尾迹的响应关系。Ａｌ－Ｌｉ合金具有优良的疲劳长裂纹门槛值和强的短裂纹效应，其物理本质是高的裂纹闭合效应和裂纹闭合效应的尾迹依赖性。２０９０合金微小裂纹的扩展抗力劣于７０７５合金，系因ΔＫｉ低；８０９０合金的ΔＫｉ与２０２４合金相当，其短裂纹扩展抗力相当或优于２０２４合金。微量稀土元素Ｃｅ可显著改善２０９０合金的本征门槛值，有益     

铝锂合金疲劳裂纹扩展行为与裂尖屏蔽效应

研究了2090、8090铝锂合金,7075、2024普通铝合金的疲劳裂纹扩展速率,门槛值,裂纹闭合效应,裂纹偏转特性和形貌,并讨论了上述各因素之间的内在联系。结果表明:在强度水平相近时,铝锂合金的疲劳裂纹扩展抗力明显优于普通铝合金,主要归因于其裂纹闭合和裂纹偏转的裂尖屏蔽(Shiekding)作用。铝锂合金特有的断裂特征(脊骨状断口)和强的氧化倾向,使之获得了高的粗糙度诱发裂纹闭合效应和氧化物诱发裂纹闭合效应。最后,本文还较系统地讨论了扁平晶粒、强织构、平面滑移、沿晶稳定相等对铝锂合金断裂特征、裂尖屏蔽效应等的影响。     

含Sc航空7075合金的疲劳裂纹萌生及扩展行为

对不同含量Sc微合金化的7075合金进行了疲劳裂纹萌生、疲劳裂纹扩展的对比研究,并对其作用机理进行了分析。结果表明,随着Sc含量的增加,合金的疲劳萌生寿命和强度比都呈现先增加而后降低趋势,在Sc含量为0.24%时,疲劳萌生寿命和强度比最大;当应力比为0.1时,在Sc含量为0.24%时合金具有最大的疲劳裂纹扩展阻力,而当应力比为0.5时,在Sc含量为0.48%时合金具有最大的疲劳裂纹扩展阻力,这可能与细小的晶粒尺寸和较多的第二相阻碍裂纹扩展有关。总而言之,在7075合金中加入0.24%Sc具有最佳的疲劳性能和断裂韧性。     

SiC颗粒对SiC_p/2009Al复合材料疲劳短裂纹扩展的影响

采用粉末冶金法制备15%(体积分数)SiC_p/2009Al复合材料,研究该材料的微观组织、力学性能、高周疲劳性能以及疲劳断口形貌.结果表明:在SiC_p/2009Al复合材料的疲劳短裂纹扩展阶段,SiC颗粒及其表面包覆的2009Al薄层在裂纹扩展面上形成"丘陵"状形貌,使疲劳断口的粗糙度增大,裂纹的闭合效应也随之增大:同时"丘陵"状形貌可以引发疲劳裂纹扩展路径偏析,使裂纹扩展的有效驱动力减小并使裂纹扩展路径增加;上述裂纹迟滞效应使SiC_p/2009Al复合材料在短裂纹扩展阶段具有较高的疲劳裂纹扩展抗力.     

显微组织对铝合金疲劳与断裂的影响

最近为宇航工业生产的铝合金和所发展的状态,与较老的合金7075-T6和2024-T3相比,在强度、断裂韧性、抗剥落腐蚀和抗应力腐蚀方面提供了良好的综合性能.然而,为了进一步提高一些飞机结构部件的可靠性或工作能力,则要求提高周期载荷裂纹扩展抗力.因此,为了弄明白显微组织如何影响疲劳裂纹的扩展,做了许多工作.对大部分结构材料来说,在提高疲劳裂纹扩展抗力的同时,不得损失其承受高的单向载荷的能力.所以,必须号虑强度和韧性.本文的目的是要说明,某些显微组织是怎     

稀土Ce对Al-Cu4.5%合金热疲劳性能的影响

在接近材料实际工况的实验条件下,以铝铜合金(Cu4.5%)为研究对象,用自约束型热疲劳实验机对铈加入量不同的合金,进行了热疲劳实验以及热疲劳裂纹萌生及扩展规律的研究。结果表明:在裂纹萌生阶段,不加铈的铝铜合金的裂纹萌生速度明显大于加铈的合金;在裂纹扩展阶段,不加铈的铝铜合金裂纹扩展速度仍是最快。加入0.5?的合金在裂纹形成后期扩展速度明显加快,这是因为合金中出现针状富稀土相,这种针状相割裂了基体,使得合金力学性能降低、热疲劳抗力下降。加入0.3?的合金不论在裂纹萌生阶段还是裂纹扩展阶段,其热疲劳抗力都优于其它成分的合金。     

航空发动机轴承钢高温碳氮共渗的渗层组织与性能

航空发动机轴承用Ｍ５０ＮｉＬ钢高温碳氮共渗,可获得均匀细小的淬火,回火组织,其疲劳裂纹扩展抗力和高温硬度高于Ｍ５０钢。分析认为,疲劳裂纹沿渗层的扩展行 残余奥氏体的诱发裂纹闭合效应,残余压应力,以及渗层塑性,韧性对断裂机制的影响有在;     

新型镍基粉末高温合金FGH98的高温疲劳裂纹扩展行为研究

测定了新型粉末高温合金FGH98在650℃空气环境中的疲劳裂纹扩展速率,与前两代粉末高温合金FGH95和FGH96的裂纹扩展速率进行了比较分析,研究了合金显微结构以及保载时间对FGH98合金裂纹扩展速率的影响.结果表明,FGH98合金的高温疲劳裂纹扩展抗力较前两代粉末高温合金有了明显提高.控制固溶后以适当的方式冷却,使得二次和三次γ’相均匀匹配析出,可以获得具有良好疲劳裂纹扩展抗力的合金组织.粗晶组织有利于降低FGH98合金的疲劳裂纹扩展速率,尤其是在近门隘区.FGH98合金的高温疲劳裂纹扩展速率随保载时间的增加而增加,其断裂模式相应地从穿晶-沿晶混合断裂变为沿晶断裂.     

时效制度对2A97铝锂合金疲劳裂纹扩展速率的影响

针对不同时效制度下2A97铝锂合金的疲劳裂纹扩展速率进行了研究;并通过光学显微镜、透射电镜和扫描电镜对合金的微观组织和疲劳断口进行了观察分析。结果表明:2A97合金薄板(T3+135℃/12 h)时效态的裂纹扩展速率最低,其次为T3,(T3+135℃/60 h)时效态的扩展速率略低于(T3+135℃/120 h)时效态,T6态的裂纹扩展速率最高。时效制度在较低的ΔK水平下对合金的裂纹扩展速率影响较大,而在较高ΔK水平下影响较小。δ′相及细小的T1相可以促进共面滑移,增加滑移可逆性,降低裂尖应力集中程度及塑性累积水平,同时使裂纹发生偏折,促进裂纹闭合效应,提高裂纹扩展抗力;粗大、密集的T1相及半共格的θ′相抑制共面滑移,易于造成应力集中,降低裂纹扩展抗力;晶界处的无沉淀析出带(PFZ)及平衡相急剧降低晶界强度,极易造成沿晶开裂,使合金裂纹扩展速率迅速提高。     

Influence Mechanism of Cerium on the Threshold of Short Fatigue Crack for Aluminum-lithium Alloy 2090

The short fatigUe cracks for aluminum-lithitim alloys are mainly in the field of mechanicalcharacteristic research [l--31. It is found thataluminum-lithium alloys eallibit the strongershort crack effect, which is difficult to overcome. In this icicle, the thresholds of short fatigUe cracks for alUminUm-lithium alloys 2090and 2090 Ce have been quantitatively stUdiedand evaluated. By adding Ce into alloy 2090, thebeneficial influence over the microstrUcture andproperties can be achieved [4, 5…     

Fatigue Crack Growth of Peened Friction Stir-Welded 7075 Aluminum Alloy under Different Load Ratios

The effect of peening on the fatigue crack growth performance of friction stir-welded 7075 aluminum alloy was investigated. The fatigue crack growth rates were assessed for laser- and shot-peening conditions at stress ratios (R) of 0.1 and 0.7. The surface and through thickness residual stress distributions were characterized for the different regions in the weld. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and the as-welded condition. The effect of the compressive stresses obtained through laser peening was deemed responsible for increasing the resistance to fatigue crack growth of the welds.     

淬火条件对砂铸A356合金疲劳裂纹扩展速率的影响

在相同固溶和时效条件下，选择3种不同温度的水对砂铸A356合金进行了淬火处理，并对这3种不同淬火态下的合金进行了拉伸及疲劳裂纹扩展性能研究。结果表明，随淬火介质温度的降低，淬火冷却速率提高，合金的强度增加，塑性下降；当应力比R较低时，淬火条件对合金门槛区的裂纹扩展有明显影响，降低淬火介质温度可以提高合金的疲劳裂纹扩展阻力。此外，不同淬火态合金的疲劳扩展均明显地表现出与应力比R的相关性，这种相关性可以用裂纹闭合来说明。     

8090Al—Li合金的疲劳裂纹扩展

研究了8090型Al—Li合?疲劳裂纹扩展行为以及组织结构和环境的影响,结果表明,在空气中,自然时效和欠时效?现出最好的疲劳裂纹扩展阻力,其次是峰时效状态,而过时效状态的疲劳裂纹扩展阻力最低,在3.5%NaCl水溶液中,时效状态对疲劳裂纹扩展行为有着同空气中相同的影响规律,但在同样的时效条件下,在3.5%NaCl水溶液中的疲劳裂纹扩展阻力要比空气中的低,在相同的应力强度因子范围△K作用下,缺口短裂纹和物理短裂纹均表现出比长裂纹高的扩展速率,用位错的平面滑移性和循环滑移可逆性解释了时效的影响,用裂纹的闭合效应和裂纹尖端塑性区尺寸说明了长短裂纹扩展行为的差别。     

7075铝合金FSW接头腐蚀疲劳性能及断裂特征

以7075-T6铝合金搅拌摩擦焊接头为研究对象,对其显微组织结构、3.5% NaCl(质量分数)溶液腐蚀疲劳寿命和腐蚀疲劳断裂特征进行了研究,分析了7075铝合金搅拌摩擦焊接头的腐蚀疲劳性能及断裂过程.结果表明,7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳S-N曲线方程为lgN=5.845-0.014S,随着应力幅增大,腐蚀疲劳寿命大幅度降低;腐蚀疲劳裂纹起源于接头的热影响区,逐渐扩展最终断裂于接头的焊核区.腐蚀疲劳断口存在多个裂纹源,且受到应力集中作用的影响,裂纹源萌生于腐蚀坑处.高应力作用加剧了试样边角部分的腐蚀损伤,导致边角比平面位置腐蚀程度更严重.裂纹扩展区出现了明显的晶间断裂和疲劳辉纹;在腐蚀介质和交变载荷的共同作用下,裂纹扩展区腐蚀程度最重,晶界处产生了阳极溶解现象并产生了“冰糖块状”和“蚁巢状”的形貌特征;瞬断区产生了大量解理台阶和二次裂纹,为脆性断裂,在第二相粒子分布区域存在孔洞形貌特征.     

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up-on the aging conditions.The naturally aged and underaged specimens showed the highest re-sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overagedspecimen is the lowest and that of the peakaged specimen in between.As compared to air,3.5% NaCl solution does not change the effect of aging conditions on the fatigue crackgrowth,but causes a decrease of the resistance to the fatigue crack growth under the same ag-ing condition.Both short cracks from notch and physically short cracks showed much higherrates of fatigue crack propagation in comparison with long cracks under the same aging condi-tion and stress intensity level.The growth behavior of the short crack depends on its type.Thegrowth rate of short crack from notch decreases first to a minimum and then increases with in-creasing △K.However,the physically short crack grows at a progressively increasing rate.The effects of aging conditions are explained in terms of the slip planarity of dislocations andthe cyclic slip reversibility.The observed short crack behavior is considered to be dependent onthe crack closure and the local plasticity near the crack tip.     

金属/金属层合板疲劳性能研究

以１４２０Ａｌ－Ｌｉ合金。ＬＹ１２爆炸层合板和ＬＦ２１热轧层合板为研究对象。考究了不同结合强度界面对Ｌ－Ｔ取向疲劳裂纹扩展的影响。结果表明：随界面结合强度的减弱分层越来越剧烈。其止裂韧化效果越来越明显。三种实验层合板材的疲劳裂纹扩展都出现了止裂现象。但其机制有所不同。Ａｌ－Ｌｉ合金的止裂主要来自事机制。爆炸层合板和热忆合层板的止裂主要来自分层机制。     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growthhave been studied,and the influences of crack tip plastic zone,crack tip blunting as well ascrack closure were discussed.Proportional(model I)overloading may cause more seriouscrack growth retardation than non-proportional(mixed mode)overloading.Therefore,forestimating the fatigue life of engineering structures to simplify a real overload which may of-ten be non-proportional as a proportional one is not always safe.     

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

It is obtained in this paper that the fatigue threshold value of mode H was 1.9 times of that ofmode Ⅰ in dual-phase steel(DPS),and the normal stress intensity factor range oJ mode Ⅱbranch crack tip was 2.2 times of that of mode Ⅰ.Above results illustrate that the resistance ofmode Ⅱ crack growth was higher than that of mode Ⅰ,the former resulting fromroughness-induced shear resistance,the latter,crack closure.The mode Ⅱ component can play two important roles in near-threshold fatigue crackgrowth:(1)increasing crack tip plasticity which accelerates the crack growth and(2)intro-ducing crack surface contact and rubbing to reduce the crack propagation rate.By means ofcrack closure,the quantity of shear resistance was easily solved in this paper.The frictionshear stress strength factor range of mode Ⅱ,K_,is still much higher than the closure stressstrength factor range of mode Ⅰ,K_(Ⅰ,cl).This illustrated that the roughness enlarged the secondrole and played a role of shielding crack tip from mode Ⅱ crack.     

喷射沉积SiC_P／Al-7Si复合材料的疲劳裂纹扩展

采用紧凑拉伸试样进行恒载和降K控制的拉--拉疲劳实验, 研究了喷射沉积SiC_p/Al-7Si复合材料及其基体的疲劳裂纹扩展行为. 通过金相显微镜和扫描电镜观察了复合材料及其基体的组织和疲劳裂纹扩展形貌, 研究了SiC颗粒对复合材料疲劳裂纹扩展机制的影响. 结果表明: 复合材料在任何相同的ΔK水平下其抗疲劳裂纹扩展能力优于基体材料, 并表现出较高的疲劳门槛值. 其原因是复合材料中裂纹裂尖遇到增强颗粒时, 裂纹发生偏转, 特别是SiC颗粒自身微裂纹萌生有效降低了裂纹尖端的应力强度因子, 复合材料的裂纹闭合效应也随之增大. 去除裂纹闭合效应的影响, 当有效应力因子ΔK_eff作为裂纹扩展的驱动力时, 复合材料的裂纹扩展速率却高于基体.