复合细化超细晶工业纯钛高周疲劳性能研究

室温下,超细晶工业纯钛光滑试样在加载频率f=25 Hz、应力比R=-1的条件下进行高周应力疲劳测试,拟合超细晶工业纯钛应力幅σ_a与疲劳断裂循环周次N_f之间的关系曲线,并对疲劳断口形貌进行观察分析。结果表明：200 ℃退火60 min超细晶工业纯钛的疲劳极限值σ_-1为376.5 MP,比未退火超细晶工业纯钛的疲劳极限值提高56.5 MPa。疲劳裂纹源萌生于超细晶工业纯钛的表面, 200 ℃退火60 min超细晶工业纯钛的疲劳辉纹间距较小,疲劳裂纹不易扩展,室温疲劳性能优良。     

2100MPa级马氏体时效钢的低周疲劳破坏特性

通过MTS试验机、扫描电镜及能谱分析,对一种2100MPa级超高强度18Ni马氏体时效钢的恒应变低周疲劳破坏特性进行了研究.结果表明,时效时间对实验钢的低周应变疲劳性能的影响不明显,其应力幅随时效时间的延长而提高;通过对疲劳试样断口形貌的SEM观察发现,实验钢的疲劳裂纹主要起源于试样表层的夹杂物,夹杂物类型主要为Ti(C,N)与氧化物.     

铌微合金化重载齿轮钢的疲劳性能

利用旋转弯曲疲劳试验方法研究了三种重载齿轮钢渗碳后的疲劳性能。结果表明,添加铌能够细化重载齿轮钢组织,提高渗碳层硬度,从而提高其疲劳强度。同时,疲劳裂纹在渗碳层沿原奥氏体晶界扩展,铌微合金化重载齿轮钢的晶粒细化,从而可以阻碍疲劳裂纹的扩展。此外,扫描电镜观察疲劳断口发现,重载齿轮钢渗碳后疲劳裂纹起源于基体或夹杂物,夹杂物尺寸越小,疲劳性能越好。     

挤压态镁合金ZK60的超高周疲劳行为

利用超声疲劳实验研究了挤压态镁合金ZK60的超高周疲劳行为.结果表明,合金的疲劳S-N曲线在5×106-108cyc范围内存在一平台,而在108-109cyc范围内,疲劳强度逐渐降低,对应于109cyc的疲劳强度为(90±5)MPa.SEM断口观察表明,在5×106-108cyc范围内,疲劳裂纹基本上萌生于试样表面或亚表面,而在108-109cyc范围内,疲劳裂纹主要萌生于试样内部的非金属夹杂物.通过测定疲劳源区的尺寸,估算的合金疲劳强度与实验结果基本一致.疲劳源的形成是由微裂纹在多个夹杂物处起裂和合并所引起的.因此,合金的疲劳强度不是由最大夹杂物尺寸决定,而是取决于由多个夹杂物组成的"缺陷区"尺寸.通过测定多个部位的"缺陷区"尺寸,可以有效的估算合金的疲劳强度.     

AZ31镁合金在高周疲劳过程中微观组织和孪晶演变特征

利用扫描电子显微镜（SEM）和电子背散射衍射（EBSD）技术研究了室温条件下AZ31镁合金在不同加载频率（3和30 Hz）和不同应力幅值（90,95,100,105,110 MPa）疲劳变形后的组织演变规律及断口形貌特征。结果表明：随着加载应力增加,基体内残余孪晶数量增加,残余孪晶主要以拉伸孪晶形式存在。随着应力幅值的增加晶粒逐渐细化,这是由于在循环过程中,拉伸孪晶演变诱导晶粒细化。随着应力幅值的增加,织构强度显著减弱,这与试样疲劳后的再结晶机制有关。通过对试样疲劳断口的分析,发现孪晶片层处容易引起裂纹萌生,随着应力的增加,试样中裂纹扩展区面积逐渐减小,在疲劳裂纹扩展区观察到明显的疲劳辉纹。最终断裂区表面粗糙,主要存在韧窝、撕裂脊以及二次裂纹等形貌。在最终断裂区可观察到韧窝,韧窝尺寸随着循环应力的增加,在较高加载频率下,韧窝的尺寸与数量均减小。     

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.     

汽车用高强度弹簧钢54SiCrV6和54SiCr6 的超高周疲劳行为

研究了54SiCrV6和54SiCr6两种洁净高强弹簧钢的超高周疲劳行为，并利用FESEM和EPMA对疲劳断口进行了观察．实验结果表明，在高应力幅区，两种弹簧钢的疲劳破坏均起源于表面基体；而在低应力幅长寿命区，疲劳开裂均发生在试样内部．54SiCrV6钢的S-N曲线为典型的台阶式曲线，在10^9循环周次内，其疲劳极限消失；而54SiCr6钢存在疲劳极限．疲劳断口分析表明，54SiCrV6钢内部破坏是由钢中小夹杂物聚集引起的，而在54SiCr6钢中则起源于碳化物的偏聚．临界夹杂物尺寸的估算表明，当高强弹簧钢中的夹杂物尺寸大于临界夹杂物尺寸时，其疲劳极限消失．     

超声冲击改善MB8镁合金焊接接头超高周疲劳性能机理探究

通过超声疲劳试验探究超声冲击对MB8镁合金焊接接头超高周疲劳性能的影响。同时,从应力集中、残余应力、晶粒细化等三个因素来探究超声冲击改善MB8镁合金焊接接头超高周疲劳性能的机理。结果表明：在1.0×108寿命下,焊态试样疲劳强度为31.62MPa,冲击态试样疲劳强度为39.81MPa,冲击态试样疲劳强度相较于焊态提高了26%。这说明超声冲击可以明显提高MB8镁合金焊接接头超高周疲劳性能。焊态试样焊趾处应力集中系数Kt1=1.95,冲击态试样焊趾处应力集中系数Kt2=1.67,应力集中系数降低了14.4%,所以超声冲击可以降低焊趾处的应力集中程度。超声冲击后试样焊趾处的应力由残余拉应力转变成残余压应力。超声冲击细化焊趾表面晶粒改善MB8镁合金焊接接头的超高周疲劳性能。     

中碳高强度弹簧钢NHS1超高周疲劳破坏行为

测试了中碳高强度弹簧钢NHS1的超高周(109 cyc)疲劳破坏行为,并利用FESEM对疲劳断口进行了观察.NHS1钢的S-N曲线呈台阶型,在109 cyc内疲劳极限消失.疲劳断口分析表明,在高应力幅区,实验钢的疲劳破坏主要起源于基体表面;而在低应力幅长寿命区,疲劳破坏主要起裂于试样内部的夹杂物,形成"鱼眼"型断裂.在夹杂物周围存在一个粗糙的粒状亮区(GBF).GBF区边界的应力场强度因子为3.6 MPa·m1/2,与疲劳寿命无关,该值与疲劳裂纹扩展的门槛值相等;"鱼眼"边界的应力场强度因子同样与疲劳寿命无关,约为10.6 MPa·m1/2.     

拉-拉载荷下2195-T8铝锂合金在N2O4中的预腐蚀疲劳研究

液体导弹在长期加注贮存的状态下,弹体与贮箱结构常因腐蚀损伤从而导致疲劳裂纹的产生乃至断裂等问题。采用疲劳寿命测试、扫描电镜以及能谱分析等方法,研究了2195-T8铝锂合金在N₂O₄中预腐蚀180天后的疲劳裂纹萌生、扩展和断裂机理,并与未腐蚀试样进行对比。结果表明：两种环境下试样的疲劳极限为145Mpa、118Mpa,循环应力降低比值约为18.62%；试样在预腐蚀的过程中,由于形成“闭塞区”,同时与富铜相粒子形成原电池,进一步加速腐蚀历程,形成多源裂纹萌生的特征,且更易从非金属夹杂区域起裂；受到晶粒间的位错、堆积等因素的影响,发现了垂直于疲劳辉纹、穿越大晶粒及途经小角度晶界的二次裂纹；两种环境中的疲劳瞬断区的断口形貌呈现出典型的沿晶韧窝和韧性断裂的特征。     

42CrMoVNb细晶高强钢的疲劳行为

研究了不同热处理制度下得到的3种晶粒尺寸的42CrMoVNb细晶高强钢的疲劳性能.结果表明，它们的光滑疲劳试样S-N曲线在10^6—10^7周期范围内无平台出现，疲劳极限消失．SEM断口观察表明，光滑试样疲劳裂纹起源位置与寿命长短密切相关，测量了长寿命试样疲劳断裂源夹杂物的位置及尺寸．通过对实验数据的初步分析，给出了夹杂物及奥氏体晶粒尺寸应控制的水平．     

超细晶粒高强度钢的延迟断裂行为

对于微合金化处理的42CrMoVNb钢，通过快速循环热处理的方法获得最小2μm的超细奥氏体晶粒，采用缺口拉伸延迟断裂实验研究了超细晶粒试样的延迟断裂行为。结果表明，随着晶粒细化，42CrMoVNb钢的强度和缺口拉伸延迟断裂抗力逐渐提高；但当晶粒细化到2μm时，强度和延迟断裂抗力均不再提高，在高温回火态，当晶粒尺寸在20—4μm范围时，断裂机制主要为穿晶断裂；但当晶粒进一步细化到2μm时，断裂机制转变为沿晶断裂，在低温回火态，不同晶粒尺寸的试样均主要为沿晶断裂，从降低应力集中和夹杂元素晶界偏聚等角度对超细晶粒高强度钢的延迟断裂行为进行了探讨。     

GCr15钢旋转弯曲超长寿命疲劳性能分析

轴承钢GCr15旋转弯曲超长寿命疲劳实验获得的应力-寿命(S-N)曲线数据分散性较大且呈连续下降趋势,不能用台阶下降的S-N曲线描述.断口分析表明,在高应力幅区疲劳破坏主要起始于试样表面的加工划痕或夹杂物;随着应力幅的降低,疲劳破坏主要起始于试样内部的夹杂物.内部破坏均带有鱼眼特征,大部分的内部夹杂物周围带有颗粒亮区(GBF区).夹杂物尺寸的较大分散程度和小尺寸夹杂物的簇集是引起GCr15钢疲劳寿命分散性较大的因素.使用推定的GBF区成长率能够定量分析夹杂物尺寸对疲劳寿命分散程度的影响.     

Failure modes and materials performance of railway wheels

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, K _ID , is obviously lower than static fracture toughness, K _IC , and has the same trend as K _IC . The ratio of K _ID /σ _YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.     

渗碳齿轮钢18CrNiMo7-6的疲劳性能及其影响因素

以18CrNiMo7-6齿轮钢为研究用基础钢,在传统真空脱气冶炼方式基础上,采用Nb微合金化和电渣重熔冶炼相结合获得一种对比试验钢,通过旋转弯曲疲劳试验表征了两种试验钢的疲劳性能,并利用显微组织、硬度分布、疲劳断口表征以及夹杂物分析等手段,探究了两种试验齿轮钢疲劳性能的影响因素。结果表明,采用电渣重熔方法冶炼并Nb微合金化的试验钢的疲劳极限较基础钢提高90 MPa,且相同载荷下寿命显著提高,渗碳层晶粒度由基础钢的7.5级细化至9级,而残留奥氏体含量的增加导致其表面硬度降低。通过Aspex夹杂物表征发现试验钢中夹杂物数量较基础钢大幅度降低,且硬质氧化物夹杂较少,与断口表征结果相一致。综合分析可知,晶粒细化和非金属夹杂物水平下降是提升试验钢疲劳性能的主要因素。     

不同循环载荷下54SiCr6钢的疲劳强度

对高强弹簧钢54SiCr6在3种循环加载条件下(旋转弯曲,超声和拉压)的疲劳性能进行了测试和比较.由于疲劳试样存在应力梯度和尺寸差异,3种循环载荷下对应的疲劳强度有较大差别.断口分析表明,3种疲劳样品也具有不同的断口形貌.通过计算高应力截面积,可以得到不同载荷下疲劳强度之间的定量关系,从而为评估不同载荷下的不同试样的高强钢疲劳强度提供依据.     

Tempered martensite embrittlement in a 32NiCrMoV125 steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.     

Critical level of intergranular fracture to affect the toughness of embrittled 2.25Cr-1Mo steels

In general, the low-temperature brittle fracture mode of unembrittled ferritic steel is transgranular cleavage. During temper embrittlement, impurity elements, such as sulfur (S), phosphorus (P), antimony (Sb), arsenic (As), and tin (Sn), segregate to prior austenite grain boundaries, which results in a decrease in the grain boundary cohesive strength. As a result, the brittle transgranular cleavage fracture mode changes to intergranular decohesion in association with the decrease in the critical fracture (stress (σ _F) as well as the fracture toughness (K). However, the appearance of intergranular facets on the fracture surface does not cause a decrease in the K and σ _F values. In this work, quenched and fully tempered 2.25Cr-1Mo steel (in an unembrittled condition that exhibits almost 100% brittle transgranular cleavage fracture) has been embrittled for 24, 96, and 210 h at 520 °C to produce different proportions of intergranular fracture. These unembrittled and embrittled steel specimens were tested to measure K (at −120 and −196 °C) and σ _F (at −196 °C). The experimental results and detailed fractographic observations show that the K and σ _F values decrease with an increase in the area fraction of intergranular fracture, provided that the area fraction of the intergranular facet on the brittle fracture surface exceeded a certain critical level, approximately 20–22%.     

Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.