特殊体育器材用2A12铝合金的疲劳裂纹扩展机制研究

研究了特殊体育器材用2A12铝合金疲劳裂纹的扩展机制。结果表明,在裂纹萌生阶段,微裂纹沿晶扩展,扩展后期微裂纹变为穿晶扩展。试样的断面裂纹成核粒子的长度和宽度较大时,疲劳寿命较短。     

钢结硬质合金热疲劳裂纹萌生扩展机理探讨

对GJW50钢结硬质合金进行了20℃-680℃的热循环试验,研究了该合金热疲劳裂纹萌生及扩展机理,重点研究裂纹在其宽度方向上的扩展机理。结果表明：经过一定次数的冷热循环后,与缺口边缘垂直方向萌生多条平行裂纹,其中在缺口尖端处与热循环方向平行的一条微裂纹发展成为主裂纹。裂纹在萌生过程中伴随着严重的氧化腐蚀,二者相互影响,氧化腐蚀促进裂纹的萌生。在主裂纹长大的过程中,与主裂纹平行方向和垂直方向同时萌生多条相互平行的微裂纹;这些微裂纹随着循环次数的增多而长大,最终与主裂纹相连相通,形成主裂纹在宽度方向上的扩展。     

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.     

ZG42CrMo缩松区疲劳裂纹的偏折和微裂纹的形成

测定了ＺＧ４２ＣｒＭｏ高周疲劳裂纹扩展速度率，对疲劳裂纹扩展过程进行了金相追踪观察，用ＥＰＭ研究了缩松及其表面微裂纹和硫化物的形态，用ＴＥＭ研究了裂纹顶端位错组态和滑移特征。讨论了缩松对裂纹捕获和偏折对疲劳裂纹扩展的双重影响及微裂纹萌生的可能机制。     

ZG42CrMo缩松区疲劳裂纹的偏析和微裂纹的形成

测定了ＺＧ４２ＣｒＭｏ高周疲劳裂纹扩展速率，对疲劳裂纹扩展过程进行了金相追踪观察．用ＥＰＭ研究了缩松及其表面微裂纹和硫化物的形态．用ＴＥＭ研究了裂纹顶端位错组态和滑移特征．讨论了缩松对裂纹捕获和偏折对疲劳裂纹扩展的双重影响及微裂纹萌生的可能机制．     

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。     

GH4169合金拘束相关的疲劳裂纹萌生寿命

以GH4169镍基高温合金为研究对象,基于低周疲劳率相关的晶体塑性本构,引入累积能量耗散和累积塑性滑移2种疲劳指示因子作为疲劳裂纹萌生判据,对不同微缺口深度和长度下的疲劳裂纹萌生寿命进行研究。并基于统一拘束参数Ap,进一步考察拘束与疲劳裂纹萌生寿命的关联。结果表明：2种疲劳指示因子均可较好地预测疲劳裂纹萌生寿命。随着微缺口深度的增加,疲劳裂纹萌生寿命逐渐减少;随着微缺口长度的增加,疲劳裂纹萌生寿命逐渐增加。在不同的微缺口深度和长度下,疲劳裂纹萌生寿命均与(A_p)^1/2存在线性关系。可以根据该线性关系,确定拘束相关的疲劳裂纹萌生寿命。     

滚珠丝杠副的接触疲劳行为

通过显微硬度、化学成分、扫描电镜和能谱仪等方法对接触疲劳试验样品进行了分析。结果表明,试样经过一定周次的循环加载,滚道表面出现点蚀、微裂纹和剥落等现象。疲劳裂纹主要从滚道表面萌生,并与表面呈一定角度向内部扩展。材料内部的非金属夹杂物对裂纹的萌生和扩展发生影响。次表面疲劳裂纹优先在Al₂O₃·MgO颗粒周边萌生和扩展,并没有在TiN颗粒周围萌生。     

GCr15钢滚珠丝杠副的接触疲劳行为

通过显微硬度、化学成分、扫描电镜和能谱仪等方法对接触疲劳试验样品进行了分析。结果表明,试样经过一定周次的循环加载,滚道表面出现点蚀、微裂纹和剥落等现象。疲劳裂纹主要从滚道表面萌生,并与表面呈一定角度向内部扩展。材料内部的非金属夹杂物对裂纹的萌生和扩展发生影响。次表面疲劳裂纹优先在Al2O3·Mg O颗粒周边萌生和扩展,并没有在Ti N颗粒周围萌生。     

DZ125高温合金超高周疲劳裂纹萌生与扩展

裂纹的萌生与扩展是研究合金材料超高周疲劳行为的重要方面。本研究分析与探讨了温度和表面状态对DZ125合金的超高周疲劳裂纹萌生与扩展特征的影响。不同温度下,DZ125合金的超高周疲劳裂纹萌生位置和扩展方式不同。室温下,裂纹均沿表面起源,裂纹扩展以拉伸模式为主;700℃下,裂纹均沿亚表面起源,裂纹扩展以剪切模式为主。室温下,DZ125合金经激光冲击处理前后的超高周疲劳裂纹萌生位置和扩展方式均存在差异。经过激光冲击处理后,裂纹萌生于合金的内部孔洞缺陷,裂纹扩展完全以剪切模式进行。     

On the nature of the low energy of special twist grain boundaries

Optical microscopy and scanning electron microscopy were used to study the plastic zone in front of the tip of a fatigue macrocrack. Slip bands and isolated microcracks were revealed in the plastic zone which reaches 300–350 μm in size. The nucleation sites, length, and distribution of slip bands in the plastic zone were studied. The angular orientation of slip bands relative to the direction of the macrocrack propagation was measured. It was found that repeated fatigue tests lead to the formation of new bands with different angular orientations. The statistical distributions of the lengths of the slip bands and their angular deviations from the macrocrack direction were determined. A correlation of the slip bands with the structural elements of steel was found. The sizes of the isolated microcracks in the plastic zones were measured, and their statistical distribution was determined. It was shown that the isolated microcracks and slip bands coincide in length only in the region of small values.     

Cu—Ni—Al形状记忆合金形变、相变以及裂纹形核的相互关系

采用金相显微镜和扫描电镜原位拉伸技术研究了Cu-Ni-Al形状记忆合金中的马氏体形变、马氏体相变以及微裂纹形核三者之间的关系。结果表明,拉伸时缺口前方首先形成可逆变的马氏体而不出现滑移带,微裂纹沿马氏体和母相界面形核,只有当裂纹扩展－段距离且应力集中足够大后,才有可能出现形变带,此时微裂纹将沿形变带和马氏体界面交替形核。     

Experimental Research on Fatigue Failure for 2219-T6 Aluminum Alloy Friction Stir-Welded Joints

The fatigue experiment was executed for the 2219-T6 aluminum alloy friction stir-welded joints at the rotation speed of 800 r/min and the welding velocity of 150 mm/min. Most fatigue failures occurred in the weld nugget zone (WNZ), the thermo-mechanical affected zone and the nearby areas. The experimental results demonstrated that the sudden hardness gradient increases sites corresponding to the fatigue failure locations. The high-angle grain boundaries with the highest concentration were scattered within the WNZ. The microcracks initiated at the intersection of the soft grains. More than one crack initiation site was observed within the WNZ and the thermo-mechanical affected zone, when the fracture occurred in these areas. The rough surface of the welding area should be one of the main reasons for the fatigue failure occurrence. The fatigue crack growth rate in the WNZ at the first stage was fastest in comparison with the fatigue crack growth rate in the other areas of the joint.     

Comparison of the Effects of Carbides and Nonmetallic Inclusions on Formation of Fatigue Microcracks in Steels

Metal Science and Heat Treatment - The effects of carbides and nonmetallic inclusions on formation of fatigue microcracks in steels for high-load bearings are determined. The running time is shown...     

Analyses of fractured implant fixture after prolonged implantation

Although fortunately rare, fracture of implants causes significant problems for both clinicians and patients. The major cause of a fractured implant may be corrosion fatigue fracture. To investigate how to increase the fatigue life and corrosion resistance of dental implants, the surface morphology of six Steri-Oss fractured implants was analyzed. The period of implantation after loading in patient jaws varied between 23 months and 37 months. The topography and surface chemical composition were studied with electron probe micro-analysis (point mapping, energy dispersive x-ray spectroscopy) and field emission scanning electron microscopy. All samples were fractured at the screw root and the crest formed a keen-edged shape. The five samples were fractured at the first thread of the fixture and one sample at the third thread of fixture. The fatigue cracks were mainly nucleated and grown at scratches occurring for the screw root and crest formation and the cervix portion of the implant having a small curvature. The pits were nucleated in the vicinity of inclusions such as SiO₂ and corrosion fatigue cracking was predominantly propagated. Corrosion products were found on the opposite side of the starting point of corrosion fatigue crack. From observations of fatigue striations, it is possible to predict the life time of fractured implants and estimate the cleavage fracture and dimple fracture of implants. In this study, analysis of fractured surfaces revealed the characteristics of the implant materials, problems of design, fatigue life, and manufacturing process. In order to protect against corrosion fatigue fractures and prolong the fatigue life of dental implants, we must consider the implant design, implant manufacturing, and surface treatment of the implant materials.     

DEFORMATION AND FRACTURE BEHAVIOUR OF LOW CARBON MARTENSITE-FERRITE DUAL-PHASE STEEL

利用扫描电镜及其动态拉伸装置,对具有板条马氏体和铁素体双相钢组织的断裂过程进行了研究.结果表明,在拉伸应力作用下,裂纹在马氏体与铁素体界面及不同马氏体取向界面处成核,或者由于马氏体板条的断裂而产生.断裂系主裂纹钝化和微裂纹长大,最后沿强烈剪切形变带迅速连接所致.其微观形态为塑坑断口,属延性断裂机制.     

In-situ deformation of TiAl PST crystals in TEM

The deformation process of TiAl polysynthetically twinned (PST) crystals was in-situ observed in a transmission electron microscope (TEM) with a tensile holder. Microcracks, resulted from different reasons, were observed to nucleate at several different sites. In many cases microcracks were observed to nucleate on lamellar boundaries and on the boundary between the deformation twin and the matrix. The intersecting parts of two deformation twins within the same TiAl lamella have been found to be favorite nucleation sites of microcracks. Microcracks were also observed to nucleate on the domain boundary of two TiAl domains coexisting in a TiAl lamella and at the triple point among TiAl domain boundary and lamellar boundary. It has revealed that cracks tend to propagate along the lamellar boundaries when the angle between tensile axis and lamellar boundaries is about 78°. When crack propagation was hindered, one or more microcracks nucleated ahead of the main crack. Then the microcracks grew up, connected each other and connected with the main crack through shear rupture of the ledgement between them.     

3Cr2W8V钢热疲劳裂纹长大方式的原位观察

在TS-2型台式扫描电子显微镜中,装入自制的微型热疲劳试验装置,观察了3Cr2W8V钢热疲劳裂纹长大过程,结果表明:热疲劳裂纹长大前,主裂纹尖端首先钝化,在其前方出现孔洞或不连续微裂纹,热疲劳裂纹的长大通过主裂纹与孔洞及微裂之间的桥接方式进行。     

Mechanical and fatigue properties of stress relieved type 302 stainless steel wire

Type 302 stainless steel wire is manufactured using a cold extrusion process. The cold working increases the ultimate strength of the wire from approximately 690 MPa to 2070 MPa. However, the cold working process creates residual stresses and surface microcracks in the wire. The residual stresses and microcracks reduce the fatigue life of the wire considerably. Elimination or minimization of residual stresses and microcracks is necessary if longer fatigue life is required. Residual stresses or microcracks in a wire can be minimized by a heat treatment process, which could improve both its mechanical properties and fatigue properties. Experimental evidence shows that the stress relieving process yields maximum mechanical properties between 316 and 482 ° (600 and 900 °F). The fatigue properties of the wire are optimum at a stress relieving temperature of 649 ° (1200 °F). However, the mechanical properties such as yield strength, modulus of resilience, modulus of toughness, and ultimate strength, etc., are reduced by as much as 30% if compared to the similar properties of 316 ° (600 °F) stress relieved wire.     

Effect of a submicroscopic structure on the fatigue life of precipitation-hardening nickel-chromium alloys

The effect of the submicroscopic-structure parameters of typical precipitation-hardening Ni-Cr alloys on their resistance to fatigue failure is studied. The presence of hardening γ′-phase (Ni₃(Al,Ti)) particles in the alloys is found to retard the nucleation kinetics of fatigue microcracks and to increase the fatigue lives of the alloys in the range of low cyclic-deformation amplitudes (high-cycle fatigue). At high cyclic-deformation amplitudes (low-cycle fatigue), the precipitated particles and the fields of elastic interfacial strains hinder plastic deformation in the mouths of cracks, which increases the rate of their propagation and decreases the fatigue lives of the alloys. The precipitation of the particles that are under hydrostatic tension during aging of the alloys substantially decreases their resistance to fatigue failure due to facilitated nucleation of fatigue microcracks at interphase boundaries and their further rapid propagation.