TiB2颗粒增强ZA—8锌合金的显微组织与疲劳裂纹扩展行为的关系

研究了ＴｉＢ２颗粒增强ＺＡ－８锌合金复合材料和基体材料和疲劳裂纹扩展行为与显微组织，断口的关系，结果表明：所研究复合材料的抗疲劳裂纹扩展能力优于基体材料，尤其在小裂纹阶段，其原因为：增强粒子与裂纹尖端的相互作用降低了裂纹尖端的应力强度；增强粒子与基体良好的界面结合力。     

电脉冲烧结法制备形状记忆合金颗粒弥散基复合材料

最近的一些研究，有将形状记忆合金纤维和颗粒嵌埋或弥散到金属或高分子材料基体中，利用形状记忆合金的相变造成的应力场抑制裂纹的扩展的新方法。即将基体中的弥散粒子或纤维进行形状记忆处理，使复合材料中产生残余应力场，或者在使用中通过变形材料的裂纹尖端附近的诱发相变产主应力场。在以形状记忆合金粒子弥散分布的场合，主要考虑下述的裂纹扩展抑制机制： （ｌ）弥散粒子周围裂纹围绕的应力缓和效应。在大多数粒子弥散材料中都是这种机制，与粒子的大小和体积分数有关。 （２）弥散材料烧结时因热应力在粒子周围产生残余压应力的效…     

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.     

TiB+La_2O_3/Ti基复合材料拉伸断裂的原位表征及其机理研究

以不同体积分数的Ti B+La_2O_3原位增强钛基复合材料为研究对象,在室温下对该材料进行SEM原位拉伸实验,通过对裂纹尖端的组织变化以及裂纹扩展路径的原位观察,分别研究了增强体对材料拉伸强度和拉伸断裂行为的影响。结果表明:增加增强体的体积分数可以提高增强体的承载作用并细化基体晶粒,从而提高颗粒增强Ti基复合材料的强度。材料的断裂行为表现为增强体断裂后微裂纹的萌生、扩展及其和滑移带的汇合。高含量的增强体可增加微裂纹的数量,使得其在萌生、扩展后更易与邻近微裂纹或滑移带相贯通,加快宏观裂纹的形成,从而导致了材料塑性的下降。     

TiC颗粒增强钛基复合材料的形变断裂

通过对ＴｉＣ颗粒增强钛基复合材料断裂研究表明：ＴｉＣ颗粒的缺陷比例对复合材料的断裂裂纹扩展速率有较大影响。形状规则且无缺陷的粒子在受力的初期不易破断,可较大提高裂纹扩展所需。反之,易破碎的粒子则增加了裂纹尖端的扩展应力,提高了裂纹扩展速率。适当的热处理可提高裂纹扩展所需塑性功,从而在一定范围内改善复合材料塑性。     

纤维增强金属基复合材料耐磨机制的研究

根据纤维增强金属基复合材料的结构特点，研究了界面在复合材料磨损过程中的作用，磨损时，复合材料的界面可消耗纹扩展能量，阻滞裂纹扩展，氧化铝短纤维增强硅合金合材料具有优异的耐磨性；基体中的合金元素有利于形成良好的界面，改善复合材料的耐磨性。     

微观结构对板条马氏体启裂和裂纹稳态扩展的影响

研究了具有典型板条马氏体组织的低碳合金结构钢启裂和裂纹早期扩展阻力与微观组织结构、断裂过程中消耗功之间关系。结果表明：板条马氏体微观组织结构决定裂纹尖端塑性约束程度；微观组织结构特征不同，裂纹尖端塑性约束程度不同，启裂过程和裂纹早期扩展过程中消耗的弹性能和塑性功不同，导致材料抵抗裂纹稳态扩展的阻有较大差异。     

GJW50钢结硬质合金热疲劳裂纹扩展的研究

在自约束型热疲劳试验机上对GJW50钢结硬质合金进行了热循环试验，用光学金相显微镜和扫描电镜研究了在热应力的作用下的热疲劳裂纹扩展方式和形态。结果表明：在合金中的两大相——硬质相和钢基体相，热疲劳裂纹优先在硬质相区中扩展；扩展的方式受硬质相粒子的尺寸及分布状态所控制。小颗粒的WC粒子聚集区及该区与钢基体区的交界而是热疲劳裂纹的主要扩展地；大尺寸的WC粒子的“自身裂纹”是热疲劳裂纹的必经之处。由于钢皋体相阻碍热疲劳裂纹的扩展，导致裂纹呈非连续性扩展；热疲劳裂纹是以“搭桥”形式穿过钢基体相；扩展路径的形态呈“折线”和“梯形”形态。     

Al/SiCp复合材料的断裂行为

采用粉末冶金和喷射沉积方法制备了Al及Al／SiCp复合材料，测试了材料的力学性能，利用金相显微镜和扫描电镜观察了材料的断裂行为。结果表明，Al／SiCp复合材料的强度比基体材料的高，但塑性低。粉末冶金方法制备的Al／SiCp复合材料与喷射沉积方法制备的比较，前者组织更致密，强度更高，塑性更好，断裂是SiC颗粒开裂，裂纹的传递主要沿颗粒中形成的裂纹扩展；后者断裂主要表现为SiC颗粒从基体中拔出，裂纹的传递大多是SiC颗粒周围形成的裂纹在基体中扩展。     

ZrC颗粒增强钨基复合材料的高温断裂行为

运用扫描电镜（ＳＥＭ）研究了ＺｒＣ颗粒增强钨基复合材料（ＺｒＣｐ／Ｗ,ＺｒＣ颗粒体积分数为３０％）在１０００℃高温的断裂行为。结果表明：高温下,复合材料断裂过程是“微裂纹萌生－裂纹连接长大一裂纹快速扩展－材料断裂：;复合材料中的ＺｒＣｐ为脆性断裂,Ｗ基体有韧性断裂迹象;随温度升高,Ｗ基体发生了脆塑转变,Ｗ基体的塑性在一定程序上延缓了裂纹的扩展速率,降低了材料对裂纹的敏感性,基体在断裂前发生一定塑性     

Fatigue crack growth under a constant amplitude loading of Al-6061-T6 welds obtained by modified indirect electric arc technique

Abstract

The characteristics of the fatigue crack growth in the base metal, weld metal and heat affected zone (HAZ) were quantified by testing compact type specimens of 6061-T6 welds obtained by the modified indirect electric arc technique. The fatigue crack growth depends on the microstructure imposed by the welding thermal cycle and it was observed that in the HAZ the crack growth rate is lower than that in the weld metal, but higher than that in the base metal. Microhardness maps revealed that this behaviour is due to the formation of a larger plastic zone around of the crack tip produced by loss of hardening. A comparison of fatigue crack growth of weld metal and HAZ for modified indirect electric arc and friction stir welding shows that the weld metal produced by friction stir welding exhibits better resistance to crack propagation, but both processes behaved similarly in the HAZ.     

Effects of laser shock processing,solid solution and aging,and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl10Fe3Mn2 alloy

The materials used in variable temperature conditions are required to have excellent thermal fatigue performance. The effects of laser shock processing (LSP), solid solution and aging treatment (T6), and cryogenic treatment (CT) on both microstructure and thermal fatigue performance of ZCuAl₁₀Fe₃Mn₂ alloys were studied. Microstructure and crack morphology were then examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The result showed that, after being subjected to the combination treatment of T6+CT+LSP, the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl₁₀Fe₃Mn₂ alloy with the tensile strength, hardness, and elongation of 720 MPa, 300.16 HB, and 16%, respectively, and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm. Moreover, the ZCuAl₁₀Fe₃Mn₂ after combination treatment shows high resistance to oxidation, good adhesion between the matrix and grain boundaries, and dramatically reduced growth rate of crack. During thermal fatigue testing, under the combined action of thermal and alternating stresses, the microstructure around the sample notch oxidized and became loose and porous, which then converted to micro-cracks. Fatigue crack expanded along the grain boundary in the early stage. In the later stage, under the cyclic stress accumulation, the oxidized microstructure separated from the matrix, and the fatigue crack expanded in both intergranular and transgranular ways. The main crack was thick, and the path was meandering.     

Nodule influence on fatigue crack growth thresholds in nodular cast iron

The influence of microstructure i.e. nodules (voids) on the fatigue threshold value in nodular cast iron is numerically investigated. It is qualitatively shown that the fatigue threshold value is unaffected by the individual nodules and that the threshold value is proportional to the bulk Young's modulus. Furthermore it is shown that nodules (voids) do not influence the crack tip conditions (stress-intensity factor and crack tip opening displacement) if the voids do not lie very close to the tip for physically long cracks. It is concluded that linear elastic fracture mechanics is valid for these types of materials and no influence of the nodules need be taken into count for fatigue calculations when dealing with long cracks.     

脉冲电流对再制造毛坯裂纹尖端组织及性能的影响

以再制造为目标,利用高能脉冲电流冲击装置对含有单边裂纹的316奥氏体不锈钢进行脉冲电流处理。采用金相显微镜、X-射线衍射仪、电子背散射衍射(EBSD)、透射电子显微镜(TEM)等表征手段,研究高密度脉冲电流对裂纹尖端区域微观组织与结构的影响,分析裂纹尖端组织加热熔化及热处理时的组织演变过程。结果表明:脉冲电流处理后,裂纹尖端组织发生了明显细化并出现组织梯度,包括柱状晶区、再结晶区及基体区。脉冲电流的快速加热和快速冷却、高速电子风的产生、电致塑性等作用降低了再结晶或相变时的热力学屏障,提高了形核率,抑制了晶粒长大过程,获得了比原始晶粒尺寸小的再结晶晶粒,材料的力学性能得到提高。     

不锈钢管道焊缝金属疲劳短裂纹行为的实验研究Ⅱ.裂纹萌生,扩展与 …

基于“有效短裂纹准则”,用复型研究了１Ｃｒ１８Ｎｉ９Ｔｉ不锈钢管道焊缝金属光滑试样表面的短裂纹萌生、扩展与交互作用行为。结果表明：有效短裂纹（ＥＳＦＣｓ）萌生于δ铁素体与奥氏体基体的交界处。ＥＳＦＣｓ扩展具有明显微观结构短裂纹（ＭＳＣ）和物理短裂纹（ＰＳＣ）两阶段特征。主导有效短裂纹（ＤＥＳＦＣ）行为是ＥＳＦＣｓ交互作用的结果,适于表征短裂纹行为。ＤＥＳＦＣ扩展率的分散性与ＥＳＦＣｓ密度的分散性有     

Microstructure-based modeling of the influence of particle spatial distribution and fracture on crack growth in particle-reinforced composites

The crack growth behavior of particle-reinforced composites is determined by several factors, such as volume fraction, particle size, particle morphology, spatial distribution and particle strength. Thus, an accurate and robust numerical model must incorporate the true microstructure of the particles. It will be shown that the strength of the reinforcement particles is also an important factor. Hence, the model must be able to simulate particle fracture. In this paper, the crack growth behavior of SiC particle-reinforced Al matrix composites was modeled using actual microstructures. Linear elastic fracture mechanics principles were used to propagate the crack and obtain the local stress intensity values. The effect of particle fracture on crack growth was studied. It will be shown that spatial distribution and shape of the particles, as well as particle fracture ahead of the crack tip, significantly affect the crack trajectory and the stress distribution at the crack tip.     

腐蚀疲劳过程中裂尖阳极溶解对裂纹扩展的作用

在模拟腐蚀疲劳裂尖介质中,分别用铂丝和40CrNiMo钢为辅助电极测定了在不同拉伸速率下中温回火的40CrNiMo钢的腐蚀电流密度i_(a)与应变速率、塑性应变量的关系,其结果为:利用疲劳裂纹尖端应变的弹塑性有限元计算,得到腐蚀疲劳裂尖阳极溶解引起的裂纹扩展与△K的关系,结果表明:中温回火的40CrNiMo钢在3.5％NaCl水容液中裂尖阳极溶解对腐蚀疲劳裂纹扩展的直接贡很小,阳极溶解的主要作用是影响为裂尖断裂区氢致开裂提供氢原子的阴极过程。     

Microstructure and fatigue properties of plasma transferred arc alloying TiC-W-Cr on gray cast iron

In this paper, TiC-W-Cr powders were alloyed on grey cast iron by plasma transferred arc (PTA). The alloying samples were characterized the microstructure, microhardness, fatigue life and fatigue crack growth. From the results, it is indicated that two distinguishing region: alloying zone, heat affected zone are formed on the surface after PTA alloying. The alloying zone mainly consists of primary austenite, martensite, a eutectic of (Fe,Cr)₇C₃ carbide and austenite as well as the uniformly distributed un-melted TiC particles. PTA alloying TiC-W-Cr eliminates the stress concentration at the edge of graphite and produced hard carbide, resulting in frequent crack deflection. As a result, the Weibull distribution of fatigue life demonstrates that PTA alloying TiC-W-Cr exhibits longer lives compared to matrix and PTA hardening without reinforcement, but more scattered. In addition, on the basis of the careful observation of fatigue crack growth, it is shown that the fatigue crack growth rate could be retarded by PTA alloying TiC-W-Cr at low stress intensity.     

Life prediction of titanium MMCs under low-cycle fatigue

Low-cycle fatigue failure in titanium metal matrix composites is caused by two separate damage mechanisms: fatigue crack growth in the Ti matrix and fiber breakage. Here, a coupled numerical model for predicting both crack growth and fiber breakage is developed and applied to predict low-cycle fatigue lives in a SiC-fiber reinforced Ti matrix composite. A three-dimensional finite element model containing a matrix crack, nucleated on the first loading cycle in the reaction layer around a fiber, that is bridged by SiC fibers is used to calculate both the matrix crack tip stress intensity factor and the local fiber stress concentrations due to the matrix crack, as a function of the crack size. The crack tip stress intensity factor is used in a Paris-law model for the growth rate of the matrix crack. The local stress distributions in the fibers are used as the effective “applied” load within a three-dimensional Greens Function method that simulates the fiber damage process at any fixed fatigue crack size. Fiber failure preferentially occurs within the matrix crack region, where the fiber stresses are comparatively high, and composite failure occurs when the damage in this region is sufficient to drive fiber failure throughout the remainder of the composite in a crack-like fracture mode. A fatigue life threshold is predicted at about 80% of the quasistatic tensile strength, where the fiber bundle can survive even with a matrix crack extending throughout the entire cross-section. Predictions for the low-cycle fatigue of Ti-matrix (IMI834) reinforced with SCS-6 SiC fibers compare well with available experimental data at high stresses using pristine fiber strengths and no adjustable parameters. Using literature values for the fatigued fiber strength beyond 10⁴ cycles and no adjustable parameters, the experimental data are also well matched at lower stresses. The model demonstrates that fatigue life can be dependent on actual composite size and can be very sensitive to initial fiber damage.