循环变形提高SiC纤维增强铝基复合材料强度及塑性Ⅰ.实验现象

对SiC纤维增强铝基复 不同周次循环变形载荷后强度与塑性的测试表明，循环变形可使其强度、塑性均有所提高。经循环变形载荷作用10周次，抗拉强度提高20％；作用100周次，强度提高30％，塑性也有类似的变化。这一现象与传统疲劳损伤理论不一致，通过对基体材料、SiC纤维体以及复合材料板的各自独立循环变形实验可知。这一现象同循环变形过程中纤维与基体的界面结合强度适度降低有关，这种降低有助于复合材料的强度与塑性的配合。     

循环变形提高SiC纤维增强铝基复合材料强度及塑性Ⅱ.理论分析

采用断裂力学方法获得了纤维增强复合材料强度与脱粘长度、纤维临界长度以及纤维体积分数的定量关系。该公式较好地预测了纤维的临界长度以及强度与纤维体积分数的关系，并再现了复合材料混合定则。该公式也较好地解释了丝状复合材料强度随短期循环变形载荷与周次增加而增加的现象。其原因是在循环变形中，纤维与基体界面结合强度发生变化，导致纤维临界长度与脱粘长度发生变化。从而使复合材料强度增加，但这种增加是有限的和有范围的。循环变形的发展最终导致强度下降。     

雾化喷射成型SiCp／2014复合材料的力学性能及断口分析

对雾化喷射成型的ＳｉＣｐ／２０１４颗粒增强金属基复合材料及其基体合金进行了力学性能及断口的分析研究。结果表明，峰时效时复合材料的室温拉绅强度高于基体合金，但塑性下降，导致塑性下降的原因，分析认为是与热压后微观区域的ＳｉＣｐ分布不十分均匀有关。     

热挤压和时效处理对SiCw/LD2复合材料拉伸性能的影响

对ＳｉＣｗ／ＬＤ２复合材料进行了热挤压,并对铸态和挤压态复合材料进行了时效处理。比较铸态、挤压态、时效态复合材料间的拉伸性能的结果表明,挤压变形可提高复合材料的强度和塑性;时效处理能够提高复合材料的强度,但却降低塑性。挤压变形后再时效处理可使复合材料达到最大的强度和模量。     

热处理对SiC_p／ZL109复合材料机械性能和物理性能的影响

试验分析了不同热处理方法对ＳｉＣｐ／ＺＬ１０９复合材料机械性能和物理性能的影响；通过金相和透射电镜分析研究了其热处理强化机理．结果表明．Ｔ６热处理可大幅度提高复合材料的强度；Ｔ４热处理在提高强度的同时可增加复合材料的延伸率；复合材料强度的提高是由于ＳｉＣｐ的均匀分布和ＳｉＣｐ与Ａｌ基体热膨胀系数差导致的高密度位错产生的．     

改善颗粒增强金属基复合材料塑性和韧性的途径与机制

评述了影响颗粒增强金属基复合材料塑性和韧性的各种因素,在此基础上深入研究了颗粒形状对ＳｉＣｐ／ＬＤ２复合材料塑性和断裂韧性的影响规律。采用有限单元法分析不同形状的ＳｉＣ颗粒增强的ＬＤ２复合材料的微区力学环境和整体力学行为,结果表明颗粒的尖锐化导致基体内应变集中和颗粒尖端断裂的可能性加剧,因而降低材料的塑性;而在外加载荷的作用下,由于复合材料基体整体均处于较高的加工硬化状态,因此颗粒形状对材料断裂韧     

颗粒形状对SiCp／LD2复合材料塑性的影响

采用经钝化处理的ＳｉＣ颗粒作为增强体制备的ＳｉＣｐ／ＬＤ２复合材料,与普通ＳｉＣｐ／ＬＤ２相比,材料明显提高了塑性,有限元与拉伸断口的扫描电镜分析表明,材料经Ｔ６处理后,断裂机制以颗粒断裂为主,塑性得以提高的原因主要是颗粒尖角钝化后,降低了尖角处热残余应变集中,并降低了颗粒尖角部在外加应低时断裂的可能性;而材料未经Ｔ６处理时,断裂机制以基体失效为主,塑性提高主要源于尖角处热残余应变集中的降低,因则     

原位SiC颗粒增强MoSi_2基复合材料的显微组织和力学性能

本文研究了原位 ＳｉＣ颗粒增强 ＭｏＳｉ２基复合材料的组织结构和力学性能。结果表明：复合材料的组织为ｔ－ＭｏＳｉ２基体上均匀分布 β－ＳｉＣ等轴颗粒,数量很少的球形小孔隙主要分布在 ＳｉＣ颗粒内, ＳｉＣ颗粒尺寸为 ２－５ μｍ．复合材料界面为直接的原子结合,无非晶层存在．复合材料的室温维氏硬度、断裂韧性、抗压强度及高温流变应力明显高于单一ＭｏＳｉ２,随着ＳｉＣ体积分数的增加,维氏硬度、断裂韧性及高温流变应力提高,而抗压强度先增加后减少． ＳｉＣ体积分数从 １０％增加到 ４５％,ＫＩＣ从 ４．３４提高到 ５．７１ ＭＰａ·ｍ１／２;与单一 ＭｏＳｉ２相比提高了 ２５％－４６％; １４００℃时,σ０．２从 ２０％ＳｉＣ的 ２３０提高到 ４５％ＳｉＣ的 ２８５ ＭＰａ,比单一 ＭｏＳｉ２提高了 ９８％－１４６％．     

弥散质点和SiC颗粒复合强化Al基复合材料 Ⅱ．性能和断裂特征

对机械合金化制备的Ａｌ_４Ｃ_３、Ａｌ_２Ｏ_３弥散质点和ＳｉＣ颗粒复合强化Ａｌ基复合材料进行了拉伸试验和断口分析，并测定了弹性模量和热膨胀系数．研究表明，在ＳｉＣ_ｐ／Ａｌ复合材料中引入弥散的Ａｌ_４Ｃ_３和Ａｌ_２Ｏ_３质点可以明显提高复合材料的室温和高温强度，随加入Ｃ含量的增加或Ａｌ粉氧化时间的加长，复合材料的强度提高．在Ａｌ_４Ｃ_３／Ａｌ复合材料的基础上加入ＳｉＣ颗粒可以提高复合材料的弹性模量并进一步降低其热膨胀系数．复合材料断口为大韧窝加细小韧窝的混合断口，随复合材料基体强度的增加，拉伸断口上断裂的ＳｉＣ颗粒数量增多．     

DISPERSOID AND SiC PARTICULATE STRENGTHENED Al COMPOSITES Ⅱ. PROPERTIES AND FRACTURE BEHAVIOUR

对机械合金化制备的Ａｌ_４Ｃ_３、Ａｌ_２Ｏ_３弥散质点和ＳｉＣ颗粒复合强化Ａｌ基复合材料进行了拉伸试验和断口分析，并测定了弹性模量和热膨胀系数．研究表明，在ＳｉＣ_ｐ／Ａｌ复合材料中引入弥散的Ａｌ_４Ｃ_３和Ａｌ_２Ｏ_３质点可以明显提高复合材料的室温和高温强度，随加入Ｃ含量的增加或Ａｌ粉氧化时间的加长，复合材料的强度提高．在Ａｌ_４Ｃ_３／Ａｌ复合材料的基础上加入ＳｉＣ颗粒可以提高复合材料的弹性模量并进一步降低其热膨胀系数．复合材料断口为大韧窝加细小韧窝的混合断口，随复合材料基体强度的增加，拉伸断口上断裂的ＳｉＣ颗粒数量增多．     

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.     

Measurement of the cyclic bridging law in a titanium matrix composite and its application to simulating crack growth

The cyclic bridging traction law for a fiber reinforced titanium matrix composite (TMC) has been measured using a multiple fiber pullout specimen geometry. The test specimens were cut from fatigued specimens containing a single fully bridged matrix crack. The two key parameters obtained from such measurements are the interface sliding stress and the fiber bundle strength. The sliding stress was found to be considerably lower than the pristine values reported for other TMCs (by a factor of 5), a result of wear of the fiber coatings. The fiber strength is also reduced following fatigue, by 25%. The strength reduction is associated with the formation of new surface flaws, also due to cyclic sliding. Additionally, simulations of fatigue crack growth have been performed using the measured sliding stress and compared with experimental measurements over a range of applied stresses. Though excellent correlations are obtained between the experiments and the predictions, it is demonstrated that similar correlations can be obtained using sliding stress values that differ from the measured one by a factor of 2 and by suitable adjustment of the Paris law parameter that characterizes the intrinsic fatigue resistance of the composite.     

In situ observation on fatigue crack growth in SCS-6/Ti-15-3 composite at elevated temperatures

Direct observation on fatigue crack growth behavior in SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite subjected to a constant tension–tension loading mode was performed by scanning electron microscope using a single edge-notched specimen in vacuum at room temperature and 550 °C. The fatigue crack growth rate at 550 °C was lower than that at room temperature, and the difference between the fatigue crack growth rates at room temperature and 550 °C increased with increasing fatigue cycles. The crack opening displacement at 550 °C was smaller than that at room temperature when the crack length exceeded a definite value, though the interface friction stress between the fiber and matrix at elevated temperature was much smaller than that at room temperature. The above results were explained qualitatively by a residual stress mechanism at the crack front and the crack closure behavior at crack wake, which could be produced by matrix creep asymmetry in tension and compression at elevated temperature during each fatigue cycle.     

超声冲击对7A52铝合金焊接接头疲劳性能的影响

文中对超声冲击处理前后的7A52铝合金焊接接头显微组织、显微硬度以及残余应力进行了分析,并对超声冲击处理前后的接头在不同循环应力比的加载条件下进行了疲劳试验.结果表明,经超声冲击处理后接头塑性变形层厚度在45~70 μm左右,母材区表面硬度提高了170%,焊缝区硬度提高了大约70%,改变了焊态残余应力分布,成功引入了压应力;在疲劳试验中,循环次数为2×106的条件下,加载应力循环比为0.1时,冲击态接头的疲劳强度为60.26 MPa,比疲劳强度为40.74 MPa的焊态试样提高了47.9%,而在加载应力循环比为0.45时,冲击态接头的疲劳强度为46.53 MPa,比疲劳强度为39.97 MPa的焊态试样提高了16.4%.     

Monitoring Damage Evolution in a Titanium Matrix Composite Shaft Under Torsion Loading Using Acoustic Emission

The damage behaviors of a titanium matrix composite shaft under torsion loading were monitored using the acoustic emission technique. The composite shaft with SiC fibers at ± 45° orientations was prepared by the solid-state fabrication process. Both the torsional rigidity and torsional strength of the TMC shaft were improved by SiC fibers. The acoustic emission responses during the loading–unloading–reloading, under quasi-static and cyclic torsion tests were investigated. Multiple acoustic emission signals were grouped as mechanical noise, matrix deformation, interface debonding and fiber fracture using amplitude, waveform shape and frequency centroid parameters. A substantial reduction of signals generated by matrix deformation was found in the reloading test. During the quasi-static torsion test, interface debonding and progressive breaks of SiC fibers occurred. According to different acoustic emission behaviors, the failure process in the torsion fatigue test can be divided into three stages: the initial stage, the fiber fracture stage and the fast fracture stage.     

FATIGUE BEHAVIOUR OF SiC_p/6061Al COMPOSITE

The fatigue of SiC_p/6061Al composite containing 15 v.-% SiC particles has been compared with 6061Al alloy.Dislocation structure and microprocess of fatigue crack initiation and propagation in the composite have been investigated by using SEM and TEM.The results in- dicate that the fatigue strength at 10~7 cycles of the composite is 196 MPa,i.e.about 25% higher than matrix alloy.The voids and microcracks initiated at and near the interface be- tween SiC_p and matrix,where has a higher density of dislocations,will propagate and link up to form the fatigue crack.It is an important evidence to note that the dislocation channels where screw dislocation can travel are formed near interface and corner region of SiC_o in the composite subjected to fatigue stress(σ_(max)=274 MPa N=2.4×10~5 cycles),demonstrating the relationship between fatigue crack initiation and dislocation movement in the SiC particles reinforced 6061 Al alloy composite.     

MgAl2O4颗粒增强型AC4C基复合材料的微观组织及力学性能

采用搅拌铸造方法制造MgAl2O4颗粒增强型AC4C基复合材料。考察MgAl2O4颗粒的尺寸及尺寸的分布对颗粒分散度的影响,并且测试复合材料的微观组织、强度与疲劳性能。5～25gm范围内的MgAl2O4颗粒可以提高复合材料的抗拉强度。疲劳性能测试表明,在250℃、10^7循环周期内,MgAl2O4颗粒使复合材料的疲劳性能提高27％。在MgAl2O4颗粒周围,观察到了大量位错,产生这种情况的主要原因是MgAl2O4与Al两者之间的热膨胀系数不匹配。在MgAl2O4颗粒周围产生裂纹,但在扩展过程中大量的裂纹发生了转向和弯曲,这有助于材料疲劳性能的提高。     

High strain rate compressive behavior of Ti-based metallic glass matrix composites

The compression behaviors of Ti-based metallic glass matrix composites with dendrites scale were tested at different loading rates. It was found that the composites exhibited not only high strength, but also large plasticity under quasi-static compression. Under the dynamic loading, however, the TZ1 alloy with fine dendrites demonstrated a catastrophic failure. Although both the strength and plasticity decreased for the TZ2 composite sample with coarse dendrite, the total strain is over 7%. Discussions on the strain rates and dendrite scale are provided by analyzing the effects of dendrite, which can present the possible deformation mechanism of the composites.     

Effect of stress level on fatigue behavior of 2D C/C composites

Laminated carbon fiber clothes were infiltrated to prepare carbon fiber reinforced pyrolytic carbon (C/C) using isothermal chemical vapor infiltration (CVI). The bending fatigue behavior of the infiltrated C/C composites was tested under two different stress levels. The residual strength and modulus of all fatigued samples were tested to investigate the effect of maximum stress level on fatigue behavior of C/C composites. The microstructure and damage mechanism were also investigated. The results showed that the residual strength and modulus of fatigued samples were improved. High stress level is more effective to increase the modulus. And for the increase of flexural strength, high stress level is more effective only in low cycles. The fatigue loading weakens the bonding between the matrix and fiber, and then affects the damage propagation pathway, and increases the energy consumption. So the properties of C/C composites are improved.