复合材料失效及其强度理论

材料的失效破坏是一个复杂的过程,迄今为止已有上百个理论模型来研究材料的强度问题,本文重点介绍了统一强度理论,它给出了一系列破坏准则,并建立了准则之间的关系.根据复合材料的特点以及基体、增强相、界面、工艺对复合材料强度的影响关系,阐述了复合材料的宏观强度理论中不同破坏准则之间的差异和特点,并指出采用宏观与细观相结合的方法研究复合材料损伤和强度理论的必要性.     

NiAl/NiCoCrAlY/8YSZ复合喷涂层的微观结构与性能研究

目的提高金属/陶瓷隔热涂层体系在海洋环境下的耐腐蚀性能。方法利用冷喷涂方法制备NiAl复合打底层和Ni CoCrAlY粘结层,与等离子喷涂制备的8YSZ陶瓷层构成适用于海洋环境的多层结构耐蚀隔热涂层体系。利用FE-SEM分别观察喷涂态粘结层和陶瓷层的表面、横截面形貌,通过EDS分析涂层元素分布;利用XRD分析表征涂层的物相组成;借助万能材料试验机,采用拉伸法测试涂层结合强度;利用热循环试验和焰流冲刷试验测试涂层的耐高温性能。结果微观分析表明,冷喷涂制备的NiAl复合打底层和Ni CoCrAlY粘结层形貌致密,涂层材料未发生明显氧化,颗粒变形程度不一,粘结层与基体间的结合强度约为18.4 MPa,粘结层与8YSZ陶瓷层界面结合紧密。陶瓷层物相结构和成分稳定,涂层经12次热震循环和1000个周期的高温焰流冲击后,表面未出现开裂、起皮和脱落。结论采用冷喷涂法和等离子喷涂法联合制备的耐蚀隔热复合涂层体系具备良好的耐热性和耐腐蚀性。冷喷涂制备的金属涂层结构致密,孔隙率低,与陶瓷层结合良好,能够有效提高涂层体系在腐蚀性环境中的耐蚀性能。NiAl复合涂层可以缓解Ni CoCrAlY粘结层和铝合金基材间的热匹配问题,增强涂层的结合性能。     

含夹杂TiC-TiB_2陶瓷强度的细观力学分析

根据超重力合成TiC-TiB_2装甲陶瓷的细观结构和断裂机理,建立TiC-TiB_2装甲陶瓷细观强度模型,分2种失效形式讨论了氧化铝杂质颗粒对材料强度的影响。首先根据相互作用直推估计法计算了TiC-TiB_2基体的等效刚度,认为氧化铝杂质颗粒稀疏分布在TiC-TiB_2形成的等效基体中,根据等效夹杂理论计算了杂质颗粒内部和附近残余应力和应力场分布,得出氧化铝杂质颗粒可能的失效形式;然后根据杂质颗粒可能出现的界面脱粘和颗粒破坏2种失效形式分别建立模型进行讨论,并以两者的最小值定义为复合材料的强度,分析了材料强度随细观结构和性能参数的变化规律,并与实验结果进行了对比。结果表明,模型能够反映材料失效机理,对于具有与本文相似细观结构的材料,杂质颗粒与等效基体的相对弹性性能和相对热膨胀性能、杂质颗粒尺寸、等效基体韧性等都可能影响杂质颗粒失效形式,但是杂质颗粒直径增加、基体韧性降低必然降低材料强度。对于含氧化铝杂质颗粒的TiC-TiB_2装甲陶瓷,界面脱粘是杂质颗粒主要的失效形式。     

SiC改性碳复合材料的微观结构分析和力学性能研究

利用SiC与碳基材料复合,采用原位合成技术制备了一种新型碳陶瓷复合材料.采用XRD和SEM技术分别表征材料的相组成和微观形貌结构,并利用万能材料试验机测试了复合材料的抗压和抗折性能.XRD测试结果表明,SiC改性碳陶瓷复合材料中没有新相产生.由SEM照片分析可知,SiC的掺杂破坏了石墨原有的层片状结构,并在碳石墨材料中观察到颗粒状晶体,随着SiC掺量的增加,散乱分布的晶粒有聚集长大的趋势,造成碳陶瓷复合材料结构的进一步破坏.力学性能测试结果表明,当掺入10%SiC(质量分数)时,材料的抗折强度最大,为58.8 MPa而在SiC掺量为5%(质量分数),其抗压强度达到最大,为157.4 MPa.     

SiC改性碳复合材料的微观结构分析和力学性能研究

利用SiC与碳基材料复合，采用原位合成技术制备了一种新型碳陶瓷复合材料。采用XRD和SEM技术分别表征材料的相组成和微观形貌结构，并利用万能材料试验机测试了复合材料的抗压和抗折性能。XRD测试结果表明，SiC改性碳陶瓷复合材料中没有新相产生。由SEM照片分析可知，SiC的掺杂破坏了石墨原有的层片状结构，并在碳石墨材料中观察到颗粒状晶体，随着SiC掺量的增加，散乱分布的晶粒有聚集长大的趋势，造成碳陶瓷复合材料结构的进一步破坏。力学性能测试结果表明，当掺入10％SIC（质量分数）时，材料的抗折强度最大，为58．8MPa；而在SiC掺量为5％（质量分数），其抗压强度达到最大，为157．4MPa。     

氧化物共晶陶瓷定向凝固研究进展

定向凝固氧化物共晶自生复合陶瓷具有优异的高温强度、抗氧化性、抗蠕变性以及良好的高温结构稳定性,被认为是新一代在高温氧化性气氛中长期工作的首选超高温结构材料之一。回顾了氧化物共晶自生复合陶瓷的发展历史,总结了目前定向凝固氧化物共晶陶瓷凝固组织和力学性能方面的研究现状,同时结合作者在本领域的研究,着重分析了高温度梯度定向凝固条件下氧化铝基共晶陶瓷的组织特征、凝固特性、力学性能以及增韧机制。最后展望了定向凝固氧化物共晶陶瓷的发展趋势。     

SiCp/A356复合材料超声波辅助钎焊

利用超声波辅助钎焊的方法实现了Zn-Al合金与体积分数55%SiCp/A356复合材料的连接,并得到了由SiC陶瓷颗粒增强的复合焊缝.通过扫描电镜、能谱等方法对焊缝的微观结构进行了描述,研究了超声波作用时间对焊接接头微观组织结构及连接强度的影响规律,建立了超声波作用下复合焊缝形成过程的物理模型.当焊接温度为475℃,超声波作用时间为20 s时钎焊焊缝的平均抗剪强度值为231 MPa.由SiC陶瓷增强的复合焊缝其室温抗剪强度与无增强相颗粒的纯合金焊缝强度相比提高了50%.结果表明,焊缝中铝含量的增加及陶瓷颗粒的增强作用是导致焊缝强度提高的两个重要因素.     

快速凝固Al-Si合金的组织形态及相结构

利用单辊旋淬法制备快速凝固共晶和过共晶铝硅合金条带,采用扫描电镜、透射电镜及XRD技术对试样的组织形貌、相结构进行表征.结果表明:快速凝固后组织明显细化,共晶和过共晶组织转变成了亚共晶组织;与平衡态相比,快速凝固有效抑制硅相的形核与生长,初生硅不能析出,α相领先共晶形核生长,形成α相和分布在α相间的羽毛针列状共晶体复合组织.     

机械镀锌层的结合强度分析

通过机械镀方法制备了镀层试样,采用冲击试验和拉伸试验研究了镀层的抗冲击性能及镀层-基体间的结合强度.研究结果表明,镀层的冲击载荷失效为镀层与基体间的剥离失效,随着镀层厚度的增加,镀层的抗冲击性能增强;镀层的拉伸破坏发生在镀层-基体界面,20 μm、30 μm、60 μm镀层的结合强度分别为3.18 MPa、2.18 MPa、3.84 MPa;薄镀层时,拉伸失效主要发生于镀层-基体的界面;厚镀层时,拉伸失效发生于在镀层-基体界面和镀层内.     

层状ZrB2基陶瓷的增韧机制分析

将传统陶瓷中的轧膜工艺应用到超高温陶瓷的制备上,制备ZrB2基层状复合陶瓷。根据层间应力需求设计材料组分。基体层组分为ZrB2+10vol%SiCp+10volSiCw;中间层组分为ZrB2+44.1vol%SiCp+37vol%MoSi2。复合陶瓷在1950℃,25MPa条件下热压烧结制成并进行机械性能测试及微结构观察。结果显示,复合陶瓷有较高的致密度,力学性能较纯ZrB2陶瓷有较大的提升。微观结构观察表明,由于层状结构的存在,裂纹在扩展过程中反复偏转,吸收了大部分能量,有效的提高了复合陶瓷的韧性。复合陶瓷的增韧机制为弱界面层对裂纹的偏折、裂纹分支、基体片层的破坏以及基体片层内部增韧的协同增韧。     

Effects of laser processing parameters on solidification microstructures of ternary Al2O3/YAG/ZrO2 eutectic in situ composite and its thermal property

Rapid surface resolidification with a high powered CO₂-laser was performed in preparing directionally solidified Al₂O₃/YAG/ZrO₂ ternary eutectic ceramic in situ composite. The effects of laser processing parameters on the solidification microstructure characteristics and thermal properties were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and synthetically thermal analysis (STA). Detailed investigations of the influence of laser power and scanning rate on the preparation and microstructural parameters of the ternary eutectic were presented. Moreover, the eutectic phase separation rule at high temperature was discussed. The results indicate that solidification microstructure of the ternary eutectic composite is greatly influenced by the laser processing parameters. The synthetically thermal analysis shows that the eutectic temperature of ternary Al₂O₃/YAG/ZrO₂ composite is 1 738 °C, well matching the phase diagram of Al₂O₃-Y₂O₃-ZrO₂.     

采用辅助脉冲电流液相扩散连接Ti(C,N)-Al_2O_3的接头形成行为及强度

采用辅助脉冲电流液相扩散连接方法,对Ti(C,N)-Al2O3陶瓷基复合材料与40Cr钢进行了焊接试验,重点研究了在辅助脉冲电流作用下,界面及焊缝组织演变、基体材料溶解等界面行为,探讨了工艺参数对接头强度及断口形态影响规律.结果表明,通过采用辅助脉冲电流液相扩散连接Ti(C,N)-Al2O3与40Cr钢,可以在较短的焊接时间条件下获得较为稳定的接头强度,四点弯曲平均强度位于231~272 MPa之间;降低焊接件整体加热温度以缓解接头残余应力,控制界面和焊缝化学冶金反应进程以较少金属间化合物数量、提高固溶体含量,抑制活性元素Zr向陶瓷基复合材料中的扩散进程以减少陶瓷颗粒向焊缝中的溶解量,是提升焊接接头强度的关键因素.     

Ag含量对纤维相强化Cu-Ag合金组织及性能的影响

通过冷拉拔结合中间热处理制备了不同Ag含量的纤维相强化Cu—Ag合金,研究了Ag含量对合金组织形态、强度和电导率的影响．Ag含量在6％-24％范围内的合金铸态组织包含初生α枝晶、共晶体和次生相．在拉拔过程中,共晶体及次生相均演变成细密的纤维形态．随Ag含量的升高,共晶体及次生相数量增多,合金强度及应变硬化速率升高,电导率下降,尤其当Ag含量增加使合金组织中的共晶纤维束增多并呈连续网状分布时,电导率下降更为明显．高Ag含量合金中共晶体纤维束的强化效应明显高于低Ag含量合金中次生相纤维的强化效应,但其对合金导电性能的损害程度也高于后者．     

高性能陶瓷内衬复合煤粉喷枪的研制

采用燃烧合成方法制备了Al2O3陶瓷内衬复合煤粉喷枪,但单一的Al2O3陶瓷内衬层韧性差,抗热震性能低,造成喷枪的寿命短.通过在(CrO3+AD燃烧体系中添加ZrO2组元,采用SHS重力分离技术,制备出了高强度、高韧性的陶瓷内衬复合弯管煤粉喷枪,陶瓷内衬层为具有共晶结构的Al2O3=ZrO2复相陶瓷.在共晶结构中,ZrO2纤维直径可达到纳米/微米尺度,从而使陶瓷基体得以强化;同时棒状共晶结构的生长取向分布,又使陶瓷因棒晶桥接与拔出得以韧化,改善了喷枪陶瓷内衬的高温强度和耐磨性,有效地提高了煤粉喷枪的使用寿命.     

Effect of gallium addition on microstructure and properties of Ag-Cu-Zn-Sn alloys

The effects of Ga on microstructures and mechanical properties of the cadmium-free silver based brazing filler metals have been investigated. The results indicated that Ga additions had the noticeable effect on the microstructure, especially on the shape of the phases. With the increase of Ga addition, the amount of eutectic structure increased, and the acicular eutectic structure changed into the fine eutectic structure with micro-vermiform and rod-like shape. When the addition of Ga was 3.0 wt.%, none of defects exist in the microstructure of the brazed joint. The tensile strength increased from 382 MPa to 511 MPa with the content of Ga increasing from 0 to 3.0 wt.%.     

Effects of Chromium on the Microstructures and Mechanical Properties of AlCoCrxFeNi2.1 Eutectic High Entropy Alloys

In the present study,a series of AlCoCrxFeNi2.1 (x=0,0.25,0.5,0.75,1.0) eutectic high entropy alloys (EHEAs) have been designed and prepared.And the effect of Cr content on the microstructures and mechanical properties of the AlCoCrxFeNi2.1 alloys was systematically investigated.The results indicate that the AlCoCrxFeNi2.1 (x ＞ 0) alloys exhibit almost complete lamellar eutectic microstructures with a mixture structure of FCC and B2 phases.And the AlCoFeNi2.1 alloy without Cr element exhibited a hypoeutectic microstructure with a primary B2 phase.In addition,the eutectic microstructures for AlCoCrxFeNi2.1 eutectic alloys do not change significantly.The room temperature compressive tests results show that with an increase in Cr content (from x =0 to x =1.0),the yield strength will first decrease,and thereafter increase.The trend is the opposite with the fracture strength and plastic strain.They show an increase trend at first,and then decrease.The AlCoCr0.5FeNi2.1 (Cr0.5) alloy shows the best comprehensive mechanical properties.The tensile yield strength,fracture strength,and elongation are 536.5 MPa,1062 MPa,and 13.8％,respectively.Furthermore,the Cr0.5 alloy also displays a high strength with a yield strength of 362 MPa at 700 ℃.In summary,by changing the Cr content,AlCoCrxFeNi2.1 eutectic high entropy alloys with excellent comprehensive mechanical properties were obtained and prepared.     

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Al-Al₂O₃ composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al₂O₃ particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al₂O₃ particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al₂O₃ content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al₂O₃ bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.     

SURFACE COMPOSITE GEAR AND PIPE MADE BY SEPARATED EUTECTIC TECHNOLOGY

1.IntroductionTheeutecticgroWthisoftenconsideredtobesimilartothatofpuremetals,becauseittakesp1aceathatemperatureneartheequilibriumtemperature.Thusthereisnoin-trinsicreasonforoccurrenceofmacroscopicsegregationofconstituentelements.However,itisrecentlyfoundthatsomeeutecticscouldbeseparatedonmacroscopicscalewhiletheothersareimpossibleI1'2].Inordertosearchouttheformativemechanismoftheseparatedeutecticphenomenon,about26kindsofbinaryeutecticsareusedtoinvestigatewhetherornottheseparatedeutecticiscon…