石墨烯纳米片增韧Al2O3基纳米复合陶瓷刀具材料

以石墨烯纳米片作为增强相,采用热压烧结工艺制备石墨烯纳米片增韧Al_2O_3基纳米复合陶瓷刀具材料。进行石墨烯纳米片分散实验,研究石墨烯纳米片添加量对刀具材料断裂韧度、抗弯强度和硬度的影响,观察其微观结构和形貌。结果表明:聚乙烯吡咯烷酮(PVP)为石墨烯纳米片的优选分散剂,当PVP添加量为石墨烯纳米片质量的60%时,分散效果最佳;当石墨烯纳米片添加量为0.75%(体积分数)时,刀具材料的断裂韧度和抗弯强度分别达到7.1MPa·m^1/2和663MPa,与未添加石墨烯纳米片的组分相比分别提高了31%和15%;石墨烯纳米片呈卷曲状结构弥散分布于基体材料中,其增韧机理为石墨烯纳米片拉断、拔出和裂纹偏转。与未添加石墨烯的刀具相比,添加石墨烯纳米片的刀具的主切削力、切削温度和前刀面摩擦因数明显降低,表现出良好的减摩、耐磨性。     

TiC含量对碳纤维编织物增韧的WC-TiC-Ni层状陶瓷刀具性能的影响

采用真空热压烧结技术,在1 500℃下制备了不同TiC含量的连续碳纤维编织物增韧的WC/TiC层状陶瓷刀具样品。研究了TiC的含量对连续碳纤维编织物增韧的WC/TiC层状陶瓷刀具材料微观组织和力学性能的影响,结果表明,随着TiC含量的增加,陶瓷刀具材料的抗弯强度、断裂韧度和硬度不断减小;当TiC含量为20%(质量分数)时,材料的致密度较高,晶粒尺寸较小,因此力学性能较好;此时,抗弯强度为516.896 MPa,断裂韧度为8.3871 MPa·m~(1/2),硬度为17.341GPa。     

TiB2-TiN复合陶瓷刀具材料的显微结构和力学性能研究

热压烧结制备了不同TiN含量的复合陶瓷刀具材料TiB2-TiN-(Ni, Mo),对其性能测试表明,随着TiN含量的增加,材料的抗弯强度和断裂韧度逐渐提高,但是材料的硬度在TiN的含量达到40%(体积分数)时却大幅度降低.利用X衍射(XRD)、扫描电镜(SEM)和能谱(EDAX)分析了复合材料的物相和显微组织,结果表明,烧结过程中生成了MoNi相;随TiN含量增加,材料从以沿晶断裂为主转变为同时有沿晶断裂和穿晶断裂的断裂模式;裂纹扩展过程中有金属颗粒桥连现象.分析认为,材料的主要增韧机制是延性相颗粒桥连和裂纹偏转.     

机械合金化后注射成形制备Cu/Al2O3复合材料的显微组织与力学性能

采用机械合金化后注射成形制备10%(体积分数,下同)Cu/Al_2O_3复合材料,研究机械合金化时间、烧结温度对复合材料显微组织和性能的影响,并分析复合材料的增韧机理。结果表明:通过机械合金化10h后注射成形、脱脂、1550℃烧结工艺制备的10%Cu/Al_2O_3复合材料具有良好的抗弯强度和断裂韧度,分别为532MPa和4.97MPa·m^1/2;烧结温度低于1550℃导致原子在固态下扩散能力不足,烧结温度高于1550℃则使颗粒边界移动速率大于孔隙逸出速率,二者都造成复合材料孔隙率增加,而导致材料的强度和韧度下降;机械合金化时间延长使复合材料晶粒细化、Cu与Al_2O_3之间的结合强度提高,材料强度和硬度提高,但断裂韧度下降;Cu粉末弥散分布于Al_2O_3基体中,抑制烧结过程中Al_2O_3晶粒粗化,且使裂纹在扩展过程中遇到延性的Cu产生裂纹桥联和偏转,提高材料的韧度。     

TiC含量对WC-TiC-TaC硬质合金材料微观组织及力学性能的影响（英文）

本研究采用真空热压烧结技术,在1600℃下制备了WC-TiC-TaC硬质合金材料,研究了TiC含量对其微观组织及力学性能的影响。结果表明,随着TiC含量的增多,硬质合金材料的晶粒显著增大。当TiC的含量从10wt%增加到25wt%时,硬质合金材料的硬度逐渐增大,最高可达19.81 GPa,这是由于TiC的硬度高于基体WC的硬度;与此同时,硬质合金材料的抗弯强度和断裂韧度逐渐减小。当TiC的含量为10wt%时,材料的抗弯强度有最大值,其值为1147.24 MPa,这是由于在材料内部形成了均匀、细小的晶粒组织;在此含量下,复合材料的增韧机理为细晶增韧、裂纹偏转、裂纹分支、裂纹桥接和韧窝增韧,其断裂韧度有最大值,为14.60 MPa·m~(1/2)。     

Fe基软磁复合材料的断裂性能研究

基于两步热处理法进行了Fe基软磁复合材料的制备研究,成功制备出具有较高抗弯强度的软磁复合材料(SMCs),研究并分析了微观组织结构与抗弯强度的内在关系。将Fe粉表面钝化增加了Fe与具有优异介电性能的有机硅环氧树脂的结合键,提高了材料的抗弯强度,其强度达到45.3 MPa。能谱(EDS)及有限元方法(FEM)分析表明,SMCs制备中产生的孔洞及裂纹主要分布在树脂层及树脂-Fe界面,在外加载荷作用下,裂纹影响孔洞外缘的应力分布,并朝着共同断裂失效的方向扩展,故对SMCs的强度产生不利影响。为了提高SMCs的强度,需要保证树脂涂覆完整,并进一步提高树脂层本身及树脂与纯铁的粘结强度,采用热压方式减少材料中的孔洞数量。     

陶瓷刀具材料断口形貌及裂纹扩展的分形特征

通过热压烧结工艺,制备了一种高性能的Si3N4基陶瓷刀具材料。利用X射线衍射仪和扫描电子显微镜分别对材料物相组成、微观形貌及裂纹扩展路径进行了分析。借助图像处理技术和分形理论,计算了断口形貌及裂纹扩展路径的分形维数,并揭示材料断裂机制。研究表明,Si3N4基陶瓷刀具材料表现为穿晶/沿晶的混合断裂模式,其裂纹扩展方式主要是偏转和桥联,断口形貌及裂纹扩展均具有明显的分形特征。当材料断口形貌越粗糙,裂纹扩展路线越不规则,分形维数值增大,表明断口微观结构的粗糙程度、裂纹扩展路线的不规则程度可用分形维数来刻画。     

预制疲劳裂纹应力强度因子幅对超高强度钢断裂韧度的影响

采用多试样法对D406A超高强度钢进行了准静态断裂韧度KⅠC试验,分析了不同应力强度因子幅预制疲劳裂纹对疲劳预裂纹扩展周期、疲劳预裂纹扩展速率、试样断口形貌以及最终断裂韧度试验结果的影响。结果表明:疲劳预裂纹扩展周期和扩展速率均与应力强度因子幅呈指数变化规律,断口上的疲劳裂纹间距及最终断裂韧度试验结果均随应力强度因子幅的增大而增大,在材料断裂韧度KⅠC的20%~30%选择最大应力强度因子进行KⅠC试验结果较为稳定。     

新型HA/Ti-Fe生物复合材料的制备及其性能研究

采用无压烧结制备出不同Ti-Fe含量的HA/Ti-Fe生物复合材料,对其组织结构和力学性能进行了研究.显微组织的观察表明:均匀分布于HA基体中的金属Ti-Fe增强颗粒呈一种新颖的蛋壳状组织结构,其中核区主要由Fe组成,壳层主要由Ti组成.力学性能测试结果显示:随着Ti-Fe含量的增加,HA/Ti-Fe复合材料的硬度有所下降,但材料的抗弯强度和断裂韧度均明显提高.当Ti-Fe含量为5％时,抗弯强度出现最大值93MPa,与纯HA相比提高了42％;当Ti-Fe含量为15％时,材料的断裂韧度达到最大值1.3MPa·m1/2,较纯HA提高了128％.良好的界面结合和分布于壳层的韧性相Ti是导致材料力学性能大幅提高的主要原因.     

颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和扩展行为的影响

通过原位扫描电子显微镜(SEM)研究了粉末冶金制备的Cu/WCp复合材料的疲劳裂纹萌生和扩展行为,分析了颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和早期扩展行为的影响。结果表明:疲劳微裂纹萌生于WCp颗粒和基体Cu的界面;微裂纹之间相互连接并形成主裂纹,当主裂纹和颗粒相遇时裂纹沿着颗粒界面扩展。在低应力强度因子幅ΔK区域疲劳小裂纹具有明显的"异常现象",并占据了全寿命的71%左右。疲劳小裂纹的早期扩展阶段易受局部微观结构和颗粒WCp的影响,扩展速率波动性较大,随机性较强;当小裂纹长度超过150μm时,裂纹扩展加快直至试样快速断裂。裂纹偏折、分叉和塑性尾迹降低了疲劳裂纹扩展速率,而颗粒界面脱粘则提高了复合材料的疲劳裂纹扩展速率。通过数值模拟也可以发现颗粒脱粘增大了材料的疲劳扩展驱动力,从而提高了疲劳裂纹扩展速率。     

烧结温度对WZV材料力学性能和显微结构的影响

为了开发一种新型刀具材料,以WC、ZrO2和VC为原料,利用热压烧结工艺,分别在1500、1550、1600℃和1650℃烧结温度下制备了4种相同成分的WC/ZrO2/VC(WZV)复合材料.分析了烧结温度与刀具材料相对密度、硬度、抗弯强度和断裂韧性之间的关系,研究了烧结温度对刀具材料力学性能和显微结构的影响,确定了该材料合理的烧结温度为1550℃.试验结果表明,ZrO2质量分数为10%的WZV复合粉末经过48 h的高能球磨,在1550℃、30 MPa的热压烧结条件下,可获得相对密度为99.2%,维氏硬度为17.6 GPa,抗弯强度为786 MPa,断裂韧性为11.51 MPa.m1/2的优异性能.此外,通过对材料显微结构和断裂方式的分析,发现烧结温度对材料的断裂方式具有重要影响.     

Research Progress of Laminated Composite Ceramic Cutting Tools

The design of the lamination structure based on bionic shell pearl layer is a successful method for toughening ceramics. Lamination with strong bonding interfaces is used to improve the mechanical property and low fracture toughness of ceramic cutting tools. Based on the idea of demand–design–preparation–analysis–failure, the development and research progress of laminated ceramic tools are reviewed herein. The research status of design, interlayer diffusion reaction, residual stress, toughening mechanism, and crack propagation path of the biomimetic laminated ceramic composite tool materials is mainly introduced. The major topics of current research include the creation of material systems, the evolution of microstructure, and the assessment of macroscopic mechanical properties. The entire mechanical properties of laminated ceramic tools are significantly influenced by the multicomposition design of the ceramic material system and the optimization design of structural parameters of layer number and layer thickness ratio. However, the research on the practical cutting application of laminated ceramic tools is limited. Cutting tool wear characteristics vary between laminated and homogeneous ceramic tools. The development of useful laminated ceramic cutting tools can greatly benefit from the study on failure mechanisms of laminated ceramic tools.     

Measurement of the fracture toughness of polycrystalline diamond using the double-torsion test

The double-torsion test was employed to study the processes of crack propagation and to measure the fracture toughness of polycrystalline diamond. The value of fracture toughness of about 13 MPa m^1/2 is surprisingly high. Inhomogeneity in microstructure may cause discontinuous crack propagation which makes it difficult to study the subcritical crack growth behaviour of this polycrystalline material. Subcritical crack growth is shown to be negligible and crack deflection is shown to be an important toughening mechanism in polycrystalline diamond.     

Microstructure and mechanical properties of yttria-stabilized tetragonal zirconia polycrystals containing dispersed TiC particles

The microstructure and mechanical properties of hot-pressed yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics containing up to 30 vol % TiC particles were studied. Adding TiC particles to Y-TZP improved the bending strength and fracture toughness. With 20 vol% TiC particles the maximum bending strength and fracture toughness reached 1073±30.4 MPa and 14.56±0.25 MPa m^1/2, respectively. The residual tensile stress induced by the thermal expansion difference between ZrO₂ and TiC must have inhibited the tetragonal-monoclinic transformation. The stress-induced phase transformation was therefore not the dominant toughening mechanism. High-densities of dislocations within TiC particles and microcracking were detected by TEM. The improved toughness of the materials is considered to be the result of crack deflection, crack bowing of TiC particles and microcracking toughening of ZrO₂.     

Fatigue crack propagation and in-situ observations in three tool steel alloys manufactured using a rapid solidification technique

By utilizing special manufacturing conditions, e.g., using only pure elements and applying a rapid cooling rate, tool materials with high quasi-static fracture toughness can be produced. However, tool materials are often subjected to cyclic loading and, hence, their lifetime is dominated by fatigue failure. This study is focused on fracture mechanics and in-situ experiments to characterize the fatigue crack propagation behavior of three newly developed tool steels at a stress ratio R of 0.05. Microstructural examinations revealed that the materials consist of the phases α′-martensite, retained austenite, and complex carbides in different amounts. Results of preliminary tests are presented, in which it was attempted to grow the crack in a plane parallel to the plane of the starter notch. The determined ?K threshold values ranged between 4 and 5 MPa√m with Paris–Erdogan exponents of 3.3–4.6. In-situ observations were performed to understand the inherent damage mechanisms and microstructural effects during fatigue loading. These observations showed that fatigue crack growth is mainly dominated by the ductility of the martensitic–austenitic matrix. Only in cases in which the primary carbides are oriented favorably (with respect to the direction of crack propagation) does the crack follow the coherent carbide network to a certain extent. Furthermore, for the first time, a phase transformation from retained austenite to α′-martensite was detected at the crack tip during fatigue crack propagation for the material group of tool steels.     

Probability of cleavage fracture for a crack propagating by fatigue

Standard fracture toughness tests use fatigue pre-cracked specimens loaded monotonically from zero to failure. Scatter in toughness (cleavage) occurs because steel is metallurgically inhomogeneous, and because each specimen has its crack tip in a different local microstructure. A probability of fracture toughness distribution can be obtained by conducting multiple repeat tests on the same steel. This is often used to make probabilistic structural fracture predictions for combinations of crack length and applied load. However, it is likely the true structural situation involves gradual extension of a fatigue crack under a cyclic load. The question then arises as to how often the probability of fracture for the structure needs to be re-calculated. It could be argued that each fatigue load cycle moves the crack tip to a new position and gives a different instantaneous probability of fracture. But if this were the case, the predicted cumulative probability of fracture would quickly tend to unity. This paper describes cold temperature, wide plate fatigue tests designed to investigate this apparent contradiction. The steel is 15 mm thick, grade A, ship plate and the tests involve propagation of a fatigue crack from 300 mm to 650 mm length under a constant amplitude fatigue cycle of 10-100 MPa at −50 °C. The cold temperature fatigue tests do not show an obviously increased probability of fracture compared with the standard monotonic load tests. Nevertheless, in view of uncertainties surrounding the issue, a cumulative probability of fracture determined at 5 mm intervals through the steel is recommended for safe structural predictions.     

Mechanical properties and microstructure of Al/Al laminated structure produced via ultrasonic consolidation process

Mechanical properties of a 2024 aluminium alloy laminated structure produced by the ultrasonic consolidation were investigated. In comparison with the monolithic aluminium alloy, the existence of laminated structure gave different fatigue and fracture mechanisms that associated with the layer interfaces. The Al/Al laminated specimens had the lower tensile strength but much higher fracture toughness than the monolithic Al specimens due to the exit of interface delaminations around the crack tip. The fatigue life of the laminated specimens was comparable to that of the monolithic Al specimens, though the initiation and propagation of the crack in the laminated specimens depended strongly on the microstructure of each material. The interface between layers could arrest the fatigue crack and impede the further propagation.     

Stellite12钴基合金的疲劳性能及其断裂机理研究

采用三点弯曲疲劳法测得光滑试样和直缺口试样的S-N曲线以研究Stellite12钴基合金的疲劳性能,并通过断口形貌观察进一步探究该钴基合金的断裂过程。结果表明:光滑试样的疲劳极限为545 MPa,约为原始抗弯强度1552 MPa的25.4%;直缺口试样的疲劳极限约为101MPa,约为静态抗弯强度517.6MPa的19.1%。对于疲劳敏感性,光滑试样与直缺口试样的疲劳敏感性分别为397和31。此外发现疲劳裂纹多萌生于近表层聚集的碳化物处,同时表面缺陷也可诱发疲劳裂纹的萌生。疲劳裂纹的扩展主要表现为碳化物的穿晶断裂,钴基体在应力比R=0.1的疲劳加载条件下虽表现出一定的韧性且呈现出较多的撕裂脊,但也呈现出一定的脆性断裂模式,因此疲劳裂纹扩展模式为真疲劳与静态疲劳的混合模式。     

Sintering behaviour and mechanical properties of Cr3C2–NiCr cermets

Cr₃C₂–NiCr cermets are used as metal cutting tools due to their relatively high hardness and low sintering temperatures. In this study, a powder mixture consisting of 75 wt% Cr₃C₂–25 wt% NiCr was sintered at four different temperatures and characterized for its microstructure and mechanical properties. The highest relative density obtained was 97% when sintered at 1350 °C. As the relative density increased, elastic modulus, transverse rupture strength, fracture toughness and hardness of the samples reached to a maximum of 314 GPa, 810 MPa, 10·4 MPa·m^1/2 and 11·3 GPa, respectively. However, sintering at 1400 °C caused further grain growth and pore coalescence which resulted in decreasing density and degradation of all mechanical properties. Fracture surface investigation showed that the main failure mechanism was the intergranular fracture of ceramic phase accompanied by the ductile fracture of the metal phase which deformed plastically during crack propagation and enhanced the fracture toughness.