Preparing damage-tolerant titanium-matrix composites

In-situ TiB-reinforced titanium alloy composites have been produced by both rapid solidification and ingot metallurgy techniques. The reinforcements have been shown to result in strength and modulus improvements, improved fracture toughness, increased fatigue endurance limit, and better fatigue crack growth and creep resistance than conventional monolithic titanium alloys. In addition to improved mechanical properties, in-situ composite structures and components with complex shapes can be fabricated using conventional ingot metallurgy, near-net shape forming techniques.     

Phase assemblage effects on the fracture and fatigue characteristics of magnesia-partially stabilized zirconia

A detailed investigation on the relationships between phase assemblage and fracture and fatigue characteristics of Mg-PSZ has been conducted. In doing so, three completely different microstructural conditions were first attained through different thermal treatments and then their flexural strength, fracture toughness and crack growth resistance and fatigue crack growth (FCG) behaviour were evaluated. The obtained results are discussed considering the interplay between microstructural features and dominant crack-microstructure interaction and its influence on the operation of given toughening and mechanical fatigue mechanisms for each phase assemblage studied. FCG resistance, under both sustained and cyclic loading, is found to be closely related to the corresponding fracture toughness of each phase assemblage. However, real mechanical fatigue effects are estimated to be, once they are rationalized with respect to particular environmental-assisted cracking behaviours, an exclusive function of crack path type. Finally, different cyclic fatigue mechanisms for Mg-PSZ are pinpointed depending upon the prevalent transgranular or intergranular FCG morphology.     

Role of the grain-boundary phase on the elevated-temperature strength,toughness, fatigue and creep resistance of silicon carbide sintered with Al,B and C

The high-temperature mechanical properties, specifically strength, fracture toughness, cyclic fatigue-crack growth and creep behavior, of an in situ toughened silicon carbide, with Al, B and C sintering additives (ABC-SiC), have been examined at temperatures from ambient to 1500°C with the objective of characterizing the role of the grain-boundary film/phase. It was found that the high strength, cyclic fatigue resistance and particularly the fracture toughness displayed by ABC-SiC at ambient temperatures was not severely compromised at elevated temperatures; indeed, the fatigue-crack growth properties up to 1300°C were essentially identical to those at 25°C, whereas resistance to creep deformation was superior to published results on silicon nitride ceramics. Mechanistically, the damage and shielding mechanisms governing cyclic fatigue-crack advance were essentially unchanged between ∼25°C and 1300°C, involving a mutual competition between intergranular cracking ahead of the crack tip and interlocking grain bridging in the crack wake. Moreover, creep deformation was not apparent below ∼1400°C, and involved grain-boundary sliding accommodated by diffusion along the interfaces between the grain-boundary film and SiC grains, with little evidence of cavitation. Such unusually good high-temperature properties in ABC-SiC are attributed to crystallization of the grain-boundary amorphous phase, which can occur either in situ, due to the prolonged thermal exposure associated with high-temperature fatigue and creep tests, or by prior heat treatment. Moreover, the presence of the crystallized grain-boundary phase did not degrade subsequent ambient-temperature mechanical properties; in fact, the strength, toughness and fatigue properties at 25°C were increased slightly.     

Novel alumina titanium-carbonitride nickel composites

Alumina-containing titanium-carbonitride nickel composites (Al₂O₃-TiCN-Ni-Mo₂C) have been synthesized with up to 20 vol.% alumina particles and nickel contents of 10–15 vol.%. Alumina in the form of platelets as well as powders have been successfully incorporated into the composites. The mechanical properties of these composites show a marked increase in toughness while retaining a good hardness and a low density when compared to TiCN-Ni cemented carbides. The presence of alumina with its superior chemical and wear resistance makes these composites very attractive for all wear parts—particularly for pieces exposed to chemically aggressive environments and high-temperature applications. Moreover, the complementary properties of these new composites (e.g., light weight, hardness, toughness, and chemical stability) allow a wide range of applications to be envisaged.     

纤维增强环氧树脂复合材料抗固体颗粒流冲蚀磨损研究进展

在生产生活中,固体颗粒流冲蚀磨损会造成经济损失,并且存在安全隐患。环氧树脂复合材料具有较好的强度和耐冲蚀性能,被广泛地应用于颗粒流冲蚀磨损工况下。为进一步提升环氧树脂的耐冲蚀性能,通常通过填料来改性环氧树脂,其中纤维增强环氧树脂表现出优异的耐固体颗粒流冲蚀性能,使得环氧树脂复合材料的应用更加广阔。根据纤维的种类可以将其分为无机纤维(玻璃纤维或碳纤维)、自然纤维及混和纤维增强环氧树脂复合材料。综述了纤维增强环氧树脂复合材料抗固体颗粒流冲蚀性能的研究现状,讨论了不同的纤维增强复合材料表现出的冲蚀行为(塑性、脆性、半塑性、半脆性),重点分析和对比了不同纤维填料特性(纤维类型、纤维含量、纤维取向)增强环氧树脂复合材料在不同工况条件(冲蚀角度、冲蚀速度、磨粒特性)下的耐冲蚀磨损性能,阐明了不同纤维增强环氧复合材料的冲蚀模式和抗冲蚀机理,指出其现存的问题并展望其发展方向和前景。     

Polymer nanocomposites: A small part of the story

Polymer nanocomposites are polymer matrix composites in which the fillers are less than 100 nm in at least one dimension. These composites have exhibited extraordinarily interesting properties. A defining feature of polymer nanocomposites is that the small size of the fillers leads to a dramatic increase in interfacial area as compared to traditional composites. This interfacial area creates a significant volume fraction of interfacial polymer with properties different from the bulk polymer even at low loadings. The properties and structure of this interfacial region are not yet known quantitatively, presenting a challenge both for controlling and predicting the properties of polymer nanocomposites. This paper provides a brief overview of polymer nanocomposites with emphasis on the impact of the interfacial region.     

微观组织对TA15 ELI钛合金损伤容限性能的影响

研究了TA15 ELI钛合金43 mm厚板的等轴组织、双态组织和片层组织的室温拉伸性能、断裂韧性(KIC)以及疲劳裂纹扩展速率(da/dN)等损伤容限性能,通过金相显微镜观测了疲劳裂纹在各类组织中的扩展规律,讨论显微组织对该合金损伤容限性能的影响.结果表明该合金等轴组织和双态组织的室温力学性能和疲劳裂纹扩展阻抗差别不大;相对等轴组织和双态组织而言,该合金片层组织在损失强度较小的前提下(Rm=992 MPa),合金断裂韧性提高,达到111 MPa·m1/2,同时该合金的疲劳裂纹扩展速率也大幅降低,其Paris公式拟合参数为c=1.08×10-8,n=3.23,具有更好的损伤容限性能.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt. % TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δα). The mechanical properties and thermal fatigue resistance of TiC/Al2O3 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a "true" cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

Fabrication and Characterization of Nickel Ferrite Based Inert Anodes for Aluminum Electrolysis

Nickel ferrite-based cermets and their relevant composites have been widely used as inert anodes for aluminum electrolysis due to the good combination of chemical resistance, thermal, and mechanical stability. In this study, various NiO/NiFe₂O₄ composites consisting 5, 10, and 15% NiO in conjunction with Cu/NiFe₂O₄ cermets containing 5, 10, and 15% Cu have been prepared by powder metallurgy method. The degradation resistance of developed inert composites has been evaluated under hot corrosion conditions by plunging the samples in the molten electrolyte at 1,000 °C for various holding times. The strength, toughness, hardness, relative density, microstructural observation, phase analysis, and electrical resistivity have been investigated in details by the 3-points bending test, Vickers hardness test, Archimedes method, scanning electron microscope, x-ray diffraction, and conventional direct current four-probe technique, respectively. The experimental results for NiO/NiFe₂O₄ composites show that a significant improvement of toughness and degradation resistance occurred in conjunction with a moderate decrease in strength by adding NiO content from 5 to 15%, while the relative density has been increased only up to 5%NiO content and then decreased. Moreover, increasing of Cu content from 5 to 15% in the cermet samples, all of the mentioned engineering properties such as strength, toughness and electrical conductivity have been improved considerably, but the degradation resistance has been decreased.     

典型冷作模具钢性能与失效关系的探讨

通过对比几种典型冷作模具钢的处理工艺及其力学性能,综合分析了该类模具钢失效的3种类型,即过载失效、磨损失效和疲劳失效的形态和特征;重点探讨了高硬度冷作模具钢的表面开裂敏感性、缺口断裂性能和断口分形维数与工艺参数的关系,提出一些提高冷作模具断裂抗力、减少模具早期失效的途径。     

Feasibility of electro-slag cast steel for hot-working tools

A study on electro-slag cast steel has been performed to assess the feasibility of its use in hot-working tools in place of traditional forged steels. In this study, some important material properties that are essential for the proper performance of hot-working tool steels have been measured. The properties of the electro-slag cast steel are compared with those of hot-working tool steels currently used in industrial practices. The use of electro-slag cast steel in specific applications has been evaluated and compared with the operational performance of currently used steels in industrial environments. The wear resistance, impact toughness, thermal fatigue resistance, and repair weldability of electro-slag cast steel have been determined. The material properties were found to be comparable to existing hot-working tool steels. The number of effective blows obtained from an industrial application test also confirms that electro-slag cast steel is a feasible material for hot-working tools.     

Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based SuperalloysEI北大核心CSCD

为满足不断攀升的两机涡轮动力系统的快速发展,表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而,高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免,由此引起的表面强韧化、抗疲劳效果的退化,成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展,分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律,探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。     

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TiB_2-B_4C CERAMIC COMPOSITES

The microstructure and mechanical properties of TiB_2 /B_4C composites have been investi-gated.It was found that both the strength and hardness for TiB_2 greatly increase with the ad-dition of 20 to 30 wt-% B_4C,and the fracture toughness K_(IC) value remaines on the originalhigh level.The flexure strength,Vicker's hardness and fracture toughness are 782 MPa,26.2GPa and 7.2 MPam~(1/2),respectively,for the TiB_2-30 wt-% B_4C composite,compared to450 MPa,21 GPa and 7.0 MPam~(1/2) for monolithic TiB_2.The toughening and strengtheningmechanisms,have also been discussed.     

Cr-N硬质镀层的断裂韧性和冲击疲劳特性研究

采用离子束辅助磁控溅射技术(IBAMS)沉积Cr-N硬质镀层,用压入法评定镀层的断裂韧性,多次冲击试验法测定镀层的多冲疲劳抗力,探讨了多冲疲劳抗力与镀层组织结构和断裂韧性等机械性能的关系。结果表明,通过调节离子束辅助轰击能量,可以提高Cr-N镀层的硬度和断裂韧性。镀层断裂韧性的提高除了与晶块尺寸减小有关外,还与离子轰击导致镀层致密度增加相关。在冲击载荷作用下Cr-N镀层的失效方式为内聚力型疲劳剥落,随硬度和断裂韧性增加,镀层的抗多次冲击疲劳能力提高。     

Improved microstructure and fracture properties of short carbon fiber-toughened ZrB2-based UHTC composites via colloidal process

Ultra-high temperature ceramics are potential materials for a variety of high temperature applications because of excellent thermo-mechanical properties and oxidation resistance. To further improve their fracture properties, a novel colloidal process was proposed to fabricate the short carbon fiber-toughened ZrB₂–ZrSi₂ composites. Microstructure analysis found that the colloidal processing route could avoid the fibers' agglomeration and alleviate the fibers' damage, which minimizes the structural defects and retains the fibers' strength. The relative density of composites achieves 98.35% and the distribution of fibers in matrix is homogeneous. Mechanical tests indicate that the flexural strength is 458 MPa and the fracture toughness is 6.9 MPa·m^1/2. In comparison to the composite obtained by conventional processing route, the fracture toughness increases by 47%. The main mechanisms for improved fracture properties could be attributed to the crack deflection, fiber sliding and fiber bridging.     

Mechanical properties evaluation of hot-pressed Si3N4–SiC(w) composites

Silicon nitride based ceramic composites have been prepared using an alternative development route, aiming to reach properties for structural applications. The prepared compositions were silicon nitride based matrix with aluminum nitride and yttrium oxide, as sintering aids, reinforced with silicon carbide whiskers. The powders were grounded/homogenized directly as received. Samples were obtained by uniaxial hot-pressing and, physical (specific mass, X-ray diffraction and microstructural analysis) and mechanical (microhardness, fracture toughness and flexural strength) properties were determined. The resulting high fracture toughness values were related to crack deflection and crack bridging toughening mechanisms, observed by SEM.     

Fabrication and fracture toughness of macro-toughening-designed composite

lINTR0DUCTIONParticleReinforcedMetalMatrixC0mp0s-ites(PMMCs)havehighspecificstrength,spe-cificmodulus,elevatedtemperatureproperties,res1stancetowearandlowcost.However,com-paniedlowductilityandtoughnessisonemainobstacletotheirapplicationforengineeringL','j.ManystudiesonSiCparticlereinforcedalu-.minum.ll.y.['v']showthattheadditionofpar-ticlenotonlyrefinesmatrixgrainbutalsoresultsinhighdensitydislocationsinthematrixneartheinterface.Particlesblocklong-distance-slipofthedislocationsinthema…     

影响P92钢焊缝冲击韧性的焊接工艺

P92钢是新型铁素体耐热钢,已广泛应用于超(超)临界燃煤发电机组。相比其他铁素体耐热钢,P92钢具有更高的高温强度和蠕变性能,其抗热疲劳性、热传导系数和热膨胀系数远优于奥氏体不锈钢,抗腐蚀性和抗氧化性优于其他9%Cr的铁素体耐热钢。P92钢的焊接技术已较为成熟,但其焊接接头易出现焊缝冲击韧性偏低的问题。影响P92钢焊缝金属冲击韧性的主要因素是焊接热输入。细焊条、薄焊层、多层多道焊,适当的预热温度、层间温度,足够的高温回火温度和恒温时间,是保证焊缝冲击韧性的有效措施。     

A Comparison Study of Wear and Fretting Fatigue Behavior Between Cr-alloyed Layer and Cr-Ti Solid-solution Layer

This work reported a comparison between the wear and fretting fatigue(FF) behaviors of a Cr-alloyed layer and a Cr-Ti solid-solution layer.The hardness and toughness of both layers were evaluated to support this comparison.The results showed that the Cr-alloyed layer had high surface hardness but poor toughness,while the Cr-Ti solid-solution layer had excellent toughness but low hardness.The FF properties of the modified Ti6A14 V alloy depended on the trade-off between two factors:wear resistance and fatigue resistance.Although the Cr-alloyed layer could effectively resist the wear in fretting areas,its poor toughness caused the fatigue resistance to drop sharply and hence led to a premature failure in FF test.Due to the relatively good fatigue resistance,the Cr-Ti solid-solution layer had slightly higher FF life than that of the Cr-alloyed layer;however,its low hardness resulted in severe wear in correspondence with the fretting area and thus a failure to improve the FF properties of Ti6A14 V alloy.When combined with shot peening post-treatment,the FF life of both layers increased by about three times compared to that of the Ti6A14 V alloy.A further study showed that the poor toughness or low hardness still exerted negative influence on combination-treated samples.     

激光原位制备复合碳化物颗粒增强铁基复合涂层及其耐磨性的研究

通过在高碳当量铁基熔覆粉末中复合添加多种强碳化物形成元素,激光原位制备的颗粒增强铁基复合材料涂层具有颗粒析出密度大、尺寸分布均匀的优点.通过在铁基熔覆粉末中单独添加Ti,复合添加Ti+Zr以及Ti+Zr+WC的方式,运用激光熔覆技术在中碳钢表面制备了颗粒增强铁基复合涂层.用X射线衍射仪、扫描电镜和透射镜等手段研究了涂层的显微组织、颗粒相结构及颗粒相与熔覆层基体相之间的界面.通过环块磨损实验,对比渗碳淬火工艺研究了颗粒增强涂层的耐磨性能,并对磨损机制进行了讨论.结果表明,涂层微观结构是典型的亚共晶介稳组织,原位合成的颗粒是一种复合碳化物,界面处结合牢固.激光原位制备复合碳化物颗粒增强铁基复合涂层具有优异的耐磨性能.