The fracture toughness of bulk metallic glasses

Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.     

Assessment of Titanium Aluminide Alloys for High-Temperature Nuclear Structural Applications

Titanium aluminide (TiAl) alloys exhibit high specific strength, low density, good oxidation, corrosion, and creep resistance at elevated temperatures, making them good candidate materials for aerospace and automotive applications. TiAl alloys also show excellent radiation resistance and low neutron activation, and they can be developed to have various microstructures, allowing different combinations of properties for various extreme environments. Hence, TiAl alloys may be used in advanced nuclear systems as high-temperature structural materials. Moreover, TiAl alloys are good materials to be used for fundamental studies on microstructural effects on irradiation behavior of advanced nuclear structural materials. This article reviews the microstructure, creep, radiation, and oxidation properties of TiAl alloys in comparison with other nuclear structural materials to assess the potential of TiAl alloys as candidate structural materials for future nuclear applications.     

LAVES PHASE ALLOYS FOR HIGH TEMPERATURE APPLICATIONS

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Microstructure and properties of sintered tungsten heavy alloys

The microstructure and properties of liquid-phase sintered tungsten heavy alloys were studied. The structure and segregation of the impurity elements at the interfacial boundaries were examined using scanning electron microscopy (SEM) and fine-probe energy dispersive spectroscopy (EDS) microanalysis. Test results of mechanical properties are presented and correlated with fracture behavior of the liquid-phase sintered tungsten alloys. It was found that the Fe-Ni-W alloy exhibits superior properties as compared with the Cu-Ni-W alloy. The detection of copper was found across tungsten grains and matrix that could be associated with inferior properties of the Cu-Ni-W alloy as compared to the Fe-Ni-W alloy. Although the fracture was predominantly brittle in both alloys, complex fracture modes seem to be operative due to the composite microstructure of the alloys. Evidence of microsegregation was observed that also contributed primarily to the brittle failure in the alloys. The impurity elements, such as sulfur and phosphorus, were detected at the tungsten matrix and tungsten-tungsten particle boundaries.     

NiAl alloys for high-temperature structural applications

If their properties can be improved, nickel aluminide alloys offer significant payoffs in gas turbine engine applications. For these materials, excellent progress has been made toward understanding their mechanical behavior as well as improving their low-temperature ductility and high-temperature strength. For example, recent work shows that room-temperature ductility can be improved dramatically by microalloying with iron, gallium or molybdenum. The next challenge is to develop an alloy which has the required balance of ductility, toughness and strength. Development of design and test methodologies for components made out of low-ductility, anisotropic materials will also be required. While significant challenges remain, the continuing developments suggest that the prognosis for using NiAl alloys as high-temperature structural materials is good.     

Gamma titanium aluminides: Their status and future

Gamma alloys, based on the gamma titanium aluminide (y-TiAl) intermetallic compound, are emerging as a revolutionary engineering material for high-temperature structural applications. This article discusses the historical background as well as the status and future prospects of gamma alloy technology in the areas of alloy development/ design, process development, and applications.     

Producing titanium aluminide foil from plasma-sprayed preforms

High-strength titanium alloy and titanium aluminide foils are required for fabricating composite structures and honeycombs for advanced aircraft engines and airframes. Titanium aluminide alloys possess limited workability, which results in significant yield loss when these materials are produced by the conventional ingot metallurgy route. This article describes the use of induction plasma spray technology to fabricate foil preforms of a titanium alloy and a titanium aluminide. These plasma-sprayed preforms were converted into 100% dense wrought titanium aluminide foil by a roll-consolidation process. The microstructure and mechanical properties of titanium aluminide foil produced from plasma-sprayed preforms were virtually identical to those of conventional ingot metallurgy foil. The plasma-spray plus roll-consolidation route may lead to the production of titanium aluminide foil as continuous coil, which would improve process efficiency and yield high-quality titanium aluminide foil at low cost.     

V4Cr4Ti合金和RAFM钢的热等静压扩散连接及其界面特性

低活化铁素体/马氏体(reduced activation ferritic/martensitic,RAFM)钢及钒合金被认为是未来核聚变反应堆第一壁的候选结构材料,性能各有优劣,可满足近中期应用要求. 采用热等静压技术在温度800 ℃、等静压压强150 MPa和保温时间2 h下实现V4Cr4Ti合金和CLF-1钢的固态扩散连接,对其界面微观组织、元素扩散特征以及抗剪强度进行了分析. 结果表明,CLF-1钢在距离连接界面120 μm区域内出现脱碳层,而V4Cr4Ti合金侧存在宽度约1.5 μm的高硬脆碳化物层;V4Cr4Ti合金/CLF-1钢连接界面无缺陷,接头室温抗剪强度最高达238 MPa. 断口分析表明,断裂发生于靠近V4Cr4Ti合金侧的高硬脆碳化物层,断口表现出整体韧性,局部脆性断裂的特征.     

预应变下高强结构钢低温断裂性能

为了测试高强结构钢Q420力学性能和断裂韧度,对其原材料和塑性变形材料分别进行了不同温度下的拉伸试验和断裂韧性试验.结果表明,结构钢的断裂韧性随着温度的降低显著减小,使其倾向于脆性断裂;而预应变虽然提高了钢材的屈服强度与抗拉强度,但显著降低了钢材的塑性及断裂韧性,进一步增加了脆性断裂发生的可能性.同时利用有限元分析得出...     

Processes of dynamic aging of VT16 titanium alloy

The use of dynamic aging in heat treatment improves the mechanical properties of alloys, first of all, the strength characteristics under conditions of static (both short- and long-term) and cyclic loading and the resistance to brittle and ductile fracture. Therefore, the processes of dynamic aging that can be treated as a variant of thermomechanical treatment have been studied in detail in several works and have been used for strengthening parts made of steels of pearlitic, martensitic, and austenitic classes, beryllium bronze. Duralumin-type alloys, etc. At the same time, data on the effect of dynarrlic aging on the properties of titanium alloys are very scarce. The present article is devoted to this problem.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 16 – 20, July, 1996.     

简单/改性铝化物涂层的研究现状

航空发动机各部件高温结构材料在苛刻环境下服役时,会遭受严重的高温氧化和热腐蚀.在合金表面施加铝化物涂层后,高温下表面能够生成一层致密且生长缓慢的Al2O3氧化膜,从而隔绝腐蚀介质,以防止合金被快速氧化腐蚀.概述了铝化物涂层的优点,包括制备简单、成本低廉.重点综述了以Ni、Fe、Ti/TiAl为合金基体的铝化物涂层微观结构.涂层的微观结构主要由渗铝工艺、基材成分及后处理工艺等因素决定,渗铝工艺包括渗剂成分、渗铝温度和渗铝时间.在高温下渗铝,Al的活度较低,涂层主要以基体元素向外扩散形成外扩散型涂层为主;在低温下渗铝,Al的活度较高,涂层主要以Al向内扩散形成内扩散型涂层为主.还归纳了不同渗铝涂层在干燥空气和水蒸气环境中的高温氧化行为,阐述了水蒸气对铝化物涂层高温氧化行为的影响,比较了Ni-Al系和Fe-Al系涂层的抗高温氧化性能.同时介绍了Cr-Al、Si-Al和Pt-Al 3种改性铝化物涂层的研究进展,包括制备方法、微观结构及抗高温氧化和腐蚀性能.最后,展望并总结了高温防护涂层的发展趋势.     

Nb-Al系金属间化合物及其复合材料研究进展

综述了Nb-Al系金属间化合物作为高温结构材料的最新研究进展和发展趋势。对目前国内外Nb-Al系金属间化合物及其复合材料的制备工艺、组织结构控制和力学性能的研究现状进行评述。结果表明:通过延性相增韧、合金化、层状结构设计、复合材料设计等方法,可以显著改善Nb-Al金属间化合物的室温脆性、抗氧化能力、高温强度及抗蠕变性能。Nb-Al系金属间化合物的研究方向应集中发展以Nb3Al及NbAl3金属间化合物为基体,以SiC、Al2O3及TiC等陶瓷相为增强相强化的陶瓷-铌基合金复合材料。     

微量元素在铝合金中的作用

总结了一些微量元素和痕量元素对形变铝合金组织、性能的影响。微量元素可以影响铝合金的时效过程,提高时效强化效果、控制合金的再结晶和晶粒长大,而Na等痕量元素可以造成铝合金的脆性断裂。     

Impurity-induced embrittlement of heat-affected zone in welded Mo-based alloys

Characteristics of strength and plasticity, fracture mode and grain boundary segregation for two Mo-based alloys with different bulk compositions, recrystallized by either furnace annealing or rapid heating followed by quenching, are studied as a function of heating temperature by mechanical test, scanning electron microscopy, Auger electron spectroscopy and computer simulation. There exists an essential difference in both segregation behaviour and mechanical properties between as-annealed and as-quenched structural states. The rapid quenching causes strong oversaturation of the grain boundaries. In this case, intergranular enrichment is approximately twice as high as that in as-annealed alloys, and spontaneous nucleation of brittle microcracks is observed at certain embrittled boundaries. The proposed high-speed heat treatments are considered as a promising method for modelling of the structural states of the heat-affected zone of weldments. The results obtained are discussed from the viewpoint of possible reasons of impurity-induced embrittlement of Mo-based alloys.     

Fracture strength and microstructure of ODS tungsten alloys

In many high temperature applications tungsten is superior to molybdenum alloys. For structural components very often joining technology is the limiting factor. If brazing or welding is used ductility at room temperature has to be considered. Particularly when handling or transporting they run the risk of brittle fracture.     

Investigation into high-temperature corrosion in a large-scale municipal waste-to-energy plant

High-temperature corrosion in the superheater of a large-scale waste-to-energy plant was investigated. A comparison of nickel-/iron-based alloys and austenitic stainless steel probes placed in the furnace demonstrated that temperature and particle deposition greatly influence corrosion. Nickel-based alloys performed better than the other metal alloys, though an aluminide coating further increased their corrosion resistance. Sacrificial baffles provided additional room for deposit accumulation, resulting in vigorous deposit-induced corrosion. Computational modelling (FLUENT code) was used to simulate flow characteristics and heat transfer. This study has shown that the use of aluminide coatings is a promising technique for minimising superheater corrosion in such facilities.     

The evolution of microstructure during the processing of gamma Ti−Al alloys

This paper describes titanium research at the Queen's University of Belfast, particularly focusing on the successful combination of experimental and computer modeling techniques. Experimental work combined x-ray diffraction, optical microscopy, and scanning-electron microscopy performed on a Ti-46Al-1.9Cr-3Nb alloy before and after various heat treatments. A phase-field model was developed to simulate the formation and evolution of lamellar microstructure in TiAl alloys. In addition, a model was developed to predict the correlation between alloy composition and microstructure and its tensile properties in gamma-based titanium aluminide alloys through the use of the artificial neural network.     

Mg-Gd-Y-Zr高强耐热镁合金的研究进展

Mg-Gd-Y-Zr合金由于具有优异的室温、高温力学性能及抗蠕变性能而成为镁合金研究的热点。本文作者总结国内外Mg-Gd-Y-Zr合金的研究进展,分析熔体纯净化技术开发现状、热变形行为、强化机制以及断裂机制,讨论Mg-Gd-Y-Zr合金蠕变机理、腐蚀机理及表面处理技术的研究情况,并对Mg-Gd-Y-Zr合金固态回收技术进行介绍,最后,对该合金未来的发展方向进行展望。     

Effect of Hydrogen and Magnetic Field on the Mechanical Behavior of High Strength AISI 4340 Steel

Presence of hydrogen in materials is known to affect their mechanical properties due to hydrogen embrittlement problem. Steels used in various applications are prone to be exposed to aqueous electrochemical environments, which may introduce hydrogen into the alloy. These alloys are also prone to be simultaneously exposed to magnetic field, which may affect the hydrogen embrittlement susceptibility of these alloys. Therefore, it is important to examine the effect of hydrogen and magnetic field on the mechanical behavior of iron-based alloys. In this work, the effect of hydrogen and magnetic field on the fracture behavior of high strength AISI 4340 steel was examined. Three-point bend test was used to study the fracture behavior. In all the cases, the samples tested with hydrogen charging show a drastic reduction in ductility and fracture stress values. The effect of magnetic field was seen to be negligible. The hydrogen embrittlement was characterized by a change in the fracture surface from a ductile-type fracture to a brittle cleavage-type fracture. Acoustic emission signals collected during the test corresponds to the fracture behavior.     

Effects of RE on microstructure and properties of AZ91 magnesium alloy

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