Nucleation of cracks from shear-induced cavities in an α/β titanium alloy in fatigue,room-temperature creep and dwell-fatigue

In titanium alloys, dwell periods during room-temperature stress-controlled fatigue tests are responsible for substantial reductions in lifetime compared to pure fatigue loading. The mechanisms of such a creep–fatigue interaction have been investigated for alloy Ti-6242. Scanning and transmission electron microscopy observations revealed crack initiation by coalescence of shear-induced cavities nucleated at α/β interfaces in large colonies of α laths nearly parallel to the loading axis. The density and average size of cavities were larger in dwell-fatigue and creep than in fatigue. A qualitative micromechanical model of cavity nucleation based on discrete dislocation dynamics was developed. The number of cycles for cavity nucleation was computed as a function of the applied stress range. A finite threshold, dependent on the size of α laths colonies with similar orientation, was found. The simulations predict earlier cavity nucleation in creep or dwell-fatigue than in pure fatigue, which is consistent with the performed experiments.     

Progress in the understanding of gamma titanium aluminides

During the last two years remarkable improvements have been made in the properties (such as toughness and creep) and processing technology of gamma titanium aluminides, making them potentially viable engineering alloys for high-temperature structural applications. These achievements were made possible by a greater understanding of both the fundamental and the practical aspects of these aluminides, such as phase relationships, the effects of alloying elements, deformation mechanisms, microstructure evolution and processing. This article reviews the current understanding of the above specific aspects and the processing-microstructure-property relationships, and identifies pacing problems and applications.     

Creep response and deformation processes in nanocluster-strengthened ferritic steels

There is increasing demand for oxide-dispersion-strengthened ferritic alloys that possess both high-temperature strength and irradiation resistance. Improvement of the high-temperature properties requires an understanding of the operative deformation mechanisms. In this study, the microstructures and creep properties of the oxide-dispersion-strengthened alloy 14YWT have been evaluated as a function of annealing at 1000 °C for 1 hour up to 32 days. The ultra-fine initial grain size (approx. 100 nm) is stable after the shortest annealing time, and even after subsequent creep at 800 °C. Longer annealing periods lead to anomalous grain growth that is further enhanced following creep. Remarkably, the minimum creep rate is relatively insensitive to this dramatic grain-coarsening. The creep strength is attributed to highly stable, Ti-rich nanoclusters that appear to pin the initial primary grains, and present strong obstacles to dislocation motion in the large, anomalously grown grains.     

Recent Development of Effect Mechanism of Alloying Elements in Titanium Alloy Design

钛合金的性能主要取决于合金成分和显微组织两方面,而合金元素的种类选择以及添加量是合金成分设计的主要组成内容,不同合金元素的添加可以一定程度上改善合金的强度,高温性能,蠕变性能以及加工性能等,满足特殊工业需求。主要探讨了合金元素在钛合金设计中的作用以及说明了各合金元素在新合金设计中应注意的一些原则,并对合金元素在钛合金设计中存在的一些问题进行探讨和归纳总结,特别对于合金中不同合金元素之间的相互作用,引起合金组织中位错取向和相组成发生变化,从而影响合金的机械性能。另外,对几种经典合金的合金组成、稳定性以及组织性能之间的相互关联进行研究,希望进一步了解合金元素在钛合金中的影响机理,为新型钛合金设计与研发提供参考。     

Application of a dispersed-reservoir concept for the development of oxidation-resistant alloys

Resistance to high-temperature oxidation is typically provided by either alloying with the desired scale-forming element or elements, or by applying a coating rich in those elements. For some alloy systems, the alloying approach requires very large additions of the oxide-forming elements and can lead to drastic reduction in the mechanical properties and melting temperature of the alloys. The dispersed-reservoir concept is based on the incorporation of a reservoir of the oxide-forming element as a second phase, which is inert to the alloy substrate, but which reacts with the environment to form a protective outer scale. The selection of the reservoir phase is primarily governed by thermodynamic considerations; its effectiveness also depends on other factors such as its size and distribution in the matrix, which are controlled by processing parameters. The research represents an examination of the applicability of this concept to the development of inherent oxidation resistance in three different classes of alloys. In each case considered, the mode of growth of the scale of the parent alloy is different, and occurs by: (i) the predominant outward diffusion of cations; (ii) the predominant inward diffusion of anions; or (iii) the formation of a volatile oxide. Observations of the high-temperature oxidation behavior of these different systems has provided some insights into the ways in which the dispersed reservoir phase exerts its effect.     

Material selection and grade optimization applied to aluminum matrix composites

A general model for the optimal use of materials based on structural optimization is derived. The competitiveness of materials is assessed with merit parameters. The competition between materials (material selection optimization) and the role of the composition and microstructure for a given material (grade optimization) are analyzed. The model is applied to aluminum matrix composites. The influence of matrix material, amount of reinforcement, and value of weight savings is studied. Mechanical properties are analyzed with the aid of published experimental data and available models. The Tsai-Halpin model is used to represent the variation of the elastic modulus with the amount of reinforcement. For yield strength the modified shear lag model is applied. It can satisfactorily describe experimental data and the variation with reinforcement for high-strength matrix alloys. For aluminum alloys of medium and lower strength, the observed increase is larger than the predicted one. This can be explained with the help of more recently developed micromechanical models that take into account the changes in microstructure in the matrix. For structural parts, large values of weight savings are usually necessary to make the particulate-reinforced composites competitive with carbon steel or their parent aluminum alloys. In other applications, combinations of properties are important to make the composites competitive.     

Mechanical properties and creep resistance of Mg-Li composites reinforced by MgO/Mg_2Si particles

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GRAIN BOUNDARY FEATURE AND CREEP BEHAVIOR OF Fe-15Cr-25Ni ALLOYS

Creep behavior of Fe-15Cr-25Ni alloys with different grain boundary features has been in-vestigated at 850 and 950℃ and in stress range of 14.7～78.5 MPa.The single phase alloyexhibits typical recovery creep characteristics,and for the alloy in which M_(23)C_6 carbidesdensely precipitate at grain boundaries,the creep rate dramatically decreases and creep behav-ior is also significantly different from the single phase alloy.When the carbides precipitate atgrain boundaries,the dislocation density are higher and the size of subgrains near grainboundaries are smaller than those of single phase alloy.The creep mechanisms for two grainboundary features have been discussed.     

Heat-resistant dispersion-strengthened copper alloys

Processing methods for producing dispersion-strengthened (DS) copper alloys with high strength, high conductivity, and good long-term stability at elevated temperature are reviewed. Particle size and stability are related to material characteristics and processing route. Physical and mechanical properties of DS copper alloys are directly associated with microstructural features such as particle volume fraction, stability, size, solubility in the matrix, and interfacial properties. New avenues for DS copper alloys design are suggested based on thermal conductivity concept and recent Rosier- Artz theory of high-temperature strength.^[1]     

Recrystallization Process in Fe-Cr-Al Oxide Dispersion-Strengthened Alloy: Microstructural Evolution and Recrystallization Mechanism

Mechanically alloyed iron-base oxide dispersion-strengthened (ODS) alloys are the class of advanced materials for application in heat exchangers tubing in which creep and oxidation resistance are paramount. The yttria dispersion in such alloys improves the high-temperature creep and stress rupture life. The strength is further enhanced by the development of a coarse-grained microstructure during recrystallization. Factors controlling the evolution of this desirable microstructure are explored in this work, focusing specifically on PM 2000. The results presented in terms of orientation imaging, transmission electron microscopy, and scanning electron microscopy indicate that the recrystallization process consists of two different stages. Before the coarse grain takes place, the alloy undergoes an extended recovery process followed by abnormal grain growth. The initial microstructure consisted of subgrains (submicrometer sizes) with a strong 〈110〉∥RD fiber texture (α fiber), which are transformed into coarse grains (mm sizes) with orientations 〈112〉∥RD. The aim of this study is to describe the mechanisms involved in the intermediate stages of recrystallization process from the submicrometer grain size to the abnormal grain size.     

Ni60Ti40合金热压缩变形行为及机理

对Ni60Ti40合金高温变形行为及变形机理进行了研究。通过计算获得了该合金在不同变形工艺下的应变速率敏感性指数m和变形激活能Q的变化规律,分别构建了Prasad、Gegel、Malas、Murty和Semiatin等不同失稳判据下的动态材料模型热加工图及包含位错数量的变形机理图。应用热加工图理论分析了该合金的适合成形加工区和流变失稳区,运用变形机理图预测了该合金高温变形过程中基于柏氏矢量补偿的晶粒尺寸和基于模量补偿的流变应力下的位错演变规律及高温变形机理。     

Multi-scale modeling of the viscoplastic response of As-fabricated microscale Pb-free Sn3.0Ag0.5Cu solder interconnects

A mechanistic multiscale modeling framework is proposed, to capture the dominant creep mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated Sn3.0Ag0.5Cu (SAC305) solder interconnects. Mechanistic creep models of dislocation climb and detachment are used to capture the dispersion strengthening mechanisms in the Sn–Ag eutectic phase. These models are combined at the next length scale, with micromechanics-based homogenization schemes, to capture the load-sharing between Sn dendrites and intermetallic phases. The next higher length scale (Sn grains) is not addressed here since secondary creep response is empirically found to be insensitive to grain microstructure. Theoretical insights into the influence of microstructural features on the viscoplastic behavior of microscale SAC305 interconnects are provided. The model effectively captures the effect of alloy composition and aging loads on SAC solders, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys.     

等通道挤压Mg2Si增强ZK60镁合金的显微组织及力学性能

研究等通道挤压(ECAP)对ZK60+2Si镁合金显微组织、室温力学性能和高温抗蠕变性能的影响。结果表明,合金铸态组织主要由-Mg基体、Mg2Si相和MgZn相组成,等通道挤压可显著碎化原粗大汉字状Mg2Si相并使其趋于弥散分布,同时基体组织也得到细化。挤压4道次后,合金的室温抗拉强度由154.8MPa增加到270MPa,伸长率由4.5%增加到17.5%。挤压6道次后,合金的伸长率进一步增加到21%,而抗拉强度却下降至261MPa;合金的高温蠕变寿命由铸态20h延长到203h,稳态蠕变速率下降了约1个数量级,这主要是因为细小颗粒状MgSi相有效阻止了晶界滑移。     

The oxidation performance of modern high-temperature alloys

The high-temperature oxidation resistance of an alloy is a key design criterion for components in a variety of industrial applications, such as advanced gas turbines, industrial heating, automotive, waste incineration, power generation and energy conversion, chemical and petrochemical processing, and metals and minerals processing. The importance of correctly assessing the long-term oxidation behavior of high-temperature alloys is illustrated. As applications move to higher temperatures, new alloys are needed. In this paper, the oxidation performance of three newly developed alloys, an alumina-forming Ni-Fe-Cr-Al alloy, a γ′-strengthened Ni-Cr-Co-Mo-(Al+Ti) alloy, and a nitride-strengthened Co-Cr-Fe-Ni-(Ti+Nb) alloy is presented. Author’s note: All compositions reported in this article are in weight percent.     

非连续增强钛基复合材料研究新进展

非连续增强钛基复合材料由于具有各向同性、比强度高、优良的高温强度、成本较低等特点而受到高度关注。TiC及TiB增强颗粒以其稳定的复合结构、良好的增强效果得到发展,成为非连续增强钛基复合材料的最终优选增强剂。从制备方法、增强体与基体的界面结构及复合材料的性能等方面概述了非连续增强钛基复合材料的最新研究进展。     

Microstructure and mechanical properties of orthorhombic alloys in the Ti---Al---Nb system

The current understanding of the metallurgy of the orthorhombic alloys in the Ti---Al---Nb system is reviewed with emphasis on tensile and creep properties of ternary alloys. It is shown that increasing the Nb content of alloys from 15 to 27 at% at a constant Al level significantly increases both the tensile and creep properties of equiaxed as well as lath structures, while small changes in Al content have a large effect on creep. For a given alloy composition, the amount of B2(β) phase and its distribution and the scale of O laths influences tensile properties, while creep properties depend on the volume fraction of equiaxed ₂/O phase present in the structure as well as the size of O laths.     

Abrasive wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Ni-P coated Si₃N₄ reinforced Al6061 composites were fabricated by vortex method. Percentage of reinforcement was varied from 6 wt.% to 10 wt.% in steps of 2. Cast matrix alloy and developed composites were hot forged at a temperature of 500 °C using a 300T hydraulic hammer. Both as cast and hot forged matrix alloy and its composites were subjected to microstructure studies, grain size analysis, microhardness and abrasive wear tests. Microstructure studies reveal uniform distribution of silicon nitride particles with good bond between matrix and reinforcement in both as cast and hot forged condition. It is observed that, increased content of reinforcement in both as cast and hot forged composites do result in significant grain refinement. However, when compared with as cast matrix alloy and its composites hot forged alloy and its composites exhibits higher extent of grain refinement. Both as cast and hot forged composites exhibit improved microhardness and abrasive wear resistance when compared with the unreinforced alloys under identical test conditions. Abraded worn surfaces were examined using scanning electron microscopy (SEM) for possible wear mechanisms. Increased abrasive particle size and load has resulted in larger extent of grooving leading to increased abrasive wear loss for both the matrix alloy and developed composites.     

Testing and Analysis of Full-Scale Creep-Rupture Experiments on Inconel Alloy 740 Cold-Formed Tubing

Full-scale pressurized creep-rupture tests were conducted on Inconel^® alloy 740 cold-formed tube bends to evaluate the effect of cold-work on the performance of tube bends for high-temperature creep applications. A new method of analysis is developed that can be used to simplify the complexities of structural (geometric) effects and material degradation due to cold-work. Results show that Inconel^® alloy 740 behaves similarly to other age-hardenable nickel-based alloys subjected to cold-work prior to creep testing with large reductions in rupture life and ductility and a corresponding moderate increase in minimum creep rate. The results also demonstrate that the full-size test method can be a beneficial to understanding the performance of large components in service.     

Environmentally friendly solders 3-4 beyond Pb-based systems

Based on environmental considerations, global economic pressures, enacted by legislations in several countries, have warranted the elimination of lead from solders used in electronic applications.Sn3.5Ag, SnAgCu, and Sn0.7Cu have emerged among various lead-free candidates as the most promising solder alloys to be utilized in microelectronic industries.However, with the vast development and miniaturization of modern electronic packaging, new requirements such as superior service capabilities have been posed on lead-free solders.In order to improve the comprehensive property of the solder alloys, two possible approaches were adopted in the current research and new materials developed were patented.One approach was involved with the addition of alloying elements to make new ternary or quaternary solder alloys.Proper addition of rare earth element such as La and Ce have rendered solder alloys with improved mechanical properties, especially creep rupture lives of their joints.Another approach, the composite approach, was developed mainly to improve the service temperature capability of the solder alloys.Composite solders fabricated by mechanically incorporating various reinforcement particles to the solder paste have again exhibited enhanced properties without altering the existing processing characteristics.The recent progress and research efforts carried out on lead-free solder materials in Beijing University of Technology were reported.The effects of rare earth addition on the microstructure, processing properties, and mechanical properties were presented.The behaviors of various Sn-3.5Ag based composite solders were also explicated in terms of the roles of reinforcement particles on intermetallic growth, steady-state creep rate, the onset of tertiary creep, as well as the overall creep deformation in the solder joints.Thermomechanical fatigue (TMF) behavior of the solder alloys and composite solders were investigated with different parameters such as ramp rate, dwell time, etc.The damage accumulation features and residual mechanical properties of the thermomechanically-fatigued composite solder joints were compared with non-composite solder joints.To match the lead-free alloys, various types of water soluble no-clean soldering flux have also been developed and their properties were presented.     

Creep studies for zircaloy life prediction in water reactors

Zirconium alloys, commonly used as cladding tubes in water reactors, undergo complex biaxial creep deformation. The anisotropic nature of these metals makes it relatively complex to predict their dimensional changes in-reactor. These alloys exhibit transients in creep mechanisms as stress levels change. The underlying creep mechanisms and creep anisotropy depend on the alloy composition as well as the thermomechanical treatment. The anisotropic biaxial creep of cold-worked and recrystallized Zircaloy-4 in terms of Hill’s generalized stress formulation is described, and the temperature and stress dependencies of the steady-state creep rate are reviewed. Predictive models that incorporate anelastic strain are used for transient and transients in creep. For more information, contact K.L. Murty, North Carolina State University, Department of Nuclear Engineering, Campus Box 7909, Raleigh, North Carolina 27695-7909; (919) 515-3657; fax (919) 515-5115; e-mail murty@ncsu.edu.