Applications for grain boundary engineered materials

Advances in automated electron diffraction techniques, microstructural modeling, and the understanding of structure-property relationships for grain boundaries have resulted in the emergence of grain boundary engineering as a formidable tool for cost-effectively achieving enhanced performance in commercial polycrystalline materials (i.e., metals, alloys, and ceramics). In this article, some applications for grain boundary engineering technology that have been developed during the past several years are presented. G. Palumbo earned his Ph.D. in metallurgy and materials science at the University of Toronto in 1989. He is currently a principal research scientist at Ontario Hydro. E.M. Lehockey earned his M.Sc. in materials engineering at the University of Western Ontario in 1988. He is currently a senior research scientist at Ontario Hydro. P. Lin earned his Ph.D. in metallurgy and materials science at the University of Toronto in 1997. He is currently a research scientist at Ontario Hydro.     

Damage-Tolerant Polymer Composite Systems

One of the reasons for the rapid growth in the application of polymer composites is the opportunity they provide for the design and construction of composite structures that are especially resistant to losses of strength or reduced life resulting from damage during service. The usefulness of such materials is enhanced by the variety of reinforcement schemes that can be chosen to reflect specific service conditions. Under cyclic loading and demanding mechanical situations (e.g., helicopter parts, vehicle springs and high-speed rotors), polymer composites are considerably superior to competing materials.     

Novel design concepts for gamma-base titanium aluminide alloys

Two-phase titanium aluminide alloys are being considered as light-weight materials to replace nickel-base superalloys for some high temperature applications in energy conversion systems. Thus, their mechanical properties have to be assessed against the high standard set by the superalloys currently in use. In this respect most titanium aluminides are particularly inferior in high temperature strength and creep resistance even if these properties are related to density. In an attempt to overcome these problems several studies have been performed on titanium aluminides which have been subjected to solid solution and precipitation hardening. The intention of the present study is to examine more closely these strengthening processes in order to assess their potential for extending the service range of the titanium aluminides towards higher temperatures. There is growing evidence that two-phase titanium aluminides, microalloyed with carbon or niobium, can provide the necessary performance. Particular emphasis will be placed on processing routes acceptable for these materials.     

Laser powder technology for cladding and welding

Laser powder technology offers several advantages compared to conventional cladding and welding techniques and is attracting increasing industrial interest. The laser materials processing group of the German Aerospace Center at Stuttgart, Germany, is currently developing these new methods for application in industrial process engineering. Key areas of the work include the design and implementation of a modular working head that can be universally used for laser welding and surface treatment, the development of powder nozzles for cladding and welding, and the construction of new systems for special applications (e.g., for inner cladding). Some of these developments are described, as well as some important examples that highlight the potential of welding and surface treatment using laser powder techniques.     

Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant

Caustic embrittlement, a kind of stress corrosion cracking (SCC), is always encountered on materials under stresses amid caustic environment. Acidic corrosion is another familiar degradation on materials contacting acidic media. However, it has been seldom studied what effect would be resulted in on materials that are exposed to an acidic/caustic alternating environment. In this paper, failure events were discovered on the carbon steel pipes under such an alternating service condition due to frequent sharp fluctuations of the heat medium's (process water) pH values in a heat exchanger. What is more, even chloride ions and sulfur element were detected, i.e., pitting corrosion was involved as well. In order to identify the causes of the failure, matrix materials of the pipes were examined, failure defects on pipe surfaces were investigated, particularly the process water was thoroughly inspected via a series of characterization methods. Based on the analysis results, a novel four‐level mechanism from microscopic scale to macroscopic scale was tentatively proposed to explain such an acidic/caustic alternating corrosion.     

Lithium-intercalation oxides for rechargeable batteries

Since the introduction of the Li_xC/LiCoO₂ cell, rechargeable lithium batteries have become the technology of choice for applications where volume or weight are a consideration (e.g., laptop computers and cell phones). The focus of current research in cathodeactive materials is on less-expensive or higher-performance materials than LiCoO₂. This article illustrates how first-principles calculations can play a critical role in obtaining the understanding needed to design improved cathode oxides. Gerbrand Ceder earned his Ph.D. in materials science and mineral engineering at the University of California at Berkeley in 1991. He is currently an associate professor at Massachusetts Institute of Technology. Dr. Ceder is a member of TMS. Anton Van der Ven earned his M.Eng. in materials science at Katholieke Universiteit Leuven in 1994. He is currently a research assistant at Massachusetts Institute of Technology. Mehmet Kadri Aydinol earned his Ph.D. in metallurgical engineering at Middle East Technical University in 1994. He is currently an assistant professor at Massachusetts Institute of Technology.     

Electrically active polymers and their application

This article reviews some of the applications of electrically active polymers. This field originated about 20 years ago with the discovery that polyacetylene became highly conductive when it was doped (i.e., oxidized). Research in the area of conductive polymers has resulted in some applications, which are discussed in this review. The field has recently, however, shifted to investigating the applications of these polymers in their undoped form, particularly in light-emitting diodes and thin-film transistors. Mary E. Galvin earned her Sc.D. in materials science from the Massachusetts Institute of Technology in 1984. She is currently a distinguished member of the technical staff at Bell Laboratories, Lucent Technologies, in Murray Hill, New Jersey.     

Dedication to Degradation: The Beauty of Materials Designed to Lay in Ruin

Degradation of materials is typically perceived to be a negative response in service. Many designs, and materials, have been and are ruined due to corrosion, fatigue, weathering, ultraviolet light, fungal attack, bacterial attack, erosion, wear, electromigration… and on the list goes. However, the carefully controlled and purposeful degradation of materials is a prerequisite for success for some designs—and such ability is a beautiful necessity when it comes to many regenerative biomaterials. In other instances, we must seek first to understand the degradation mechanisms before we can achieve degradation prevention—and the resistance of some materials to degradation is also beautiful. Regardless of whether we try to prevent or elicit degradation, our dedication to degradation of materials is ever present in materials design.     

A Review of Self-healing Metals: Fundamentals,Design Principles and Performance

Self-healing metals possess the capability to autonomously repair structural damage during service. While self-healing concepts remain challenging to be realized in metals and metallic systems due to the small atomic volume of the mobile atoms, the slow diffusion unless at high temperatures and the strong isotropic metallic bonds, the scientific interest has increased sharply and promising progress is obtained. This article provides a comprehensive and updated review on the developments and limitations associated with the various modes of potentially healable damage induced in metals and alloys, i.e., stressinduced damage, irradiation-induced damage in bulk materials and contact damage in corrosion protective coatings. The spontaneous intrinsic healing mechanisms not requiring external assistance other than the material operating at the right temperature and an assisted healing mechanism with external intervention are reviewed. Promising strategies to achieve self-healing in metals are identified. Finally, we give some prospects for future research directions in self-healing metals.     

Thermal Spray Maps: Material Genomics of Processing Technologies

There is currently no method whereby material properties of thermal spray coatings may be predicted from fundamental processing inputs such as temperature-velocity correlations. The first step in such an important understanding would involve establishing a foundation that consolidates the thermal spray literature so that known relationships could be documented and any trends identified. This paper presents a method to classify and reorder thermal spray data so that relationships and correlations between competing processes and materials can be identified. Extensive data mining of published experimental work was performed to create thermal spray property-performance maps, known as “TS maps” in this work. Six TS maps will be presented. The maps are based on coating characteristics of major importance; i.e., porosity, microhardness, adhesion strength, and the elastic modulus of thermal spray coatings.     

The combustion synthesis of advanced materials

This article discusses the versatility of utilizing exothermic combustion synthesis reactions to produce ceramic, intermetallic, and composite materials in a one-step process. The emphasis is placed on controlling the reaction parameters of these exothermic synthesis reactions as a means of controlling the microstructure and properties of the synthesized materials and composites. The application of this technology to the synthesis and control of a selection of advanced materials is demonstrated (i.e., dense composite materials, uniformly porous or “foamed” ceramics, and functionally graded materials). In addition, the control of product morphology (e.g., submicrometer particles or whiskers by gas-solid reactions that use vapor phase species deliberately generated at the reaction front) is also demonstrated, as is the effect of microgravity processing on some of these reactions.     

Thermal Spray Coatings on Orthopedic Devices: When and How the FDA Reviews Your Coatings

Thermal spray coatings have been commonly applied on medical devices for various reasons, e.g., surface roughening, biological fixation, and similarity of chemical composition to bone minerals. Generally, to introduce a thermal spray-coated device to the US market, a premarket review of the coated device is necessary by the US Food and Drug Administration (FDA). This article aims to improve understanding regarding FDA review of thermal spray coatings in orthopedic medical device marketing applications and expectations for information to be submitted as part of this process. While different thermal spray technologies and materials have been used for coatings on medical devices, thermal spray coatings often seen by the FDA on orthopedic devices include plasma-sprayed titanium (Ti) coatings and hydroxyapatite (HA) coatings as well as Ti/HA dual coatings. The coated devices are mostly metals (e.g., Ti alloy, cobalt-chromium alloy, stainless steel alloys) and some polymers (e.g., polyetheretherketone). The FDA does not clear or approve individual coatings or materials; rather, coatings and materials are evaluated as part of the final, finished medical device in the context of the specific device technological characteristics and intended use. The FDA has two current guidance documents for orthopedic implants with modified metallic surfaces and hydroxyapatite coatings, which outline the FDA’s recommendations for full characterization and testing of these two types of coatings, respectively. Additionally, the standards organizations (e.g., ISO and ASTM) have developed many materials and testing standards for these coatings, some of which are recognized by the FDA. It is helpful that the coating companies reference these standards for appropriate material/coating specifications, testing methods, and acceptance criteria. Depending on the intended use of the coated device, it is important that coating properties also address some items specific to that device type. Additionally, the impact of cleaning, sterilization, and packaging/shelf-life processes on the coating properties is also considered to ensure that the coated device is safe for its intended use.     

Effects of Carbon Allotropic Forms on Microstructure and Thermal Properties of Cu-C Composites Produced by SPS

Combination of extreme service conditions and complex thermomechanical loadings, e.g., in electronics or power industry, requires using advanced materials with unique properties. Dissipation of heat generated during the operation of high-power electronic elements is crucial from the point of view of their efficiency. Good cooling conditions can be guaranteed, for instance, with materials of very high thermal conductivity and low thermal expansion coefficient, and by designing the heat dissipation system in an accurate manner. Conventional materials such as silver, copper, or their alloys, often fail to meet such severe requirements. This paper discusses the results of investigations connected with Cu-C (multiwall carbon nanotubes (MWNTs), graphene nanopowder (GNP), or thermally reduced graphene oxide (RGO)) composites, produced using the spark plasma sintering technique. The obtained composites are characterized by uniform distribution of a carbon phase and high relative density. Compared with pure copper, developed materials are characterized by similar thermal conductivity and much lower values of thermal expansion coefficient. The most promising materials to use as heat dissipation elements seems to be copper-based composites reinforced by carbon nanotubes (CNTs) and GNP.     

The intelligent processing of materials: An overview and case study

The intelligent processing of materials is an emerging methodology for simulating and controlling the processing and manufacture of materials. It involves model-based process optimization, in-situ microstructure sensing, and the control of both the process variables and the performance-defining microstructural attributes of a material during its synthesis and processing. It is finding widespread application in the manufacture of electronic, photonic, and composite (i.e., high-performance) materials, as well as primary metals. Authors’ Note: This article is based on AGARD SMP lecture series 205, Smart Structures and Materials: Implications for Military Aircraft of New Generation, held in Philadelphia, Pennsylvania, on October 30–31, 1996; in Amsterdam, Netherlands, on November 18–19, 1996; and in Paris on November 21–22, 1996. Haydn N.G. Wadley earned his Ph.D. in physics at the University of Reading, England, in 1979. He is currently the Edgar A. Starke, Jr., research professor in materials science and associate dean for research at the School of Engineering and Applied Science at the University of Virginia. Dr. Wadley is a member of TMS. Ravi Vancheeswaran earned his Ph.D. in mechanical and aerospace engineering at the University of Virginia in 1996. He is currently a research assistant professor at the School of Engineering and Applied Science at the University of Virginia. Dr. Vancheeswaran is also a member of TMS.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

Performance of lubricants in the rolling friction zone and kinetics of their tribochemical transformations

There are two viewpoints on tribology: (1) an independent science that occupies a certain niche in physical chemistry, thermodynamics, and mechanics of interacting bodies and (2) a technology aimed to enhance the reliability and service life of machine elements in contact during relative motion. In any case, this requires the knowledge of physicochemical processes within the real contact zone of interacting bodies, i.e., in the lubricant itself and at its phase boundaries. This predetermines the importance of studying not only tribology, which is recognized for a long time, but also the tribochemistry of lubricant materials. In this study, we systematically analyze the mechanism and kinetics of tribochemical transformations of lubricant materials (oils and greases) in the rolling friction zone and the main directions of their consumption, which determine the break of metal bonds in the structural material of bearings. Based on the proposed kinetic mechanism, one can predict the depth of tribochemical processes and the service life of friction units with accuracy tolerable for practice.Translated from Zashchita Metallov, Vol. 41, No. 2, 2005, pp. 168–181.Original Russian Text Copyright © 2005 by Melnikov.     

New sealing concept for planar solid oxide fuel cells

A key element in developing high-performance planar solid oxide fuel-cell stacks is the hermetic seal between the metal and ceramic components. Two methods of sealing are commonly used: (a) rigid joining and (b) compressive sealing. Each method has its own set of advantages and design constraints. An alternative approach is currently under development that appears to combine some of the advantages of the other two techniques, including hermeticity, mechanical integrity, and minimization of interfacial stresses in the joint substrate materials, particularly the ceramic cell. The new sealing concept relies on a plastically deformable metal seal; one that offers a quasi-dynamic mechanical response in that it is adherent to both sealing surfaces, i.e., non-sliding, but readily yields or deforms under thermally generated stresses. In this way, it mitigates the development of stresses in the adjacent ceramic and metal components even when a significant difference in thermal expansion exists between the two materials. The pre-experimental design of the seal, initial proof-of-principle results on small test specimens, and finite-element analyses aimed at scaling the seal to prototypical sizes and geometries are described herein. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Low-thermal-expansion copper composites via negative CTE metallic elements

Thermal management is an important issue in electronic packaging due to the increasing complexity, miniaturization, and high density of components in modern devices. In high-power-dissipation packages, heat sinks are essential for preventing thermal damage to heat-sensitive components on the silicon chip. However, commonly used heat-sink materials (e.g., copper and aluminum alloys) have a much higher coefficient of thermal expansion (CTE) than silicon. CTE mismatch between the various materials in an electronic package can lead to stresses that can trigger complex failure mechanisms like component distortion, stress rupture, thermomechanical fatigue, and creep, thereby seriously degrading device reliability and lifetime. Therefore, it is highly desirable to minimize the CTE mismatch by developing new heat-sink materials having CTEs that are close to the CTE of silicon. In this work, low-thermal-expansion copper composites with CTEs as low as 4 ppm/°C have been fabricated by employing a negative thermal-expansion alloy—equiatomic Ni-Ti, which has a CTE of approximately −21 ppm/°C. The use of negative CTE elements, especially those with very large negative CTE values, offers an attractive route for controlling the thermal-expansion behavior of various metallic and nonmetallic materials. H. Mavoori earned his Ph.D. in materials science and engineering from Northwestern University in 1996. He is a post-doctoral member of the technical staff of the Applied Materials and Metallurgy Research Group at Bell Laboratories, Lucent Technologies. Dr. Mavoori is also a member of TMS. S. Jin earned his Ph.D. in materials science at the University of California at Berkeley in 1974. He is currently technical manager at Bell Laboratories, Lucent Technologies. Dr. Jin is also a member of TMS. Author’s Note: Unless otherwise indicated, compositions are in weight percent.     

Recent Progress of Synchrotron X-Ray Imaging and Diffraction on the Solidification and Deformation Behavior of Metallic Materials

Characterizing the microstructure and deformation mechanism associated with the performances and properties of metallic materials is of great importance in understanding the microstructure-property relationship. The past few decades have witnessed the rapid development of characterization techniques from optical microscopy to electron microscopy, although these conventional methods are generally limited to the sample surface because of the intrinsic opaque nature of metallic materials. Advanced synchrotron radiation (SR) facilities can produce X-rays with strong penetrability and high spatiotemporal resolution, and thereby enabling the non-destructive visualization of full-field structural information in three dimensions. Tremendous endeavors were devoted to the 3rd generation SR over the past three decades, in which X-ray beams have been focused down to 100 nm. In this paper, recent progresses on SR-related characterization technologies were reviewed, with particular emphases on the fundamentals of synchrotron X-ray imaging and synchrotron X-ray diffraction, as well as their applications in the in situ observations of material preparation (e.g., in situ dendrite growth during solidification) and service under extreme environment (e.g., in situ mechanics). Future innovations toward next-generation SR and newly emerging SR-based technologies such as dark-field X-ray microscopy and Bragg coherent X-ray diffraction imaging were also advocated.     

Adhesion and coalescence of ductile metal surfaces and nanoparticles

Much more is known about material failure, such as fracture and crack propagation, than the reverse effect of material formation, i.e., how bulk materials form or consolidate during material processing or crack healing. Using the Surface Forces Apparatus, optical interferometry, optical and scanning probe microscopy, and x-ray diffraction, we have studied how gold and platinum films sinter or cold-weld at the nano-scale to form continuous bulk films when two initially rough surfaces composed of nanometer-scale asperities are pressed together. We find that coalescence of these ductile materials occurs abruptly, like a first order phase transition, once a critical local pressure or interparticle separation is reached. Simple thermodynamic and kinetic considerations suggest that it may be a more general phenomenon for ductile materials interacting at the nano-scale. We also make some qualitative comparisons with the very different behavior observed with hard, brittle materials.