Research
Surfaces and Interfaces
Interfaces between phases are being increasingly recognized as critical elements in the understanding of nucleation, growth kinetics and morphological development during phase transformations as well as properties in materials. Current research includes a study of the role of interphase boundaries during lamellar growth in ferrous and non-ferrous eutectoid transformations and the discontinuous reaction in the Cu-Ti, and Ni-In systems. Rather than considering interfacial structure relatively unimportant we find it critical in interpreting growth phenomena such as lamellar curvature and branching. These studies are coupled with theoretical models of interfacial structure. Such models are also being applied to interfaces between elemental semiconductors and compound semiconductors to obtain nearly perfect crystal layers. These principles and those of epitaxy are being applied to grow multilayered structures. A scientifically rewarding, as well as technologically important, research area is "tribology," i.e. the science of sliding interfaces, concerned with friction and wear. The amount of money annually lost through friction and wear amounts to tens of billions of dollars in this country alone. Correspondingly even minor advances in the better understanding of friction and wear can lead to large money savings, besides opening avenues for new products or improvements in existing ones that can be very major. For example, the present projected advantage of optical data or signal storage and transmission, e.g. via compact disks, could be more than offset through increased wear resistance of magnetic heads and tapes. Yet, fundamental knowledge in tribology is woefully inadequate, mainly for the reason that one cannot observe what is going on at the interface while sliding occurs and we must rely on indirect evidence. In order to expand the knowledge of such interfacial processes, extensive use is made of correlated mechanical and electrical measurements. Also, the conditions of sliding interfaces are similated via extensive shearing of samples subjected to high superimposed pressures in a Bridgman anvile apparatus. The simulation samples are subsequently studied by a wide variety of methods and very interesting interfacial alloying and amorphization has been discovered. An independent but also important aspect of sliding interfaces is the interfacial electrical resistance. This is crucial for the design and use of electrical brushes. In fact, a number of major technological advances which would otherwise be possible have been delayed for several years now due to the lack of electrical brushes capable of conducting currents with sufficiently low electrical resistance at sufficiently high densities and/or at sufficiently high speeds and low electrical noise. Research is in progress to develop superior electrical brushes based on a velvet of very fine metal fibers. [top]
Light Metals
There is a need for structural materials which have a high strength and/or stiffness to weight ratio and at the same time are able to perform satisfactorily in hostile environments. The department is involved in a wide range of research on light materials including alloy processing, mechanical properties and microstructural characterization, deformation mechanisms and environmental effects of light metals.
Grain structure controls superplastic properties of all types of alloys including high-strength aluminum alloys used in aerospace applications. Grain size and morphology are established during thermomechanical processing treatments, a series of deformation and heat treatment steps. Previous research on this subject in the department concentrated on thermomechanical processing of superplastic alloys which are fully recrystallized before superplastic forming. That research culminated in development of a model capable of predicting recrystallized grain size resulting from thermomechanical processing treatments for a wide variety of commercial aluminum alloys. Present research efforts are focused on superplastic aluminum alloys which recrystallize in situ during the superplastic forming operation. Many types ofadvanced materials exhibit this recrystallization behavior, but many fundamental questions remain about the in situ recrystallization process. New techniques for microtexture analysis are currently being developed to improve our understanding of the relationships between thermomechanical processing methods and in situ recrystallation mechanisms.
Another investigation in light metals is an attempt to understand and ultimately control the precipitation and deformation mechanism of the particles in Al-Li-Cu-Mg alloys. This system is unique in age-hardening aluminum systems because the two strengthening precipitates have large coherency strains and compete for the available nucleation sites. We are attempting to understand this competition from a fundamental level by studying nucleation sites, mechanisms of growth and coarsening both theoretically and experimentally using high resolution electron microscopy. The goal is to enhanve properties through controlling precipitation type, density and distribution.
The microstructure and mechanical properties of a variety of (Al,X)3Ti intermetallic alloys are being investigated. These alloys are potentiall useful in many advanced aerospace applications becuase they have moderate density, high modulus, retain their strength to 600 degrees C and have excellent oxidation resistance. Unfortunately, the toughness of all of the (Al,X)3Ti alloys investigated thus far is very low. Since this characteristic extends across the entire family of intermetallic alloys, the investigation has focussed on gaining a fundamental understanding of the factors controlling toughness. In addition, detailed microstructural analyses including TEM image simulation methods have allowed 2 previously unknown types of stacking faults to be characterized in the Al67Ni8Ti25 intermetallic alloy.
Current high speed aircraft have titanium skins because friction with the air causes the skin to heat up above the maximum temperature at which age-hardening alloys can be used. However, because titanium is expensive and is difficult to machine, designers would prefer to use an aluminum-base material instead of titanium. A research program sponsored by the Air Force is investigating several types of aluminum-base alloys. [top]
Transformations
The synthesis of rapidly solidified metastable metallic alloys and the study of their stability and unusual symmetry properties are of interest within the department. The structural relationship between the new phases and their glassy and equilibrium counterparts are investigated by convergent beam electron diffraction, x-ray diffraction, by lattice and structural imaging, and by computer simulation of structures. In particular, the competition between the glassy and quasi-crystalline order in some systems and the temporal evolution and morphological development (nucleation and growth) of the various phases are examined.
The notion of interface during solid-state phase transformations in systems far from equilibrium is being studied both experimentally and theoretically. Particular emphasis is placed on interface evolution where impediments to establishing equilibrium exist at the interface. Equations are derived and used to predict interface motion and composition profiles including the effects of elastic deformation.
Materials scientists have long been aware that there is much to be learned about the behavior of grain and interphase boundaries from a detailed understanding of their structure. Problems such as intergranular embrittlement, intergranular corrosion, segregation, recrystallization texture, and boundary migration rates are all intimately connected to the nature of atomic bonding across a grain boundary. Studies of the relationship between grain boundary segregation, structure and nucleation/precipitation are carried out with the principal objective being to characterize active heterogeneous nucleation sites and preferred growth centers. [top]
Electronic and Magnetic Materials
Current programs in electronic materials include (i) Fundamental studies of strain relief mechanisms in growth and processing of lattice-mismatched semiconductor heteroepitaxial layers, with particular emphasis on dislocation dynamics at ultra-high stresses; (ii) Studies of process-induced stresses in microelectronic and optoelectronic devices; (iii) Studies of electronic device degradation mechanisms by real-time imaging during device operation in the electron microscope, (iv) Studies of growth and processing of multi-layer interconnect structures in microelectronic circuits and (v) Studies of the chemistry, structure and mechanical properties of oxidized compound semiconductor materials. The common theme in these programs is correlation of atomic-scale structure and chemistry with electronic and optical properties of semiconductor materials and devices. These programs are funded by a range of government and industrial sponsors. [top]
Composites
Due to increased demands on materials performance in extreme environments, metals and alloys are reaching their limitation. In order to attain property goals for new structural systems, alternate concepts must be utilized. Metal matrix composites are presently being evaluated as replacements for conventional materials. The department has been active in metal matrix composite research for over 25 years.
Fundamental investigations are being made on metal matrix composites including high temperature mechanical property determination, deformation and fracture for correlation with processing and microstructural features. Understanding the interaction between composite constituent phases and crack initiation and propagation is critical for defining failure mechanisms. [top]
Electrochemical Science and Environmental Failure
Corrosion costs the U.S. economy on the order of 4% of the Gross National Product annually. In 1991, this amounted to over $100 billion. Every type of industry is adversely affected by the degradation of materials' properties due to interactions with the environment. Advances in understanding the controlling processes are required for the development of new technologies as well as the extension ofthe useful life of the nation's infrastructure. Many corrosion reactions have an electrochemical nature. Charge transfer reactions, on the other hand, are also used to convert stored chemical energy to useful electrical energy in batteries and fuel cells. These related areas are the subject of major research emphasis in the department and include fundamental and applied programs.
Research areas include studies of passivity, localized corrosion, protective coatings, environmental effects on composites, aging aircraft, and degradation of reinforced concrete. Studies of the localized corrosion of metals and alloys include both experimental and computational projects aimed at better understanding the links among alloy metallurgy, environment, and resistance to attack. The degradation of steel-reinforced concrete plagues many parts of the U.S. as well as many other nations. Projects are underway that seek to better understand the controlling processes as well as develop cost-effective means of monitoring, inhibiting and mitigating the corrosion of the embedded steel. Several aspects of the problem of aging aircraft are under study including the development of predictive corrosion models, understanding organic coating failure, and the corrosion and environmental fracture of conventional and advanced aluminum alloys.
An area of emphasis is the environmental fracture of alloys resulting from the interaction of mechanical strain and electrochemical or gas-metal reactions. Interdisciplinary research focuses on the mechanisms of damage at the crack tip due to localized micromechanical, metallurgical, chemical and electrochemical conditions. New probes are being developed to characterize the controlling processes and better delineate the responsible mechanisms. Specific projects include studies of advanced aluminum, titanium and nickel based alloys. The deleterious effects of absorbed hydrogen on the mechanical properties of titanium, aluminum and high strength steel are being studied in order to better understand hydrogen embrittlement. [top]
Fatigue and Fracture Techniques
State-of-the-art fracture mechanics experimental methods, coupled with micromechanical modeling, are being employed to establish the cracking behavior of light aerospace alloys based on aluminum and titanium, as well as nickel-based superalloys and steels. The unifying goal of this work is to define fundamental mechanisms for fatigue and fracture. Successful work in this regard provides the foundation necessary for developments of high performance materials, and for damage tolerant component life prediction. Major research areas in mechanical behavior include ductile fracture, hydrogen embrittlement, high cycle fatigue, and environmental effects on fatigue crack growth.
Several projects are investigating the ductile fracture of advanced aluminum alloys. Microvoid nucleation at small dispersoid and precipitate particles is of particular interest, with both high resolution characterization and modeling efforts underway. A major variable is void-nucleating particle size and strength with respect to the dislocation interactions. Second, studies are using crack tip deformation and damage modeling to predict the temperature dependence of the initiation fracture toughness in the next generation aluminum alloys. The approach here is to separate and understand the effect of temperature on crack tip stress and strain distributions versus temperature-dependent resistance to microvoid damage in the crack tip process zone.
Research on the fatigue and fracture of high strength metastable beta-titanium alloys is being conducted in support of the NASA High Speed Civil Transport Program. We aim to understand and optimize the microstructural dependence of both fracture toughness and high cycle fatigue life. The interactive factors of alpha-phase microstructure and strength, as well as localized slip deformation, are emphasized. A project is examining the effect of dissolved hydrogen on these properties, with the aim of understanding long-term alloy durability. Since hydrogen may be introduced to titanium alloy microstructures during processing, fabrication and component-service, it is necessary to understand the varied effects of this solute on the next generation of high strength alloys, in both sheet and thick-section forms.
Paralleling extensive research on environmental cracking, a fatigue study is ongoing to investigate the role of environmental reactions on crack closure in 7000 series aluminum alloys. Here, we are investigating the effects of environment chemistry and alloy microstructure on the time-dependent formation of corrosion products that contribute crack closure forces.
Figure A
Fig. A: SEM micrograph is combined with ordination map obtained by electron back-scattered diffraction technique.
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Figure B
Fig. B: Multiple fatigue crack facets are present within a single grain. Near-{111} facet crystallography represented in a stereographic projection that superimposes the grain orientation (D for {111}) and facet normals (×).
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Processing
The department has numerous activities in the synthesis and processing of materials. Rapid solidification methods are used to investigate amorphous metal alloy tranformations as well as research on new processes for synthesizing high performance coatings (e.g. thermal barrier coatings) and thin films (such as those used for Giant Magnets Resistive Multilayers). All aspects of the processing and manufacture of materials including the development and use of in situ sensor techniques, the application of mathematical and computational modeling methods for process simulation, the development of novel process control methodologies and the development of new process technologies such as directed vapor deposition and a variety of techniques for synthesizing high performance metal matrix composites and ultralight cellular metals and alloys are being investigated. [top]
Polymer Physics
The interest in macromolecules in the Department spans several areas of polymer physics. One major area is transport properties as a function of structure, morphology, pressure, temperature, and other variables. Of special interest are thermal conductivity, electrical conductivity, and gas permeation. Research has included a unified investigation of such transport properties in an exciting new class of stiff chain polymers, as well as comparative studies in other important types of polymers.
The interest in transport phenomena is strongly coupled to interests in the dielectric and morphological properties of extended chain and other anisotropic polymers.
Another major area is in the investigation of the structures and properties of semicrystalline polymers. Through x-ray diffraction and molecular modeling techniques, the structures of polymers and the changes they undergo can be determined or predicted. Changes in structure with temperature, molecular defects, pressure, applied electric field, or molecular architecture can be investigated. Computer workstations, and a wide assortment of modeling software are central tools in these studies. Specific polymer systems of current interest include rigid-rod and stiff chain polymers, polytetrafluoroethylene, polyvinylidene fluoride, and a family of silicon-based polymers having a variety of organic sidechains. Studies of liquid crystalline and biologically-based materials having interesting and useful optical properties are also underway. [top]
Theoretical Studies
Considerable recent theoretical work has been in the line of investigating the ability of mathematical models to explain or predict physical phenomena. This research involves describing some particular situation mathematically, which in general leads to equations which cannot be solved analytically, and then studying the model by numerical methods.
For example, the physical properties of a region of metal crystal lattice can be simulated by a mathematical model based on atom-atom interactions. The energies and associated configuration for lattice defects such as vacancies, interstitials, and dislocations can then be calculated by introducing these defects into the model and minimizing the energy of the system. The most pressing need for progress in this field is a better understanding of the interatomic forces which are simulated by the model. Research into both interatomic forces and the application of these forces to the study of defects in metals are presently being carried out.
Stresses are introduced in crystals where surface atomic planes, which are normally in a state of strain, terminate in steps on the surface. Properties such as the distribution of strain, dilatation, stress and strain energy density at the crystal surface and within its interior are of importance in a variety of physical phenomena occurring at, or near, crystal surfaces. Surface strains modify surface lattice parameters and surface electronic states. These together affect adatom-substrate bonding and adatom mobility along the surface. Adsorption on the terrace edge, rather than at the bottom of the step, may be partly related to the fact that the strains in the two positions have different signs. Stored strain energy may influence chemical reactivity in catalysis. Modification of the lattice parameter and bonding also influence surface diffusion kinetics involved in crystal growth and epitaxy. We are developing a model and mathematical analysis of these problems resulting from atomic steps on an otherwise smooth surface.
In addition, continuum representations of stressed interfaces are being used to obtain equations governing the evolution of interfaces when the system is far from equilibrium. Both analytical and numerical solutions are being obtained in order to predict the growth of intermediate phases in thin-film diffusion couples. [top]
