Showing papers in "Advanced Engineering Materials in 2003"

Understanding Magnesium Corrosion—A Framework for Improved Alloy Performance

Abstract: The purpose of this paper is to provide a succinct but nevertheless complete mechanistic overview of the various types of magnesium corrosion. The understanding of the corrosion processes of magnesium alloys builds upon our understanding of the corrosion of pure magnesium. This provides an understanding of the types of corrosion exhibited by,magnesium alloys, and also of the environmental factors Of most importance. This deep understanding is required as a foundation if we are to produce magnesium alloys much more resistant to corrosion than the present alloys. Much has already been achieved, but there is vast scope for improvement. This present analysis can provide a foundation and a theoretical framework for further, much needed research. There is still vast scope both for better fundamental understanding of corrosion processes, engineering usage of magnesium, and also on the corrosion protection of magnesium alloys in service.

Titanium Alloys for Aerospace Applications

Abstract: There is probably no other material more closely related to aerospace than titanium and its alloys. With a density of 4.5 g/cm{sup 3}, titanium alloys are only about half as heavy as steel or Ni-based superalloys, yielding an excellent strength-to-weight ratio. Furthermore, they have exceptional corrosion resistance. The use of titanium alloys in the aerospace sector will be highlighted including airframe, engine, helicopter, and space applications. (Abstract Copyright [2003], Wiley Periodicals, Inc.)

ARB (Accumulative Roll-Bonding) and Other New Techniques to Produce Bulk Ultrafine Grained Materials

Abstract: Accumulative roll-Bonding (ARB) is a severe plastic deformation (SPD) process invented by the authors in order to fabricate ultrafine grained metallic materials. ARB is the only SPD process applicable to continuous production of bulky materials. In the process, 50 % rolled material is cut into two, stacked to be the initial dimension and then rolled again. In order to obtain one-body solid material, the rolling in ARB is not only a deformation process but also a bonding process (roll-bonding). By repeating this procedure, SPD of bulky materials can be realized. In this review paper, various kinds of new SPD mechanical properties of the ARB processed materials are indicated.

Creep Resistant Magnesium Alloys for Powertrain Applications

Abstract: Creep resistant magnesium alloys are candidate materials for automotive powertrain applications. Since the 90’s, a number of new creep-resistant magnesium alloy systems have been investigated and developed. These are for the most part based on rare-earth, alkaline earth, and silicon additions. This paper gives an overview of creep resistance in magnesium and a review of creep resistant magnesium alloys for power-train applications.

Physical Aspects of Process Control in Selective Laser Sintering of Metals

Abstract: Direct selective laser sintering (SLS) is a layered manufacturing technique that can produce fully dense, functional components in high performance metals. In this review paper, a first step is taken towards identifying and understanding some of the important physical mechanisms in direct SLS. This study not only provides an insight into phenomena observed during direct SLS processing of a variety of metallic materials but also helps in selecting those materials that are most amenable to direct SLS processing. The physical mechanisms discussed include oxidation, non-equilibrium wetting, epitaxial solidification, metal vaporization, and oxide purification. Understanding these mechanisms is crucial for the design of direct SLS machines, process development, and process control.

Grain Refinement of Aluminium Alloys by Inoculation

Abstract: A review is given of the studies of aluminium inoculation undertaken within the EU Network “Microstructural Engineering by Solidification Processing” (MEBSP). A wide range of studies of industrial practice of grain refining and of the fundamental mechanisms of nucleation and growth have contributed to improved understanding. Particular advances have been made in understanding the relative performance of different grain refiners.

Polymer nanocomposites for aerospace applications: Properties

Abstract: Polymer nanocomposites may provide significantly increased modulus, gas barrier, thermal performance, atomic oxygen resistance, resistance to small molecule permeation and improved ablative performance when compared to typical traditional carbon-fiber-reinforced polymeric composites. This presentation gives a review on both theoretical and experimental investigations extracting valuable fundamental elements including field emission, thermal stability, and electrical, optical and mechanical properties of polymer nanocomposites for aerospace applicability.

The Role of Hydrostatic Pressure in Severe Plastic Deformation

Abstract: The contribution presents several experimental examples which show that the presence of an enhanced hydrostatic pressure—as compared to conventional large deformation modes—is one of the main features of severe plastic deformation (SPD). At the example of systematic high pressure torsion experiments with Cu at room temperature, strength measurements after deformation showed that the onset strains of deformation stages III, IV, and V are not affected by the pressure applied; however, the related onset flow stresses increase by at least 10 % of the values of low pressure torsion, per GPa of pressure increase. During deformation, increases of flow stresses by at least 40 % of the values of low pressure torsion, per GPa of pressure increase, have been found. From comparisons with tests on Ni, the increases appear to grow with the materials melting temperature. For a theoretical explanation of flow stress increases the pressure induced changes of i) the elastic moduli, and ii) the formation energy of lattice defects. While contribution i) is almost negligible, contribution ii) accounts for an increase of flow stress during deformation by about 15 % per GPa of pressure increase. The difference left to experiment has to be attributed to a third contribution, i.e., the pressure specific evolution of the structure. For this contribution, a modification of the model of Zehetbauer and Les[1–3] is introduced which is based on the pressure caused decrease of lattice diffusion. The latter is thought to restrict the diffusion controlled annihilation of dislocations, thus leading to a higher density of vacancies, dislocations and/or grain boundaries causing the higher stress level observed.

Inorganic–Organic Hybrid Polymers for Information Technology: from Planar Technology to 3D Nanostructures

Abstract: Sol-gel synthesis allows inorganic–organic hybrid polymer materials (ORMOCER®s) to be produced, which can be functionalized to tailor their physical and chemical properties such as refractive index or optical loss. A particular material system is discussed here, which is synthesized without addition of water and is applied in optical communications. As examples for 2D and 2.5D technology, planar waveguides, stacked waveguides, and microlenses are shown. Using two-photon polymerization initiated by femtosecond laser pulses, arbitrary 3D structures can be made in the submicrometer range. In particular, 3D photonic crystal structures are described and discussed.

Paradoxes of Severe Plastic Deformation

Abstract: Severe plastic deformation (SPD) can lead to emergence of microstructural features and properties in materials which are fundamentally different from the ones well known for conventional cold deformation. In particular, the instances of unusual phase transformations resulting in development of highly metastable states associated with formation of supersaturated solid solutions, disordering or amorphization and their further decomposition during heating, high thermal stability of the SPD-produced nanostructures, and the paradox of strength and ductility in some SPD-processed metals and alloys are discussed.

Diffusion in Nanocrystalline Metals and Alloys – A Status Report

Abstract: Diffusion is a key property determining the suitability of nanocrystalline materials for use in numerous applications, and it is crucial to the assessment of the extent to which the interfaces in nanocrystalline samples differ from conventional grain boundaries. The present article offers an overview of diffusion in nanocrystalline metals and alloys. Emphasis is placed on the interfacial characteristics that affect diffusion in nanocrystalline materials, such as structural relaxation, grain growth, porosity, and the specific type of interface. In addition, the influence of intergranular amorphous phases and intergranular melting on diffusion is addressed, and the atomistic simulation of GB structures and diffusion is briefly summarized. On the basis of the available diffusion data, the diffusion-mediated processes of deformation and induced magnetic anisotropy are discussed.

Layered Organic–Inorganic Materials: A Way towards Controllable Magnetism†

Abstract: The search for new materials for tailor-made applications and new devices involves not only solid-state chemists, physicists or materials engineers, but also the area molecular and organo-metallic chemistry, and even biochemistry. This is especially clear in the field of organic–inorganic multifunctional materials, whose design necessitates to investigate new concepts and principles developed in these different disciplines. Here, the authors review the structure-magnetic property relationships in layered structures, made of organic and inorganic subunits.

Ductility in Intermetallic Compounds

Abstract: Intermetallic compounds are comprised of two or more metallic elements, but unlike ordinary metals, they have bonding that is part metallic, part covalent, and part ionic. Because of their mixed bonding, they are often lighter, stronger, stiffer, and more corrosion-resistant than ordinary metals, particularly at high temperatures. Yet their uses are limited because they are usually brittle at room temperature (RT), making them difficult to fabricate and vulnerable to fracture. These materials hold great promise to improve efficiency in the transportation, electric power generation, and chemical process industries; however, persistent problems with low ductility and poor fracture toughness have severely limited their use in engineering systems. This article presents an overview of the progress in improving the RT ductility and fracture toughness of intermetallic compounds and describes prospects for their near-term engineering use.

Titanium in Automotive Production

Abstract: Although titanium has meanwhile made its way into serial production of automotive parts, its use has been widely limited to niche applications and was not driven by weighing the technical and economic aspects Cost of semi-finished titanium products is still limiting; indeed, new and cheaper processes for production of Ti itself are urgently needed However, if one extrapolates the current status of Ti technology based on the historic development of the knowledge bases on other automotive metals (steel, Al, and Mg), the future of titanium use in cars looks quite bright

Fatigue of Severely Deformed Metals

Abstract: In this brief communication, we would like to review present data on fatigue performance of ultra-fine grain materials fabricated by severe plastic deformation (SPD) and to discuss the possible mechanisms of their plastic deformation and degradation in light of currently available experimental data. The most prominent effect of SPD is often associated with significant grain refinement down to the nanoscopic scale. The other evident effect, which accompanies intensive plastic straining, is the dislocation accumulation up to limiting densities of 1016 m–2. Since namely these two factors, the grain size and the dislocation density, govern the strengthening of polycrystalline materials, we shall primarily confine ourselves to their role in cyclic deformation of severely pre-deformed metals.

Microstructural Design for Enhanced Elevated Temperature Properties in Sand‐castable Magnesium Alloys

Abstract: The low pressure and gravity casting processing techniques offer the component designer an opportunity to produce pieces with increased complexity over those able to be fabricated via the high pressure die casting route. The design of appropriate microstructures for the enhancement of elevated temperature properties is dependent on the chosen processing route, with the low pressure and gravity techniques allowing for post-cast manipulation of the microstructure through heat treatment. There are competing microstructural requirements for strength and creep resistance in casting alloys, and the optimised microstructure must, of necessity, be a compromise. The contributions from solid solution strengthening, grain boundaries and precipitation processes are described, with particular reference to elevated temperature magnesium alloys.

The Meaning of Size Obtained from Broadened X‐Ray Diffraction Peaks

Abstract: X-ray diffraction peak profile analysis (DPPA) is a powerful tool for the characterisation of microstructures either in the bulk or in loose powder materials. The evaluation and modelling procedures have been developed together with the experimental techniques. The dislocation density and structure, as obtained from peak profile analysis, is in good correlation with TEM observations. As for the crystallite size, in some cases a good correlation, however, in other cases definite discrepancies with TEM results can be observed. In the present work those literature data are critically reviewed where crystallite size or grain size have been determined by DPPA and TEM simultaneously. The correlation and discrepancies between DPPA and TEM results are discussed in terms of the microstructures in different types of specimens.

Texture Evolution in Severe Plastic Deformation by Equal Channel Angular Extrusion

Abstract: The majority of the techniques of severe plastic deformation calls for simple shear deformation mode. This is why a special interest is given in this paper to textures that develop in simple shear. The evolution of texture in Equal Channel Angular Extrusion (ECAE) is also discussed in detail. The classical “simple shear model” of ECAE is examined as well as a new, more precise flow field which uses an analytical flow function. The proposed function is inspired from finite element calculations. The velocity gradient that follows from the analysis is incorporated into the self consistent viscoplastic polycrystal code. The evolution of deformation texture is predicted up to two passes in the A-route ECAE deformation of copper polycrystal when the extrusion angle is 90°.

Continuous Fiber Reinforced Titanium Matrix Composites: Fabrication, Properties and Applications

Abstract: Titanium matrix composites (TMCs) have been developed as high performance materials for light weight structural applications. The materials are comprised of a silicon carbide (SiC) fiber embedded in a titanium matrix, thus making use of the high strength of the SiC fibers, their high stiffness and creep resistance at elevated temperatures combined with the damage tolerance of titanium alloys. Since materials properties are closely related to the quality of the fabrication process, TMC processing is of major importance. Moreover, reinforcement-induced anisotropy of the material and thermal residual stresses formed during the consolidation process must be considered and understood for proper component design. Finally, modeling using FE methods reduces the time needed for component development and helps to elucidate failure mechanisms of the material.

Titanium and Titanium Alloys—From Raw Material to Semi-finished Products

Abstract: With a total of 0.6 %, titanium ranks fourth among metals, behind iron, aluminum, and magnesium, and ninth among all elements in the Earth's crust. However, even today it has not yet lost the air of an exotic and expensive material. The majority of titanium is used as oxide (about 95 %) in the paint industry as whitener or filler material. Not many people are aware of the fact that titanium as oxide is part of everyday life, e.g., in toothpaste, white paint, sun blockers, etc. The main reason behind the late use of titanium as a metal is the difficult and costly reduction of oxide to metal.

Equivalent strains in severe plastic deformation

Abstract: A concept that allows to compare strains achieved by different SPD methods, designated equivalent strain, is introduced and discussed by the author. Their definition may be based on the shape of the specimen, or on the plastic work spent. Complications arise when strain is accumulated in increments with variable strain path. In this case, it may be helpful to introduce efficiency factors which depend on the material property under discussion.

Unique Features and Properties of Nanostructured Materials

Abstract: In this introductory paper an attempt is made to give an overview of the area of nanostructured materials irrespective of the synthesis process. The various microstructural features such as clusters or isolated nanoparticles, agglomerated nanopowders, consolidated nanomaterials and nanocomposite materials as well as all materials classes are considered. As an important component of modern research on nanomaterials a section describes the various characterization tools available. Based on these remarks some properties of nanostructured materials will be summarized emphasizing the property–microstructure relationships. Finally, a brief outlook on applications and initial industrial use of nanomaterials is presented.

Commercialization of Nanostructured Metals Produced by Severe Plastic Deformation Processing

Abstract: The promise of nanotechnology is increasingly being realized as governments, universities, public and private research laboratories, and the various industrial sectors devote resources to this emerging area. Estimates for the economic impact of nanotechnology on existing global markets exceed 700 billion by the year 2008. Nanomaterials are projected to be one of the earliest components of nanotechnology to appear in commercial applications. Amongst the emerging new nanomaterials, bulk nanostructured metals produced by severe plastic deformation (SPD) have shown promise in a wide range of application areas. In this paper, we overview developments in severe plastic deformation technology, emphasizing progress since the international workshop “Investigations and Applications of Severe Plastic Deformation” held 2–8 August 1999 in Moscow, Russia. Then, we overview some of principal areas of application for SPD metals and alloys.

Wrought Ni‐Base Superalloys for Steam Turbine Applications beyond 700 °C

Abstract: The development of a new steam turbine generation for use in advanced coal fired power plants with prospective operating temperatures beyond 700 °C and a projected thermodynamic efficiency of about 55 % requires, amongst other innovations, the partial substitution of ferritic steels by wrought Ni-base superalloys. Although Ni-base alloys are already widely used in the aerospace industry, they are faced with demands regarding component size and operation temperature, which by far exceed current aero-engine requirements. In this article, the potential of selected alloys for 700 °C steam turbine applications is discussed with respect to their manufacturability and mechanical performance. Hereby, the focus is on the steam turbine rotor, which probably is the most critical component. It is concluded that material solutions are available for operation conditions around 600 °C but not for temperatures of 700 °C and above. Based on these results, alloy development strategies are suggested in order to close this gap and two new alloys, DT 706 and DT 750, are introduced.

Influence of Rolling Conditions on the Microstructure and Mechanical Properties of Magnesium Sheet AZ31

Abstract: Two aspects of the deformation behavior at magnesium sheets are the subject of this paper. First it will give a mechanical characterization of a commercial magnesium with a focus on the anisotropy and the possible microstructural causes of this behavior. The rolling process will be part of the second aspect. The aim is to show the influence of the parameters on the microstructure and texture and how this could affect the anisotropy in the rolled sheets.