Showing papers in "Advanced Engineering Materials in 2008"

Solid‐Solution Phase Formation Rules for Multi‐component Alloys

Abstract: The factors of the atomic size difference Delta and the enthalpy of mixing ΔH mιx of the multi-component alloys were summarized from the literatures. The formation zones of solid-solution phases, intermediate phases, and bulk metallic glasses were determined and the validity was verified by experimental results. For forming the solid solution, the alloys should have high entropy of mixing, lower Delta, and not too negative and positive enthalpy of mixing.

Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues

Abstract: Freeze-casting of porous ceramics have seen a great deal of efforts during the last few years. The objective of this review is to provide a first understanding of the process as of today. This analysis highlights the current limits of both the understanding and the control of the process. A few perspectives are given, with regards of the current achievements, interests and identified issues. 2 Abstract Freeze-casting, the templating of porous structure by the solidification of a solvent, have seen a great deal of efforts during the last few years. Of particular interest are the unique structure and properties exhibited by porous freeze-casted ceramics, which opened new opportunities in the field of cellular ceramics. The objective of this review is to provide a first understanding of the process as of today, with particular attention being paid on the underlying principles of the structure formation mechanisms and the influence of processing parameters on the structure. This analysis highlights the current limits of both the understanding and the control of the process. A few perspectives are given, with regards of the current achievements, interests and identified issues.

Porous Metals and Metallic Foams: Current Status and Recent Developments†

Abstract: Porous metals and metallic foams are presently the focus of very active research and development activities. There are currently around 150 institutions working on metallic foams worldwide, most of them focussing on their manufacture and characterisation. Various companies are developing and producing these materials which are now being used in numerous industrial applications such as lightweight structures, biomedical implants, filters, electrodes, catalysts, and heat exchangers. This review summarizes recent developments on these materials, with particular emphasis on research presented at the latest International Conference on Porous Metals and Metallic Foams (MetFoam 2007).

Progress and Challenge for Magnesium Alloys as Biomaterials

Abstract: Magnesium alloys are very biocompatiable and show promise for use in orthopaedic implant. Significant progress of research on bioabsorbable magnesium stents and orthopaedic bones has been achieved in recent years. The issues on degradation, hydrogen evolution, and corrosion fatigue and erosion corrosion of magnesium alloys and various influencing factors in simulated body fluid (SBF) are discussed. The research progress on magnesium and its alloys as biomaterials and miscellaneous approaches to enhancement in corrosion resistance is reviewed. Finally the challenges and strategy for their application as orthopaedic biomaterials are also proposed.

Stimuli-Responsive Polymeric Systems for Biomedical Applications

Abstract: Smart polymeric-based devices and surfaces that reversibly alter their physico-chemical characteristics in response to their environment are the center of many studies related to the development of materials and concepts in a broad-range of biomedical fields. Although the initial interests were more focused in systems for the delivery of therapeutic molecules, other applications have been raised in topics ranging from actuators to biomaterials for tissue engineering and regenerative medicine. The general aspects of the different types of stimuli that can be used to modulate the response are reviewed mainly for the case of hydrogels and surfaces, based on natural-origin or biodegradable macromolecules. Thermosensitive or light responsive surfaces that can modulate cell adhesion or protein adsorption are addressed as well as less conventional smart surfaces, such as substrates onto which biomineralization may be triggered. Injectable liquids that turn to gels by the action of heating (sol-gel thermo-reversible hydrogels) or by changing pH or the ionic milieu (bioinspired self-assembling systems) may find great applicability as temporary scaffolds in non invasive procedures to deliver drugs or cells to particular places in the body. Examples of systems that recognize independently or simultaneously more than one stimulus will also be presented. Besides the typical response to temperature and pH, recent developments on materials that react to biochemical stimuli, including specific enzymes, antibodies or cells, are also highlighted.

Fracture of Ceramics

Abstract: The paper reviews the current state of art of the fracture of brittle ceramic materials. Typical loading situations (thermal shock, contact damage) are analysed and the resulting fracture modes are discussed. In focus of the paper are the brittle fracture and the resulting probabilistic aspects. The delayed failure of brittle materials (sub critical crack growth and cyclic fatigue) is also discussed.

Corrosion of Pure Mg as a Function of Grain Size and Processing Route

Abstract: This study is a discrete effort towards optimization of microstructure for enhanced corrosion resistance by understanding the largely unknown corrosion - grain size relationship for magnesium. This is particularly important for magnesium that commonly displays poor corrosion resistance. A significant variation in corrosion resistance with grain size exists, which is of key significance; however these trends were strongly dependent upon the specific thermo-mechanical processing route used to prepare the specimens.

Nanostructured titanium for biomedical applications

Abstract: Metallic materials, for example, stainless steel, titanium and its alloys, and tantalum, are widely used for medical implants in trauma surgery, orthopedic and oral medicine. Successful incorporation of these materials for design, fabrication and application of medical devices require that they meet several critical criteria. Paramount is their biocompatibility as expressed by their relative reactivity with human tissues. Another is their ability to provide sufficient mechanical strength, especially under cyclic loading conditions to ensure the durability of the medical devices made therefrom. Finally the material should be machinable and formable thereby enabling device fabrication at an affordable cost. In this paper we show that nanostructured commercial purity titanium produced by severe plastic deformation (SPD) opens new avenues and concepts for medical implants, providing benefits in all areas of medical device technology. Numerous clinical studies of medical devices fabricated from commercial purity (CP) titanium for trauma, orthopaedic and oral medicine has proven its excellent biocompatibility. However the mechanical strength of CP titanium is relatively low compared to other metals used in biomedical devices. Whereas the strength of this material can be increased by either alloying or secondary processing, for example rolling, drawing, etc., these enhancements normally come with some degradation in biometric response and fatigue behaviour. Recently it has been shown that nanostructuring of CP titanium by SPD processing can provide a new and promising alternative method for improving the mechanical properties of this material. This approach also has the benefit of enhancing the biological response of the CP titanium surface. This paper reports the results of the first developments and studies of nanostructured titanium (n-Ti), produced as long-sized rods with superior mechanical and biomedical properties and demonstrates its applicability for dental implants. The effort was conducted using commercially pure Grade 4 titanium [C – 0.052 %, O2 – 0.34 %, Fe – 0.3 %, N – 0.015 %, Ti-bal. (wt. pct.)]. Nanostructuring involved SPD processing by equal-channel angular pressing followed by thermo-mechanical treatment (TMT) using forging and drawing to produce 7 mm diameter bars with a 3 m length. This processing resulted in a large reduction in grain size, from the 25 lm equiaxed grain structure of the initial titanium rods to 150 nm after combined SPD and TMT processing, as shown in Figure 1. The selected area electron diffraction pattern, Figure 1(c), further suggests that the ultra fine grains contained predominantly high-angle non-equilibrium grain boundaries with increased grain-to-grain internal stresses. A similar structure for CP Ti can be produced in small discs using other SPD methods, for example – high pressure torsion (HPT) as studied in detail. In the present work it was essential to produce homogeneous ultrafine-grained structure throughout a three-meter length rod to enable the pilot production of implants and provide sufficient material for thorough testing of the mechanical and bio-medical properties of the nanostructured titanium. Table 1 illustrates mechanical property benefits attainable by nanostructuring of CP titanium, for example, the strength of the nanostructured titanium is nearly twice that of conventional CP titanium. Notably this improvement has been achieved without the drastic ductility reductions (to below C O M M U N IC A IO N S

Aluminium Foam Sandwich Panels: Manufacture, Metallurgy and Applications

Abstract: Sandwich panels consisting of a highly porous aluminium foam core and aluminium alloy face sheets are manufactured by roll-bonding aluminium alloy sheets to a densified mixture of metal powders – usually Al-Si or Al-Si-Cu alloys with 6–8% Si and 3–10% Cu – and titanium hydride, and foaming the resulting three-layer structure by a thermal treatment. We review the various processing steps of aluminium foam sandwich (AFS) and the metallurgical processes during foaming, compare the process to alternative ways to manufacture AFS, e.g. by adhesive bonding, and give an overview of the available literature. Two ways to treat AFS after foaming are presented, namely forging and age-hardening. Some current and potential applications are described and the market potential of AFS is assessed.

Shape-Memory Polymer in Response to Solution

Abstract: Significant advances have recently been made in the development of shape-memory polymers (SMP) that are able to undergo solution-induced shape changes. The main challenge in the development of such polymer systems is the conversion of solution-induced effects at the molecular level to macroscopic movement of working pieces. This paper presents a systematic study on the effects of solution on the glass transition temperature (Tg). In addition, it provides a new approach that the SMP can be induced by applying non-energy stimulus.

Selective Electron Beam Melting of Cellular Titanium: Mechanical Properties

Abstract: Cellular titanium seems to be a promising material for medical implant applications due to an elastic modulus comparable with human bone and an interconnected porosity which facilitates bone ingrowth. This paper reports the mechanical properties of non-stochastic cellular Ti-6Al-4V structures fabricated by Selective Electron Beam Melting depending on different unit cell sizes and varying energy input per unit length of the electron beam.

A New Era in Porous Metals: Applications in Orthopaedics

Abstract: The development of porous metals and coatings has revolutionized the field of orthopaedics. However, most implants are fabricated utilizing traditional materials (i.e. sintered beads, fiber metal, plasma spray), which have several inherent limitations. Several new porous metals have been recently introduced to improve upon the biomaterial properties of these traditional metals. Tritanium (Stryker, Mahwah, NJ), Regenerex (Biomet, Warsaw, IN), Stiktite (Smith and Nephew, Memphis, TN), and Trabecular Metal (Zimmer, Warsaw, IN) are currently available for use in orthopaedic surgery, all with a characteristic appearance similar to cancellous bone. The open-cell structure of these materials affords several intriguing properties, including; high volumetric porosity (60–80 %), low modulus of elasticity, and high frictional characteristics. The following represents a review of the biomaterial properties and applications in orthopaedic surgery for this new class of highly porous metals.

Heterogeneous and Architectured Materials: A Possible Strategy for Design of Structural Materials

Abstract: Facing increasing demands for multifunctional solutions, the classical strategy of the metallurgist to improve properties, using microstructural refinement, reaches its limits: very often the function is not provided by the property only, but by the interplay between the shape, the properties, and possible association of materials. The purpose of the present paper is to outline new strategies for structural materials development offered by new degrees of freedom and by their combination: not only playing with the microstructure or with the macroscopic shape, but allowing a new scale for materials organization, the "architecture", and controlling a new degree of freedom, the "spatial heterogeneity". For these ideas to be effective, the question of processing such "heterogeneous architectured materials" in an affordable manner has to be kept in mind. Very often the development of architectured materials will require new processing methods.

Influence of Microstructure on Corrosion of As‐cast ZE41

Abstract: Corrosion of ZE41 initiated in the -Mg matrix adjacent to the eutectic micro-constituent and was attributed to micro-galvanic corrosion of the -Mg matrix coupled to the eutectic micro-constituent. The eutectic micro-constituent did not act as a corrosion barrier and did not stop the advance of the corrosion. As a consequence, the -Mg matrix corroded over the whole surface with little corrosion of the inter-connected eutectic micro-constituent.

Deformation Limits in Shape-Memory Polymers

Abstract: This paper examines the link between polymer structure, temperature, and thermomechanics of shape-memory polymer networks. Strain to failure of the networks was extremely dependent on test temperature and reached a significant maximum near the onset of glass transition; a phenomenon denoted the deformability peak. A new protocol for maximizing stored strain in shape-memory polymers is presented and the results are discussed in regards to biomedical applications.

Record Superplastic Ductility in a Magnesium Alloy Processed by Equal‐Channel Angular Pressing

Abstract: The Mg-5.5 % Zn-0.5 % Zr alloy exhibits large superplastic elongations after processing by Equal-Channel Angular Pressing and these are strongly dependent on the number of passes through the die. Grain refinement improves the elongations up to a peak of 3050 % after 2 passes but grain growth limits ductility after larger numbers of passes.

Colours and Metallic Sheen in Beetle Shells : A Biomimetic Search for Material Structuring Principles Causing Light Interference

Abstract: Visual aesthetic has always played a vital role for the success of many products. This includes colours and glossiness and metal appearance which is often achieved using surface coatings. Present coating techniques do, however, have limitations. It is difficult to reach very bright and brilliant colours, colours tend to fade over time and many of the materials and coating technologies pollute and have other environmental problems. Beetles in nature have many of the desired properties: They have appealing brilliant colours and some even with metallic appearance. It is noticeable that the colours are long lasting as some of the beetles we have studied at the zoological museum are more than 200 years old and have colours and brightness as if they were still alive. Furthermore, the beetles in nature are part of sustainable ecosystems, which means that they are made from renewable materials that are broken down and recycled when the beetle dies. Beetles also possess another and very attractive property: Their metallic look originates from structures in organic materials which is both electrically and thermal insulating. The industrial perspective is to be able to manufacture products with attractive metallic surfaces that do not feel so cold to touch as their metallic counterparts and that do not represent an electrical shock hazard.

Multi‐walled Carbon Nanotube‐Reinforced Hydroxyapatite Layers on Ti6Al4V Medical Implants by Electrophoretic Deposition (EPD)

Abstract: Sol-gel synthesised nano-size hydroxyapatite (HA) powders were dispersed in water-based suspensions with the addition of multi-walled carbon nanotubes. Ti6A14V medical alloys were coated with monolithic and carbon nanotube-reinforced HA using electrophoretic deposition (EPD) in an attempt to control deposit structure and thickness. It was shown that the sintering temperature of the deposited HA layers was significantly lowered by the use of sinter active nano-powders. Moreover the addition of carbon nanotubes increased the bonding strength of the EPD-formed layers to the metallic substrate. The cost-effective EPD technique used in the present work has high industrial potential for coating metallic medical implants with composite bioactive layers.

Increasing Lubricant Film Lifetime by Grooving Periodical Patterns Using Laser Interference Metallurgy

Abstract: A new laser surface patterning technique called Laser Interference Metallurgy is used to structure metallic samples, attempting to increase the lubricant film lifetime. Lifetime tests were carried out for several pattern geometries and two laser intensities. In some cases, lifetime improvements were remarkable, up to around 1500 %.

Analytical Modelling of the Radiative Properties of Metallic Foams: Contribution of X‐Ray Tomography

Abstract: Two metallic foams exhibiting a similar porosity but different cell sizes have been characterized using X-ray tomography. The images have been processed and analysed to retrieve the morphological properties required for the calculation of the radiative properties such as the extinction coefficient. The multiple possibilities of using the X-ray tomography method rather than conventional optical methods like SEM have been quantified. The extinction coefficient has then been determined from two approaches. First, the resulting morphological properties have been used as the input data of the conventional independent scattering theory. A special emphasis is put on the determination of morphological properties and their influence on the results. In the second approach, an original method is also proposed in order to determine the extinction coefficient of highly porous open cell metal foams, from the tomographic images and without any calculation or hypothesis. Results show a good agreement with the extinction coefficient obtained from experimental measurements. Our novel method enables to reduce uncertainties considerably.

Effect of Hydrogen on the Mechanical Properties of Stainless Steels

Abstract: Fuel cell vehicles running on hydrogen are seen as the long term solution to enable sustainable mobility. Compressed hydrogen gas storage systems are a promising route for storing hydrogen on board of vehicles, provided that a reliable and cheap material capable of withstanding hydrogen embrittlement is found. In this paper, the physicochemical behaviour of stainless steel in the presence of hydrogen with special focus on a ductility minimum near room temperature is reviewed.

Influence of Homogenization Annealing of AZ91 on Mechanical Properties and Corrosion Behavior

Abstract: The influence of homogenization annealing (HA) of magnesium alloy AZ91 on mechanical properties and corrosion behavior has been examined. The microstructure of the alloy was investigated by scanning electron microscopy (SEM) after metallographic preparation. The dependence of the mechanical properties, including Vickers hardness, yield strength, ultimate tensile strength, and elongation to failure on HA time at 380°C and 410°C were examined. It was observed that for each temperature, the hardness increased with increasing time up to a maximum and then slightly decreased for longer times. The ultimate tensile strength also exhibited the similar trend. The mechanical properties of AZ91 were significantly improved by HA. When the alloy samples are immersed in 1 N NaCl solution, it has been observed that intensive hydrogen evolution preferentially takes place at certain sites and the corrosion gradually advances.

Parametric Investigation of Laser-Assisted Machining of Commercially Pure Titanium†

Abstract: Turning of commercially pure (CP) titanium was carried out with the assistance of a laser beam. The effects of laser beam arrangement laser power, and cutting speed on the reduction of cutting forces was also examined. A 2.5 kW Nd:YAG laser was used for heating the workpiece. The laser beam was delivered by a 15 m long optical fiber and focused by an optical lens with a focal length of 200 mm. The relative position of the laser beam with the workpiece and tool was adjusted by changing the angle between work-piece axis, tool-beam distance on the surface of workpiece, and lens workpiece distance on chamber surface. The effect of laser power on the reduction of cutting forces was examined at the cutting speed of 20 m/min, with the distance between lens and workpiece of 196 mm, and beam incident angle with the workpiece axis of 40°. The result demonstrated that with increasing cutting speed in LAM, the interaction time between the laser beam and workpiece decreases and the temperature rise at cutting edge due to laser heating is lower.

Enhanced Superplasticity in a Magnesium Alloy Processed by Equal-Channel Angular Pressing with a Back-Pressure

Abstract: An investigation was initiated to evaluate the feasibility of using equal-channel angular pressing (ECAP) to obtain high superplastic elongations in the AZ31 alloy with a back pressure producing a bimodal grain structure. Processing by ECAP was performed using a die with an angle of 90 ° between the two parts of the channel and a ram velocity of 15-20 mm/sec. Some pressing were conducted with a back-pressure by making use of a backward punch in the exit channel of the die. Molybdenum disulphide and a graphite spray were used as lubricants and billets were pressed using processing route B c in which each billet is rotated by 90 °. The pressing were conducted at temperatures in the range from 423 to 523 K and every billet was quenched in water after each pass. The significance of the bimodal microstructure is attributed to the ability of the larger grains to more easily accommodate grain boundary sliding through intragranular slip and twinning and to contribute to the strain hardening capability.

Microstructure and Mechanical Properties of an AA6181‐T4 Aluminium Alloy to HC340LA High Strength Steel Friction Stir Overlap Weld

Abstract: Friction stir welding (FSW) of dissimilar joints offers serveral advantages over conventional fusion joining techniques since no associated melting is involved. In this study, FSW was applied to join AA6181-T4 aluminium alloy to HC340LA high strength steel and a systematic investigation of the microstructure formation, the development of Fe x Al y intermetallic compounds and their influences on the mechanical behaviour is presented. Based on fractography analyses, a possible failure scenario for the joint during shear test is also proposed.

Amorphous, Elastic AB Copolymer Networks from Acrylates and Poly[(L‐lactide)‐ran‐glycolide]dimethacrylates

Abstract: Three series of amorphous AB copolymer networks with shape-memory capabilities were obtained from poly[(L-lactide)-ran-glycolide]dimethacrylate (DM) as crosslinker and ethylacrylate-(AEt), butylacrylate-(ABu), or hexyl acrylate (AHe) as comonomers. The switching temperature T SW of the shape-memory effect could be adjusted between 9 °C to 45 °C. Strain fixity rates and strain recovery rates above 96 % could be reached. Compared to the homonetwork from DM, the incorporation of polyacrylate segments into the network led to a significant improvement of mechanical properties. Reaching higher elasticity and the possibility to adjust T SW to a temperature between room temperature and body temperature are considered to be substantial steps to improve the applicability of these polymer networks.

Multi-Scale Analysis of IN-718 Microstructure Evolution During Linear Friction Welding

Abstract: The present investigation focuses on Linear Friction Welding of a widely used Ni-based superalloy: IN-718. Blocks of commercial IN-718 alloy were linear friction welded under optimized processing conditions and the evolution of the weld joint microstructure was investigated using optical and scanning electron microscopy. In particular, visual examinations and macroscopic observations of microstructure evolution in the transverse cuts through the weld revealed a specific shape and a dual nature for the flash.