Progress in Materials Science

Piezoelectric films have recently attracted considerable attention in the development of various sensor and actuator devices such as nonvolatile memories, tunable microwave circuits and ultrasound transducers. In this paper, an overview of the state of art in piezoelectric films for high frequency transducer applications is presented. Firstly, the basic principles of piezoelectric materials and design considerations for ultrasound transducers will be introduced. Following the review, the current status of the piezoelectric films and recent progress in the development of high frequency ultrasonic transducers will be discussed. Then details for preparation and structure of the materials derived from piezoelectric thick film technologies will be described. Both chemical and physical methods are included in the discussion, namely, the sol-gel approach, aerosol technology and hydrothermal method. The electric and piezoelectric properties of the piezoelectric films, which are very important for transducer applications, such as permittivity and electromechanical coupling factor, are also addressed. Finally, the recent developments in the high frequency transducers and arrays with piezoelectric ZnO and PZT thick film using MEMS technology are presented. In addition, current problems and further direction of the piezoelectric films for very high frequency ultrasound application (up to GHz) are also discussed.

The micromechanics of ductile fracture has made enormous progress in recent years. This approach, which was mostly developed in the context of structural integrity analysis, is becoming a key tool for materials scientists to optimize materials fracture properties and forming operations. Micromechanical models allow quantitatively linking fracture properties, microstructure features at multiple lengths scales, and manufacturing conditions. After briefly reviewing the state of the art, this paper illustrates the application of the micromechanics-based methodology by presenting the results of an investigation on the damage resistance of 6xxx Al produced by extrusion.

Studies at high pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as, metals, semiconductors, superconductors, or minerals. In particular, the phase behaviour of ABX4 scintillating materials is a challenging problem with many implications for other fields including technological applications and Earth and planetary sciences. A great progress has been done in the last years in the study of the pressure-effects on the structural and electronic properties of these compounds. In particular, the high-pressure structural sequence followed by these compounds seems now to be better understood thanks to recent experimental and theoretical studies. Here, we will review studies on the phase behaviour of different ABX4 scintillating materials. In particular, we will focus on discussing the results obtained by different groups for the scheelite-structured orthotungstates, which have been extensively studied up to 50 GPa. We will also describe different experimental techniques for obtaining reliable data at simultaneously high pressure and high temperature. Drawbacks and advantages of the different techniques are discussed along with recent developments involving synchrotron X-ray diffraction, Raman scattering, and ab initio calculations. Differences and similarities of the phase behaviour of these materials will be discussed, on the light of Fukunaga and Yamaoka’s and Bastide’s diagrams, aiming to improve the actual understanding of their high-pressure behaviour. Possible technological and geophysical implications of the reviewed results will be also commented.

Bulk metallic glasses (BMGs) are of current interest worldwide in materials science and engineering because of their unique properties. Exploring BMGs materials becomes one of the hottest topics in the materials science field. To date, there is very active worldwide development of new BMGs, and extensive efforts have been carried out to understand and improve the glass-forming ability of metallic materials supported by large government and industry programs in North America, Asia, and Europe. Minor addition or microalloying technique, which has been widely used in other metallurgical fields, plays effective and important roles in formation, crystallization, thermal stability and property improvement of BMGs. This simple approach provides a powerful tool for the BMG-forming alloys development and design. In this paper, we present a comprehensive review of the history and the recent developments of this technique in the field of BMGs. The roles of the minor addition in the formation and the properties of the BMGs and the BMG-based composites will be discussed and summarized within the framework of thermodynamics, kinetics and microstructure. The empirical criteria, or the principles and guidelines for the applications of the technique in BMG field are outlined.

This paper provides an overview of the research on the use of high-energy mechanical milling in processing advanced materials. The focus is on the major understanding achieved on each of the major topics in this area. This overview demonstrates that high-energy mechanical milling can be used to produce several different types of materials, including amorphous alloy powders, nanocrystalline powders, intermetallic powders, composite and nanocomposite powders, and nanopowders. Good understanding of the mechanisms related to the process for each of these purposes has been achieved at the phenomenological level. However, accurate quantification and modelling of high-energy mechanical milling is still not available, even though several research groups have investigated these topics. The whole area of mathematically describing and modelling the high energy milling process deserves the close attention of all materials scientists and engineers working in the area of processing powder materials using high-energy mechanical milling. Without mastering the art of optimising, controlling and predicting the process, there is no hope of developing this powerful process into a main-stream industrial scale materials processing process as has happened for metallurgical processes, such as melting, casting, or heat treatment. Issues related to the consolidation of the mechanically milled powders and future development in this area of high-energy mechanical milling have also been commented on.

Friction-stir welding is a refreshing approach to the joining of metals. Although originally intended for aluminium alloys, the reach of FSW has now extended to a variety of materials including steels and polymers. This review deals with the fundamental understanding of the process and its metallurgical consequences. The focus is on heat generation, heat transfer and plastic flow during welding, elements of tool design, understanding defect formation and the structure and properties of the welded materials.

Bond-order potentials are an appealing way to describe the cohesive energy of materials, because they are based on the rigourous quantum mechanics of electrons, they can be derived for semiconductors as well as transition metals, and they are suitable for large scale atomistic simulations, yielding insights that previous, simpler models of interatomic forces could not. The concept grew out of Coulson’s definition of bond orders in molecules published in 1939, and was developed into a workable scheme by David Pettifor and co-workers, starting in the 1980s. This article is an introduction to the ideas and their implementation.

The Hume–Rothery matching rule 2kF=Khkl has been theoretically investigated by performing the LMTO-ASA (Linear Muffin-Tin Orbital–Atomic Sphere Approximation) band calculations for the three electron compounds: the γ-phase Cu5Zn8 compound or γ-brass, the nearly-free-electron-like Frank–Kasper-type Al30Mg40Zn30 1/1–1/1–1/1 approximant and the Mackay–Icosahedral-type Al68Cu7Ru17Si8 1/1–1/1–1/1 approximant. The zone planes responsible for the formation of the pseudogap across the Fermi level are identified. In the free-electron-like Al–Mg–Zn approximant, the Fermi surface-Brillouin zone interaction participating in the Hume-Rothery matching rule solely gives rise to a sizable pseudogap at the Fermi level. In the case of the γ-brass and the Al–Cu–Ru–Si approximant, where d-states are involved in the middle of the valence band, we could demonstrate that the particular Fermi surface-Brillouin zone interactions are strongly coupled with the sp-d hybridization to produce a deep pseudogap across the Fermi level.

In the low-temperature range, b.c.c. alloys exhibit a lower stress-temperature dependence than the pure base metals. This effect often leads to a phenomenon that is called “alloy softening”: at low temperatures, the yield stress of an alloy may be lower than that of the base metal. Various theories are reviewed: the most promising are based either on extrinsic or intrinsic models of low-temperature deformation. Some other aspects of alloy softening are discussed, among them the effects on teh ductile-brittle transition temperature.

Semi-continuous direct-chill (DC) casting holds a prominent position in commercial aluminium alloy processing, especially in production of large sized ingots. Macrosegregation, which is the non-uniform chemical composition over the length scale of a casting, is one of the major defects that occur during this process. The fact that macrosegregation is essentially unaffected by subsequent heat treatment (hence constitutes an irreversible defect) leaves us with little choice but to control it during the casting stage. Despite over a century of research in the phenomenon of macrosegregation in castings and good understanding of underlying mechanisms, the contributions of these mechanisms in the overall macrosegregation picture; and interplay between these mechanisms and the structure formation during solidification are still unclear. This review attempts to fill this gap based on the published data and own results. The following features make this review unique: results of computer simulations are used in order to separate the effects of different macrosegregation mechanisms. The issue of grain refining is specifically discussed in relation to macrosegregation. This report is structured as follows. Macrosegregation as a phenomenon is defined in the Introduction. In “Direct-chill casting – process parameters, solidification and structure patterns” section, direct-chill casting, the role of process parameters and the evolution of structural features in the as-cast billets are described. In “Macrosegregation in direct-chill casting of aluminium alloys” section, macrosegregation mechanisms are elucidated in a historical perspective and the correlation with DC casting process parameters and structural features are made. The issue of how to control macrosegregation in direct-chill casting is also dealt with in the same section. In “Role of grain refining” section, the effect of grain refining on macrosegregation is introduced, the current understanding is described and the contentious issues are outlined. The review is finished with conclusion remarks and outline for the future research.

Plutonium and plutonium-based alloys containing Al or Ga exhibit numerous phases with crystal structures ranging from simple monoclinic to face-centered cubic. Only recently, however, has there been increased convergence in the actinides community on the details of the equilibrium form of the phase diagrams. Practically speaking, while the phase diagrams that represent the stability of the fcc δ-phase field at room temperature are generally applicable, it is also recognized that Pu and its alloys are never truly in thermodynamic equilibrium because of self-irradiation effects, primarily from the alpha decay of Pu isotopes. This article covers past and current research on several properties of Pu and Pu-(Al or Ga) alloys and their connections to the crystal structure and the microstructure. We review the consequences of radioactive decay, the recent advances in understanding the electronic structure, the current research on phase transformations and their relations to phase diagrams and phase stability, the nature of the isothermal martensitic δ → α′ transformation, and the pressure-induced transformations in the δ-phase alloys. New data are also presented on the structures and phase transformations observed in these materials following the application of pressure, including the formation of transition phases.

The microstructural evolution and the high-strength age-hardening of copper-titanium alloys were analyzed. It was shown that the decomposition of Cu-Ti alloys involved complex interplay between clustering and ordering effects. The overaging in Cu-Ti age hardening alloys was observed to be associated with the emergence of a coarse lamellar microconstituent. It was found that the activation energy for the growth of the cells consumed the metastable, fine-scale coherent/semicoherent phase mixtures which led to rapid degradation of mechanical properties.

The kinetics of oxide formation in the presence of water vapour are discussed and compared with oxidation in dry atmospheres. The main protective oxide systems are considered, i.e. alumina, chromia, silica, titania and iron and nickel oxides, and with the possible exceptions of alumina and nickel oxide, oxidation rates are increased by the presence of water vapour. Scale morphology is also influenced by water vapour, and an important observation is that whisker formation is encouraged; this is believed to be due to the more rapid dissociation of water vapour compared to oxygen. In general, water vapour promotes the formation of a more porous scale. This is related to an increase in cation diffusion and consequent vacancy condensation, thereby developing a porous structure. The thermochemistry of oxide formation is discussed, and here oxide stability and hydroxide formation are considered. A significant observation is that where hydroxides or oxyhydroxides form, they generally have higher volatility than the corresponding oxide, and this leads to loss of protection.

The latest development of Pettifor’s bond-order approach – the analytic bond-order potentials (ABOPs) – represents a significant improvement over the empirical potentials of the Abell–Tersoff–Brenner type. This article aims at a critical evaluation of this promising novel scheme for the hydrocarbon system and assesses its applicability to realistic large-scale atomistic simulations. It is shown that ABOP reproduces the underlying orthogonal tight-binding model accurately for both hydrocarbon molecules and carbon crystalline phases in their ground-state configurations. However, in order to reproduce also non-equilibrium configurations it is necessary to extend the σ bond-order expression to account for the non-negligible sp atomic energy level separation of carbon. While the Brenner hydrocarbon potential exhibits several deficiencies in the description of amorphous hydrocarbon films, the extended ABOP model comes closer to results of accurate non-orthogonal tight-binding calculations. Remaining discrepancies of ABOP can be traced back to the limitations of the underlying orthogonal tight-binding model and its parameterization.

Ball milling induces self-sustaining reactions in many sufficiently exothermic powder mixtures. The process begins with an activation period, during which size reduction, mixing, and defect formation take place. The MSR (mechanically induced self-propagating reaction) is ignited when the powder reaches a well defined critical state. Once started, the reaction propagates through the powder charge as a combustion process. In this paper, the current knowledge on MSR is reviewed from both experimental and theoretical points of view. Experimental results on a broad variety of systems are examined and compared. The variation of the ignition time with composition and milling conditions is investigated. Some unusual phenomena, such as the mutual suppression of combustion in mixed metal-chalcogen systems, are discussed. The mechanism of MSRs is extremely complex, with important processes on several length and time scales. The key objective is to understand ignition and the changes during the activation process that lead to ignition. Combining models with systematic empirical studies appears to be the most realistic approach to a detailed understanding of MSR processes.

The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. Because of differing manufacturing approaches and operating scenarios, several specific mechanisms are involved. These mechanisms have begun to be understood. This article reviews this understanding and presents relationships between the durability, the governing material properties and the salient morphological features. The failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface. This amplification induces an energy release rate at cracks emanating from the imperfections that eventually buckle and spall the TBC.

The durability of thermal barrier systems is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. This sequence is governed by the σzz stresses that develop normal to the substrate, around imperfections, as the thermally grown oxide (TGO) thickens. Their effect is manifest in the stress intensity factor, K, caused by the σzz stresses acting on cracks emanating from them. In turn, these events are governed by scaling laws, ascribed to non-dimensional groups governing σzz and K. In this article the basic scaling relations are identified and used to gain some understanding of the relative importance of the various mechanisms that arise for application scenarios with minimal thermal cycling. These mechanisms are based on stresses that develop because of TGO growth strains in combination with thermal expansion misfit. The results are used to identify a critical TGO thickness at failure and express it in terms of the governing material variables. The changes in behavior that arise upon extensive thermal cycling, in the presence of TGO ratcheting, are elaborated elsewhere.

Research on silicon oxide thin films developed as gas-barrier protection for polymer-based components is reviewed, with attention paid to the relations between (i) coating defects, cohesive strength and internal stress state, and (ii) interfacial interactions and related adhesion to the substrate. The deposition process of the oxide from a vapor or a plasma phase leads in both cases to the formation of covalent bonds between the two materials, with high adhesion levels. The oxide coating contains nanoscopic defects and microscopic flaws, and their respective effect on the barrier performance and mechanical resistance of the coating is analyzed. Potential improvements are discussed, including the control of internal stresses in the coating during deposition. Controlled levels of compressive internal stresses in the coating are beneficial to both the barrier performance and the mechanical reliability of the coated polymer. An optimal coating thickness, with low oxygen permeation and high cohesive strength, is determined from experimental and theoretical analyses of the failure mechanisms of the coating under mechanical load. These investigations are found relevant to tailor the interactions and stress state in the interfacial region, in order to improve the reliability of the coating/substrate assembly.

In Ni-based superalloys, microtwinning is observed as an important deformation mechanism at intermediate temperature and low stress and strain rate conditions. Current knowledge concerning this unusual deformation mode is comprehensively reviewed, and fundamental aspects of the process are further developed using state of the art experimental and modeling techniques. The nature of microtwins and the microtwinning dislocations at the atomic level have been determined using High Angle Annular Dark Field Scanning Transmission Electron Microscopy imaging. The results unambiguously confirm that the operative twinning dislocations are identical Shockley partials a/6〈1 1 2〉, and that they propagate through the γ′ precipitates in closely-separated pairs on consecutive {1 1 1} planes. The rate-limiting process of the microtwinning deformation mechanism is the diffusion-controlled reordering in γ′-phase. It is shown that reordering requires very simple, vacancy-mediated exchange between Al and Ni atoms. The energetic aspect of the vacancy-mediated exchanges is studied for the first time using ab initio calculations. The concept of reordering as a rate-limiting process is generalized and shown to be relevant for other, previously reported deformation mechanisms in superalloys such as a〈1 1 2〉 dislocation ribbons, and superlattice intrinsic and superlattice extrinsic stacking fault formation. Other diffusion phenomena associated with microtwinning, such as segregation of heavy elements, is also discussed and supported by experimental evidence. The influence of the γ/γ′ microstructure on microtwinning deformation mode is also discussed in light of observations and phase-field dislocation modeling results.

The changes of sample shape are caused by plastic deformation or by martensitic phase transformations. In both cases the mechanisms of atomic rearrangements are based on collective displacements of atomic aggregates. The internal structure of dislocations, carriers of plastic deformation, can be examined using the energies of generalized stacking faults displayed by so called γ-surfaces calculated for bcc metals by Vasek Vitek already more than 40 years ago. This approach can be extended to the shuffling of atomic planes that plays a crucial role in martensitic phase transformations. Similarities and differences between displacive processes of lattice shearing and atomic plane alternate shuffling are discussed.

Many materials of engineering interest have highly heterogeneous microstructures. To a first approximation, the response of multi-phase materials to external stimuli such as mechanical loading depends on global parameters such as average particle size or phase volume fraction. Most classical models of materials behaviour are based on such an assumption. It is clear however that an accurate description must include parameters that characterize the distribution of phases. Moreover, some processes that we wish to model are inherently stochastic in nature. This adds considerable complexity. First, the quantitative description of microstructure containing higher order moments is fraught with difficulties — both analytical and experimental. Second, the inclusion of clustering into analytical models is prone to assumptions and approximations. In this paper we will restrict ourselves to phenomena for which a continuum approach is adequate. For these, self-consistent approaches are especially valuable. The two examples that we discuss in some depth are related to (i) damage in porous, brittle films such as thermal barrier coatings and (ii) the simultaneous effects of damage and particle clustering on the elasto-plastic response of metal matrix composites.

Whilst the dislocation core structure was investigated in the early days of the dislocation theory, the importance of core effects for understanding the basic features of plastic behaviour was first recognized in the case of bcc metals. At this time the first extensive computer modelling studies of dislocation cores were initiated. In this paper we show how the atomistic studies of dislocations advanced since these early calculations and how the basic ideas, developed at this time, apply when analysing deformation properties of other materials. Since a description of atomic interactions is the precursor of any atomistic calculations we first briefly describe the present status in this area, in particular the recently developed N-body potentials. Next, we discuss the concept of generalised stacking faults and associated energy-displacement surfaces (γ-surfaces). In this part we demonstrate how the symmetry considerations can be used to assess the existence of possible metastable planar faults which play a role in dislocation splitting. A general discussion of planar and non-planar dislocation cores then follows in which the dislocation splitting and core phenomena are combined into one notion. These general concepts are illustrated by results of recent studies of the dislocation cores in hcp metals and intermetallic compounds with L12 and DO22 structures. Using these results we discuss the physical reasons for the following phenomena: preference for the prism slip in some hcp metals, anomalous positive temperature dependence of the yield stress for prism slip in beryllium, similar anomalous yield behaviour observed in L12 intermetallic compounds, such as Ni3Al, existence of another class of L12 compounds with a strong temperature dependence of the yield stress at low temperatures, and brittleness of DO22 compounds.

We describe a formalism to predict diffusion coefficients of substitutional alloys from first principles. The focus is restricted to vacancy mediated diffusion in binary substitutional alloys. The approach relies on the evaluation of Kubo-Green expressions of kinetic-transport coefficients and fluctuation expressions of thermodynamic factors for a perfect crystal using Monte Carlo simulations applied to a cluster expansion of the configurational energy. We make a clear distinction between diffusion in a perfect crystal (i.e. no climbing dislocations and grain boundaries that can act as vacancy sources) and diffusion in a solid containing a continuous distribution of vacancy sources that regulate an equilibrium vacancy concentration throughout. A variety of useful metrics to characterize intermixing processes and net vacancy fluxes that can result in the Kirkendall effect are described and are analyzed in the context of thermodynamically ideal but kinetically non-ideal model alloys as well as a realistic thermodynamically non-ideal alloy. Based on continuum simulations of diffusion couples using self-consistent perfect-crystal diffusion coefficients, we show that the rate and mechanism of intermixing in kinetically non-ideal alloys is very sensitive to the density of discrete vacancy sources.

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life and the science and technology associated with this field has now led to multi-million dollar business. The paper focuses its attention mainly on titanium-based alloys, even though there exists biomaterials made up of ceramics, polymers and composite materials. The paper discusses the biomechanical compatibility of many metallic materials and it brings out the overall superiority of Ti based alloys, even though it is costlier. As it is well known that a good biomaterial should possess the fundamental properties such as better mechanical and biological compatibility and enhanced wear and corrosion resistance in biological environment, the paper discusses the influence of alloy chemistry, thermomechanical processing and surface condition on these properties. In addition, this paper also discusses in detail the various surface modification techniques to achieve superior biocompatibility, higher wear and corrosion resistance. Overall, an attempt has been made to bring out the current scenario of Ti based materials for biomedical applications.

Chitin and chitosan are natural biopolymers that are non-toxic, biodegradable and biocompatible. In the last decade, chitin and chitosan derivatives have garnered significant interest in the biomedical and biopharmaceutical research fields with applications as biomaterials for tissue engineering and wound healing and as excipients for drug delivery. Introducing small chemical groups to the chitin or chitosan structure, such as alkyl or carboxymethyl groups, can drastically increase the solubility of chitin and chitosan at neutral and alkaline pH values without affecting their characteristics; substitution with carboxyl groups can yield polymers with polyampholytic properties. Carboxymethyl derivatives of chitin and chitosan have shown promise for adsorbing metal ions, as drug delivery systems, in wound healing, as anti-microbial agents, in tissue engineering, as components in cosmetics and food and for anti-tumor activities. This review will focus on the preparative methods and applications of carboxymethyl and succinyl derivatives of chitin and chitosan with particular emphasis on their uses as materials for biomedical applications.