Crystallography in the Life Sciences

Abstract

Modern crystallographic techniques and computer graphics are surveyed as applied to the determination of the structures of large molecules of biological importance, such as DNA-binding proteins, photochemical reaction centers and viruses. A discussion of protein stability is also presented. In an epilogue, some brief remarks are given concerning W. H. Bragg's work in Australia and a comparison is made between the excitement of research then and now.     

Corrosion of Mechanical Joints in Aluminium and Aluminium-Steel Structures

Abstract

This paper is primarlly concerned with the problem of poor performanee of mechanical joints, the main reason for which is inadequacy of published informaiion on joint design, construction procedure,, and maintenance. This contention is illustrated by discussion of corrosion problems that arise with mechanical joints, particularly with aluminium-stell joints which are commonly used in shipbuilding.

The need for closer co-operation amongst designers, constructors and corrosion specialists is strongly emphasised. Some methods of making practical corrosion informaiion available to industry are suggested.     

An Enhanced Methodology for Antiferromagnetic Structure and Form-factor Determinations

Abstract

It seems that it might be due to a combination of practical and historical circumstances that the unpolarised neutron diffraction method, applied usually to powder samples, still dominates structure determinations of antiferromagnets. Polarised-beam methods are used for ferromagnets. There is also a more advanced polarised-neutron single-crystal method for antiferromagnets, which is at last beginning to emerge, and this paper outlines the conceptual changes that are involved in adapting to this little-known revision of methodology, for multidomain antiferromagnetic structures. Further analyses are presented of initial UND-FFI-NPA_(A) results, for a tP3z antiferromagnet t ₁, AuMn for example, and the discussion is extended to the possibilities for further study, such as with a different UND-FFI-NPA_(D) experimental arrangement. By introducing additional techniques to determine the moment directions in individual coherent domains, the studies can proceed to more rigorous and detailed investigations of the form-factors in antiferromagnets of various symmetries. Some appropriate form-factor expressions are presented.     

Recommendations for Controlling the Application of Organic Protective Coatings: Report of Working Party No. 2 of the European Federation of Corrosion

Abstract

A draft of this paper prepared by the authors during the Spring of 1973 has been submitted to a detailed discussion at the Annual Meeting of Working Party NO. 2 (Delft, 14 June, 1973) whose participants are listed below.?

All modifications proposed by the members of the Working Party received the agreement of the authors who then prepared the present version. Final approval was then required and obtained from all the members of the Working Party and from the Scientific Advisory Committee of the E.F.C.

R. Scimar—President of W.P.2     

Optimum Conditions for Blast Cleaning of Steel Plate

Abstract

Technological investigations are further described in which practical blast cleaning of steel plate was simulated in the laboratory.

Removal of rust is fastest when blasting is perpendicular to the surface. The smaller grain size of abrasive leads to higher rates of cleaning. Work-hardening of the surface depends on grain size and extends much deeper than can be deduced from microscope observation of cross-section.

A discussion of the concept ‘properly cleaned surface’ leads to the conclusion that 100% clean is a fallacy and that for the moment the use of a magnifying glass is the only practical method of inspection.

From a few exploratory tests on the cleaning of welds the conclusion is justified that blasting perpendicular to the weld surface leads to the highest cleaning rates. Small grain sizes are to be preferred.

The velocity of abrasive grains in flight was determined by high-speed photograph. Values found were somewhat higher than the ones reported for centrifugal machines. Influence of air pressure and specific gravity of the abrasive could be demonstrated.     

Mechanism and estimation of porosity defects in ductile cast iron

Abstract

Although there are many works on the formation mechanism of porosity defects due to solidification in ductile cast iron, the formation mechanism is still not clear and decreasing the porosity defects is still a main issue in the industry. This paper critically reviews conventional explanations for the porosity formation including estimation methods. Based on the discussion the authors propose a formation theory where it considers gas and oxide entrapment during mould filling, expansion of outer part of casting due to graphite formation and pressure decrease in the inner part, followed by the growth of entrapped small gas bubbles. This mechanism can explain various facts in practice and be usable to estimate the defects. It also gives a good way to design effective risers. Future challenges are also discussed including the effect of inoculation on the fluidity of the mushy region.     

A new perspective for Pulse Plating

Abstract

The 1st European Pulse Plating Seminar was held in Vienna on 10th March 2006. The 10 presentations from international authors and the parallel exhibition focused on both the scientific and industrial approach to the pulse plating technique. 82 participants coming from 13 different European countries contributed to an intensive and very productive discussion on the topic that will be continued in 2007 at the 2nd European Pulse Plating Seminar in Vienna.     

Macrosegregation in directionally solidified dendritic alloys

In this article, gravity-driven flow and its subsequent effect of promoting macrosegregation during unidirectional solidification of dendritic alloys is presented. Examples of macrosegregation that arise during the controlled directional solidification of hypo- and hypereutectic Pb−Sn alloys are shown, and a method of preventing macrosegregation is demonstrated. The experimental work is discussed in terms of how current knowledge of solute redistribution in a dendritic array can be promoted as well as how the processing technique might be applied to improve microstructural homogeneity during controlled directional solidification. Authors' Note: Excellent sources for historical information are Cyril Stanley Smith's booksA Search for Structure, A History of Metallography, andThe Science of Steel 1532–1786. We acknowledge that there were many early investigations regarding segregation phenomena that took place in countries other than the United States and Great Britain that have not been referenced. Furthermore, conference proceedings and reports are not included (e.g., prior to 1938, the British Iron and Steel Institute published seven reports on the heterogeneity of stel ingots). In trying to keep the discussion within the context of dendritic growth we do not, unfortunately, recognize the many important crystal growth/planar/cellular front investigations. Finally, there are certainly many recent and relevant contributions of which we are unaware and apologize for the lack of their inclusion. R.N. Grugel earned his Ph.D. in metallurgical engineering at Michigan Technological University in 1983. He is currently a staff scientist at the Universities Space Research Association. He is also a member of TMS. L.N. Brush earned his Ph.D. in metallurgical engineering and materials science at Carnegie Mellon University in 1988. He is currently associate professor at the University of Washington.     

Sintered aluminum nitride ceramics for high-power electronic applications

This article provides an overview of some of the critical materials issues involved in the utilization and commercialization of aluminum-nitride ceramics in electronic applications. Critical material defects and microstructural issues that control thermal, electrical, and mechanical properties and the status of metallization technology are reviewed. Finally, a discussion of current and future potential applications of sintered AlN and how these applications tie into key material properties is provided. Jonathan H. Harris earned his Ph.D. in solid state physics at Brown University in 1983. He is currently vice president of technology at Crystalline Materials Corporation.     

Thermodynamics of constrained and unconstrained equilibrium systems and their phase rules

The two types of phase equilibria, the normal unconstrained one and the constrained one, and their thermodynamics are discussed. The concepts of potential and phase rule, which have recently been discussed in the literature, are reconsidered, and their formal definitions are analyzed in some detail. It is realized that in the unconstrained equilibrium system, the properly defined chemical potentials of all components must be constant across the phase interface in both the hydrostatically and nonhydrostatically stressed systems. It is demonstrated that in a constrained equilibrium system, e.g. with a coherent equilibrium or paraequilibrium, the discussion of the phase rule is rather meaningless even though it is possible to find a relationship between the number of independent potentials and phases. On the other hand, a constrained equilibrium system may be treated as a normal equilibrium by describing its equilibrium features with a different definition of the system.     

The influence of copper on microstructure and mechanical properties of compacted graphite iron

Abstract

The influence of copper content (0·26 to 1·31 wt-%) on microstructure formation and mechanical properties of compacted graphite iron (CGI) has been evaluated through standard metallographic analysis, colour etching techniques and tensile testing of machined test bars. The properties investigated are yield strength, tensile strength and elongation. The castings were made in an industrial environment from a combination of CGI returns, pig iron, cast iron- and steel scrap. A total of four heats were cast in specially designed sampling cups (3 different cooling rates), chill wedges as well as tensile test bars machined from sand moulded cylinders (20, 45 and 85 mm in diameter). The results clearly illustrate the combined effect of copper and cooling rate on nodularity, chilling tendency as well as pearlite content. A discussion concerning the effect of graphite morphology on the ferrite growth is also included.     

Electrochemical rehabilitation methods for reinforced concrete structures: advantages and pitfalls

Abstract

There are still many unresolved issues concerning the efficiency of electrochemical realkalisation and electrochemical chloride removal as electrochemical rehabilitation methods for corroding reinforced concrete structures. The present paper seeks to answer a number of questions which, though seemingly elementary, continue to arouse controversy in scientific, technical and economic communities, despite the vast amount of work that has been devoted to research on corrosion in concrete embedded steel, such as whether corrosion can be stopped once it has started, whether corroded reinforced concrete structures can be repassivated, and whether it is sufficient to remove the sources of corrosion in order to stop rusting. A discussion is conducted on the relationship between the prerusting grade of rebars and the possibility of their repassivation; on whether electrochemical rehabilitation methods treatments are efficient, and if so, when and on whether a simple potential measurement can determine the passive or active state of a rebar. For this purpose an analysis is made, using electrochemical, gravimetric and metallographic techniques, of the response of clean and previously corroded steel electrodes in a Ca(OH)₂ saturated solution with and without a sodium nitrite corrosion inhibitor and in cement mortar. The effectiveness of electrochemical realkalisation and electrochemical chloride removal for repassivating prerusted steel in concrete is found to depend heavily on the prerusting grade.     

Managing the transition to hexavalent chromium free anti-corrosion coatings

Abstract

Hexavalent chromium is a well established material used in the commercial production of sacrificial zinc coatings for anti-corrosion applications. However, environmental and legislative pressures have been the catalyst for adopting alternatives based upon trivalent chromium and chromium-free materials. This change has wide implications within the global supply chain. End users and applicators must consider strategic issues such as product availability, cost implications and specific application needs. Experience with the change has illustrated the need for cooperation along the entire supply chain. The chemical supplier becomes the conduit between applicator and end user, and can facilitate a managed change of the process. Suppliers are ideally placed to establish best practice techniques at the applicator, and can work in partnership to achieve lowest operating cost with maximum performance. Developments in trivalent passivates for blue and iridescent colour films will be described, followed by discussion of the use of an audit approval programme to manage the transition to hexavalent chromium free products.     

Flattening of composite powder particles during thermal spraying

An investigation is conducted of the time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of composites (agglomerated) powder particles at smooth and rough surfaces during thermal spraying. Considerations include the splat solidification, droplet mass loss, solid volume fraction, and variations of splat- surface friction and splat solidification velocity due to the presence of the solid phase. Effective values of the droplet viscosity, impact velocity, and Reynolds number taking into account characteristic features of the flattening process are introduced, and analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are established. Results can be used to predict the splat flattening parameters.     

Solidification paths in modified Inconel 625 weld overlay material

Abstract

Inconel 625 is commonly used for overlay welding to protect the base metal against high temperature corrosion. The efficiency of corrosion protection depends on effective mixing of the overlay weld with the base metal and the subsequent segregation of alloy elements during solidification. Metallographic analysis of solidified samples of Inconel 625 with addition of selected elements is compared with thermodynamic modelling of segregation during solidification. The influence of changes in the melt chemistry on the formation of intermetallic phases during solidification is shown. In particular, focus is put on how the composition of the dendrite core is affected by modifications to the alloy. It has previously been shown that when the overlay material corrodes, the corrosion take place in the dendrite core. Therefore, the discussion will be directed towards explaining the extent to which the variations in chemical compositions influence the composition of the dendrite core of the weld overlay.     

The steady-state corrosion kinetics of single-phase binary alloys with a solubility gap forming the most-stable oxide

The steady-state, high-temperature oxidation kinetics of single phase alloys rich in a most-reactive componentB in binaryA-B systems presenting a limited solubility of the two components (beta phase alloys) have been examined assuming the exclusive formation of the most-stable oxideBO _v. Alloys sufficiently rich inB can form externalBO _v scales directly in contact with the beta phase, while below a criticalB content the growth ofBO _v involves also the appearance of an intermediate layer ofB-depleted solid solution ofB inA (alpha phase). The parabolic rate constants for the oxidation of single-phase beta alloys are lower than those of alloys of identicalB content which are single-phase over the whole range of composition (solid-solution alloys) but higher than for two-phase alpha + beta alloys under the same values of all the relevant parameters. Moreover, the tendency of single-phase beta alloys to form the most-stable oxide simultaneously as an external scale and internally to the alloy is greater than for solid-solution alloys but smaller than for two-phase alloys.     

An assessment of climate change effects on atmospheric corrosion rates of steel structures

Abstract

This paper presents a discussion on assessing the potential impacts of climate change on the atmospheric corrosion rates of exposed steel structures. The effects on atmospheric corrosion due to changes in the environmental temperature, carbon dioxide, relative humidity, wind, rainfall and pollution are considered. The limitations and complexities of these assessments are discussed. To demonstrate the use and limitations of this science to evaluate effects related to climate change, a model developed in Australia to predict corrosion is combined with climate change models to project the change in the corrosion rates of steel components and protective zinc coatings in constructions. The method is applied to constructions located along the coastal areas of two Australian cities: Melbourne and Brisbane. These assessments are made using the A1FI scenario, the highest emission scenario defined by the Intergovernmental Panel on Climate Change, applied to nine general circulation models. The projected changes in corrosion rates were found to be an increase of ～14% for both zinc and steel in Brisbane and a decrease of ～14% for steel and 9% for zinc in Melbourne. It was also found that the uncertainties associated with the climate change models were small compared to those involved in modelling corrosion for engineering purposes.     

Liquid-liquid equilibrium in immiscible In-Se alloys suffering metal-nonmetal transition

Electroconductivity σ(T measurements for liquid immiscible In_1-xSe_x (x = < 1/3) alloys are carried out under excess pressure of argon gas (up to 50 MPa) at temperatures up to 1200 K. In the transition region, the electroconductivities of coexisting liquids vary greatly. The liquid-liquid coexistence curve on the phase diagram of the In-Se system is determined from σ(T measurements. The coordinates of the critical point were found to bex _c = 19.2 ± 0.1 at % Se andT _c = 949 ± 1.8 K. The critical indices β and 1 — α are estimated to be 0.35 ± 0.02 and 0.87 ± 0.02, respectively. An asymmetry of the coexistence curve and the nonlinearity of its diameter are analyzed in comparison with available data for fluid metals in the vicinity of the liquid-vapor critical point.