J.W. Christian

Bio: J.W. Christian is an academic researcher from University of Oxford. The author has contributed to research in topic(s): Crystal & Nucleation. The author has an hindex of 10, co-authored 16 publication(s) receiving 8273 citation(s).

The theory of transformations in metals and alloys

Abstract: Part I General introduction. Formal geometry of crystal lattices. The theory of reaction rates. The thermodynamics of irreversable processes. The structure of real metals. Solids solutions. The theory of dislocations. Polycrystalline aggregates. Diffusion in the solid state. The classical theory of nucleation. Theory of thermally activated growth. Formal theory of transformation kinetics. Part II Growth from the vapour phase. Solidification and melting. Polymorphic Changes. Precipitation from supersaturated solid solution. Eutectoidal transformations. Order-disorder transformations. Recovery recrystalisation and grain growth. Deformation twinning. Characteristics of martensic transformations. Crystallography of martensitic transformations. Kinetics of martensitic transformations. Rapid solidification. Bainite steels. Shape memory alloys.

CHAPTER 4 – The Thermodynamics of Irreversible Processes

Abstract: Classical thermodynamics is concerned primarily with the interdependence of certain well-defined macroscopic concepts (temperature, pressure, entropy, energy, composition, etc.) possessed by a closed assembly. The usual thermodynamic equations are valid only for assemblies at equilibrium and for reversible transitions among such equilibrated assemblies. When thermodynamic considerations are applied to irreversible (i.e. “natural”) processes, the equations become inequalities, and are much less useful. For example, the principle of increase in entropy during an adiabatic irreversible process provides information only about the direction of the change. The theory of transformations is a description of a particular class of irreversible processes. This type of theory is the main concern of this chapter and may appropriately be described as kinetic. The connection between the microscopic properties of the systems of the assembly and the macroscopic (measurable) properties is made by statistical mechanics. The thermodynamics of irreversible processes has also been applied to chemical reactions, though only in the limiting case of very close approach to equilibrium.

Stabilization of metallic supercooled liquid and bulk amorphous alloys

Abstract: Bulk metallic materials have ordinarily been produced by melting and solidification processes for the last several thousand years. However, metallic liquid is unstable at temperatures below the melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys are composed of a crystalline structure. Recently, this common concept was exploded by the findings of the stabilization phenomenon of the supercooled liquid for a number of alloys in the Mg-, lanthanide-, Zr-, Ti-, Fe-, Co-, Pd-Cu- and Ni-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multicomponent systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the pro- duction of bulk amorphous alloys in the thickness range of 1-100 mm by using various casting processes. Bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good cor- rosion resistance and have been used for sporting goods materials. The stabilization also leads to the appearance of a large supercooled liquid region before crystallization and enables high-strain rate super- plasticity through Newtonian flow. The new Fe- and Co-based amorphous alloys exhibit a large super- cooled liquid region and good soft magnetic properties which are characterized by low coercive force and high permeability. Furthermore, homogeneous dispersion of nanoscale particles into Zr-based bulk amor- phous alloys was found to cause an improvement of tensile strength without detriment to good ductility. The discovery of the stabilization phenomenon, followed by the clarification of the stabilization criteria of the supercooled liquid, will promise the future definite development of bulk amorphous alloys as new basic science and engineering materials. # 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Physical metallurgy of Ti–Ni-based shape memory alloys

Abstract: Ti–Ni-based alloys are quite attractive functional materials not only as practical shape memory alloys with high strength and ductility but also as those exhibiting unique physical properties such as pre-transformation behaviors, which are enriched by various martensitic transformations. The paper starts from phase diagram, structures of martensites, mechanisms of martensitic transformations, premartensitic behavior, mechanism of shape memory and superelastic effects etc., and covers most of the fundamental issues related with the alloys, which include not only martensitic transformations but also diffusional transformations, since the latter greatly affect the former, and are useful to improve shape memory characteristics. Thus the alloy system will serve as an excellent case study of physical metallurgy, as is the case for steels where all kinds of phase transformations are utilized to improve the physical properties. In short this review is intended to give a self-consistent and logical account of key issues on Ti–Ni based alloys from physical metallurgy viewpoint on an up-to-date basis.

Mechanical Behavior of Materials

Abstract: A balanced mechanics-materials approach and coverage of the latest developments in biomaterials and electronic materials, the new edition of this popular text is the most thorough and modern book available for upper-level undergraduate courses on the mechanical behavior of materials To ensure that the student gains a thorough understanding the authors present the fundamental mechanisms that operate at micro- and nano-meter level across a wide-range of materials, in a way that is mathematically simple and requires no extensive knowledge of materials This integrated approach provides a conceptual presentation that shows how the microstructure of a material controls its mechanical behavior, and this is reinforced through extensive use of micrographs and illustrations New worked examples and exercises help the student test their understanding Further resources for this title, including lecture slides of select illustrations and solutions for exercises, are available online at wwwcambridgeorg/97800521866758

Changes in portlandite morphology with solvent composition: Atomistic simulations and experiment

Abstract: Experimental work has been done to determine changes in the particle shape of portlandite grown in the presence of different ions. To quantify the experimentally observed changes in morphology a new analysis tool was developed, allowing the calculation of the relative surface energies of the crystal facets. The observed morphology in the presence of chlorides and nitrates was facetted particles of a similar shape, the addition of sulfates leads to hexagonal platelet morphology and the addition of silicates leads to the formation of large irregular aggregates. In addition to the experimental work, the surfaces of portlandite were studied with atomistic simulation techniques. The empirical force field used has first been validated. The equilibrium morphology of portlandite in vacuum and in water was then calculated. The results indicate that the presence of water stabilizes the [20.3] surface and changes the morphology. This is consistent with the experimental observation of [20.3] surfaces.

Fine phase mixtures as minimizers of energy

Abstract: Solid-solid phase transformations often lead to certain characteristic microstructural features involving fine mixtures of the phases. In martensitic transformations one such feature is a plane interface which separates one homogeneous phase, austenite, from a very fine mixture of twins of the other phase, martensite. In quartz crystals held in a temperature gradient near the α-β transformation temperature, the α-phase breaks up into triangular domains called Dauphine twins which become finer and finer in the direction of increasing temperature. In this paper we explore a theoretical approach to these fine phase mixtures based on the minimization of free energy.