Principles of equal-channel angular pressing as a processing tool for grain refinement

Publisher Summary This chapter focuses on measurements and theories pertaining to homogeneous nucleation under cooled liquids and glasses that transform to phases of the same composition. The dynamical model describing the cluster evolution appears to be quantitatively correct. Although there are some experimental and theoretical difficulties with the classical theory, there are also many approaches to rectify these problems. At present, there are no experiments that can demonstrate or can refute conclusively the validity of the classical theory in the regime where it is expected to be valid. This is partially because of a lack of knowledge of the correct values for the parameters that are fit by the theory. Better measurements of the steady-state and time-dependent nucleation rates and the free-energy difference as a function of temperature, and the design of new experiments such as the multistep annealing treatments discussed in the chapter are used to test more rigorously nucleation theories. Time-dependent nucleation and the interface kinetics are incorporated into the field-theoretic approaches to give theories that are as widely applicable and, hence, testable as the classical theory.

Crystal Nucleation in Liquids and Glasses

This highly readable, popular textbook for upper undergraduates and graduates comprehensively covers the fundamentals of crystallography and symmetry, applying these concepts to a large range of materials. New to this edition are more streamlined coverage of crystallography, additional coverage of magnetic point group symmetry and updated material on extraterrestrial minerals and rocks. New exercises at the end of chapters, plus over 500 additional exercises available online, allow students to check their understanding of key concepts and put into practice what they have learnt. Over 400 illustrations within the text help students visualise crystal structures and more abstract mathematical objects, supporting more difficult topics like point group symmetries. Historical and biographical sections add colour and interest by giving an insight into those who have contributed significantly to the field. Supplementary online material includes password-protected solutions, over 100 crystal structure data files, and Powerpoints of figures from the book.

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Structure of Materials: An Introduction to Crystallography, Diffraction and Symmetry

We express the cubic (Al, Zn)49Mg32 structure as a periodic packing of the rhombohedra1 cells from the three-dimensional Penrose tiling, decorated by atoms. This decoration is extended to the aperiodic quasicrystal packing of the same cells to give a hypothetical structure for the recently discovered icosahedral phase with the same composition. This is the first structure of a real system which is related both to quasicrystals and to Frank-Kasper icosahedral close packings.

Quasicrystal structure of (Al, Zn)49Mg32

Publisher Summary This chapter reviews the experimental and simulation evidence for polytetrahedral order in small clusters, liquids, glasses, and periodic or quasiperiodic crystals and shows that it is a powerful unifying concept in their theoretical description. Discussions of polytetrahedral order have recently received new impetus from the experimental discovery of quasi crystal that have a macroscopic icosahedral symmetry and which, in many cases, are closely related to Frank–Kasper compounds. Coxeter introduced a mean field theory of dense-random packing called the “statistical honeycomb model” and emphasized the importance of “polytope {3, 3, 5},” a regular four-dimensional Platonic solid that provides a regular tetrahedral tiling of S3, the curved, three-dimensional surface of a sphere embedded in four dimensions. The 120 atoms of polytope {3 , 3 , 5} form a regular lattice where each atom sits in an identical icosahedral coordination shell and serves as a paradigm for the kind of global order prevented by frustration in flat space.

Polytetrahedral Order in Condensed Matter

It has been established that quasicrystals with icosahedral point group symmetry occur in a rapidly solidified Mg32 (Al, Zn)49 alloy chosen on the basis of its equilibrium crystal structure. This alloy has a natural tendency to form icosahedral atomic clusters stabilised by size difference amongst constituent atoms. Results highlight the relationship between equilibrium crystal structure and the tendency to form quasicrystals.

A basis for the synthesis of quasicrystals

Flow behavior of many metals and alloys in the region of uniform plastic strain is normally expressed by the simple power law relationship (σ = Kepn). However, deviations have been observed from such behavior in several fcc metallic materials with low stacking fault energy. The age-hardenable nickel-iron base superalloy IN718 is also observed not to obey the simple power law relationship either in the solution treated as well as in the different age-hardened conditions. Various flow relationships were used to obtain the best fit for the experimental data, and different relationships were identified for the different heat-treated conditions. Detailed transmission electron microscopy (TEM) examination of the tensile-tested samples, interrupted after different amounts of strain, revealed inhomogeneous deformation by planar slip in all the heat-treated conditions. Cold rolling, however, was found effective in eliminating the unusual plastic flow behavior, observed in the as-heat-treated conditions.

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Work-Hardening Behavior of the Ni-Fe Based Superalloy IN718

A new quasi-crystalline phase in rapidly solidified Mg4CuAl6

Mechanical properties of aluminium–copper–lithium alloy AA2195 at cryogenic temperatures

Advanced Ultra Super Critical power plants (AUSC) with intended capabilities of higher efficiency and less emissions of CO2 compared with those of conventional power plants, require materials with high creep strength and good corrosion resistance. They are also required to withstand steam at high temperatures of 700–780 °C and pressures upto 35 MPa. Inconel 617 alloy is a candidate material for super heater and re-heater parts of boilers. In the present study, tensile behaviour of the Inconel 617 is investigated in the temperature range from RT to 900 °C at different strain rates from 5 × 10−4 s−1 to 1 × 10−2 s−1 and the deformed microstructures are characterised by optical and electron microscopy. Serrations are observed in the temperature range 300–700 °C at all the three strain rates which indicated the occurrence of dynamic strain aging (DSA). The amplitude of serrations increased with increase in the temperature and decrease in the strain rate. Temperature regime of DSA is confirmed to be from 300 °C to 700 °C by the occurrence of plateau in the yield strength, ductility minima and negative strain rate sensitivity in this regime. Activation energies for the serrated flow are found to be 65, 80 and 110 kJ/mol for the three types of serrations, namely B, (A+B) and C respectively. The controlling mechanism of the DSA is found to be diffusion of carbon through dislocation cores in the lower temperature range and diffusion of substitutional elements Cr and Mo in the higher temperature range. TEM studies revealed increase in the number of slip bands as well as interaction of dislocations with solute atoms with increase in the temperature up to 700 °C. Precipitation of carbides and subgrain formation was observed at 700 °C, at the ductility minima. SEM examination of the fracture surfaces revealed ductile fracture with dimples and facets at room temperature, ductile fracture with essentially dimples at 400–600 °C, at intense serrations in the flow curve. At 700 °C, at ductility minima, there was mixed mode of fracture with dimples, facets and intergranular cracks associated with some grains boundaries. At 800 °C and 900 °C, there was completely ductile fracture with large and deep dimples.

G.V.S. Sastry

Papers

A basis for the synthesis of quasicrystals

Work-Hardening Behavior of the Ni-Fe Based Superalloy IN718

A new quasi-crystalline phase in rapidly solidified Mg4CuAl6

Mechanical properties of aluminium–copper–lithium alloy AA2195 at cryogenic temperatures

Dynamic strain aging, deformation and fracture behaviour of the nickel base superalloy Inconel 617