Engineering Physics of High-Temperature Materials. Nirmal K. Sinha. Читать онлайн. Newlib. NEWLIB.NET

Автор: Nirmal K. Sinha
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119420460
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2.4a has both a liquid phase and a solid phase present. The composition of the phases can be determined by drawing a tie‐line from the point to the liquidus and solidus curves and then reading the percent composition at the intersections. The ratio of the liquid/solid present can then be determined using the two phase compositions and the overall composition, using the lever rule:

      (2.2)Fraction of liquid phase present equals StartFraction percent-sign normal upper X in solid minus percent-sign normal upper X overall Over percent-sign normal upper X in solid minus percent-sign normal upper X in liquid EndFraction

      Figure 2.4b presents a typical phase diagram of a eutectic system. A eutectic system is a homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of any of the constituents. At a eutectic point, the liquid and two solid solutions all coexist in chemical equilibrium. Cooling through this temperature results in intricate macrostructures that can take several different forms, including lamellar, rod‐like, globular, or acicular (needle‐like) structures. Other critical points in a binary system include:

       Eutectoid: Transformation of a solid phase to yield two solid phases.

       Peritectic: Transformation of a liquid phase and a solid phase to yield a single solid phase.

       Peritectoid: Transformation of two solid phases in an alloy system to yield a new solid phase.

      Binary systems can be intricate with multiple types of transformations available to the system at different compositions. Further increasing the number of components can quickly make materials' systems very complex. Phase diagrams can thus be incredibly useful in designing materials to have specific phase compositions.

      Phase diagrams can be incredibly important in engineering materials, but they do not always tell the whole story. Phase diagrams generally present equilibrium phases. Nonequilibrium phases, such as phases stabilized through quenching or even a more complex environmental history, can often be critical to achieving properties of interest. Time–temperature–transformation diagrams that plot the time required for isothermal transformation provide kinetic information about transformation processes and can be highly useful in developing synthesis processes; however, these diagrams must still be interpreted with the assistance of phase diagrams.

      

      An individual crystal or grain in the solid phase consists of atoms aligned in space in definite patterns over long distances until it impinges on the border of another crystal. In reality, solids are not perfect. There are many different kinds of imperfections within a solid that play an important, and often determining, role in the properties of the material. Crystal imperfections thus play an important role in materials science and its use across disciplines.

      Understanding imperfections and their impact on materials' properties is extraordinarily complex and dependent on the system under question. Here, we strive only to give a brief overview of some high‐level key features that will enable us to understand aspects presented later in this text.

      2.3.3.1 Point Defects

      Point defects can often occur in pairs or sets. For example, in ionic crystals, point defects can occur in pairs that maintain stoichiometry or charge neutrality, such as Schottky defects (vacancies of ions with opposite charge) or Frenkel defects (a vacancy and an interstitial). A high density of point defects, particularly vacancies, can give rise to nonstoichiometric compounds. Such compounds often consist of transition metals, lanthanides, and actinides paired with polarizable anions, such as oxides and sulfides, e.g. FexO and CuxS. Point defects can be naturally occurring, such as the substitutional impurities that give rise to beautiful gems, e.g. sapphires and rubies. Point defects can also be designed into the material to tailor its properties, such as doping semiconductors to control their electrical conductivity.

Schematic illustration of common point defects in crystals, including a vacancy, an interstitial, and a substitution.

      2.3.3.2 Line Defects/Dislocations

      Line defects or dislocations are imperfections in the linear arrangement of atoms in the crystal. There are two main types of dislocations that represent the extreme forms of line defects: edge dislocations and screw dislocations. Dislocations play an important role in plastic deformation and allow deformation to occur at lower stresses than would be anticipated in a perfect crystal as only a few atoms are moved from their equilibrium positions at a given time.

Schematic illustration of the basic motion or propagation of an edge dislocation. (a) Dislocation glide or slide as a result of shear stress. (b) Dislocation climb involving material transfer from the dislocation core.

      In edge dislocation climb, as shown in Figure 2.6b, the dislocation moves up or down out of its slip plane to a parallel slip plane. This involves the creation or annihilation of a row of vacancies and is energetically difficult.

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