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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display preferred crystal orientation as a result of dislocation processes due to natural forces on the material.

      In the extreme case of DS processes, such as in the Czochralski process, only one crystal will survive. This process is well known in the semiconductor industry to produce high‐grade silicon wafers but is also used in casting creep‐sensitive components, such as turbine blades. However, single‐crystal (SC or SX) formation can be a very expensive process.

      Once initially solidified, the microstructure of a material does not remain static, and the mechanical response and physical properties can likewise change as a result of a material's history. For practical reasons, primarily to avoid sudden high‐temperature fractures, operating temperatures for most metallic solids are restricted to temperature regimes well below 0.4 T m. In nuclear and power generation industries, and for the components of gas turbine engines, exposure to temperatures higher than 0.4 T m cannot be avoided. This being said, the approaches for developing complex alloys used in aerospace and gas turbine applications have taken bold steps and the operating temperatures are now pushed upward to 0.8 T m.

      In the geophysical arena, the mechanical properties of naturally occurring materials and their dependence on microstructure close to T m are paid only passing attention even though the temperatures of the lithosphere–asthenosphere boundary are extremely high.

      To better understand phase transitions as well as the forces that may be at play as one approaches the transition temperature, it is necessary to have a better understanding of how a material system behaves on exploring its phase diagram.

      A material system is composed of both phases and components. “Phases (P)” in a system are homogeneous in chemical composition and physical state. “Components (C)” in a system represent a pure element or compound. The number of components in a system is thus the number of independent species necessary to define the composition of all the phases. Phase diagrams map the preferred or equilibrium phases of a material at different thermodynamic variables.

      The degrees of freedom, F, is the number of thermodynamic parameters that may vary independently while maintaining the same phase/phases. F can be derived from the number of components (C) and phases (P), according to the phase rule (Gibbs 1874–1878 ):

      (2.1)upper F equals upper C en-dash upper P plus 2

Schematic illustration of typical pressure versus temperature of a one-component system.

      Another key point on such phase diagrams is the end point of a phase equilibrium curve – called a “critical point.” Figure 2.3 demonstrates a common critical point where the liquid and gaseous states become indistinguishable and are often referred to as a supercritical fluid. In the vicinity of a critical point, the physical properties of the liquid and vapor can dramatically change as they become more similar. Historically, a solid–liquid critical point has generally not been accepted (Landau and Lifshitz 1980); however, this has recently been challenged – largely through molecular dynamics simulations (Elenius and Dzugutov 2009; Mochizuki and Koga 2015).

      It should be mentioned that the phase diagram of a single compound is not necessarily a single‐component system. Polymorphism can give rise to intricate phase diagrams for single compounds. For example, the phase diagram of water is quite complex and discussed further in Section 2.6.

Schematic illustration of sample phase diagram schematic for a binary XY system.

      Phase diagrams for binary systems contain a large amount of information. Not only do they give an indication of the phases present, but also provide information about the composition of the phases and fraction of the phases present in the mixture. For example, the point of interest highlighted in Скачать книгу