Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 49

Most inorganic solids, like minerals, rocks, and ice, on Earth's surface are solidified or frozen from a liquid phase. The temperature at which a material transitions between the solid and liquid phases is denoted as the melting point (T _m) of the material. This transformation temperature represents the point where the free energy of the solid phase and that of the liquid phase are equal. Unless otherwise noted, this T _m is assumed to be at standard pressure. However, there are several environmental aspects involved in the solidification process from liquid to solid. These are all contributing factors that determine the microstructure of the solid.

At a high level, during the solidification process, atoms/molecules from the fluid begin to bond together to form a nucleus. Homogeneous nucleation involves the clustering of liquid molecules/atoms to form a critical‐sized nucleus without an external interface. If a seed crystal of a desired structure is exposed to the melt, it can act as a nucleus and support in controlling the final structure. The nucleation process can also be triggered by the surface of an impurity (or structural material) that lowers the critical free energy required to form a stable nucleus. This process is called heterogeneous nucleation and, in practice, can control the solidification process.

Generally, multiple nuclei simultaneously develop in the fluid. The nuclei define the crystallographic phase of the grain. The crystals grow by the progressive addition of molecules from the fluid until they impinge upon adjacent crystals. The border between crystals is called the crystal boundary or grain boundary (GB). In metals, crystal growth from the liquid often follows a dendritic pattern where there is a main branch with many side branches caused by the growth of defined crystal planes in the lattice. As the crystal grows and impinges on its neighbors, the interdendritic spaces fill in and the original dendrites may not be apparent in the final microstructure. The bonding structures within the grains will depend not only on the temperature, composition, atmosphere, and pressure, but also on the speed of temperature changes and time at various temperatures. Certain crystals or grains may grow at the expense of others such that with slow cooling the individual grain sizes tend to be larger. Impurities, differing elemental compositions (e.g. due to overall concentration and equilibrium phases), and pores may be segregated into different grains and even into grain and subgrain boundaries. The size/shape/composition of the containment vessel and flow patterns for heat exchange can also play important roles. Microstructural properties, such as grain/crystal size, shape, crystallographic orientation, and intragranular/intergranular impurities, are just a few of the parameters that influence the mechanical response and physical properties of a material.

The physical properties of a material can also depend on the orientation of the material. The “texture” of a material is used to describe the distribution of crystallographic orientations and can have a strong influence on directionally dependent properties. A perfectly random polycrystalline material will have isotropic properties when the size of the sample is sufficiently larger than the grain size. This is due to the averaging effect across the different grains in different orientations. Some common textures include:

 Wire/fiber: Orientational alignment in the axial direction with nearly random radial orientation.

 Sheets: Compression and rolling in sheet preparation can orient grains in both axes through grain flow; however, annealing can change the texture.

 Thin films: “Fiber textures,” where certain lattice planes preferentially align with the substrate, and “biaxial textures,” where alignment with the substrate occurs.

 Rocks: Constituent minerals that often display preferred crystal orientation as a result of dislocation processes due to natural forces on the material.

Solidification of a fluid can also produce textured solids if time and activation energy enable atoms to find desired energy sites on existing crystals allowing some crystal facets to grow faster than others. The process of careful unidirectional freezing or popularly known “directional solidification” (DS) can be employed to align the grain boundaries parallel to the solidification direction and hence along the long axis of the columnar grains. The resultant structure is similar to a bundle of pencils with each grain having the low modulus <001> axis aligned parallel to the long axis of the grains. This kind of texturing can be very important to material properties. In the case of nickel‐base superalloys, Duhl (1987) mentioned that VerSnyder and Guard (1960) and Versnyder and Shank (1970) demonstrated for the “first time” that by aligning the grain boundaries parallel to the principal stress axis, the stresses acting at elevated temperatures on the weak grain boundaries could be minimized, thus delaying failure initiation and enhancing creep‐rupture life.

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.