Many metals, nonmetals, and minerals exhibit an interesting feature called poly­morphism (poly means “many” and morphism means “structure”). Polymorphism (sometimes called allotropism) is known as the ability of a material to be present in more than one type of crystal structure as determined by temperature and/or pres­sure. Many phase transformations are based on this unique feature of the materi­als. As a general trend, more close-packed crystal structures are favored at lower temperatures, whereas open structure such a BCC crystal structure is most favored at higher temperatures. So, many metals show polymorphic transformation from the HCP to BCC crystal structure as the temperature increases and these phases are commonly referred to by Greek alphabets (a, b, y, etc.). For example, zirconium (Zr) assumes a HCP structure (a-Zr) at <865 °C, but becomes BCC (b-Zr) above that temperature. Similarly, hafnium (Hf) exhibits HCP structure below 1950 °C, but assumes BCC crystal structure above 1950 °C until its melting point (2233 °C). The example of iron (Fe) is interesting and bit of an exception. a-Fe (BCC) trans­forms to y-Fe (FCC) at about 912 °C, and then back to a BCC allotrope known as 6-Fe (with slightly larger lattice constant than that of a-Fe) above about 1394 °C. Thus, with the example of a-Fe and 6-Fe, polymorphism may not necessarily mean the presence of entirely different crystal structures, but the definition needs to be more related to the difference in crystal parameters such as lattice constants. There is also a HCP crystal structure (є-Fe) that is formed only under very high pressures. Polymorphic transformations of iron as a function of temperature make the heat treatment of steels possible leading to multitude of resulting microstructures and properties. In general, no FCC or BCC metal transforms to a HCP phase without an exception. Nuclear fuels like uranium and plutonium show multiple crystal structures at different temperature regimes as a result of polymorphism, and will be mentioned in more detail in Chapter 7.

54 I 2 Fundamental Nature of Materials

2.1.5