4. Structure of Metals and Alloys

The metal from liquid state solidifies at a fixed temperature characteristic of the metal and atoms take up a definite geometrical position, the smallest of which is called a crystal. Most of the metals solidify in three crystal forms of face centered cubic (f.c.c.), body centered cubic (b.c.c) and close packed hexagonal (c.p.h.). The crystal undergoes no change till it melts except in some cases like iron where changes before melting occur in the order of b.c.c. (alpha ferrite) to f.c.c. (austenite) and again to b.c.c. (delta ferrite). On being alloyed, depending on the solvent and solute, complete or partial solid solution and intermetallic phases are formed. The phases formed as a function of solute concentration and temperature is depicted in phase diagrams. One important feature of all metals or alloys is the presence of grain boundaries. The different crystal systems, solidification and the process of evolution of grain boundaries, the factors governing the type of alloy formation, the phase rule and phase diagrams have been discussed briefly in this chapter.