Phase Diagrams and Microstructures

A phase diagram is a graphical representation of the thermodynamic conditions under which different phases of a material can exist. It shows the temperature, pressure, and composition at which different phases are stable.

An equilibrium diagram is a phase diagram that shows the phase boundaries of a material at equilibrium. It shows the temperature, pressure, and composition at which different phases are in equilibrium with each other.

A phase transformation is a change in the crystal structure, composition, or physical properties of a material. Phase transformations can be caused by changes in temperature, pressure, or composition.

A solid solution is a mixture of two or more elements that are completely miscible in each other. This means that the atoms of the different elements are randomly distributed throughout the crystal lattice. A compound is a mixture of two or more elements that are not completely miscible in each other. This means that the atoms of the different elements are not randomly distributed throughout the crystal lattice, but instead form ordered structures.

In a eutectic reaction, two liquids react to form a solid. The temperature at which this reaction occurs is called the eutectic temperature. In a peritectic reaction, a liquid and a solid react to form a new solid. The temperature at which this reaction occurs is called the peritectic temperature.

A hypoeutectic alloy contains less of the alloying element than the eutectic composition. This means that the alloy will solidify completely to a solid solution. A hypereutectic alloy contains more of the alloying element than the eutectic composition. This means that the alloy will solidify to a mixture of a solid solution and a second phase.

A grain is a region of a material that has the same crystal structure and orientation. This means that the atoms in a grain are arranged in the same way. A grain boundary is the boundary between two grains. At a grain boundary, the atoms are not arranged in the same way as they are in the grains on either side of the boundary.

A recrystallized microstructure has a finer grain size than an annealed microstructure. This is because recrystallization occurs at a higher temperature than annealing. The higher temperature causes the atoms in the material to move more quickly, which allows them to rearrange themselves into smaller grains.

A quenched microstructure has a higher hardness than a tempered microstructure. This is because quenching traps the atoms in the material in a metastable state. This metastable state is harder than the equilibrium state that is achieved by tempering.

A martensitic transformation is a diffusionless transformation. This means that it occurs without the atoms in the material moving very much. A bainitic transformation is a diffusional transformation. This means that it occurs by the atoms in the material moving to new positions.

A pearlite microstructure consists of alternating layers of ferrite and cementite. Ferrite is a phase of iron that is body-centered cubic, while cementite is a phase of iron that is iron carbide (Fe3C). A spheroidite microstructure consists of spheroidal particles of cementite in a ferrite matrix. The spheroidal particles of cementite are formed by annealing a pearlite microstructure.

A Widmanstätten microstructure consists of lath-shaped ferrite grains. Lath-shaped ferrite grains are long, thin, and flat. A bainitic microstructure consists of acicular ferrite grains. Acicular ferrite grains are needle-shaped.

A martensitic microstructure consists of lath-shaped martensite grains. Lath-shaped martensite grains are long, thin, and flat. A bainitic microstructure consists of acicular bainite grains. Acicular bainite grains are needle-shaped.

A tempered martensite microstructure has a higher hardness than a tempered bainite microstructure. This is because martensite is a harder phase than bainite. Tempering softens both martensite and bainite, but it softens martensite more than it softens bainite.