Material Science-Chapter 11

All vocab for chapter 11

transformation rate

The reciprocal of the time necessary for a reaction to proceed halfway to its completion.

phase transformation

A change in the number and/or character of the phases that constitute the microstructure of an alloy.

nucleation

The initial stage in a phase transformation. It is evidenced by the formation of small particles (nuclei) of the new phase that are capable of growing.

growth (particle):

During a phase transformation and subsequent to nucleation, the increase in size of a particle of a new phase.

kinetics

The study of reaction rates and the factors that affect them.

supercooling

Cooling to below a phase transition temperature without the occurrence of the transformation.

superheating

Heating to above a phase transition temperature without the occurrence of the transformation.

isothermal transformation (T-T-T) diagram

A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to determine when transformations begin and end for an isothermal (constant-temperature) heat treatment of a previously austenitized alloy.

coarse pearlite

Pearlite for which the alternating ferrite and cementite layers are relatively thick.

fine pearlite

Pearlite in which the alternating ferrite and cementite layers are relatively thin.

bainite

An austenitic transformation product found in some steels and cast irons. It forms at temperatures between those at which pearlite and martensite transformations occur. The microstructure consists of α-ferrite and a fine dispersion of cementite.

spheroidite

Microstructure found in steel alloys consisting of spherelike cementite particles within an α-ferrite matrix. It is produced by an appropriate elevated-temperature heat treatment of pearlite, bainite, or martensite and is relatively soft.

martensite

A metastable iron phase supersaturated in carbon that is the product of a diffusionless (athermal) transformation from austenite.

athermal transformation:

A reaction that is not thermally activated, and usually diffusionless, as with the martensitic transformation. Normally, the transformation takes place with great speed (i.e., is independent of time), and the extent of reaction depends on temperature.

plain carbon steel

A ferrous alloy in which carbon is the prime alloying element.

alloy steel

A ferrous (or iron-based) alloy that contains appreciable concentrations of alloying elements (other than C and residual amounts of Mn, Si, S, and P). These alloying elements are usually added to improve mechanical and corrosion-resistance properties.

continuous-cooling-transformation (CCT) diagram

A plot of temperature versus the logarithm of time for a steel alloy of definite composition. Used to indicate when transformations occur as the initially austenitized material is continuously cooled at a specified rate; in addition, the final microstructure and mechanical characteristics may be predicted.

tempered martensite

The microstructural product resulting from a tempering heat treatment of a martensitic steel. The microstructure consists of extremely small and uniformly dispersed cementite particles embedded within a continuous α-ferrite matrix. Toughness and ductility are enhanced significantly by tempering.

solution heat treatment:

The process used to form a solid solution by dissolving precipitate particles. Often, the solid solution is supersaturated and metastable at ambient conditions as a result of rapid cooling from an elevated temperature.

precipitation heat treatment

A heat treatment used to precipitate a new phase from a supersaturated solid solution. For precipitation hardening, it is termed artificial aging.

overaging

During precipitation hardening, aging beyond the point at which strength and hardness are at their maxima.

natural aging

For precipitation hardening, aging at room temperature.

artificial aging

For precipitation hardening, aging above room temperature.

melting temperature

The temperature at which, upon heating, a solid (and crystalline) phase transforms into a liquid.

glass transition temperature (T_g):

The temperature at which, upon cooling, a noncrystalline ceramic or polymer transforms from a supercooled liquid into a rigid glass.