Materials Science Glossary - G

galvanic corrosion. The preferential corrosion of the more chemically active of two metals that are electrically

coupled and exposed to an electrolyte.


galvanic series. A ranking of metals and alloys as to their relative electrochemical reactivity in seawater.


gas constant (R). Boltzmann’s constant per mole of atoms. R= 8.31 J/mol K (1.987 cal/mol K).


Gibbs phase rule. For a system at equilibrium, an equation that expresses the relationship between the number of phases present and the number of externally controllable variables.


glass-ceramic. A fine-grained crystalline ceramic material that was formed as a glass and subsequently crystallized.


glass transition temperature (Tg). The temperature at which, upon cooling, a noncrystalline ceramic or polymer transforms from a supercooled liquid to a rigid glass.


graft copolymer. A copolymer wherein homopolymer side branches of one monomer type are grafted to homopolymer main chains of a different monomer type.


grain. An individual crystal in a polycrystalline metal or ceramic. Note that spherulites in polymers are not single crystals.


grain boundary. The interface separating two adjoining grains having different crystallographic orientations. 


grain Boundary segregation. Solute enrichment at grain boiundaries. An approximation for the segregation tendency of a solute to a grain boundary is its bulk solubility: The smaller the bulk solubility, the higher is the enrichment factor of that element at the grain boundary. The thermodynamics of grain boundary segregation has close analogy to monolayer gas adsorption at solid surfaces according to Gibbs and can be formalized in terms of the adsorption isotherm. In this concept the grain boundary excess concentration is proportional to the reduction in the interfacial energy per molar concentration change at constant temperature T and volume V. The value for the boundary excess concentration can be obtained by measuring the change in interfacial energy as a function of concentration changes in logarithmic presentation. Although the Gibbs adsorption isotherm enables quantitative analysis of grain boundary segregation, the actual measurement of the interfacial energy as a function of bulk composition and temperature is experimentally rather challenging. Therefore, it is pertinent to use the Langmuir-McLean isotherm which approximates segregation by balancing adsorption and desorption rates.


grain growth. The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary. Note that the driving force for grain growth is the capillary driving pressure pushing the system towards a state of lower total interface energy.  Although the term grain growth implies that the grains are becoming larger it must be considered that this is an average observation, many of the grains are actually becoming smaller and vanish.  In three dimensions a stable grain structure cannot be realized due to the fact that the equilibrium of the forces stemming from the interfacial energies at the connection points (e.g., triple points etc.) requires the grain boundaries to the curved, hence, creating local capillary forces that aim at reducing the total planar size of the defect.  While in normal grain growth the average grain size increases in abnormal or discontinuous grain growth only few grains much larger than the others.


grain size. The average grain diameter as typically determined from a line intersect metallographic method.


gray cast iron. A cast iron alloyed with silicon in which the graphite exists in the form of flakes. A fractured surface appears gray.


green ceramic body. A ceramic piece, formed as a particulate aggregate, that has been dried but not fired.