Materials Science Glossary - M

macromolecule. A huge molecule made up of thousands of atoms.


magnetic field strength (H). The intensity of an externally applied magnetic field.


magnetic flux density (B). The magnetic field produced in a substance by an external magnetic field.


magnetic susceptibility ( ). The proportionality constant between the magnetization M and the magnetic field strength H.


magnetization (M). The total magnetic moment per unit volume of material. Also, a measure of the contribution to the magnetic flux by some material within an H field.


malleable cast iron. White cast iron that has been heat-treated to convert the cementite into graphite clusters; a relatively ductile cast iron.


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


matrix phase. The phase in a composite or two-phase alloy microstructure that is continuous or completely surrounds the other (or dispersed) phase.


Matthiessen’s rule. The total electrical resistivity of a metal is equal to the sum of temperature-, impurity-, and cold-work-dependent contributions.


melting point (glass). The temperature at which the viscosity of a glass material is 10 Pa s (100 P).


melting temperature. That temperature at which, upon heating, a solid (and crystalline) phase transforms to a liquid.


metal. The electropositive elements and alloys based on these elements. The electron band structure of metals is characterized by a partially filled electron band.


metallic bond. A primary interatomic bond involving the nondirectional sharing of nonlocalized valence electrons (“sea of electrons”) that are mutually shared by all the atoms in the metallic solid.


metal-matrix composite (MMC). A composite material that has a metal or metal alloy as the matrix phase. The dispersed phase may be particulates, fibers, or whiskers that normally are stiffer, stronger, and/or harder than the matrix.


metastable. A nonequilibrium state that may persist for a very long time.


microconstituent. An element of the microstructure that has an identifiable and characteristic structure. It may consist of more than one phase, such as with pearlite.


microelectromechanical system (MEMS). A large number of miniature mechanical devices that are integrated with electrical elements on a silicon substrate. Mechanical components act as microsensors and microactuators and are in the form of beams, gears, motors, and membranes. In response to microsensor stimuli, the electrical elements render decisions that direct responses to the microactuator devices.


microscopy. The investigation of microstructural elements using some type of microscope.


microstructure. The structural features of an alloy (e.g., grain and phase structure) that are subject to observation under a microscope.


Miller indices. A set of three integers (four for hexagonal) that designate crystallographic planes, as determined from reciprocals of fractional axial intercepts.


mixed dislocation. A dislocation that has both edge and screw components.


mobility (electron,_e, and hole,_h). The proportionality constant between the carrier drift velocity and applied electric field; also, a measure of the ease of charge carrier motion.


modulus of elasticity (E). The ratio of stress to strain when deformation is totally elastic; also a measure of the stiffness of a material.


molarity (M). Concentration in a liquid solution, in terms of the number of moles of a solute dissolved in one liter (103 cm3) of solution.


molding (plastics). Shaping a plastic material by forcing it, under pressure and at an elevated temperature, into a mold cavity.


mole. The quantity of a substance corresponding to 6.022 x 1023 atoms or molecules.


molecular chemistry (polymer). With regard only to composition, not the structure of a repeat unit.


molecular structure (polymer). With regard to atomic arrangements within and interconnections between polymer molecules.


molecular weight. The sum of the atomic weights of all the atoms in a molecule.


monomer. A stable molecule from which a polymer is synthesized.


MOSFET. Metal-oxide-semiconductor field-effect transistor, an integrated circuit element.