Materials Science Glossary - D

deep drawing.  With respect to DIN 8584-3 deep drawing refers to a mixed tensile and compression deformation of metallic sheets to build a hollow body or a continued mixed tensile and compression deformation to built a hollow body with a smaller diameter. The basic principle of deep drawing can be seen in the lower picture by the example of the cupping test. The blank sheet is fixed by the drawing ring and the blank holder. During the deformation, material continuously flows through the gap between the drawing ring and the blank holder, forming a cup. There are two important parameters controlling the deformation process. The ratio between punch and sheet together with the blank holder force. By varying these values, a range of validity can be achieved whether a desired deformation can be realized under specific conditions or not. During industrial application, very complex parts with a huge range of different local strains are produced. The introduced cupping test is an important tool to find possible design concepts and processing windows altogether with making material selections.

defect structure. Relating to the kinds and concentrations of vacancies and interstitials in a ceramic compound.

degradation. A term used to denote the deteriorative processes that occur with polymeric materials. These processes include swelling, dissolution, and chain scission.

degree of polymerization (DP). The average number of repeat units per polymer chain molecule.

Delayed Cracking. Material failure after an incubation time with or without external loading, caused by the interaction of stress or residual stress with hydrogen. Delayed Cracking is a kind of material failure frequently observed in Advanced High Strength Steels (AHSS) with tensile strength above 800 MPa. The AHSS are strengthened by multiphase structure or deformation mechanisms such as TRIP, TWIP, etc. Therefore, the deformed components obtain high residual stress and high density of lattice defects. The stress gradient drives hydrogen accumulation in the stress concentrated regions. Hydrogen also interacts with various lattice defects (hydrogen trapping) during the redistribution process (hydrogen diffusion). As a result, material failure occurs with time delay when the material strength is lowered by the local hydrogen concentration. Deep drawing test is a typical test to reveal the material susceptibility to delayed cracking. The successfully deformed specimens show delayed cracking after different incubation periods (varying from minutes to months), indicating their delayed cracking susceptibility.  High Manganese steels are known to be under certain conditions susceptible to delayed cracking. The underlying fracture mechanisms have been investigated on specimens after mechanical testing in combination with ab-initio calculation of hydrogen interactions with different lattice defects, deformation mechanisms and fracture analysis.

design stress (_d). Product of the calculated stress level (on the basis of estimated maximum load) and a design factor (which has a value greater than unity such as a stress-intensity factor). Used to protect against unanticipated failure.

diamagnetism. A weak form of induced or nonpermanent magnetism for which the magnetic susceptibility is negative.

die. An individual integrated circuit chip with a thickness on the order of 0.4 mm (0.015 in.) and with a square or rectangular geometry, each side measuring on the order of

6 mm (0.25 in.).

dielectric. Any material that is electrically insulating.

dielectric constant  The ratio of the permittivity of a medium to that of a vacuum. Often called the relative dielectric constant or relative permittivity.

dielectric displacement. The magnitude of charge per unit area of capacitor plate.

dielectric (breakdown) strength. The magnitude of an electric field necessary to cause significant current passage through a dielectric material.

diffraction (x-ray). Constructive interference of x-ray beams that are scattered by atoms of a crystal.

diffusion.  Diffusion means atoms moving and changing places, i.e. directed or undirected mass transport by atomic motion. Atomic diffusion in solids is far slower than in gases or liquids. Diffusion is essential to understand such different processes as creep, solute solution alloys, oxidation, nucleation, recrystallization, precipitation and phase transformation. 

diffusion coefficient. The constant of proportionality between the diffusion flux and the concentration gradient in Fick’s first law. Its magnitude is indicative of the rate of

atomic diffusion.

diffusion flux. The quantity of mass diffusing through and perpendicular to a unit cross-sectional area of material per unit time.

digital image correlation (DIC). is a digital image analysis method which is based on the recognition of kinematic changes in the gray scale or surface feature distribution of surface patterns before and after a series of subsequent straining steps. Both, the natural characteristics of an unprepared sample surface or an artificial (stochastic) color spray applied to a polished sample surface can be used as input and reference pattern. In order to measure the three dimensional surface coordinates digital stereo pair images of the sample are acquired from these patterns before and after straining using two high resolution CCD cameras.

diode. An electronic device that rectifies an electrical current—that is, allows current flow in one direction only.

dipole (electric). A pair of equal yet opposite electrical charges that are separated by a small distance.

dislocation. A linear crystalline defect around which there is atomic misalignment. Plastic deformation corresponds to the motion of dislocations in response to an applied

shear stress. Edge, screw, and mixed dislocations are possible.

dislocation density. The total dislocation length per unit volume of material; alternately, the number of dislocations that intersect a unit area of a random surface section.

dislocation line. The line that extends along the end of the extra half-plane of atoms for an edge dislocation, and along the center of the spiral of a screw dislocation. The dislocation line can be also seen as a line that separates the sheared area from the non-sheared area of a crystal. It is an interesting feature that the total dislocation line length in a heavily deformed crystal of a size of 1 cubic meter can amount to up to 1 light year.

dispersed phase. For composites and some two-phase alloys, the discontinuous phase that is surrounded by the matrix phase.

dispersion strengthening. A means of strengthening materials wherein very small particles (usually less than 0.1 um) of a hard yet inert phase are uniformly dispersed within a load-bearing matrix phase.

domain. A volume region of a ferromagnetic or ferrimagnetic material in which all atomic or ionic magnetic moments are aligned in the same direction.

donor state (level). For a semiconductor or insulator, an energy level lying within yet near the top of the energy band gap, and from which electrons may be excited into the conduction band. It is normally introduced by an impurity atom.

doping. The intentional alloying of semiconducting materials with controlled concentrations of donor or acceptor impurities.

drawing (metals). A forming technique used to fabricate metal wire and tubing. Deformation is accomplished by pulling the material through a die by means of a tensile force applied on the exit side.

drawing (polymers). A deformation technique wherein polymer fibers are strengthened by elongation.

driving force. The impetus behind a reaction, such as diffusion, grain growth, or a phase transformation. Usually attendant to the reaction is a reduction in some type of energy (e.g., free energy).

ductile fracture. A mode of fracture that is attended by substantial plastic deformation, dissipating additional energy prior to fracture.

ductile iron. A cast iron that is alloyed with silicon and a small concentration of magnesium and/or cerium and in which the free graphite exists in nodular form. Sometimes called nodular iron.

ductile-to-brittle transition. The transition from ductile to brittle behavior with a decrease in temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests.

ductility. A measure of a material’s ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation (%EL) or percent reduction in area (%RA) from a tensile test.