Materials Science Glossary - H

Hall effect. The phenomenon whereby a force is brought to bear on a moving electron or hole by a magnetic field that is applied perpendicular to the direction of motion. The force direction is perpendicular to both the magnetic field and the particle motion directions.

hardenability. A measure of the depth to which a specific ferrous alloy may be hardened by the formation of martensite upon quenching from a temperature above the upper critical temperature.

hard magnetic material. A ferromagnetic or ferromagnetic material that has large coercive field and remanence values, normally used in permanent magnet applications.

hardness. A qualitative measure of a material’s resistance to deformation by surface indentation or by abrasion.  It is worth noting that many different types of indentation tool designs exists which also deliver different hardness levels.  Furthermore, when hardness is being probed at very small scales, for instance at the nanoscale, and the hardness that is being measured becomes typically size dependent in any materials.  This effect is referred to as indentation size effect.

This shows that the term hardness of a material is a rather ill-defined quantity which has many meanings depending upon the material and method of probing. In general, hardness usually implies a resistance to deformation, and for metals the property is a measure of their resistance to permanent or plastic deformation. To a person concerned with the mechanics of materials testing, hardness is most likely to mean the resistance to indentation, and to the design engineer it often means an easily measured and specified quantity which indicates something about the strength and heat treatment of the metal.

There are three general types of hardness measurements depending on the manner in which the test is conducted, namely, scratch hardness, indentation hardness, and rebound, or dynamic, hardness.Mainly indentation hardness is of major engineering interest for metals. Scratch hardness is of primary interest to mineralogists. With this measure of hardness, various minerals and other materials are rated on their ability to scratch one another. Scratch hardness is measured according to the Mohs’ scale. This consists of 10 standard minerals arranged in the order of their ability lo be scratched. The softest mineral in this scale is talc (scratch hardness 1), while diamond has a hardness of 10. The Mohs’ scale is not well suited for metals since the intervals are not widely spaced in the high-hardness range. Most hard metals fall in the Mohs’ hardness range of 4 to 8.

In dynamic-hardness measurements the indenter is usually dropped onto the metal surface, and the hardness is expressed as the energy of impact. The Shore seleroscope, which is the commonest example of a dynamic-hardness tester, measures the hardness in terms of the height of rebound of the indenter.

heat capacity (Cp, Cv). The quantity of heat required to produce a unit temperature rise per mole of material.

hexagonal close-packed (HCP). A crystal structure found for some metals. The HCP unit cell is of hexagonal geometry and is generated by the stacking of close-packed planes of atoms.  One should note in this context that for pure metals with hexagonal crystal structure the ideal packing density which requires a ratio between the hexagonal length axis a and the basal axis a of 1.633 is not observed.  The chemical elements that are closest to this ideal stacking sequence are cobalt and magnesium.  Hence, strictly speaking no hexagonal close packed pure metal exists.

high polymer. A solid polymeric material having a molecular weightgreater than about 10,000 g/mol. 

high-strength, low-alloy (HSLA) steels. Relatively strong, low-carbon steels, with less than about 10 wt% total of alloying elements.

hole (electron). For semiconductors and insulators, a vacant electron state in the valence band that behaves as a positive charge carrier in an electric field.

homopolymer. A polymer having a chain structure in which all repeat units are of the same type.

hot working.  This term is of more of practical relevance and not so strictly defined: in many practical situations it would describe a metal-forming operation performed above a metal’s recovery or recrystallization temperature.  A key idea of warm deformation in this field lies in obtaining sufficient softening for enabling high thickness reductions.

hybrid composite. A composite that is fiber reinforced by two or more types of fibers (e.g., glass and carbon).

hydrogen bond. A strong secondary interatomic bond that exists between  a bound hydrogen atom (its unscreened proton) and the electrons of adjacent atoms.

hydrogen embrittlement. The loss or reduction of ductility of a metal alloy (often steel) as a result of the presence of atomic hydrogen in the material.  The effect of hydrogen embrittlement is often observed at high strength levels of the affected metal.  Different types of theories or discussed as underlying reasons regarding the cause of hydrogen embrittlement, such as for instance the hydrogen enhanced decohesion and hydrogen enhanced local plasticity.  The problem of hydrogen embrittlement is an essential obstacle for the wider use of high strength metallic alloys in the transportation industry as well and the oil and gas industries.

hydroplastic forming. The molding or shaping of clay-based ceramics that have been made plastic and pliable by adding water.

hypereutectoid alloy. For an alloy system displaying a eutectoid, an alloy for which the concentration of solute is greater than the eutectoid composition.

hypoeutectoid alloy. For an alloy system displaying a eutectoid, an alloy for which the concentration of solute is less than the eutectoid composition.

hysteresis (magnetic). The irreversible magnetic flux densityversus- magnetic field strength (B-versus-H) behavior found for

ferromagnetic and ferromagnetic materials; a closed B–H loop is formed upon field reversal.