Elif Temiz, 030070195, 11th Week Definitions-bonus Words-Part 2

2-Dispersion Strengthening

 New Definition (Material)(Better)

Most engineered materials are composed of more than one phase, and many of these materials are designed to provide improved strength. In simple dispersion-strengthened alloys, small particles of one phase, usually very strong and hard, are introduced into a second phase, which is weaker but more ductile. The soft phase, usually continuous and present in larger amounts, is called the matrix. The hard-strengthening phase may be called the dispersed phase or the precipitate, depending on how the alloy is formed. In some cases, a phase or a mixture of phases may have a very characteristic appearance—in these cases, this phase or phase mixture may be called a microconstituent. For dispersion strengthening to occur, the dispersed phase or precipitate must be small enough to provide effective obstacles to dislocation movement, thus providing the strengthening mechanism.

In most alloys, dispersion strengthening is produced by phase transformationsThis will be called a eutectic reaction and is of particular importance in cast irons and many aluminium alloys. When increased strength and toughness are the goals of incorporating a dispersed phase, the guidelines below should be followed (Figure I I-I). 

1.The matrix should be soft and ductile, while the dispersed phase should be hard and strong. The dispersed phase particles interfere with slip, while the matrix provides at least some, ductility to the overall alloy.

2. The hard dispersed phase should be discontinuous, while the soft, ductile matrix should be continuous. If the hard and brittle dispersed phase were continuous, cracks could propagate through the entire structure.

3. The dispersed phase particles should be small a rid numerous, increasing the likelihood that they interfere with the slip process since the area of the interphase interface is increased significantly.

4. The dispersed phase particles should be round, rather than needle-like or sharp edged, because the rounded shape is less likely to initiate a crack or to act as a notch.

5. Higher concentrations of the dispersed phase increase the strength of the alloy.

(Donald R. Askeland,Pradeep P. Fulay,Wendelin J. Wright, The Science and Engineering of Materials, p.414)

Previous Definition
In the case of dispersion strengthening (hardening), the fine strengthening particles are a discontinuous second phase without atomic continuity with the matrix. The behavior of such particles is shown schematically in Figure 12.1.7a as a function of increasing aging time or aging temperature (fixed time) which result in larger, more widely spaced dispersed-phase particles. Under stress, dislocations must move around (bypass) such particles, so that yield strength decreases with increased aging. Long aging imes may be used to decrease yield strength (“soften”) of the metal for fabrication. A short aging time, would be used for maximum strength. The dispersed phase can also provide some enhancement of ductility. A dispersion-strengthened metal for which the dispersed phase is stable at elevated temperatures can provide both high-temperature strength and creep resistance (subsection on high-temperature effects).
Surface diffusion treatments usually produce dispersion hardening.

(Frank Kreith, CRC Press Mechanical Engineering Handbook 1999, sec.12 pg.10)

3-Graphitization

 New Definition (Material) (Better)

The transformation of amorphous carbon into artificial (synthetic) graphite by heat treatment is known as the "graphitization process." Graphitization can be described in a series of steps that occur as the temperature is raised from approximately 1000 up to about 3000°C. A typical graphitization process involves the graphitization heat treatment of a composite material composed of filler carbon held in a pitch-based binder matrix.

 As the filler carbon's calcining temperature (1000 to 1300°C) is surpassed, the incipient graphitic structure slowly develops. The binder evolves hydrogen, sulfur, and other heteroatoms in the range of 1500 to 2000°C and undergoes a volume expansion (0.2 to 0.6 %). As 1800°C is surpassed, the graphitic structure grows more rapidly and grain size increases as temperatures exceeds 2200°C. At approximately 2600°C, a volume contraction takes place and crystallite growth predominates.

 One can follow the development of crystallites by X-ray diffraction analysis. Crystallite size increases from 50 to about 1000 A, while interlayer spacing decreases from 3.44 A, typical of amorphous carbon, to 3.35 A, typical for graphitic structure.

Graphitization involves a displacement and rearrangement of layer planes and small groups of planes to achieve a three-dimensional ordering. Movements of individual atoms or single carbon rings to fill vacancies or improve perfection in existing crystallites (annealing) may supplement the growth of such planes. The probability of rearrangement is related to the existing degree of disorientation and the degree of carbon-to-carbon bonding between layer planes (cross-linking). A high degree of inter-planar cross-linking is known to inhibit graphitization.

In addition to the structure of the precursor, maximum temperature and residence time at temperature are important factors in the achievement of desirable properties. The presence of oxygen and carbon dioxide enhances the beneficial effects of graphitization. The crystallites interlayer spacing reaches a limiting value at each temperature, even after long residence periods. When the temperature is increased, the rate approach to the limiting value is also increased. Where graphitization temperatures are higher than 2500°C and the times are longer than 12 h, the effects of kinetics are minimal.

Mechanical properties follow a similar behaviour with respect to the ultimate graphitization temperature. As temperatures are increased from 2000 to 2500/2600°C, the tensile strength remains constant or increases slightly to a maximum. From 2600 to 2900°C, a decrease in tensile strength is observed, followed by a flat region from 3000 to 3200°C. Electrical resistivity typically decreases as calcining temperature is raised to about 1000°C and then may stay constant or increase as temperatures are increased. Not all physical properties achieve optimum values at any one temperature.

At ordinary temperatures, carbon is one of the most inert of the elements, but at high temperatures, it becomes one of the most reactive. Mantell describes the reactions with the various compounds and chemical elements. Mantell also gives information on other properties: contact resistance, creep, crystal structure, elasticity, electrical conductivity, electrochemical equivalent, entropy, hardness, heat capacity, heat of combustion, heat of vaporization, magnetic susceptibility, melting and boiling points, and radiation constants.

(Salvatore J. Rand, Significance of Tests for Petroleum Products,p.99)

Previous definition 

Graphitization is a solid-state transformation of thermodynamically unstable non-graphitic carbon into graphite by thermal treatment. The degree of graphitization is a measure of the extent of long-range 3D crystallographic order, as determined by diffraction studies alone. The degree of graphitization significantly affects many properties, such as thermal conductivity, electrical conductivity, strength, and stiffness.
 A common, but incorrect, use of the term graphitization is to indicate a process of thermal treatment of carbon materials above 2200 C regardless of any resultant crystallinity. The use of the term graphitization without reporting confirmation of long-range 3D crystallographic order determined by diffraction, studies should be avoided, as it can be misleading.

 (Scheffler M., Colombo P., Cellular Ceramics: Structure, Manufacturing, Properties and Applications, p.139)