Sezgin Koçak-030070026-8th Week

Stress Corrosion Cracking:

Stress corrosion is an interaction of sustained tension stress and corrosive attack causing cracking that can result in premature brittle failure of a ductile material. Before stress corrosion can occur, there must be the proper combination of a chemical environment and metallurgical condition of the material. Failures of this type have been identified with certain alloy systems of practically all menas, and in most instances the failures have been shown to be associated with electrochemical activity. In aluminum alloys, stress corrosion can be promoted by a film of moisture on the surface of metal exposed to the atmosphere or by contaminants such as chlorides. Only a prolonged surface tension stress will cause SCC, and sustained compression stresses actually prevent it. The tensile stress required to cause SCC are small, usually below the macroscopic yield stress. These stresses can be externally applied, but rsidual stresses often cause SCC failures. Susceptically to SCC limits the static strength of a material in certain applications just as fatigue cracking does in others. Both types of failure are caused by tension stress, but there the similarity ends. While fatigue failures occur under dynamic loads, stress-corrosion failures occur under dynamic loads, stress-corrosion failures are caused only by sustained static tension loads. An important difference with aluminum alloys is that stress-corrosion cracks are predominantly intergranular, most fatigue cracks are transgranular. It should be noted,however, that transgranular SCC has been observed for a few alloys under highly specific environmental conditions.

(Joseph R. Davis, Corrosion of aluminum and aluminum alloys, p.99)

Chemical Vapor Deposition:

Chemical vapor deposition (CVD) is a widely used materials- processing technology. The majority of its applications involve applying solid thin-film coating to surfaces, but it is also used to produce high-purity bulk materials and powders, as well as fabrikating composite materials via infiltration techniques. It has been used to deposit a very wide range of materials. The majority of the elements in the periodic table have been deposited by CVD techniques, some in the form of the pure elements, but more often combined to form compounds.

(Jong-Hee Park,T. S. Sudarshan, Chemical vapor deposition, p. 1)