Materials that can have more than one crystal structure are called allotropic or polymorphic. The term allotropy is normally reserved for this behavior in pure elements, while the term polymorphism is used for compounds. Some materials such as iron and titanium, have more than one crystal structure. At low temperatures, iron has BCC structure, but at higher temperatures, iron transforms to an FCC structure. These transformations result in changes in properties of materials and form the basis for the heat treatment of steels and many other alloys.
(Askeland D.R., Phulé P.P., The Science and Engineering of Materials, pg.62, Kayra Ermutlu)
Die Geometry Angle
The die geometry directly influences material flow. and thereforei it affects the distribution of the effective strain and flow stress in the deformation zone. In forward extrusion, for a given reduction, a larger die angle increases the volume of metal undergoing shear deformation and results in an increase in shear deformation load Pds. On the other hand, the length of the die decreases, which results in a decrease in die friction load, Pdf. Consequently, for a given reduction and given friction conditions, there is an optimum die angle that minimizes the extrusion load.
(Altan T., Ngaile G., Shen G., Cold and Hot Forging:Fundamentals and Applications- Part1, pg.219, Kayra Ermutlu)
Microshrinkage
A casting imperfection, not detectable microscopically, consisting of interdendritic voids. Microshrinkage results from contracting during solidification where the opportunity to supply filler material is inadequate to compansate for shrinkage. Alloys with wide ranges in solidification temperature are particularly susceptible.
(American Society for Metals, ASM Metals Reference Book, pg.59, Kayra Ermutlu)
Acetate (Cellulose)
An amber-colored, transparent material made by the reaction of cellulose with acetic acid or acetic anhydride in the presence of sulfuric acid. In Germany it was made by treating beech-wood pulp with acetic acid in the presence of an excess of zinc chloride. It is employed for lacquers and coatings, molding plastics, rayon, and photographic film. Cellulose acetate may be the triacetate C6H7O2, but may be the tetracetate or the pentacetate, or mixture. It is made in different degrees of acetylation with varying properties.
Unlike nitrocellulose, it is not flammable, and it has better light and heat stability. It has a refractive index of 1.47 to 1.50, and a sheet 0.125 in (0.32 cm) thick will transmit 90% of the light. The specific gravity is 1.27 to 1.37, Brinell hardness 8 to 15, tensile strength 3500 to 8000 lb/in^2 (24 to 55 MPa), compressive strength up to 20000 lb/in^2 (138 MPa), elongation 15 to 80%, dielectric strength 300 to 600 V/mil (12*10^6 to 24*10^6 V/m), and softening point 122 to 205F (50 to 96C). It is thermoplastic and is easily molded. The molded parts or sheets are tough, easily machined, and resistent to oils and many chemicals. In coatings and lacquers, the material is adhesive, tough and resilient, and it does not discolor easily.
(Brady G.S., Clauser H.R., Vaccari J.A., Materials Handbook,15th Ed., pg. 204, Kayra Ermutlu)
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