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Pure titanium is a silver-colored metal, which has grown as a metal of strategic importance in the last 50 years. Its density is approximately 55% of steel with similar strength. Titanium is a durable, biocompatible metal, which is commonly used in artificial joints, dental implants, and surgical equipment. It also finds use in aerospace and some automotive applications because its operating temperature limit is nearly 500°C. For aerospace structures manufacturing it is supplied in any one of the following forms: titanium billet, titanium bar, titanium plate, titanium sheet, titanium tubes, titanium extrusions or titanium forgings.
Apart from its use as a structural metal it is also added in small quantities to steels and other alloys to increase hardness and strength by the formation of carbides and oxides. Titanium can exist in two allotropic forms: alpha and beta. Its mechanical properties are closely related to these allotropic phases, with the beta phase being much stronger but more brittle than the alpha phase. Hence, titanium alloys are commonly classified as alpha, beta, and alpha-beta alloys. Titanium alloys have attractive engineering properties, which include a desirable combination of moderate weight and high strength, property retention at elevated temperatures, and good corrosion-resistance. These properties provide high values of specific strength, Sylp, which is desirable for transportation systems.
Apart from the commercially pure forms of titanium, there are three principal types of alloys: alpha, beta, and alpha-beta alloys which are available in wrought and cast forms. In recent years some also have become available in powder forms. The system of designations for titanium alloys vary from one standard to another; however, the most prevalent and commonly used system is to name the alloy by its composition. For instance, Ti-4A1-3V, which means its major alloying elements are 4% aluminum and 3% vanadium. There are five grades of what are known as commercially pure or unalloyed titanium, ASTM Grades 1 through 4, and 7. Each grade has a different impurity content, with Grade 1 being the most pure. Tensile strengths vary from 172 GPa for Grade 1 to 483 GPa for Grade 4.
Titanium carbide is an important product of titanium and is made by reacting titanium dioxide and carbon black at temperatures above 1800°C. It is compacted with cobalt or nickel for use in cutting tools and for heat-resistant parts and it is lighter weight and less costly than tungsten carbide, but it is more brittle in cutting tools.
One of the primary uses of titanium is as titanium oxide in the form of a white pigment. It is also widely used as titanium carbide for hard facings and for cutting tools. Primarily because of their high strength-to-weight ratio (specific strength), titanium and its alloys are widely used for aircraft structures requiring greater heat-resistance than aluminum alloys. Owing to their exceptional corrosion-resistance they are also used for chemical processing, desalination, power generation equipment, marine hardware, valve and pump parts, and prosthetic devices. Alloy Ti-6A1-4V is widely used in medical applications supplied to manufacturers as titanium billet, titanium bar, titanium plate, titanium sheet, titanium tubes, titanium extrusions or titanium forgings.
Titanium is also found in a shape memory alloy (SMA) material called Nitinol, which is a titanium alloyed with nickel that exhibits superelastic behavior. It is a corrosion resistant, biocompatible material that has a shape memory property, making it useful for implantable devices requiring an initial shape for insertion and a final shape once in place. The properties of Nitinol rely on its dynamic crystalline structure, which is sensitive to external stress and temperature. The alloy has three defined temperature phases that influence its behavior:
Austenite Phase: This temperature is above the transition temperature and varies depending upon the exact composition of the Nitinol alloy; commercial alloys usually have transitional temperatures between 70 and 130°C (158 to 266°F). The yield strength with which the material tries to return to its original shape is considerable; 35,000 to 70,000 psi. Crystalline structure is cubic. Martensitic Phase: In this low-temperature phase the crystal structure is needle-like, with the crystals aligned. The alloy may be bent or formed easily using a deformation pressure of 10,000 to 20,000 psi. Bending transforms the crystalline structure of the alloy by producing internal stresses.
Annealing Phase: In this high-temperature phase the alloy will reorient its (cubic) crystalline structure to "remember" its present shape. The annealing phase for Nitinol wire is in the range of 540°C.
When at room temperature Nitinol is in the martensitic phase and can be deformed as required. When the new shape is heated above its transitional temperature (austenite phase), the crystalline structure changes from needle-like to cubic. The resulting cubic structure does not fit into the same space as the needle-like structures formed when the alloy was bent.
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