Titanium (Ti) is an allotropic metal; it can exist in two different crystallographic forms. At room temperature, it has a close-packed hexagonal structure (the alpha phase). At ~884oC, the alpha phase transforms to a body-centered cubic structure (the beta phase), which is stable up to titanium’s melting point (~1677oC). Titanium alloys can therefore be classified into three groups: the alpha, beta, and alpha-beta alloys. Titanium's properties are closely related to these phases. For example, the beta phase is stronger-- but more brittle-- than the alpha phase. Alloying elements promote formation of one phase or the other. Alluminum (Al) stabilizes the alpha phase (it raises the alpha to the beta transformation temperature), while Vanadium (V) and other beta stabilizers lower the transformation temperature, allowing the beta phase to remain stable even at room temperature. Titanium has excellent strength properties. It's approximately 40% lighter than steel (60% heavier than aluminum), and it has the highest strength-to-weight ratio of any structural metal-- ~30% more than aluminum or steel. This exceptional strength-to-weight ratio is maintained from -216oC (-420oF) up to 538oC (1000oF). Titanium is also highly resistant to both atmospheric and seawater corrosion, as well as to a wide range of chemicals including chlorine (Cl) and chloride-containing organic materials. The excellent corrosion resistance of titanium alloys results from the formation of stable, continuous, highly adherent and protective titanium oxide (TiO) films on metal surfaces. Because titanium is highly reactive and has a high affinity for oxygen, these beneficial surface oxide films form spontaneously and instantly when fresh metal surfaces are exposed to air and/or moisture. In fact, a damaged oxide film can generally reheal itself instantaneously if at least traces of oxygen or water are present in the environment. However, anhydrous conditions in the absence of oxygen may result in titanium corrosion, because the protective film may not be regenerated if damaged. The nature, composition, and thickness of these protective TiO films depend on environmental conditions. In most aqueous environments, the oxide is TiO2, but may include Ti2O3 and TiO. High-temperature oxidation tends to generate the crystalline form of TiO (known as rutile), whereas lower temperatures often generate the more amorphous form (anatase), or a mixture of the two. Although these naturally-formed oxide films are thin and invisible to the eye, titanium oxide is highly chemically resistant and is attacked by very few substances, including hot, concentrated HCl, H2SO4, NaOH, and HF. This thin surface oxide is also a highly effective barrier to hydrogen. Finally, titanium has a higher melting point than iron, has low thermal conductivity, low coefficient of expansion, and high electrical resistivity. The titanium alpha-beta alloys are the alloys most widely used. These alloys are a two-phase combination of alpha and beta alloys; their characteristics fall in a range between the two single-phase alloys. They can be heat-treated, are useful up to 427oC (800oF), and are easier to form (but more difficult to weld) than alpha alloys. Titanium alloy 6Al-4V is by far the most popular titanium alloy; over 70% of all titanium alloys made are a sub-grade of Ti6Al4V. This alpha-beta material (titanium alloyed with 6% aluminum, 4% vanadium, 0.25% (max) iron, and 0.2% (max) oxygen) has an ultimate tensile strength of 950 MPa (138,000 PSI), has good impact and fatigue strength, and can be welded. It is sometimes known as "Titanium Grade 5". The addition of 0.05% palladium, (grade 24), 0.1% ruthenium (grade 29) and 0.05% palladium and 0.5% nickel (grade 25) significantly increase corrosion resistance in reducing acid, chloride and sour environments, raising the threshold temperature for attack to well over 200°C (392°F). Titanium Grade 2 is considered unalloyed (sometimes called "commercially pure titanium") and has similar mechanical properties to Grades 1, 3, 4, 7, 11, and 12. This is the most commonly used and widely available grade of unalloyed titanium. Grade 2 combines excellent corrosion resistance and weldability with good strength, ductility and formability. It is 99.2% pure, and alloyed with just trace amounts of carbon (C, 0.1% max), iron (Fe, 0.3% max), hydrogen (H, 0.015% max), nitrogen (N, 0.03% max) and oxygen (O, 0.25% max). Titanium Grade 2, like the other commercially pure titanium grades, can be considered an alpha-phase alloy-- not nearly as strong as the more highly alloyed types. It has an ultimate tensile strength of 344 MPa (49,900 PSI).