| Titanium has a relatively low density, just over half | | | | alpha. Such alloys are achieved by adding small |
| that of steel. It has a relatively low strength when | | | | amounts, about 1 to 2%, or beta-stabilising elements |
| pure, but alloying gives a considerable increase in | | | | such as molybdenum and vanadium to what is |
| strength. Because of the low density of titanium, its | | | | otherwise an alpha-stabilised alloy. An example of |
| alloys have a high strength to weight ratio. It has a | | | | such an alloy is 90% titanium. 8% aluminium. 1% |
| high melting point (1660°C) and excellent | | | | molybdenum and 1% vanadium. This alloy is normally |
| corrosion resistance. However, titanium is an | | | | used in the annealed condition. There are two forms |
| expensive metal, its high cost reflecting the difficulties | | | | of annealing; mill annealing and duplex annealing. Mill |
| experienced in the extraction and formation of the | | | | annealing involves heating the alloy to 790°C, |
| material; the ores are quite plentiful | | | | soaking for eight hours and then furnace cooling. |
| Titanium can exist in two crystal forms, alpha, which | | | | Duplex annealing involves mill annealing followed by |
| is a hexagonal close-packed structure and beta, which | | | | reheating to 790°C, soaking for quarter of an |
| is a body-centred cubic. In pure titanium, the alpha | | | | hour and then air cooling. The result of such annealing |
| structure is the stable phase up to 883°C and is | | | | is beta particles dispersed throughout an alpha matrix. |
| transformed into the beta form above this | | | | Titanium alloy in the annealed state is used for |
| temperature. This beta form then remains stable up | | | | airframe and jet engine parts which require high |
| to the melting point. | | | | strengths, good creep resistance and toughness up |
| Commercially pure titanium ranges in purity from 99 | | | | to temperatures of about 850°C. The alloy has |
| to 99.5%, the main impurities being iron, carbon, | | | | good weldability. |
| oxygen. nitrogen and hydrogen. Such material is lower | | | | Alpha-beta-titanium alloys |
| in strength than titanium alloys but more corrosion | | | | These contain sufficient quantities of beta-stabilising |
| resistant. The properties of the commercially pure | | | | elements for there to be appreciable amounts of |
| titanium are largely determined by the oxygen | | | | beta phase at room temperature. An example of |
| content. Because of its excellent corrosion resistance, | | | | such an alloy is 90% titanium-6% aluminium-4% |
| commercially pure titanium is used for aircraft engine | | | | vanadium. The aluminium stabilises the alpha phase |
| parts. | | | | while the vanadium stabilises the beta phase. These |
| Titanium alloys can be grouped into categories | | | | alloys can be solution treated, quenched and aged for |
| according to the phases present in their structure. | | | | increased strength. The microstructure of the alloys |
| The addition of elements such as aluminium, tin, | | | | depends on their composition and heat treatment. |
| oxygen or nitrogen results in the enlargement of the | | | | Thus, a fast cooling rate from a temperature where |
| alpha phase, such elements being referred to as | | | | the material was all, beta, e.g. quenching in cold |
| alpha-stabilising elements. The alpha phase exists to | | | | water, produces a martensitic structure with some |
| much higher temperatures. Other elements, such as | | | | increase in hardness. Ageing can then produce some |
| vanadium, molybdenum, silicon and copper, enlarge | | | | further increase in strength as a result of beta |
| the beta phase region and are termed beta-stabilising | | | | precipitates. |
| elements. Increasing the amounts of beta stabiliser | | | | Beta-titanium alloys |
| means that beta phase can exist at room | | | | When sufficiently high amounts of beta-stabilising |
| temperature. Other elements added to titanium | | | | elements are added to titanium, the resulting |
| alloys, e.g. zirconium, can contribute solid solution | | | | structure can be made entirely beta at room |
| strengthening. | | | | temperature after quenching, in some cases by air |
| Alpha-titanium alloys | | | | cooling. Unlike alpha-titanium alloys. beta-titanium alloys |
| These are composed entirely of alpha phase. An | | | | are readily cold worked in the solution treated and |
| example of such an alloy is 92.5% titanium-5% | | | | quenched condition, and can be subsequently aged to |
| aluminium-2.5% tin. Both the aluminium and tin are | | | | give very high strengths. In the high-strength |
| alpha stabilisers. Such alloys have the hexagonal | | | | condition the alloys have low ductilities. They can also |
| close-packed structure and, as a consequence, are | | | | suffer from poor fatigue performance. The alloys are |
| strong, maintain their strength at high temperatures | | | | thus not so widely used as the alpha-beta alloys. |
| but are difficult to work. This type of titanium alloys | | | | A typical beta-titanium alloy has 77% titanium-13% |
| have good weldability and are used where high | | | | vanadium-11% chromium-3% aluminium. The alloy is |
| temperature strength is required, e.g. turbine blades. | | | | usually used in the solution treated, quenched and |
| Near alpha-titanium alloys | | | | aged condition in order to obtain the very high tensile |
| These are composed of almost all alpha phase with a | | | | strength. It is used for aerospace components, |
| small amount of beta phase dispersed throughout the | | | | honeycomb panels and high strength fasteners. |