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