Titanium exists in two crystal forms. At room temperature, unalloyed (commercially pure) titanium has a hexagonal close-packed (hcp) crystal structure called the α (α) phase. When the temperature of pure titanium reaches 885°C (called the β transition temperature of titanium), the crystal structure changes to a bcc structure called β (β) phase. Alloying elements will increase or decrease the temperature of α to β transformation, so the alloying elements in titanium are classified as α stabilizers or β stabilizers. For example, vanadium, niobium, and molybdenum will lower the α to β transition temperature and promote the formation of β phase.
Alpha alloy. Alpha alloys contain elements such as aluminum and tin, and are more suitable for high temperature applications due to their excellent creep characteristics. These α-stabilizing elements work by suppressing the change of the phase transition temperature or increasing it. Beta alloys do not have a characteristic of toughness to brittleness transition, which makes alpha alloys suitable for low temperature applications.
On the other hand, they cannot be strengthened by heat treatment because α is a stable phase, so their strength is not as good as β alloys.
Beta alloy. Beta alloys contain transition elements such as vanadium, niobium, and molybdenum, which tend to lower the temperature of the alpha to beta phase transition. Beta alloy has excellent hardenability and is easy to heat treatment. These materials are highly malleable and have high fracture toughness. For example, the ultimate tensile strength of the high-strength titanium alloy TI-10V-2Fe-3Al is about 1200MPa.
Alpha + Beta alloy. The composition of the α + β alloy supports a mixture of α and β phases, and may contain 10% to 50% β phase at room temperature. The most common α+β alloy is Ti-6Al-4V. The strength of these alloys can be improved and controlled by heat treatment. Examples include: Ti-6Al-4V, Ti-6Al-4V-ELI, Ti-6Al-6V-2Sn, Ti-6Al-7Nb.
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