Grade 5 (commonly designated as Ti-6Al-4V in the industry) is a type of wrought titanium alloy that falls under the category of alpha-beta (α+β) titanium alloys based on its phase composition and microstructure. It is one of the most widely used titanium alloys globally, with its designation defined by standards such as ASTM B348 (for titanium and titanium alloy bars, billets, and forgings) and ASTM B265 (for titanium and titanium alloy sheet, strip, and plate).
Beyond its primary classification as an α+β wrought alloy, Grade 5 is also classified as a heat-treatable titanium alloy. Its mechanical properties can be tailored through heat treatment processes (e.g., solution annealing followed by aging), which allows it to achieve a balance of strength, ductility, and toughness suitable for diverse industrial applications, including aerospace, medical devices, marine engineering, and automotive components.
Grade 5 is explicitly an α+β type titanium alloy, not an α-type or β-type alloy. The reason for this classification lies in its chemical composition and the phases that form at different temperatures:
Chemical basis: Grade 5 contains approximately 6% aluminum (Al) and 4% vanadium (V) as key alloying elements. Aluminum is a strong α-stabilizer (it promotes the formation of the hexagonal close-packed α-phase and raises the α/β transus temperature), while vanadium is a β-stabilizer (it stabilizes the body-centered cubic β-phase and lowers the α/β transus temperature, typically around 995°C for Ti-6Al-4V).
Microstructural behavior:
Below the α/β transus temperature, the alloy's microstructure consists of a mixture of primary α-phase grains and transformed β-phase (which forms secondary α laths or plates within the prior β grains), giving it the α+β designation.
When heated above the transus temperature, it transforms into a fully β-phase microstructure, but this high-temperature state is not the equilibrium phase at room temperature or typical service temperatures.
In contrast, α-type titanium alloys (e.g., Grade 2, Grade 3) contain only α-stabilizers (or neutral elements) and have a single α-phase microstructure at all service temperatures, with no β-phase present. β-type titanium alloys (e.g., Ti-10V-2Fe-3Al) are dominated by β-stabilizers and retain a fully β-phase structure at room temperature, offering higher formability but lower creep resistance compared to α+β alloys like Grade 5.
