Mechanical and electrochemical behavior of Ti-6Al-4V alloys

Abstract

Titanium (Ti) alloys find wide applications in civilian and defense industries owing to their high specific strength (strength/density), high corrosion resistance, ability to withstand moderately high temperatures without creeping, good fracture toughness, and superior fatigue resistance. It is forecasted that the global Ti alloys market may reach US$5.9 billion by 2027 with a compound annual growth rate of 3.7% during 2022-2027. Having a density of 4.5 g/cc, Ti is known to be the heaviest of lightweight metals and its alloys have a high specific strength than most structural steels and super alloys. Pure Ti exists in two allotropic forms: the Hexagonal close-packed (HCP) phase which is also known as 𝛼 phase exists below 882°C and the Body-centered cubic (BCC) phase or the 𝛽 phase exists above 882°C. Both these phases exhibit unique properties: for example, 𝛼-Ti has high tensile strength, creep resistance, and fatigue strength, while the 𝛽-Ti has good ductility and weldability. Alloying pure Ti with certain alloying elements results in a change in 𝛽 transus temperature. Those alloying elements which increase the 𝛽 transus temperature or promote the stability of 𝛼 phase at higher temperatures are known as 𝛼 stabilizers and some of them are Al, O, N, and C. On the other hand, alloying elements such as V, Mo, Fe, Cu, etc., reduce the 𝛽 transus temperature and stabilize the 𝛽 phase at room temperatures are known as 𝛽 stabilizers. The nature of the alloying elements used determines the presence of dominating phases in a given Ti alloy and based on them the Ti alloys are classified as 𝛼, 𝛼 + 𝛽, and 𝛽 alloys. Each of these alloy types has certain advantages and disadvantages. Of all the alloys, the unique advantage of 𝛼 + 𝛽 alloys is the presence of cumulative properties of both phases. One such 𝛼 + 𝛽 alloy is Ti-6Al-4V having 6 wt. % Al and 4 wt. % V (also known as Ti64 alloy). This alloy has gained maximum popularity in terms of application and is therefore known to be the workhorse alloy as more than 60% of the Ti goes into making this alloy. The widespread use of Ti64 alloy is found in aerospace applications where it is used in jet engine components like external fan blades, fan casing, compressor blades, and the disc of gas turbine engines. In airframe components, it is used in tubing, plates, sheets, fasteners, brakes, landing gear, etc. Other than aerospace applications Ti64 is also used in other critical applications like lightweight armor (because of its high specific strength and toughness), bioimplants, and sea valves of submarines (because of its corrosion resistance).