Titanium and titanium-based alloys are used in a variety of applications due to the relatively high strength, low density, and good corrosion resistance of these materials. For example, titanium and titanium-based alloys are used extensively in the aerospace industry because of the materials' high strength-to-weight ratio and corrosion resistance. One groups of titanium alloys known to be widely used in a variety of applications are the alpha/beta (α+β) Ti-6Al-4V alloys, comprising a nominal composition of 6 percent aluminum, 4 percent vanadium, less than 0.20 percent oxygen, and titanium, by weight.
Ti-6Al-4V alloys are one of the most common titanium-based manufactured materials, estimated to account for over 50% of the total titanium-based materials market. Ti-6Al-4V alloys are used in a number of applications that benefit from the alloys' combination of high strength at low to moderate temperatures, light weight, and corrosion resistance. For example, Ti-6Al-4V alloys are used to produce aircraft engine components, aircraft structural components, fasteners, high-performance automotive components, components for medical devices, sports equipment, components for marine applications, and components for chemical processing equipment.
Ti-6Al-4V alloy mill products are generally used in either a mill annealed condition or in a solution treated and aged (STA) condition. Relatively lower strength Ti-6Al-4V alloy mill products may be provided in a mill-annealed condition. As used herein, the “mill-annealed condition” refers to the condition of a titanium alloy after a “mill-annealing” heat treatment in which a workpiece is annealed at an elevated temperature (e.g., 1200-1500° F./649-816° C.) for about 1-8 hours and cooled in still air. A mill-annealing heat treatment is performed after a workpiece is hot worked in the α+β phase field. Ti-6Al-4V alloys in a mill-annealed condition have a minimum specified ultimate tensile strength of 130 ksi (896 MPa) and a minimum specified yield strength of 120 ksi (827 MPa), at room temperature. See, for example, Aerospace Material Specifications (AMS) 4928 and 6931A, which are incorporated by reference herein.
To increase the strength of Ti-6Al-4V alloys, the materials are generally subjected to an STA heat treatment. STA heat treatments are generally performed after a workpiece is hot worked in the α+β phase field. STA refers to heat treating a workpiece at an elevated temperature below the β-transus temperature (e.g., 1725-1775° F./940-968° C.) for a relatively brief time-at-temperature (e.g., about 1 hour) and then rapidly quenching the workpiece with water or an equivalent medium. The quenched workpiece is aged at an elevated temperature (e.g., 900-1200° F./482-649° C.) for about 4-8 hours and cooled in still air. Ti-6Al-4V alloys in an STA condition have a minimum specified ultimate tensile strength of 150-165 ksi (1034-1138 MPa) and a minimum specified yield strength of 140-155 ksi (965-1069 MPa), at room temperature, depending on the diameter or thickness dimension of the STA-processed article. See, for example, AMS 4965 and AMS 6930A, which is incorporated by reference herein.
However, there are a number of limitations in using STA heat treatments to achieve high strength in Ti-6Al-4V alloys. For example, inherent physical properties of the material and the requirement for rapid quenching during STA processing limit the article sizes and dimensions that can achieve high strength, and may exhibit relatively large thermal stresses, internal stresses, warping, and dimensional distortion. This disclosure is directed to methods for processing certain α+β titanium alloys to provide mechanical properties that are comparable or superior to the properties of Ti-6Al-4V alloys in an STA condition, but that do not suffer from the limitations of STA processing.
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