Patent Application
20130014865
1. Field of the Invention
The present invention relates to a method of improving mechanical properties of beta titanium alloys, and more specifically, a method of increasing strength and stiffness of Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy without debit in ductility.
2. Description of the Background Art
Beta titanium alloys offer improved performance via higher specific strength (strength normalized with density) which enables weight reduction. These alloys find applications in the aerospace industry, e.g., for the structure, landing gear assemblies, and helicopter rotor systems.1 In these applications, titanium alloys replace steels such as high strength low alloy steel and 4340M steel, providing weight savings along with reduced maintenance due to superior corrosion resistance. The alloy Ti-5Al-5Mo-5V-3Cr 1[Ref. 1]: R. R. Boyer and R. D. Briggs. The Use of Beta Titanium Alloys in the Aerospace Industry, Journal of Materials, Engineering and Performance, Volume 14(6), 2005, pp. 681-685.](Ti-5553) (all compositions expressed in weight percent) has recently gained an increasing interest as an alternative to the more established alloy Ti-10V-2Fe-3Cr. Ti-5553 alloy offers improved processibility, ability to heat treat in section sizes up to 6 inches and more favorable combination of strength-ductility-toughness. Typical target properties of Ti-5553 in the heat treated condition are ultimate tensile strength of 180 ksi, tensile elongation of 5%, and tensile elastic modulus of 16.2 Msi. Improvements in strength and stiffness of beta titanium alloys would offer improved performance and provide further weight reduction benefit.
There is a need, therefore, for a new and improved method of increasing the mechanical properties of beta titanium alloys without debits in tensile elongation. The method of present invention meets this need.
In accordance with the new and improved method of present invention, titanium boride (TiB) precipitates are incorporated into a beta titanium alloy such as Ti-5553, the alloy is then subjected to process steps of homogenization, hot work, and final heat treatment to achieve improvements in mechanical properties compared to the baseline alloy. The boron is introduced into the titanium alloy composition to produce TiB precipitates by a suitable method, such as a pre-alloyed powder metallurgy technique. As an illustrative example, the method of the present invention may be used to increase mechanical properties of Ti-5553 alloy produced via a gas atomized pre-alloyed powder approach.
A new and improved method of increasing mechanical properties of multi-component beta titanium alloys such as Ti-5553 is described hereinafter.
The method described in this disclosure encompasses four critical elements:
Introduction of boron into the titanium alloy composition to produce TiB precipitates can be accomplished by several different methods, such as casting, cast-and-wrought processing, powder metallurgy techniques such as gas atomization and blended elemental approach. Homogenization heat treatment above the beta transus temperature (temperature at which alpha to beta phase transformation is complete) produces equilibrium microstructure that possesses good strength-elongation combination. Conventional hot metalworking operations such as forging, rolling, and extrusion below the beta transus temperature can be used to produce fine-grained microstructure. Final heat treatment comprising solution treatment to precipitate a desired volume fraction of coarse alpha plates followed by ageing to precipitate fine alpha platelets, both conducted below the beta transus temperature, provides the desired strength-elongation combination in the final product. Solution treatment in general is well known to those skilled in the art.2 2“Titanium”, G. Lutjering and J. C. Williams, Second Edition, Springer, 2007, page 289.
The present approach has been practiced by a gas atomization powder metallurgy process flowchart as shown in
By a series of experiments, for a given boron enhancement content, it has been determined that homogenization and ageing are critical steps for achieving improved mechanical property combinations in accordance with the method of the present invention. The influence of homogenization heat treat on room temperature tensile properties of extruded Ti-5553-1B is shown in
The influence of ageing treatment on room temperature tensile properties of extruded Ti-5553-1B for different homogenization temperatures is demonstrated in Table 1 hereinafter. The hot work temperature (1500° F.), solution treatment (1500° F./1 hour), and ageing time (6 hours) were kept constant in this study. Upon ageing, tensile strength increased by 50-60 ksi, tensile modulus increased by 4-5 Msi without debit in tensile elongation compared to the no post heat treat condition. By a suitable choice of homogenization temperature and ageing temperature, optimum strength-modulus-ductility combinations can be achieved as shown in Table 1.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
