Beta alloys

Many different titanium alloys have been developed for a large variety of applications. Titanium alloys are generally classified according to the equilibrium phases present in their microstructure at room temperature (Ref 1). They can be classified as commercially pure (CP) alloys and alpha alloys that mainly contain the hep phase, alpha-beta alloys that contain both phases, and metastable beta alloys and beta alloys that consist largely of the bcc phase. 

Alloys with compositions less than the point where the a transus meets the composition axis are termed a alloys. The CP alloy discussed in this chapter is an example of an a alloy. Those with compositions greater than the point where the 3 transus meets the axis are termed 3 alloys. Those with compositions in between have a microstructure of a and 3 phases at ambient temperature under equilibrium conditions. Two types of these alloys can be identified. One type has composition limits between the a transus and the Mj curve and can be described by the term a-P alloy. The T1-6A1-4V alloy discussed later in this chapter is a common a-p alloy. The second type is given the name metastable p alloy. Composition limits for metastable p alloys fall between the and the p transus. Metastable beta alloys can best be described as alpha-beta alloys that contain an appreciable level of beta stabilizers. The low difhisivity of the beta stabilizers promotes complete retention of beta phase to room temperature at moderate cooling rates. The Ti-15V-3Cr-3Al-3Sn and Beta 21-S alloys are common metastable beta alloys. 

Ti-6A1-4V is an alpha-beta alloy that can be modified extensively by both thermal and thermomechanical processing to produce a large variety of microstructures and hence a wide spectrum of mechanical properties. The beta-transus temperature is approximately 1000 °C (1830 °F) and is a function of interstitial content (Ref 1). Samples of Ti-6A1-4V cooled at relatively slow rates from elevated temperatures contain mainly the alpha and beta phases as a result of diffusional transformations, while those cooled rapidly may also contain martensitic phases such as the cc (hep structure) or the a" (orthorhombic structure) phases. 

Beta 21-S. The Beta 21-S alloy is a relatively new metastable beta alloy (Ref 1). It was designed to have good formability, similar to Ti-15-3, but also has improved oxidation resistance, creep resistance, and high-temperature strength relative to Ti-15-3. Composition ranges and room-temperature tensile properties for Beta 21-S are listed in Tables 7.1 to 7.3. The alloy contains approximately 15% Mo, 3% Al, and 2.8% Nb, with additions of silicon (Ref 1). It is normally provided in the beta solution-treated condition. Beta 21-S has an elastic modulus close to that of bone and hnds use in prosthetic application. It has excellent high-temperature stability and can be used at temperatures up to 290 °C (550 °F). 

Industrial grades of pure titanium have relatively low tensile strength in comparison to alpha-beta stabilized alloys such as Ti6A14V. Therefore, the majority of industrial uses for titanium incorporate the alpha-beta alloys and as such these are the focus of most adhesion studies.As with other metals, there is a large range of pre-treatments available for titanium and its alloys, as indicated in Table 1. 

Timetal I Metastable beta alloy produced in bar or rod form and targeted at titanium spring and other high-strength-requirement applications. 

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