There is no clear-cut definition for beta titanium alloys. Conventional terminology usually refers to near-beta alloys and metastable-beta alloys as classes of beta titanium alloys. A near-beta alloy is generally one which has appreciably higher beta stabilizer content than a conventional alpha-beta alloy such as Ti-6Al-4V, but is not quite sufficiently stabilized to readily retain an all-beta structure with an air cool of thin sections. For such alloys, a water quench even of thin sections is required. Due to the marginal stability of the beta phase in these alloys, they are primarily solution treated below the beta transus to produce primary alpha phase which in turn results in an enriched, more stable beta phase. This enriched beta phase is more suitable for aging. The Ti-10V-2Fe-3Al alloy is an example of a near-beta alloy.
On the other hand, the metastable-beta alloys are even more heavily alloyed with beta stabilizers than near-beta alloys and, as such, readily retain an all-beta structure upon air cooling of thin sections. Due to the added stability of these alloys, it is not necessary to heat treat below the beta transus to enrich the beta phase. Therefore, these alloys do not normally contain primary alpha since they are usually solution treated above the beta transus. These alloys are termed “metastable” because the resultant beta phase is not truly stable—it can be aged to precipitate alpha for strengthening purposes. Alloys such as Ti-15-3, B120VCA, Beta C, and Beta III are considered metastable-beta alloys.
Unfortunately, the classification of an alloy as either near-beta or metastable beta is not always obvious. In fact, the "metastable" terminology is not precise since a near-beta alloy is also metastable-i.e., it also decomposes to alpha plus beta upon aging.
There is one obvious additional category of beta alloys—the stable beta alloys. These alloys are so heavily alloyed with beta stabilizers that the beta phase will not decompose to alpha plus beta upon subsequent aging. There are no such alloys currently being produced commercially. An example of such an alloy is Ti-30Mo.
The interest in beta alloys stems from the fact that they contain a high volume fraction of beta phase which can be subsequently hardened by alpha precipitation. Thus, these alloys can generate quite high-strength levels (in excess of 200 ksi) with good ductilities. Also, such alloys are much more deep hardenable than alpha-beta alloys such as Ti-6Al-4V. Finally, many of the more heavily alloyed beta alloys exhibit excellent cold formability and as such offer attractive sheet metal forming characteristics.
5.5.1 Ti-13V-11Cr-3AL
5.5.1.0
Comments and Properties — Ti-13V-11Cr-3Al is a heat-treatable alloy possessing good workability and toughness in the annealed condition and high strength in the heat-treated condition. It is noted for its exceptional ability to harden in heavy sections (up to 6-inch diameter or greater) to tensile strength of 170 ksi .
Manufacturing Considerations — This alloy possesses very good formability at room temperature; stretch forming is usually conducted at 500°F. Ti-13V-11Cr-3Al is readily fusion or spot welded. Arc-welded joints are very ductile in the as-welded condition, but have low strengths.
Environmental Considerations — Ti-13V-11Cr-3Al is stable for times up to 1000 hours in the annealed condition at 550 °F and in the solution treated and aged condition up to 600 °F. Prolonged exposure above these temperatures may result in ductility losses. If welding is employed, the stability of the weld should be investigated under the particular exposure conditions to be encountered. While the material is not noted for good creep performance, Ti-13V-11Cr-3Al has exceptional short-time strength at temperatures to 1200 °F and above. Oxidation resistance is satisfactory at such temperatures for short-time exposure and for long-time exposure at the lower elevated temperatures. Hot-salt stress corrosion has been shown to be possible in this alloy at temperatures as low as 500 °F in highly stressed applications (e.g., rivet heads). It is generally thought that the
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5.5.1.1
Annealed Condition — Elevated temperature curves for annealed Ti-13V-11Cr-3Al are shown in Figures 5.5.1.1.1 through 5.5.1.1.4. Typical tensile stress-strain curves for annealed material at temperatures ranging from room temperature to 1000 °F are shown in Figure 5.5.1.1.6. Unnotched and notched fatigue data at room and elevated temperatures for annealed sheet are shown in Figures 5.5.1.1.8(a) through (d).
5.5.1.2
Solution-Treated and Aged Condition — Elevated temperature curves for solution-treated and aged Ti-13V-11Cr-3Al are shown in Figures 5.5.1.2.1 through 5.5.2.1.4. Typical tensile stress-strain curves at various temperatures are shown in Figure 5.5.1.2.6. Unnotched fatigue data at room and elevated temperatures for solution-treated and aged sheet are shown in Figures 5.5.1.2.8(a) through (c).
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5.5.2 Ti-15V-3Cr-3Sn-3Al (Ti-15-3)
5.5.2.0
Comments — Ti-15V-3Cr-3Sn-3Al is a solute rich (metastable) beta titanium alloy. It was developed primarily to lower the cost of titanium sheet metal parts by reducing materials and processing cost. Contrary to conventional alpha-beta alloys, this alloy is strip producible and has excellent room temperature formability characteristics. It can also be aged to a wide range of strength levels to meet a variety of application needs. Although this alloy was originally developed as a sheet alloy, it has expanded into other areas such as fasteners, foil, plate, tubing, castings, and forgings.
Manufacturing Considerations — Ti-15V-3Cr-3Sn-3Al is usually supplied in the solution-annealed condition. In this condition, the alloy has a single phase (beta) structure and, hence, is readily cold formed. After cold forming, the alloy can be resolution-treated in the 1450 °F to 1550 °F range and subsequently aged in the 900 °F to 1100 °F range, depending upon desired strength. Care should be exercised to ensure that no surface contamination results from the solution treatment. The alloy can be directly aged after forming; however, strength will vary depending upon the amount of cold work in the part. The alloy can also be hot formed. Heating times prior to hot forming should be minimized in order to prevent appreciable aging prior to forming. Ti-15V-3Cr-3Sn-3Al alloy is readily welded by standard titanium welding techniques.
Environmental Considerations — In the aged condition, Ti-15V-3Cr-3Sn-3Al appears to be immune to hot-salt stress corrosion cracking below the 500 °F to 440 °F range. However, some susceptibility has been noted after 100-hour stressed exposures at 600 °F. The presence of salt water does not appear to affect the room temperature crack growth behavior of aged material. Alloy Ti-15V-3Cr-3Sn-3Al should not be used in the solution treated condition. Long time exposure of solution treated and cold worked material to service temperatures above approximately 300 °F or solution treated material to service temperatures above approximately 400 °F can result in low ductility. Under certain conditions, titanium, when in contact with cadmium, silver, mercury, or certain of their compounds, may become embrittled. Refer to MIL-S-5002 and MIL-STD- 1568 for restrictions concerning such applications.
Heat Treatment — This alloy should be solution treated for 10-30 minutes in the 1450°F to 1550°F range, cooled at a rate approximating an air cool of 0.125 inch thick sheet and subsequently aged. Aging is generally conducted in the 900°F to 1100°F range, followed by an air cool. Aging times will vary depending upon aging temperature. The material can be used in service in the solution treated condition subject to the temperature limitations described above.
Specifications and Properties — A material specification for Ti-15V-3Cr-3Sn-3Al is shown in Table 5.5.2.0(a). Room-temperature mechanical properties for Ti-15V-3Cr-3Sn-3Al are shown in Table 5.5.2.0(b). The effect of temperature on physical properties is shown in Figure 5.5.2.0.
5.5.2.1 Solution-Treated and Aged (1000°F) Condition—Typical tensile and compressive Table 5.5.2.0(a). Material Specification
for Ti-15V-3Cr-3Sn-3Al
| Specification | Form |
|---|---|
| AMS 4914 | Sheet and strip |
stress-strain and compressive tangent-modulus curves are presented in Figures 5.5.2.1.6(a) and (b).
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5.5.3 Ti-10V-2Fe-3Al (Ti-10-2-3)
5.5.3.0
Comments and Properties — Ti-10V-2Fe-3Al is a solute lean beta (near beta) titanium alloy that was developed primarily as a high-strength forging alloy. It has excellent forging characteristics, possessing flow properties at 1500°F similar to Ti-6Al-4V at 1700°F. This characteristic provides advantages, such as lower die cost and better die fill capability. This alloy also provides the best combination of strength and toughness of any of the commercially available titanium alloys. For example, at the 180 ksi tensile ultimate strength level, the alloy has a value of minimum.
In addition to this high-strength condition, the alloy can also be processed to intermediate strength levels for higher fracture toughness. This alloy has also been reported to exhibit a shape-memory effect.
Manufacturing Considerations — Ti-10V-2Fe-3Al is usually supplied as bar or billet product which has been finish forged (or rolled) in the alpha-beta field. In order to optimize the microstructure for the high-strength condition, the forging is usually given a pre-form forge above the beta transus, followed by a 15 to 25 percent reduction below the beta transus. Ideally, the beta forging operation is finished through the beta transus, followed by a quench. The intent of the two-step forging process is to develop a structure without grain boundary alpha, but with elongated primary alpha needles in an aged beta matrix. The alloy is considered to be deep hardenable, capable of generating high strengths in section thicknesses up to approximately 5 inches. The alloy is also readily weldable by conventional titanium welding techniques.
Environmental Consideration — In the solution treated plus aged condition, the material exhibits excellent resistance to stress corrosion cracking, typically exhibiting a K_{\text {Isc }} >0.8 \mathrm{~K}_{\mathrm{lc}} . In the solution-treated condition, the material should not be subjected to long-term exposure in the 500^{\circ} \mathrm{F} to 800^{\circ} \mathrm{F} range, since such exposure could result in high-strength, low-ductility conditions. Exposure to cadmium, silver, mercury, or certain other compounds should be avoided. Refer to MIL-STD-1568 and MIL-S-5002.
Heat Treatment — For the high-strength condition, the alloy is generally solution treated approximately 65 °F below the beta transus (which is typically 1460 to 1480°F), followed by a water quench and an 8-hour age at 900 °F to 950 °F. Overaging in the 950 °F to 1150 °F range may also be used to obtain lower strength levels.
Beta Flecks — Ti-10V-2Fe-3Al is a segregation prone alloy which can exhibit a microstructural phenomenon known as “beta-flecks”. Certain areas may possess a lower beta transus than the matrix (due primarily to beta stabilizer enrichment) and, as such, can fully transform during heat treatment just below the matrix transus. In severe cases, this condition can lead to lower ductility and a reduction in fatigue strength due to grain boundary alpha formation in the “flecked” region. Care should be exercised to procure only material which has been melted under strict control to prevent severe “fleck” formation.
Specifications and Properties — Material specifications for Ti-10V-2Fe-3Al are shown in Table 5.5.3.0(a). Room temperature mechanical properties for Ti-10V-2Fe-3Al are presented in Table 5.5.3.0(b) and (c) for die and hand forging.
5.5.3.1
Solution Treated and Aged (900 to 950°F) Condition — Typical tensile and compressive stress-strain and compressive tangent-modulus curves are presented in Figure 5.5.3.1.6.
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5.5.3.2
Solution Treated and Aged (950 to 1000°F) Condition—Typical tensile and compressive stress-strain and compressive tangent-modulus curves are shown in Figure 5.5.3.2.6.