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Fig. 4.6. Large Titanium Heat Exchanger

Near-alpha alloys are those which contain some beta phase dispersed in an otherwise all alpha matrix. The near-alpha alloys generally contain 5-8% aluminum, some zirconium, and tin, along with some beta stabilizer elements. Because these alloys retain their properties at elevated temperature and posses good creep strength, they are often specified for elevated temperature applications. Silicon in the range of 0.10-0.25% enhances the creep strength. High temperature near-alpha alloys include Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.25Si) and IMI 829 (Ti-5.5Al-3.5Sn-3Zr-1Nb-0.3Si), which can be used to 1000° F, and IMI 834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si) and Ti-1100 (Ti-6Al-2.8Sn-4Zr-0.4Mo-0.4Si), a modification of Ti-6242S, which can be used to 1100° F. Ti-8Al-1Mo-1V is an older alloy that has rather poor stress corrosion resistance (due to its extremely high aluminum content) but has a low density and a higher modulus than other titanium alloys.

4.2.3 Alpha-Beta Alloys

The alpha-beta alloys are heat treatable to moderate strength levels but do not have as good elevated temperature properties as the near-alpha alloys. Their weldability is not as good as the near-alpha alloys but their formability is better. The alpha-beta alloys, which include Ti-6Al-4V, Ti-6Al-6V-2Sn, and Ti-6Al-2Sn-4Zr-6Mo, are all capable of higher strengths than the near-alpha alloys. They have a good combination of mechanical properties, rather wide processing windows, and can be used in the range of about 600-750° F. They can be strengthened by STA but the strength obtainable decreases with section thickness. The lean alloys, such as Ti-6-4, are weldable. Alpha-beta alloys contain elements, in particular aluminum, to strengthen the alpha phase and beta stabilizers to provide solid solution strengthening and response to heat treatment. As the percentage of beta stabilizing elements increases, the hardenability increases and the weldability decreases. Ti-6-4 accounts for approximately 60% of the titanium used in aerospace and up to 80-90% for airframes. There are four different heat treatments that are often used for wrought Ti-6-4:1

(1) Mill Annealed.(MA). Mill annealed is the most common heat treatment. It produces a tensile strength of approximately 130 ksi, good fatigue properties, moderate fracture toughness (KIC = 60ksi^/in.), and reasonable fatigue crack growth rate.

(2) Recrystallization Anneal (RA). A recrystallization anneal can be used for parts requiring increased damage tolerance, as the KIC value for Ti-6-4 goes from around 60 to 70 ksi in. This heat treatment produces slightly lower strength and fatigue properties and improved fracture toughness and slower fatigue crack growth rates.

(3) Beta Anneal (BA). Beta annealing is used where it is important to maximize fracture toughness (minimum KIC ~ 80ksi^/in.) and minimize fatigue crack grow rate. However, the fatigue strength is significantly degraded. Alpha-beta alloys for fracture critical applications are often beta annealed to develop a transformed beta structure. A transformed beta structure produces tortuous crack paths with secondary cracking which enhances fracture toughness and slows fatigue crack growth; however, the equiaxed alpha-beta microstructure has twice the ductility and better fatigue life than the transformed beta microstructure.

(4) Solution Treated and Aged (STA). STA provides maximum strength but full hardenability is limited to sections 1 in. thick or less. This heat treatment is used for mechanical fasteners with a minimum tensile strength of 160 ksi. The STA treatment is not normally used for structural components due to its limited hardenability and warping problems during heat treating.

Ti-6-4 ELI, with a maximum oxygen content of 0.13%, is used for fracture critical structures and for cryogenic applications. Oxygen is a potent strengthening element in Ti-6-4 and must be held to low limits in order to develop high fracture toughness. The commercial grade of Ti-6-4 has an oxygen content of 0.16-0.18% while the ELI grade is limited to 0.10-0.13%. With higher oxygen content, the commercial grade has higher strength and slightly lower ductility, while the ELI grade has about a 25% higher fracture toughness. A comparison of the properties of commercial and ELI Ti-6-4 is given in Fig. 4.7. The beta transus temperature is also influenced by the oxygen content with the

YS = Yield Strength KIC = Fracture Toughness

UTS = Ultimate Tensile Strength FL = Fatigue Life

El = Elongation CS = Creep Strength RA = Reduction in Area

YS = Yield Strength KIC = Fracture Toughness

UTS = Ultimate Tensile Strength FL = Fatigue Life

El = Elongation CS = Creep Strength RA = Reduction in Area

YS UTS El RA KIC

Fig. 4.7. Room Temperature Properties of Commercial vs. ELl Ti-6Al-4V5

YS UTS El RA KIC

Fig. 4.7. Room Temperature Properties of Commercial vs. ELl Ti-6Al-4V5

beta transus being in the range of 1850-1870° F for the commercial grade and 1760-1800° F for the ELI grade.

Ti-6-22-22S (Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si) is similar to Ti-6-4 but is a higher strength alloy with a tensile strength of 150 ksi in the MA condition and 170 ksi in the STA condition. Ti-6-22-22S2 was originally developed as a deep-hardening high strength alloy for moderate service temperatures. Due to its higher alloying content, it has better room and elevated temperature static and fatigue strength than Ti-6-4. It can be given a triplex heat treatment to maximize damage tolerant properties. Typical properties are a tensile strength of 150 ksi with a minimum fracture toughness of KIC = 70ksi>/in. The heat treatment consists of solution treating above the beta transus followed by a solution treatment below the beta transus and then a lower temperature aging treatment. This alloy also exhibits excellent SPF characteristics and can be formed at temperatures lower than that for Ti-6-4 and yet have higher strengths.

SP-700 (Ti-4.5Al-3V-2Mo-2Fe) was developed as a lower temperature SPF alloy which can be formed at temperatures as low as 1300° F compared to around 1650° F for Ti-6-4. This reduces energy costs, lengthens die life, and reduces the amount of alpha case that must be removed after part fabrication. Since alpha case is an oxygen-enriched surface layer that is extremely brittle and causes a significant reduction in fatigue life, it must be removed by either chemical milling or machining. Ti-3Al-2.5V is a lean version of Ti-6-4 that is often used for aircraft hydraulic tubing in sizes ranging from 0.25 to 1.5 in. diameter and for honeycomb core. Ti-6246 (Ti-6Al-2Sn-4Zr-6Mo) and Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) are engine alloys that are used up to 600 and 750° F respectively, where higher strengths than Ti-6-4 are needed. Where higher temperature creep resistance is required, the near-alpha alloy Ti-6242S is normally specified.

4.2.4 Beta Alloys

Beta alloys contain high percentages of the BCC beta phase that greatly increases their response to heat treatment, provides higher ductility in the annealed condition, and provides much better formability than the alpha or alpha-beta alloys. In general, they exhibit good weldability, high fracture toughness, and a good fatigue crack growth rate; however, they are limited to about 700° F due to creep.

The beta alloys, including Ti-10-2-3 (Ti-10V-2Fe-3Al), Ti-15-3 (Ti-15V-3Al-3Cr-3Sn), and Beta 21S (Ti-15Mo-3Al-2.7Nb-0.25Si), are high strength alloys that can be heat treated to tensile strength levels approaching 200 ksi. In general, they are highly resistant to stress corrosion cracking. The fatigue strength of these alloys depends on the specific alloy; for example, the fatigue strength is of Ti-10-2-3 is very good but that of Ti-15-3 is not so good. The beta alloys possess fair cold formability which can eliminate some of the hot forming operations normally required for the alpha-beta alloys.

Ti-10-2-3 is the mostly widely used of the beta alloys in airframes. Ti-10-2-3 is a popular forging alloy because it can be forged at relatively low temperatures, offering flexibility in die materials and forging advantages for some shapes. It is used extensively in the main landing gear of the Boeing 777.6 It is used at three different tensile strength levels; 140, 160, and 170 ksi. At the 170 ksi level, it has a fairly narrow processing window to meet the strength, ductility, and toughness requirements. The lower strength levels have wider processing windows and improved toughness. It exhibits excellent fatigue strength but only moderate fatigue crack growth rates. Although Ti-10-2-3 is not as deep hardening or as fatigue resistant as Ti-6-22-22S, it is capable of being heat treated to higher strengths than Ti-6-22-22S. A Russian near-beta alloy, Ti-5-5-5-3 (Ti-5Al-5V-5Mo-3Cr), is receiving quite a bit of consideration for new applications. Forged bar that has been STA heat treated has a tensile strength of 175 ksi, a yield strength of 162 ksi, and an elongation of 10%. Although the fracture toughness is somewhat lower than comparatively processed Ti-10-2-3, it is not a concern for the applications being considered. Although primarily a wrought alloy, it has also been evaluated for investment castings with promising results.

Heat treated Beta C (Ti-3Al-8V-6Cr-4Mo-4Zr) is often used for springs. Ti-15-3 can also be used for springs when it is heat treated to a tensile strength of 150 ksi. An advantage of Ti-15-3 is the ability to cold form the material in thin gages and then STA to high strengths. Ti-15-3 can also be used for castings where higher strength (UTS = 165 ksi) than Ti-6-4(UTS = 120 ksi) is required. Beta 21S (Ti-15Mo-3Al-2.7Nb-0.25Si) was originally developed as an oxidation resistant alloy for high temperature metal matrix composites for the National Aerospace Plane. Even though it is a beta alloy, it has reasonable creep properties, better than Ti-6-4. It can be heat treated to a tensile strength of 125 ksi for temperature uses between 900 and 1050° F or to higher strength levels (UTS = 150 ksi) for lower temperature usage. One of the distinguishing features of Beta 21S is its resistance to hydraulic fluids, presumably due to the synergistic effects of molybdenum and niobium.

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