P

Ti -► Mn, Cr, Co, Fe, Co, Ni, Cu, Si Ti -► Sn, Zr

P Eutectoid Stabilizers Neutral

Fig. 4.2. Phase Diagrams for Binary Titanium Alloys for all practical purposes, this will not happen so they are often referred to as just beta alloys.

Alpha and near-alpha alloys usually contain 5-6% aluminum as the main alloying element with additions of the neutral elements tin and zirconium and some beta stabilizers. Since these alloys retain their strength at elevated temperatures and have the best creep resistance of the titanium alloys, they are often specified for high temperature applications. The addition of silicon, in particular, improves the creep resistance by precipitating fine silicides, which hinders dislocation climb. Aluminum also contributes to oxidation resistance. These alloys also perform well in cryogenic applications. Due to the limitation of slip systems in the HCP structure, the alpha phase is less ductile and more difficult to deform than the BCC beta phase. Because these are single phase alloys containing only alpha, they cannot be strengthened by heat treatment.

The alpha-beta alloys have the best balance of mechanical properties and are the most widely used. In fact, one alpha-beta alloy, Ti-6Al-4V, is by far and away the most widely utilized alloy. In this alloy, aluminum stabilizes the alpha phase while vanadium stabilizes the beta phase; therefore, alpha-beta alloys contain both alpha and beta phases at room temperature. In contrast to the alpha and near-alpha alloys, the alpha-beta alloys can be heat treated to higher strength levels, although their heat treat response is not as great as that for the beta alloys. In general, the alpha-beta alloys have good strength at room temperature and for short times at elevated temperatures, although they are not noted for their creep resistance. The weldability of many of these alloys is poor due to their two-phase microstructures.

The metastable beta alloys contain a sufficient amount of beta stabilizing elements that the beta phase is retained to room temperature. Since the BCC beta phase exhibits much more deformation capability than the HCP alpha phase, these alloys exhibit much better formability than the alpha or alpha-beta alloys. Where the alpha and alpha-beta alloys would require hot forming operations, some of the beta alloys can be formed at room temperature. The beta alloys can be solution treated and aged (STA) to higher strength levels than the alpha-beta alloys while still retaining sufficient toughness. The biggest drawbacks of the beta alloys are increased densities due to alloying elements such as molybdenum, vanadium, and niobium; reduced ductility when heat treated to peak strength levels; and some have limited weldability.

Titanium has a great affinity for interstitial elements, such as oxygen and nitrogen, and readily absorbs them at elevated temperature. Oxygen tends to increase the strength and decrease the ductility. As the amount of oxygen and nitrogen increases, the yield and ultimate strengths increase and the ductility and fracture toughness decreases. Titanium absorbs oxygen at temperatures above 1300° F, which complicates the processing and increases the cost, since many hot working operations are conducted at temperatures exceeding 1300° F. However, oxygen, in controlled amounts, is actually used to strengthen the commercially pure (CP) grades. Some alloys are available in an extra low interstitial (ELI) grade that is used for applications requiring maximum ductility and fracture toughness. Hydrogen is always minimized in titanium alloys because it causes hydrogen embrittlement by the precipitation of hydrides, so the maximum limit allowed is about 0.015%.

Titanium alloys derive their strength from the fine microstructures produced by the transformation from beta to alpha. If alpha-beta or beta alloys are solution heat treated and aged, titanium martensite can form during the quenching operation; however, the martensite formed in titanium alloys is not like the extremely hard and strong martensite formed during the heat treatment of steels. For example, the tensile strength of Ti-6-4 only increases from 130 to 170 ksi on STA, while the tensile strength of 4340 steel can be increased from 110 to 280 ksi by heat treatment. While the grain size does not normally affect the

Multiple Deformation Cycles

Multiple Deformation Cycles

Time

Deformation

Solution Heat Treat

• Temperature

• Temperature

• Temperature

Fig. 4.3. Thermomechanical Treatment of Titanium Alloys ultimate tensile strength, the finer the grain size, the higher the yield strength, ductility, and fatigue strength. However, to resist grain boundary sliding and rotation, larger grain sizes are actually preferred for some applications requiring creep resistance.

The microstructure of titanium alloys is determined by both alloy composition and the thermomechanical treatment (TMT) history that the alloy undergoes during processing. TMT, as shown in Fig. 4.3, is a combination of mechanical deformation sequences and heat treatments. TMT for wrought alloys consists of three processing steps: (1) a series of elevated temperature deformations, (2) solution heat treatment, and (3) aging. This is a highly simplified explanation of TMT processing in that the elevated temperature deformation may occur in several steps and many titanium alloys, Ti-6Al-4V being a good example, are used in the mill annealed condition more than they are in the solution treated and aged condition.

A key element in microstructure and mechanical property development is whether or not the TMT treatments are conducted above or below the beta transus temperature. Lamellar structures (Fig. 4.4) are a result of cooling from the beta phase field in which the alpha phase nucleates at prior beta grain boundaries and then grows into the beta grains. If the cooling rate is fairly fast, then the microstructure will be fine (Widmanstatten or basketweave), whereas if it is slow, it will be coarse (colony structure). Lamellar structures offer a Growing From P Grain Boundaries a Growing From P Grain Boundaries

^ Grain Boundaries

Early Stage of Growth

Later Stage of Growth

^ Grain Boundaries

Early Stage of Growth

Later Stage of Growth

Fig. 4.4. Lamellar Structures Produced by Cooling from above the Beta Transus4

the highest fracture toughness and the lowest fatigue crack growth rates. On the other hand, equiaxed structures, as shown in Fig. 4.5, are a result of a recrystallization process and therefore require previous cold work to initiate recrystallization. This can be accomplished by deformation in the two-phase alpha + beta field followed by solution treating in the two-phase field. Equiaxed structures produce the highest strength and ductility and the best fatigue strength. The difference between the terms "fatigue strength" and "crack growth rate" should be explained. Fatigue strength is the total fatigue life before failure. It consists of crack initiation and then crack growth until final failure. The solution

Fig. 4.5. Equiaxed Grain Structure in Titanium4

treating temperature determines the volume fraction of the primary alpha phase with temperatures just below the beta transus leading to duplex structures in which equiaxed alpha is dispersed in an alpha + beta lamellar matrix. Duplex structures produce a balance of properties between the equiaxed and lamellar structures.

Although the melting point of titanium is higher than 3000° F, it is not possible to create titanium alloys that operate at temperatures approaching their melting point. Titanium alloys are usually restricted to maximum temperatures of 600-1100° F depending upon alloy composition.

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