Heat Treatment of Titanium Alloys
“Oh, you must process titanium alloys,” surmised the scholar.
How did he know? Let’s learn more.
Titanium has many attributes that are useful in today’s modern society. It is a relatively lightweight, corrosion-resistant structural material that can be strengthened dramatically through alloying and, in some cases, by heat treatment. Among its many advantages for aerospace, military and commercial: good strength-to-weight ratio, low density, low coefficient of thermal expansion, good corrosion resistance, good oxidation resistance at intermediate temperatures, good toughness and (relatively) low heat-treatment temperatures.
Titanium alloys are typically classified as pure titanium, alpha, beta and alpha-beta alloys. There are also so-called near-alpha-phase and near-beta-phase (i.e. metastable beta) alloys. Under equilibrium conditions, pure titanium and alpha (a) phase have hexagonal-closed-packed (HCP) structures up to 882˚C (1620˚F). Above this temperature, they transform to beta (b) phase having a body-centered-cubic (BCC) structure up to the alloy’s melting point. Near-alpha alloys typically have a small amount of beta-phase (1-2%) stabilizing elements. In near-beta alloys, significant additions of beta stabilizer suppress the Ms temperature to below ambient, and the beta phase is retained at room temperature by rapid cooling or quenching from the alpha-beta phase. The inherent properties of all these structures are quite different.
Alloying elements (Fig. 1) generally stabilize one or the other of these phases. The alpha phase is stabilized by aluminum, gallium, germanium, boron, carbon, oxygen and nitrogen, while the beta phase is stabilized by molybdenum, vanadium, tantalum, niobium, iron and hydrogen.
The boundary between the alpha and beta phase and the two-phase alpha-beta region (Fig. 2) is called the alpha transus and beta transus respectively. The beta-stabilized system has Ms and Mf temperatures associated with it.
The titanium grades most commonly used have compositional speciﬁcations determined by ASTM E120 (Standard Test Methods for Chemical Analysis of Titanium and Titanium Alloys) for the commercially important alloys. Military speciﬁcations are found under MIL-T-9046 and MIL-T-9047, and aerospace material speciﬁcations for bar, sheet, tubing and wire are under AMS speciﬁcation numbers 4900-4980. In addition, large aircraft companies have their own set of alloy speciﬁcations. Because titanium alloys are used in a variety of applications, several different material and quality standards are speciﬁed. Among these are ASTM, ASME, AMS, the U.S. military and a number of proprietary sources.
Titanium alloys may be divided into two principal categories: corrosion resistant and structural.[3-9] The corrosion-resistant alloys are generally based on a single-phase (alpha) with dilute additions of solid-solution strengthening and alpha-stabilizing elements such as oxygen (interstitial), palladium, ruthenium and aluminum (substitutional). These alloys are used in the chemical, energy, paper and food-processing industries to produce highly corrosion-resistant tubing, heat exchangers, valve housings and containers. The single-phase alpha alloys provide excellent corrosion resistance, good weldability, and easy processing and fabrication but a relatively low strength.
The beta phase is stabilized by additions such as molybdenum, vanadium, niobium, iron (substitutional) and hydrogen (interstitial). A dispersion of alpha in the beta matrix along with solid-solution strengthening of both the alpha and beta phases lead to higher-strength alloys referred to as structural alloys. The structural alloys can be divided into four categories: the near-alpha alloys, the alpha-beta alloys, the beta alloys and the titanium aluminide (ordered) intermetallics (based on TixAl where x = 1 or 3). With titanium alloys used in structural applications, optimization of mechanical properties is very important. Therefore, processing and microstructure control are critical.
Not all heat treatments are applicable to titanium alloys because of the differences in composition and microstructure. Alpha alloys generally are not heat treatable, having medium strength, good notch toughness and good creep resistance.
The response of titanium alloys to heat treatment depends on their composition and the effect of heat treatment on the alpha-beta phase balance. Strength of annealed alloys increases gradually and linearly with increasing alloy contents.
Alloys of the beta type respond to heat treatment, are characterized by higher density than pure titanium and are more easily fabricated. The purpose of alloying to promote the beta phase is either to form an all-beta-phase alloy having commercially useful qualities, to form alloys that have duplex alpha and beta structure to enhance heat-treatment response (i.e. changing the alpha and beta volume ratio) or to use beta eutectoid elements for intermetallic hardening.
Quenching from the beta-phase field gives a martensitic transformation with improved strength (depending on composition). Rapid quenching of titanium with relatively few alloying elements from the beta-phase field gives maximum strength at Mf. For highly alloyed titanium, rapid quenching from beta-phase field gives lowest strength, but the maximum strength is obtained after aging. The most important beta alloying element is vanadium.
Beta and alpha-beta alloys are heat treated to enhance specific properties. The general classifications for these heat treatments include:
• Stress relief – to reduce residual stress due to fabrication (e.g., forming, machining, welding) or heat treatment
• Process annealing – to optimize microstructure, manufacturability, dimensional stability and service life
• Solution treat and age – for mechanical-property development (e.g., strength, ductility, fracture toughness, creep resistance, fatigue strength)
Heat treatments specific to alpha-beta alloys include stress relief; mill or full anneal; recrystallization annealing (RA); duplex annealing (DA); beta annealing (BA); solution treat and age (STA); solution treat and overage (STOA); beta solution treat and age (ßT STA); beta solution treat and overage (ßT STOA); and beta plus alpha-beta solution treat and age (TRIPLEX STA).
Heat treatments applicable to metastable beta titanium alloys include solution treating, sub-transus solution treating, supra-transus solution treating, direct aging, solution treat plus single age, solution treat plus duplex age and pre-age solution treat and age (PASTA).
In some applications, titanium alloys have been case hardened (nitrided and carburized) to enhance certain surface properties. Laser surface alloying and ion implantation are possible but not commonly used.
One of the biggest challenges faced by the heat treater is to understand and comply with the large number of specifications applicable to a particular titanium alloy grade. By way of example, some of the common AMS heat-treat specifications for Ti-6Al-4V alloys (including legacy specifications) include: AMS 2801, AMS 4901, AMS 4904, AMS 4905, AMS 4911, AMS 4928, AMS 4931, AMS 4935, AMS 4943, AMS 4945, AMS 4965, AMS 4967, AMS 4975, AMS 4979, AMS 4984, AMS-H-81200 and AMS-T-9046 to name a few. IH
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