How many titanium alloy grades are there? The literature gives a number: more than 100. Of those, 40–50 are genuinely used in industry, and only a dozen or so are in common use. With that many alloys, memorizing grade designations alone will never get you there. So metallurgists did the most fundamental thing possible — they classified them by microstructure.

The "microstructure" here refers to which crystal structure actually dominates in a titanium alloy at room temperature: the close-packed hexagonal α phase, the body-centered cubic β phase, or a mixture of both. Do not underestimate this classification — what the room-temperature microstructure is basically determines whether the alloy welds well, can be cold-formed, resists high temperatures, and how high its strength can go. You could say that microstructure is titanium's innate character.

Today we will layout the four schools of titanium alloys — α, near-α, α+β, and β — in one go. In the Chinese national standard they use the prefixes TA, TC, and TB respectively (commercially pure titanium and the α type use TA; the α+β type uses TC; the β type uses TB), followed by a registration number, such as TA7, TC4, and TB2.

Spend a minute recalling the underlying logic covered earlier. Titanium undergoes an allotropic transformation at 882 °C: at high temperature it is the body-centered cubic β phase, and at low temperature the close-packed hexagonal α phase. Adding alloying elements changes the spheres of influence of these two phases:

• α-stabilizing elements (aluminum, oxygen, nitrogen, etc.) expand the α

phase field, making the room-temperature microstructure predominantly α;

• β-stabilizing elements (molybdenum, vanadium, niobium, iron,

chromium, etc.)

expand the β phase field; add enough of them and the β phase can be retained

even at room temperature;

• Add both, with the β phase accounting for 10%–50%, and you get an

α+β dual-phase alloy.

Thus, ranked by β-phase content at room temperature from low to high, titanium alloys naturally fall into four schools: α (almost entirely α) → near-α (β not exceeding 10%) → α+β (β at 10%–50%) → β (fully β at room temperature). This line is essentially a line of increasing molybdenum equivalent from low to high.

And the tempers of the α and β phases are worlds apart: the α phase is close-packed hexagonal with few slip systems, giving it high strength, high-temperature resistance, corrosion resistance, and good weldability — but it is hard to cold-form. The β phase is body-centered cubic with many slip systems, so it is soft, tough, easy to cold-form, and can be substantially strengthened by heat treatment. Whichever phase dominates, the alloy follows its disposition.

α titanium alloys contain mainly α-stabilizing elements, and in the stable room-temperature condition are essentially 100% α phase. The representatives are commercially pure titanium (TA0, TA1, TA2, TA3) and TA7 (Ti-5Al-2.5Sn).

This school's character keywords: corrosion-resistant, readily weldable, good hot workability — but not heat-treatable for strengthening, relatively low in strength, and only moderate in cold formability.

Because the microstructure is single-phase α with a uniform composition and none of the complications brought by phase transformations, α alloys have the best welding performance among titanium alloys — the strength coefficient of a welded joint can approach 100%. Precisely because it is single-phase and cannot be strengthened through phase transformation, its strength comes mainly from interstitial elements (oxygen, nitrogen) and the solid-solution strengthening of aluminum and tin, so its ceiling is not high.

The literature points out specifically that α alloys are used primarily in the chemical, petrochemical, and process industries, where the first consideration is not strength but corrosion resistance and deformability. Commercially pure titanium (TA0–TA3) is therefore the first choice, as are the corrosion-resistant enhanced versions such as palladium-bearing TA9 (Ti-0.2Pd) and TA10 (Ti-0.3Mo-0.8Ni) with small additions of molybdenum and nickel — these are the specific remedy for hot hydrochloric acid, sulfuric acid, and other strongly reducing acids, and are the mainstay material for reactors, heat exchangers, piping, and chlor-alkali equipment.

In one sentence: the α school does not pursue brute strength; it pursues “never rusting, welding soundly, and performing stably for a lifetime.”

Near-α titanium alloys add small amounts of β-stabilizing elements on an α base; in the annealed microstructure the β phase or intermetallic compounds generally do not exceed 10%. Do not scorn this less-than-10% of β: it lets the alloy, while retaining the α school’s high-temperature resistance and weldability, take high-temperature strength and creep resistance up a notch.

This school exists almost entirely for the hot sections of aero-engines. The literature names three representatives:

– TA11 (Ti-8Al-1Mo-1V): developed in the United

States, intended for high-temperature service. But its aluminum content is as

high as 8%, which brings hot-salt stress corrosion problems — it is prone to

cracking in high-temperature salt-containing environments.

– TA15 (Ti-6.5Al-1Mo-1V-2Zr): this is the Russian BT20

alloy. It is a “similar alloy” to TA11, but cleverly lowers aluminum and adds

zirconium, preserving heat resistance while improving hot-salt stress corrosion

behavior. It is a classic high-temperature titanium alloy for long-term service

at around 500 °C, used extensively in aircraft airframes and engine

load-bearing parts.

– TA13 (Ti-2.5Cu): the British IMI230 alloy, of

the “α + compound” type, strengthened by Ti₂Cu precipitation — one of the few

near-α alloys that can be strengthened by heat treatment.

The near-α school also has a contemporary mainstay — TA19 (Ti-6Al-2Sn-4Zr-2Mo, international designation Ti-6242S). With an α matrix plus tin, zirconium, molybdenum, and trace silicon, its long-term service temperature can reach 550 °C; creep strain at 550 °C over 100 hours can be held within 0.1%, while it retains the α school's good weldability (argon-arc welded joint strength coefficient can reach 100%). Today it is used in aero-engine high-pressure compressor casings, blades, and blisks, where it substitutes for nickel-based superalloys in domestic engines and in C919/C929-related structures, achieving weight reductions of 25%–30% (according to technical data from Litai Metal and Kehui Titanium).

The design philosophy of the near-α school is very clear: keep the β phase to a mere touch (under 10%), retain the high-temperature-resistant α matrix almost entirely, and use elements such as tin, zirconium, molybdenum, and silicon for high-temperature solid-solution and creep strengthening. It is the optimal solution for “needing heat resistance, plus weldability, plus a bit more strength.”

α+β titanium alloys contain a relatively large amount of β-stabilizing elements; at room temperature the α and β phases coexist, with β content generally at 10%–50%. This is the most important and most heavily used school among titanium alloys.

Its character keywords: medium strength, heat-treatable for strengthening, excellent comprehensive performance — but relatively poor weldability.

Because there are two phases, α+β alloys can be "tuned" over a wide range among strength, plasticity, and toughness by adjusting the heat treatment (solution treatment + aging / duplex annealing) and the two-phase ratio — something single-phase α alloys cannot do. In Table 6-1 this school has a luxurious lineup:

– TC1 (Ti-2Al-1.5Mn): annealed condition, low cost,

good formability;

– TC4 (Ti-6Al-4V): annealed condition, the

absolute star;

– TC6 (Ti-6Al-1.5Cr-2.5Mo-0.5Fe-0.35Si): duplex annealed;

– TC10 (Ti-6Al-6V-2Sn-0.5Cu-0.5Fe): annealed

condition, higher strength;

– TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si): duplex annealed,

heat-resistant type.

A few more words must be said about TC4: Ti-6Al-4V was successfully developed by Watertown Arsenal in the United States in 1954 and is widely used in the aerospace industry; its products account for 55%–65% of total titanium alloy output, and it can be produced as large-size aviation forgings of every kind. It has excellent comprehensive performance, is the most thoroughly researched, has been in service the longest, and has the widest range of applications — "half a century after its birth it still retains vigorous vitality."

It is also a "onegrade travels the world" international citizen, with a corresponding grade in every country:

• China: TC4

• Timet Division, USA: Ti-6Al-4V; Reactive Metals Inc., USA:

RMI 6Al-4V; corresponding to ASTM Grade 5

• UK, IMI: IMI 318

• Russia: BT6

• Japan, Sumitomo: ST-A140

• France: TA6V

• Germany, Krupp: LT31

Why does the whole world use it? 6% aluminum stabilizes α and 4% vanadium stabilizes β, giving a two-phase ratio that is just right — high enough strength (annealed tensile strength ≥ 895 MPa), good enough plasticity, corrosion resistance, weldability (though not as good as α alloys), heat-treatability, and biocompatibility: it has virtually no shortcomings. It is the “all-subject top scholar” of titanium alloys.

Even so, the textbook also points out the α+β school’s soft spot: relatively poor weldability. Because under the rapid heating and cooling of welding, the β phase in the two-phase microstructure undergoes martensitic transformation, making the joint prone to embrittlement and loss of plasticity. For this reason, critical welded parts often prefer α / near-α alloys, or require dedicated post-weld heat treatment.

β titanium alloys contain enough β-stabilizing elements that, at an appropriate cooling rate, the room-temperature microstructure can be entirely β phase. The textbook further divides them into two categories: heat-treatable β titanium alloys (metastable β) and thermally stable β titanium alloys.

This school has the most “split” and most remarkable character:

After quenching, metastable β alloys are a soft single-phase β at room temperature, with excellent processing plasticity; sheet can be cold-formed, cold-bent, and cold-headed much like aluminum alloys, and the bend radius can approach 0.5 times the sheet thickness (α+β TC4 typically needs 3–4 times the sheet thickness and still tends to crack). For complex-shaped sheet-metal parts, springs, and fasteners, this is tremendously good news.

After forming, aging treatment lets nanoscale α phase precipitate dispersedly from the β matrix, and strength can reach a room-temperature tensile strength as high as 1,300–1,400 MPa — the highest among the four schools.

The representative in the textbook’s Table 6-1 is TB2 (Ti-5Mo-5V-8Cr-3Al), whose condition is marked “quenched / quenched and aged” — the very model of “form it soft first, then harden it by aging.”

The classic applications of the β school are springs and fasteners. Take the famous Beta C alloy (Ti-3Al-8V-6Cr-4Mo-4Zr, US Grade 19): developed in the 1960s to replace the hard-to-process Ti-13V-11Cr-3Al, it is extremely soft in the solution-treated condition and can be cold-coiled into shape; after aging its tensile strength can reach 1,100–1,200 MPa. At 40%–50% lighter than steel springs and with higher energy-storage density, it is widely used in aircraft landing-gear springs, high-performance valve springs, and high-strength corrosion-resistant components for oil and gas wells (meeting NACE MR0175 requirements for sour H₂S environments). NASA even verified, in a Mars sample-return mission, that Beta C springs remained reliable after 3,000 cycles at a deep-space low temperature of −135 °C (according to NASA technical reports and Titanium Industries data).

The price the β school pays: many alloying elements (molybdenum, vanadium, and chromium are all expensive), relatively high density, high cost, and sensitivity to heat-treatment practice. So it is used where "the performance is worth paying for."

Once you understand this table, you have grasped the "first principles" of titanium alloy selection: for corrosion resistance and weldability choose α; for high-temperature resistance choose near-α; for all-round performance choose α+β (with TC4 as the safe default); for cold formability plus ultra-high strength choose β.

From that heat of Ti-6Al-4V at Watertown Arsenal in 1954 to today's AI-designed 1,400 MPa-class β titanium alloys, what changes are the strength numbers and manufacturing processes — what does not change is this underlying logic of classification by microstructure. Among the four schools there is no high or low, no noble or base; there is only "using it in the right place."