How are high-temperature titanium alloys prepared and heat-treated?

Vacuum Consumable Arc Melting (VAR)
Vacuum consumable arc melting (VAR) is a melting technology that uses arc heating. In a vacuum environment, an arc is generated between the raw material and the electrode. The high temperature of the arc rapidly melts the raw material, forming a molten pool. The molten metal then flows into the mold below under gravity and solidifies into an ingot. Its advantages include mature technology, low equipment investment, and high production efficiency. Although VAR is widely used in titanium alloy melting, problems such as high/low-density inclusions and composition segregation remain, which can affect the mechanical properties of the alloy.

Electron Beam Cold Hearth Melting (EBCHM)
Electron beam cold hearth melting (EBCHM) uses a high-energy electron beam to heat the metal. In a vacuum environment, the kinetic energy of high-speed electrons serves as the heat source. The electron beam, emitted from an electron gun, is focused onto the metal feedstock, producing intense heat that melts the material. The molten metal solidifies on the cooling bed, forming an ingot. Due to its high energy density, EBCHM enables rapid heating and precise temperature control. However, the technology has drawbacks: the equipment is costly to manufacture and maintain, especially the electron gun and vacuum system, which limits its use in small-scale production. Moreover, while heating is rapid, controlling molten pool flow and temperature can be difficult, which may complicate alloy composition control.

Plasma Cold Hearth Melting (PACHM)
Plasma cold hearth melting (PACHM) employs a plasma arc as the heat source, using inert or reducing gases as the medium. The high temperature generated by the plasma arc melts the metal feedstock, which then solidifies on the cold bed to form an ingot [58,59]. The advantage of PACHM is that it operates in an inert atmosphere at near-atmospheric pressure, which prevents volatilization of highly volatile elements and allows precise control of composition. However, disadvantages include expensive equipment, short plasma gun lifespan, and high consumption of inert gases and refractory materials.

 

(2) Powder Metallurgy (PM)
Powder metallurgy (PM), i.e., powder mixing → sintering, is a preparation method characterized by controllable composition, high material utilization, uniform microstructure, reduced segregation, low cost, and a relatively simple process flow. Researchers have developed a near-α high-temperature titanium alloy (Ti-6Al-4Zr-0.5Mo-0.6Si, wt.%) using PM. The resulting alloy exhibited a uniform α+β two-phase microstructure, with finely dispersed silicides and no coarse silicide segregation or precipitation. Other researchers prepared TA15 high-temperature titanium alloy via PM and studied its high-temperature softening mechanism, further demonstrating the feasibility of PM for titanium alloy production.

The sintering process is central to PM material manufacturing, as it directly determines the relative density and mechanical properties of the sintered bulk. Mechanical ball milling (BM) before sintering can enhance powder surface quality and activity, improving sintering behavior. Spark plasma sintering (SPS), an advanced PM technique, promotes particle contact through external pressure, eliminating porosity, enhancing densification, and reducing the required sintering temperature. Pulsed current applied to the conductive graphite die produces plasma, enabling rapid heating. Compared with traditional hot pressing (HP), hot isostatic pressing (HIP), and microwave sintering (MS), SPS offers faster heating rates, shorter sintering times, and lower energy consumption. Researchers successfully used SPS to rapidly sinter SiCNWs-reinforced Ti60 titanium alloy at a heating rate of 100 ± 5 °C/min, mitigating reactions between SiC and Ti and improving high-temperature strength. Some researchers also combined BM with SPS, adjusting the ball-to-powder ratio to control microstructure, refine grains, and achieve uniform grain distribution. This approach produced a new Ti–O alloy with excellent yield strength.

 

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2. Heat Treatment of High-Temperature Titanium Alloys

Heat treatment is a key method for optimizing microstructure and improving mechanical properties. By adjusting the proportion, size, and morphology of primary and secondary α phases, both the strength and toughness of titanium alloys can be enhanced. Solution treatment followed by aging is the most common process for near-α high-temperature titanium alloys.

One study investigated the effect of solution temperature on the microstructure and mechanical properties of a Ti-6Al-3.5Sn-4.5Zr-0.5Mo-0.4Si-0.7Nb-2.0Ta-0.1Er-0.06C (wt.%) alloy. As the solution temperature increased, the amount of primary α decreased, while the β-transformed structure (βt) increased. At 1000 °C, the alloy's ultimate tensile strength increased by 119 MPa and elongation by 18.7%.

Another study examined a two-step solution treatment plus aging for Ti-6Al-4Sn-8Zr-0.8Mo-1W-1Nb-0.25 (wt.%) alloy. The first solution treatment introduced a high-temperature β-Ti phase, which transformed into martensite during cooling. This martensite decomposed in the α+β region to form a β-transformed matrix. The second solution treatment coarsened equiaxed and lamellar α phases, and aging stabilized the structure. The resulting alloy exhibited multiple microstructures, improving creep resistance.

A Ti-5.86Al-3.69Sn-3.56Zr-0.99Mo-0.36Nb-0.38Si-0.18Ta-0.13O-0.011C (wt.%) alloy subjected to different solution and aging temperatures showed that solution temperature strongly influenced microstructure and strength. Treatments at 900 °C and 950 °C for 2h produced dual-scale silicides, which enhanced dislocation motion and improved strength. Aging temperature, however, had little effect on mechanical properties.

Studies on Ti60 (Ti-5.8Al-4.8Sn-2Zr-1Mo-0.35Si-0.85Nd, wt.%) found that higher solution temperatures reduced equiaxed α content and increased Al concentration in both α and β phases, while Mo concentration in β remained stable. Lower cooling rates produced coarser α lamellae with narrower spacing.

For the near-α titanium alloy IMI 834 (Ti-5.8Al-4Sn-3.5Zr-0.5Mo-0.7Nb-0.35Si-0.05C, wt.%), researchers optimized solution temperature and cooling rate to maximize strength and toughness. They found that higher solution temperatures reduced both volume fraction and size of equiaxed α, increasing interparticle spacing. Lower cooling rates produced more and larger α particles. Optimal performance was achieved at a solution temperature of 1029 °C and a cooling rate between oil and air cooling (1769 °C/min).

(Reference: Microstructure and Strengthening and Toughening of Powder Metallurgy Near-α High-Temperature Titanium Alloys – Zheng Dongyang)