Alpha-Beta Titanium Alloys: Heat Treatment and Properties

Alpha-beta titanium alloys contain both phases at room temperature and combine the formability of beta with the stability and strength of alpha. Their final properties depend strongly on solution temperature, deformation history, cooling rate, and aging. Ti-6Al-4V is the best-known example.

Beta Annealing and Lamellar Structures

Heating above the beta transus removes primary alpha. During cooling, alpha nucleates at prior-beta grain boundaries and grows inward as colonies of parallel plates. Slow cooling produces coarse alpha plates; faster cooling refines the plates and may form alpha-prime martensite. Coarser lamellar structures generally improve fracture toughness and creep resistance, while finer structures increase strength and fatigue resistance but may reduce ductility.

Cooling Rate and Phase Transformation

Cooling rate determines whether beta decomposes diffusively into alpha plus retained beta or transforms into martensitic products. It also changes colony size and alpha-lath thickness. The resulting strength-ductility balance must therefore be selected together with section size and quenching capability.

Alpha-Beta Annealing and Bimodal Structures

Annealing in the alpha-beta field retains a controlled fraction of equiaxed primary alpha. The remaining beta transforms during cooling, producing secondary lamellar alpha within a beta matrix. This bimodal structure combines equiaxed-alpha fatigue resistance and ductility with the fracture toughness and creep performance of transformed beta.

Two-stage annealing can first establish primary-alpha fraction and morphology, then stabilize the transformed beta. Solution treatment and aging strengthen the alloy through controlled precipitation, but excessive temperature, holding time, or slow cooling can coarsen the structure. Process design should therefore specify temperature relative to the beta transus, cooling path, and aging response rather than relying on a heat-treatment label alone.

Property Tradeoffs

• Equiaxed structures favor ductility and high-cycle fatigue resistance.
• Lamellar structures favor crack-growth resistance, fracture toughness, and creep.
• Bimodal structures provide a practical balance for many aerospace parts.
• Fine transformed products raise strength but can reduce tensile elongation.
• Prior-beta grain size, primary-alpha fraction, and alpha-lath spacing are critical control variables.