Duplex Heat treatment of Titanium Alloys: Part One

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Introduction to Titanium Alloy Classification and Properties

High strength, low density, and excellent corrosion resistance represent the primary properties that make titanium attractive for a variety of applications including aircraft components, biomedical devices, and components in chemical processing environments. Commercial titanium alloys are classified conventionally into three different categories as α, α+β, and β alloys according to their equilibrium constitution, which varies with the types and concentrations of alloying elements.

Commercially pure titanium possesses an all-alpha structure and demonstrates superior corrosion resistance but exhibits inferior mechanical properties compared to titanium alloys. When compared with beta titanium alloys, alpha titanium alloys are superior in heat resistance and weldability but inferior in strength and workability characteristics.

Beta titanium alloys are solution-strengthened alloys created by adding beta structure stabilizers. An all-beta structure at room temperature can be obtained by rapidly cooling the specimen through solution treatment. Alpha phase precipitates within an all-beta structure through aging treatment. Alloys having a beta structure with precipitated alpha phase exhibit excellent strength characteristics. Two-phase α+β alloys with a dispersion of the beta form in the alpha phase exhibit properties characteristic of each individual phase.

Comprehensive Heat Treatment Objectives for Titanium Alloys

Titanium and titanium alloys undergo heat treatment to achieve several critical objectives. Stress relief treatments reduce residual stresses developed during fabrication processes. Annealing treatments produce an optimum combination of ductility, machinability, and dimensional and structural stability. Solution treating and aging processes increase strength properties significantly. Additionally, specialized heat treatments optimize specific properties such as fracture toughness, fatigue strength, and high-temperature creep strength.

Within the family of titanium alloys, the α+β alloys are the most widely used because of the great variety of microstructures and mechanical properties that can be obtained by varying their composition and thermomechanical treatments. One of these α+β alloys has the greatest commercial importance, namely Ti-6Al-4V, which represents more than half of all titanium alloy sales worldwide.

Microstructural Control in Alpha-Beta Titanium Alloys

To control the microstructure of titanium alloys effectively, most studies in the α+β field focus on the evolution of the primary α phase. Additionally, understanding the effect of processing parameters on the precipitation mechanism of secondary α phase is important for controlling microstructure through processing parameters in high-temperature deformation of titanium alloys.

For α+β alloys as well as β alloys, the mechanical properties relevant for particular applications are optimized through microstructural control. A prerequisite for property optimization is profound knowledge of microstructure-property relationships and understanding of the microstructure's evolution as a result of processing conditions.

Annealing heat treatments below β transus temperature followed by different cooling rates are applied to hot-deformed samples in the α+β field. Experimental results demonstrate the influence of deformation temperature on the final microstructure after annealing. The subsequent effect of aging treatment after annealing shows strong interdependence of the resulting cooling rate on the final microstructure. Additionally, combinations of isothermal heat treatments below β transus followed by controlled cooling rates are applied to samples deformed in the α+β field with the intention of obtaining recrystallized structures.

Duplex Heat Treatment Studies of Ti-6Al-4V Alloy

Experimental Investigation of Short-Time Duplex Treatment

The effect of short-time duplex heat treatment on the microstructure and mechanical properties of Ti-6Al-4V alloy was investigated in comprehensive studies by T. Morita et al. This duplex heat treatment consisted of solution treatment at 1203K for 60 seconds followed by water quenching, plus aging at 753K, 853K, and 953K for 40 seconds.

The yield strength and tensile strength of the alloy were significantly increased by this heat treatment protocol, with maximum improvement rates reaching approximately 25%. This strengthening was attributed to the formation of α'-martensite phase through quenching after short-time solution treatment and the precipitation of fine α-phase in the retained β-phase during short-time aging.

Material Preparation and Heat Treatment Procedures

The starting material consisted of mill-annealed Ti-6Al-4V rods with 14mm diameter. The specimens were solution-treated at 1203K for 60 seconds in air and water-quenched, then aged at temperatures ranging from 753K to 953K for durations from 40 seconds to 16.2 kiloseconds. After heat treatments, the test sections were mechanically polished with emery papers (100-2000 mesh) and alumina powders (diameter: 0.03 mm).

Figure 1: Conditions of heat treatments

The specimens were heat-treated in air under the conditions illustrated in the figure. The solution treatment and quenching process is designated as "STQ treatment," while the duplex heat treatment consisting of STQ treatment and aging is called "STA treatment."

Hardness Evolution During Duplex Heat Treatment

Figure 2: Aging curves

The hardness measurements revealed significant increases through the STQ treatment alone. This hardness increase was primarily attributed to the formation of α'-phase through quenching. With subsequent aging for short durations (40 seconds), hardness increased further beyond the STQ condition values, except for aging at 953K.

The time required to reach maximum hardness was shortened by elevating the aging temperature, and hardness reduction began earlier at higher temperatures. Particularly noteworthy is that aging at 953K resulted in continuous hardness decrease from 40 seconds without any initial hardness increase. This acceleration of aging with temperature elevation probably resulted from the rapid decomposition of the α'-phase.

Optimal Aging Conditions and Implications

A significant finding was that the hardness of material aged at 853K for 40 seconds was approximately equal to that obtained by aging at 753K and 803K for longer durations. This result suggests that aging at 853K can remarkably improve the alloy's strength in a short time, making it particularly valuable for industrial applications where processing time is critical.

The experimental focus on aging for 40 seconds demonstrated the potential for achieving substantial property improvements through carefully controlled and time shortened duplex heat treatments, offering significant advantages in terms of energy efficiency and production throughput while maintaining or enhancing mechanical properties.

Advanced Applications and Future Perspectives

The development of duplex heat treatment processes for titanium alloys represents a significant advancement in materials processing technology. These treatments enable the optimization of multiple properties simultaneously, making titanium alloys even more attractive for demanding applications in aerospace, medical, and industrial environments.

The ability to achieve 25% strength improvements through carefully controlled duplex treatments while maintaining other desirable properties opens new possibilities for weight reduction and performance enhancement in critical applications. Future research directions may focus on extending these principles to other titanium alloy systems and optimizing treatment parameters for specific application requirements.

Conclusion

Duplex heat treatment of titanium alloys, particularly the Ti-6Al-4V system, demonstrates remarkable potential for achieving enhanced mechanical properties through controlled microstructural evolution. The formation of α'-martensite phase during quenching followed by fine α-phase precipitation during aging provides the mechanism for significant strength improvements. Short-time duplex treatments offer practical advantages for industrial implementation while achieving substantial property enhancements, making this technology valuable for advancing titanium alloy applications in critical engineering sectors.