Is Titanium Alloy Stronger Than Titanium?

Multi-dimensional Performance Analysis of Titanium and Titanium Alloys: Strength, Modulus, and High and Low Temperature Performance

1. Strength and Hardness Properties of Titanium and Titanium Alloys

△ Strength Index

Titanium alloys are highly sought after in many fields primarily due to their exceptional strength. While the tensile strength of pure titanium ranges from 265 to 353 MPa, the strength of common titanium alloys is significantly higher, reaching 686 to 1176 MPa. Some specialized titanium alloys even reach 1764 MPa, demonstrating a specific strength comparable to many steel grades. This not only enhances the material’s load-bearing capacity but also demonstrates its exceptional durability in various engineering applications.

In compression testing, titanium and titanium alloys demonstrate strength comparable to their tensile strength. For commercially pure titanium, the compressive yield strength is similar to the tensile yield strength, while titanium alloys such as Ti-6AI-4V and Ti-5AI-2.5Sn exhibit slightly higher compressive strength than their tensile strength. Furthermore, shear strength, a key material property indicator, is typically approximately 60% to 70% of tensile strength, further highlighting the high strength characteristics of titanium alloys.

At room temperature and pressure, processed and annealed titanium and titanium alloys exhibit a high ultimate tensile strength, reaching 0.5 to 0.65 times the ultimate tensile strength. Under specific fatigue testing conditions, for example, annealed Ti-6AI-4V can achieve an ultimate tensile strength of 0.2 times the ultimate tensile strength in a notched state, further demonstrating its exceptional durability.

△ Hardness

Hardness is an important indicator of a material’s hardness. The highest purity grades of processed industrial-pure titanium have a relatively low hardness, typically less than 120 HB (Brinell hardness), while other industrial-pure titanium grades range from 200 to 295 HB. In contrast, pure titanium castings have a slightly higher hardness of 200 to 220 HB. For titanium alloys, the hardness in the annealed state is 32 to 38 HRC (Rockwell hardness), equivalent to 298 to 349 HB. Notably, the hardness of as-cast Ti-5Al-2.5Sn and Ti-6Al-4V, as well as Ti-6Al-4V castings with low interstitial inclusions, is higher than that of the annealed state.

2. Modulus and Toughness of Titanium and Titanium Alloys

△ Elastic Modulus

The tensile modulus of commercially pure titanium ranges from 105 to 109 GPa, while most titanium alloys reach a tensile modulus of 110 to 120 GPa in the annealed state, demonstrating high rigidity. Notably, age-hardened titanium alloys exhibit a slightly higher tensile modulus, while their compressive modulus is equal to or higher than the tensile modulus. The specific elastic modulus of titanium alloys is comparable to that of aluminum alloys and is second only to beryllium, molybdenum, and certain high-temperature alloys, demonstrating their excellent mechanical properties.

△ Toughness and Impact Resistance

Many titanium alloys, such as Ti-6Al-4V, exhibit good fracture toughness, meaning they effectively resist crack propagation. For example, annealed Ti-6AI-4V exhibits excellent toughness, with a notch-to-unnotched tensile strength ratio greater than 1 at a notch concentration factor (Kt) of 25.4 mm. The Charpy notched impact strength of modified commercially pure titanium varies depending on the type and state of the titanium alloy, ranging from 15 to 54 J/m2. Cast titanium alloys, such as Ti-5AI-2.5Sn, have a Charpy V-notched impact strength of 10 J/m2, while Ti-6AI-4V has a Charpy V-notched impact strength of 20 to 23 J/m2.

3. High- and Low-Temperature Properties of Titanium and Its Alloys

△ High-Temperature Performance

Titanium alloys maintain stable properties even at high temperatures. While general industrial titanium alloys can be used for short periods at 540°C, their optimal long-term operating temperature range is 450-480°C. Currently, titanium alloys capable of operating at 600°C have been developed. Furthermore, titanium alloys are widely used in missile materials, capable of long-term operation at 540°C and even short-term use at 760°C.

Low-Temperature Performance

Titanium and its alloys retain their inherent mechanical properties at low temperatures. Strength gradually increases with decreasing temperature, while ductility decreases. However, many annealed titanium alloys maintain adequate ductility and fracture toughness at -195.5°C. Notably, the Ti-5AI-2.5Sn titanium alloy, which contains very low levels of interstitial elements, can even operate at -252.7°C, maintaining a notched to unnotched tensile strength ratio of 0.95 to 1.15 at -25.7°C.

Liquid oxygen, liquid hydrogen, and liquid fluorine are key propellants in missiles and spacecraft, and their low-temperature properties are crucial for the manufacture of cryogenic gas containers and structural components. Titanium alloys, particularly those with an equiaxed microstructure and extremely low levels of interstitial elements (such as oxygen, helium, and hydrogen), can retain over 5% ductility even at extremely low temperatures of -252.7°C. The elongation of Ti-6AI-4V titanium alloy at the same temperature is as high as 12%, showing its excellent low-temperature performance.

Keywords：

Pure Titanium

Titanium Alloy