鈦is an element with the atomic number 22 in the periodic table. It belongs to the group IVB in the fourth period, along with zirconium and hafnium. These elements are characterized by high melting points and the formation of stable oxide films on their surfaces at room temperature.
Major Properties of Titanium
1. Low Density, High Strength, High Specific Strength
Titanium has a density of 4.51 g/cm³, 這是 57% that of steel.
It is less than twice as heavy as aluminum but three times stronger.
The specific strength (strength-to-density ratio) of titanium alloys is the highest among common industrial alloys.
Titanium alloys have a specific strength:
3.5 times that of stainless steel
1.3 times that of aluminum alloys
1.7 times that of magnesium alloys
These properties make titanium alloys essential structural materials for the aerospace industry.
2. Excellent Corrosion Resistance
Titanium’s corrosion resistance is due to the presence of an oxide film.
It performs better in oxidizing media compared to reducing media, where high-rate corrosion can occur.
Titanium is resistant to many corrosive environments, such as:
Seawater
Wet chlorine gas
Hypochlorite and perchlorate solutions
Nitric acid
Chromic acid
Metal chlorides
Sulfides
Organic acids
In environments that generate hydrogen through reaction (e.g., hydrochloric and sulfuric acids), titanium typically has a higher corrosion rate.
Adding small amounts of oxidizing agents to the acid can form a passivating film on titanium.
Thus, titanium is resistant to corrosion in:
Strong sulfuric-nitric acid mixtures
Hydrochloric-nitric acid mixtures
Hydrochloric acid containing free chlorine
Its protective oxide film forms when the metal contacts water, even in small amounts of water or steam.
In completely water-free strong oxidizing environments, rapid oxidation and violent reactions, including spontaneous combustion, can occur.
Such phenomena have been observed in reactions with:
To prevent these reactions, a certain amount of moisture must be present.
3.Good Heat Resistance
While aluminum loses its original properties at 150°C and stainless steel at 310°C, titanium alloys maintain good mechanical properties at around 500°C.
For aircraft flying at Mach 2.7, the surface temperature reaches 230°C, making aluminum and magnesium alloys unsuitable.
Titanium alloys can meet the requirements for these high-temperature conditions.
Titanium’s heat resistance makes it suitable for:
Compressor discs and blades in aircraft engines
The skin of the rear fuselage.
4. Excellent Low-Temperature Performance
Certain titanium alloys (e.g., Ti-5Al-2.5SnELI) increase in strength as temperature decreases, with only a slight reduction in plasticity.
These alloys maintain good ductility and toughness at low temperatures.
These properties make titanium suitable for use in ultra-low temperature environments, such as:
Liquid hydrogen and liquid oxygen rocket engines
Ultra-low temperature containers and storage tanks of spacecraft.
5. Non-Magnetic
Titanium is non-magnetic, making it useful for submarine hulls to prevent triggering mines.
6. Low Thermal Conductivity
Titanium has low thermal conductivity, only:
1/5 that of steel
1/13 that of aluminum
1/25 that of copper
While poor thermal conductivity can be a disadvantage, it can also be advantageous in certain applications.
7. Low Elastic Modulus
The elastic modulus of titanium is only 55% that of steel, which can be a disadvantage when used as a structural material.
8. Tensile Strength Close to Yield Strength
The tensile strength of Ti-6Al-4V鈦合金is 960 MPa, with a yield strength of 892 MPa, a difference of only 58 MPa.
9. Prone to Oxidation at High Temperatures
Titanium strongly binds with hydrogen and oxygen, requiring precautions to prevent oxidation and hydrogen absorption.
Welding titanium must be done under argon protection to avoid contamination.
Titanium pipes and thin sheets need to be heat-treated under vacuum.
Titanium forgings require controlled mildly oxidizing atmospheres during heat treatment.
10. Low Damping Capacity
If you make clocks of the same shape and size from titanium and other metals (copper, steel), and strike them with equal force, the sound from the titanium clock will resonate longer, indicating low damping capacity.
Three Special Functions of Titanium
Shape Memory Function
Ti-50%Ni (atomic) alloys can return to their original shape under certain temperature conditions. These materials are called shape memory alloys.
Superconducting Function
Nb-Ti alloys lose electrical resistance and allow any amount of current to pass without heating or energy loss when the temperature approaches absolute zero. Nb-Ti is known as a superconducting material.
Hydrogen Storage Function
Ti-50%Fe (atomic) alloys can absorb large amounts of hydrogen, allowing safe storage without the need for steel high-pressure cylinders. Under certain conditions, Ti-Fe alloys can also release the stored hydrogen, making them energy storage materials.
What are the Disadvantages of Titanium?
High Cost
Titanium is 5-10 times more expensive than stainless steel.
Difficult to Process
As an active metal, titanium requires a more stringent processing environment compared to stainless steel.
Poor Thermal Conductivity
It easily scorches and sticks when used in cookware, but it enhances insulation when used in thermos flasks.
Low Surface Hardness
Compared to stainless steel, which typically achieves a surface hardness of HRC56-58, pure titanium has a surface hardness of about HRC20, making it more prone to scratches during use.
Poor Polishing Performance
Titanium is stickier during polishing processes, making mirror polishing more challenging than with stainless steel.
Easily Shows Fingerprints
Compared to copper and stainless steel, titanium surfaces are more likely to retain fingerprints, which are also harder to remove.
Reference
Materials for Mechanical Engineering. (n.d.). Materials for Mechanical Engineering. ISSN 1000-3738, CN 31-1336/TB.