1. Performance Differences of Pure Titanium with Different Purity Levels
1.1 Mechanical Properties
Strength and Hardness
Low-purity commercial titanium (Grade 3/Grade 4 CP titanium): With higher impurity content (e.g., oxygen content up to 0.35% in Grade 4), interstitial atoms (O, N, C) act as "hardening agents" that distort the titanium lattice, significantly enhancing tensile strength and hardness. Grade 4 CP titanium has a tensile strength of 550–650 MPa and a Brinell hardness of 150–180 HB, which is nearly twice the strength of Grade 1 titanium. However, this comes at the cost of reduced ductility, with an elongation at break of only 10–15%.
Medium-purity commercial titanium (Grade 2 CP titanium): With a balanced impurity content (oxygen content ~0.25%), it achieves a trade-off between strength and ductility, boasting a tensile strength of 345–450 MPa, an elongation of 18–25%, and is the most widely used grade in general engineering applications.
High-purity titanium (purity ≥99.9%): With impurity content (total interstitial elements <0.05%) minimized, the lattice structure remains highly regular, resulting in extremely low strength (tensile strength ~200–250 MPa) and hardness (Brinell hardness <100 HB). However, it exhibits exceptional ductility, with elongation at break exceeding 30%, and can even be cold-formed into ultra-thin sheets or fine wires without cracking.
Fatigue and Impact Resistance
Low-purity CP titanium (Grade 3/4) has higher fatigue strength (up to 200–250 MPa for 10⁷ cycles) due to its increased strength, but its impact toughness is poor (Charpy impact energy <20 J at room temperature) because impurities cause stress concentration and reduce crack resistance.
High-purity titanium has lower fatigue strength (~100–120 MPa) but superior impact toughness (Charpy impact energy >40 J at room temperature), as the absence of impurity-induced defects allows the material to absorb more energy during deformation or impact.
1.2 Physical Properties
Thermal and Electrical Conductivity
Impurities in low-purity titanium scatter heat carriers (phonons and free electrons), leading to lower thermal conductivity (18–20 W/(m·K) for Grade 4) and electrical conductivity (1.8–2.0 MS/m).
High-purity titanium has minimal electron and phonon scattering, with thermal conductivity increased to 25–30 W/(m·K) and electrical conductivity up to 3.0–3.5 MS/m, making it more suitable for applications requiring stable thermal or electrical transmission.
Thermal Stability and Melting Point
Impurities in low-purity titanium lower its recrystallization temperature (to 500–550°C) and cause grain coarsening at high temperatures, reducing high-temperature structural stability.
High-purity titanium has a higher recrystallization temperature (600–650°C) and maintains a fine, uniform grain structure at elevated temperatures (up to 400°C), ensuring consistent performance under long-term high-temperature conditions.
1.3 Chemical and Corrosion Resistance
General Corrosion Resistance
All pure titanium grades form a dense, self-healing TiO₂ passivation film, providing excellent resistance to most acids (except concentrated HF) and alkalis. However, impurity levels affect passivation film quality:
Low-purity titanium (Grade 4) may form localized impurity-rich regions that weaken the passivation film, leading to pitting corrosion in high-chloride environments (e.g., seawater with high salinity).
High-purity titanium forms a uniform, defect-free passivation film, with corrosion rates <0.001 mm/year in 10% H₂SO₄ at room temperature and near-zero corrosion in most ultra-pure chemical media.
Hydrogen Embrittlement Susceptibility
Impurities like iron and carbon in low-purity titanium accelerate hydrogen absorption, causing hydrogen embrittlement at temperatures above 200°C, which leads to brittle fracture under stress. High-purity titanium has minimal hydrogen absorption (hydrogen content <0.001%) and is immune to hydrogen embrittlement even in high-temperature hydrogen-containing environments.
1.4 Processing Performance
Cold Working Machinability
High-purity titanium's high ductility allows for deep cold drawing, stamping, and spinning, enabling the production of complex-shaped components (e.g., thin-walled tubes with a wall thickness of <0.1 mm). In contrast, low-purity Grade 4 titanium has poor cold formability and requires intermediate annealing during processing to avoid cracking.
Weldability
High-purity titanium has low impurity content, eliminating the risk of impurity-induced weld cracking and forming weld joints with mechanical properties nearly identical to the base metal. Low-purity titanium may produce brittle intermetallic phases (e.g., TiC, TiN) in weld zones, reducing joint toughness by 30–40%.
2. Main Applications of High-Purity Titanium
2.1 Semiconductor and Electronic Industry
Wafer Processing Equipment: High-purity titanium is used to fabricate components for chemical vapor deposition (CVD) chambers, etchers, and ion implanters. Its low impurity content prevents contamination of silicon wafers, while its corrosion resistance withstands aggressive etchants (e.g., HF/HNO₃ mixtures).
Vacuum Electronic Components: It is used to make electron tube cathodes, getter materials, and thin-film substrates. Its high electrical conductivity and thermal stability ensure stable electron emission and heat dissipation in high-vacuum, high-temperature environments.
Printed Circuit Board (PCB) Substrates: Ultra-thin high-purity titanium foils serve as flexible conductive substrates for high-frequency PCBs, as they do not interfere with signal transmission and have excellent fatigue resistance under repeated bending.
2.2 Biomedical Engineering
High-Precision Implants: For specialized applications like cochlear implants, neurostimulation electrodes, and microvascular stents, high-purity titanium's superior biocompatibility (no cytotoxic impurities) and ductility allow it to be processed into micro-scale, flexible structures that adapt to human tissue movement without causing inflammation or rejection.
Medical Device Components: It is used in surgical instrument tips, dental implant abutments, and diagnostic sensor casings, where its corrosion resistance in bodily fluids (blood, saliva) and low magnetic susceptibility (compatible with MRI scans) are critical advantages.
2.3 Aerospace and Aviation
Aerospace Propulsion Systems: High-purity titanium is used for cryogenic fuel tank liners and liquid oxygen/liquid hydrogen pipeline components in rocket engines. Its low hydrogen embrittlement susceptibility and ductility at cryogenic temperatures (-253°C) prevent cracking under extreme thermal cycling, while its chemical purity avoids reactions with propellants.
Satellite Structural Parts: For lightweight, high-precision satellite components (e.g., antenna supports, thermal control panels), its stable thermal expansion coefficient (10.8×10⁻⁶/°C) and high purity ensure dimensional stability in the extreme temperature fluctuations of space.
2.4 Chemical and Nuclear Industry
Ultra-Pure Chemical Processing: In the production of high-purity chemicals (e.g., pharmaceutical intermediates, electronic-grade reagents), high-purity titanium reactors and pipelines prevent impurity leaching into the product, ensuring compliance with strict purity standards (e.g., USP Class VI for pharmaceuticals).
Nuclear Reactor Components: It is used for cladding of research reactor fuel rods and neutron moderator containers. Its low neutron absorption cross-section (reducing neutron loss) and resistance to corrosion by liquid sodium or boric acid coolant make it ideal for nuclear applications.
2.5 Advanced Material Manufacturing
Titanium Alloy Master Alloys: High-purity titanium serves as a base material for producing high-performance titanium alloys (e.g., Ti-6Al-4V ELI for medical use), as its low impurity content ensures the alloy's consistent mechanical and biocompatible properties.
Thin-Film and Coating Technology: High-purity titanium sputtering targets are used in depositing titanium films for solar cells, optical coatings, and hard disk drive (HDD) magnetic layers, where uniform purity guarantees film adhesion and performance.
