Commercially pure (CP) titanium is defined by its high purity-it contains ≥99% titanium by weight, with only trace amounts of "interstitial impurities" (primarily oxygen, nitrogen, carbon, and hydrogen). These impurities are not added intentionally; instead, they are tightly controlled during manufacturing to prevent brittleness or performance loss. No other metallic elements (e.g., aluminum, vanadium) are purposefully included to modify properties.
Common grades of CP titanium (e.g., Grade 1, Grade 2, Grade 3, Grade 4) differ mainly in their oxygen content: higher oxygen levels slightly increase strength but reduce ductility, but the material remains fundamentally "pure titanium" at its core.
Titanium alloy is a blended material: it uses titanium as the base metal (the "balance" of the composition) but intentionally incorporates other metallic elements ("alloying elements") to enhance specific properties. These alloying elements typically make up 1–15% of the total composition, and their selection is tailored to target performance goals.
Common alloying elements include:
Aluminum (strengthens the material and stabilizes the alpha-phase, improving high-temperature stability).
Vanadium (stabilizes the beta-phase, boosting toughness and enabling heat treatment for further strengthening).
Tin (enhances strength at elevated temperatures).
Molybdenum (improves corrosion resistance in aggressive environments like strong acids).
The most widely used titanium alloy is Ti-6Al-4V (Grade 5), which contains ~6% aluminum and ~4% vanadium-these elements transform its performance far beyond that of pure titanium.
The presence of alloying elements creates a dramatic gap in mechanical performance between CP titanium and titanium alloys:
CP titanium cannot be significantly strengthened by heat treatment. The only common heat treatment for pure titanium is "annealing" (heating to ~650–700°C, then slow cooling), which relieves internal stress from manufacturing (e.g., cold working) and restores ductility-but does not increase strength. Any strength gains for CP titanium must come from cold working (e.g., rolling, drawing), which hardens the material but reduces its ductility.
Most titanium alloys respond strongly to heat treatment, allowing manufacturers to "tune" their properties for specific needs. The most common process for alloys like Ti-6Al-4V is solution treatment and aging (STA):
Heat the alloy to a temperature in the alpha-beta phase region (e.g., 920–960°C) to dissolve alloying elements into the titanium matrix.
Quench (rapidly cool) it to trap elements in a metastable state.
"Age" it at a lower temperature (e.g., 480–650°C) to precipitate tiny, uniform particles that block dislocation movement-drastically increasing strength without sacrificing too much toughness.
This heat treatability is a key reason titanium alloys are used in high-performance applications.
CP titanium has excellent corrosion resistance in mild to moderate environments, thanks to a dense, self-healing titanium dioxide (TiO₂) passive film on its surface. It performs well in freshwater, seawater, neutral/weak acids, and biocompatible settings (e.g., medical devices). However, it is vulnerable to strong reducing acids (e.g., concentrated hydrochloric acid) and high-temperature oxidizing environments (above ~300°C), where the passive film breaks down.
Titanium alloys retain good corrosion resistance but are often tailored for specific aggressive environments via alloying:
Alloys with palladium (e.g., Ti-Pd, Grade 7) have enhanced resistance to crevice corrosion in hot, concentrated chloride solutions (e.g., seawater-based chemicals).
Alloys with molybdenum (e.g., Ti-15Mo, Grade 13) excel in strong reducing acids like sulfuric acid.
Ti-6Al-4V (Grade 5) has good general corrosion resistance but is slightly less resistant to crevice corrosion in hot chlorides than CP titanium-its strength benefits outweigh this tradeoff for most high-load uses.
CP titanium is highly formable, especially lower-grade variants like Grade 1. Its low strength and high ductility allow it to be cold-worked into complex shapes (e.g., thin foils, tight-radius bends, small-diameter tubes) with minimal force and low cracking risk. Machinability is moderate: its softness reduces tool wear, though titanium's low thermal conductivity still requires coolants to prevent overheating.
Titanium alloys are poorly formable compared to CP titanium, especially in their heat-treated (high-strength) state. Cold forming requires extreme force and often pre-heating (to ~300–500°C) to avoid fracturing; complex shapes are typically made via forging or casting, not bending/rolling. Machinability is difficult: their high hardness and low thermal conductivity cause rapid tool wear, requiring specialized carbide tools, slow cutting speeds, and heavy coolants-making machining 2–3x more costly than for CP titanium.
Cost: Lower (among the most affordable titanium materials), as it requires no expensive alloying elements and simpler manufacturing.
Applications: Ideal for low-to-moderate strength needs where formability or purity matters, such as:
Chemical processing: Thin-walled tubes, tanks for non-aggressive fluids.
Medical devices: Flexible components (e.g., catheter shafts, surgical staples).
Consumer goods: Decorative parts (e.g., jewelry, watch bands), lightweight fasteners.
Cost: Higher (2–3x more expensive than CP titanium), due to alloying elements, complex smelting, and difficult machining/heat treatment.
Applications: Reserved for high-performance, high-load scenarios where strength and strength-to-weight ratio are critical, such as:
Aerospace: Aircraft frames, engine blades, landing gear.
Medical: Load-bearing implants (e.g., hip/knee replacements, spinal rods).
Automotive: High-performance parts (e.g., racing engine valves, exhaust components).
Industrial: Heavy-duty pressure vessels, turbine components for power generation.
