Nitinol - NiTi / F2063
Description
Nitinol is a nickel titanium alloy based on the NiTi system and is specified in medical material standards such as ASTM F2063 as a nickel titanium shape memory alloy. The alloy Nitinol according to ASTM F2063 consists primarily of nickel and titanium in a near equiatomic ratio and belongs to the group of shape memory alloys with reversible martensitic phase transformation.
The chemical composition of the nickel titanium alloy Nitinol according to ASTM F2063 is approximately 55 mass percent nickel and 45 mass percent titanium. Maximum allowable impurity levels include carbon 0.05 percent, cobalt 0.05 percent, copper 0.01 percent, chromium 0.01 percent, hydrogen 0.05 percent and iron 0.02 percent. This chemical composition is defined for medical nickel titanium alloys specified under ASTM F2063.
Nitinol according to ASTM F2063 exhibits a reversible phase transformation between martensitic and austenitic phases. This phase transformation forms the basis of the shape memory effect and the superelastic properties of the nickel titanium alloy Nitinol. The reversible transformation enables elastic deformation followed by recovery of the original shape through martensitic transformation and reverse transformation to the austenitic phase.
The density of Nitinol is approximately 6.5 g/cm3. The melting temperature is approximately 1310 °C. The modulus of elasticity of the austenitic phase is approximately 75 to 80 GPa, while the modulus of elasticity of the martensitic phase is approximately 28 to 40 GPa. The electrical resistivity at 20 °C is approximately 82 ×10⁻⁸ Ωm for austenite and approximately 76 ×10⁻⁸ Ωm for martensite. Thermal conductivity at 20 °C is approximately 18 W/mK for austenite and approximately 9 W/mK for martensite. The coefficient of thermal expansion at 20 °C is approximately 11 ×10⁻⁶/K for austenite and approximately 6.6 ×10⁻⁶/K for martensite.
The mechanical and functional properties of Nitinol according to ASTM F2063 are controlled by thermo-mechanical processing and heat treatment. Test methods for determining superelastic properties of nickel titanium alloys include ASTM F2516. Delivery conditions for Nitinol according to ASTM F2063 include annealed, cold worked or heat treated conditions.
Typical applications of Nitinol according to ASTM F2063 and nickel titanium NiTi alloys include self expanding stents, medical guidewires, orthodontic archwires, vena cava filters, surgical instruments and miniature actuators.