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Description

Nitinol is a shape memory alloy composed of nickel (Ni) and titanium (Ti) in an approximately 1:1 atomic ratio. Owing to its unique shape memory effect and superelasticity, it has become a core material in high-tech fields such as medical devices and aerospace.

Key Characteristics

1. Shape Memory Effect: The alloy can be arbitrarily deformed at low temperatures (martensitic phase). Upon heating above the transformation temperature (Af), it automatically recovers its original shape at high temperatures (austenitic phase).

2. Superelasticity (Pseudoelasticity): Above the transformation temperature, the material can withstand an ultra-high elastic strain of 8%–10% (compared to <1% for ordinary metals) and immediately returns to its original shape upon removal of the applied stress.

3. Biocompatibility: A titanium dioxide (TiO₂) passivation film forms on the surface, which inhibits the release of nickel ions. The material is non-toxic, corrosion-resistant, and compatible with human tissue.

4. Other Excellent Properties

Corrosion Resistance: Superior to stainless steel, resistant to bodily fluids, seawater, acids, and alkalis.
Vibration Damping: Internal phase transformation dissipates energy, providing excellent damping capacity.
Physical Parameters: Density 6.45 g/cm³, melting point approx. 1310°C.

Main Application Areas

1. Medical

Interventional Devices: Vascular stents, heart occluders, filters.
Orthodontics: Superelastic archwires, orthodontic wires.
Orthopedics / Surgery: Intramedullary nails, bone plates, surgical suturing needles.

2. Aerospace

Satellite Antennas: Folded during launch and automatically deployed when exposed to sunlight in orbit.
Fasteners / Seals: Temperature-controlled self-tightening, anti-loosening, high-temperature sealing.

3. Industrial & Civil

Electronics / Automotive: Temperature switches, sensors, shock absorbers.

Common Grades

NiTi-01 (55-Nitinol): Approximately 55 wt% Ni, transformation temperature 0–40°C. The most widely used grade.
NiTi-02: With niobium (Nb) addition, offering a wider transformation temperature range and larger hysteresis, used for temperature-control components.

Processing Key Points

Use vacuum arc/induction melting → hot forging/cold rolling → heat treatment (solution treatment + aging) to precisely control the transformation temperature.
Annealing must be performed after cold working; otherwise, the material becomes brittle and prone to cracking.

Complete Physical & Mechanical Properties Table of Nitinol (55NiTi)

Standard Medical Austenitic State (Af ≈ 37°C, body temperature type)
(The table data are typical engineering averages, suitable for direct use in drawings, reports, and material selection.)

Basic Physical Properties

Property	Value	Remarks
Alloy Composition	Ni ≈ 55 wt%, Ti ≈ 45 wt%	Standard nickel-titanium shape memory alloy
Density	6.45 g/cm³ / 6450 kg/m³	Lighter than titanium alloy and stainless steel
Melting Point	1280–1310 °C
Thermal Conductivity	18–20 W/(m·K)	Poor thermal conductivity, prone to overheating during processing
Electrical Resistivity	80–100 μΩ·cm	Poor electrical conductivity; can be electrically heated for phase transformation
Coefficient of Linear Expansion (Austenite)	11×10⁻⁶ /°C	Significant volume change during phase transformation
Magnetic Properties	Non-magnetic (weakly paramagnetic)	MRI compatible, no artifacts

Mechanical & Physical Properties (Core)

Property	Austenite (Above body temperature)	Martensite (Low temperature)
Young's Modulus	60–80 GPa	20–30 GPa
Yield Strength	500–800 MPa	Very low, easily deformable
Tensile Strength	900–1200 MPa	—
Maximum Superelastic Recoverable Strain	6%–8%	Can be plastically bent arbitrarily
Fatigue Performance	Excellent, withstands millions of cycles without failure	Poor
Transformation Temperature Af (Medical Grade)	32–37 °C	Below this temperature, martensite (soft)

Corrosion Resistance & Biophysical Properties

Property	Value
Corrosion Resistance	Far superior to 316L stainless steel and pure titanium
Surface Passive Film	Stable and dense TiO₂ film
Nickel Ion Release	Extremely low, biocompatible
Biocompatibility	Safe for medical implant applications

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