Nitinol Terminology – Kellogg’s Research Labs

Nitinol Terminology refers to the terms used in technical applications of Nitinol

Age: A heat treating process at relatively low temperatures that results in grains precipitating out of solution.  For nitinol, during the aging process, nickel and titanium combine to form Ti₄Ni₃ grains.  This, of course changes the ratio of nickel to titanium in the nitinol grains, thereby raising the transformation temperature.  The effects of aging may be undone by solution treating.

Anneal: A heat treating process that rearranges the structure of the material, relieving any stress.  This results in setting a new, distinct shape.  Annealing typically results in a softer material (easier to scratch).

Austenite: the microstructure that exists in nitinol at “high” temperatures.  Chemically, it is a B2 cubic structure that results in a stiff but flexible material.

Heat treatment profile: The prescribed range of temperatures and times necessary to achieve the desired properties.  A heat treatment profile may be divided up into multiple stages, ramps, quenches, and atmospheres.  Since the performance of nitinol is highly dependent on the heat treatment profile, it is highly recommended to use a PID controlled furnace.

Martensite: The microstructure that exists in nitinol at “low” temperatures. Chemically, it is a B19’ twinned structure.  Since the chemical bonds in the twinned structure can easily rotate about the atoms, martensite appears to be an easily deformable or ductile material.

PID: A method of process control commonly used by digital and computer controlled systems.  PID is an acronym for Proportional, Integral, and Derivative.  Controllers may be programed to use P, PI, or PID control in order to meet the necessary requirements of your process.  For more information on the intricities of PID control, insert book title hereis an excellent resource.

Pseudoplasticity: the term used to describe the false-permanent deformation observed in martensitic nitinol.  Plastic deformation refers to permanent deformation that remains after all forces are removed.  Pseudoplasicity exists in nitinol because, while it appears to be permanent, the deformation is recovered upon heating.  This is also called thermoelasticity.

Quench: The rapid cooling of a sample by immersion in a liquid. While water and oil the two most common quenching fluids, researchers sometimes use liquid nitrogen (LN2) as a quenching fluid, especially when trying to obtain very tiny grain structures or completely amorphous nitinol.

Ramp: The controlled increase or decrease of temperature inside of a furnace.  Ramps are typically specified in °C/min between two temperatures (i.e. ramp from 750°C to 375°C at a rate of 25°C/min).

R-Phase: the term used to refer to the rhombohedral structure that is found between martensite and austenite.

Solution treating: Heat treating at a temperature high enough for any precipitated material to be reabsorbed by the bulk.  For nitinol alloys, this generally is 850℃ (1,550℉) or higher.  Solution treating should take place under an argon shielding gas to prevent carbon, oxygen, and nitrogen from reacting with the titanium.

Stage: A section of a heat treatment profile that has like conditions.

Stress Relief Heat Treatment: A heat treatment used to set the material into a new shape.  While this typically occurs at temperatures similar to annealing, the time is generally shorter to avoid excessively influencing the thermal performance of the nitinol.  Stress relief heat treatments can be conducted in succession to gradually form nitinol into very difficult to form shapes.

Thermoelasticity: Elastic deformation is deformation that is recovered once the force is removed. Thermoelastic deformation is deformation that isn’t recovered when force is removed but after heating.

Tube Furnace: A furnace that is generally long and cylindrical in shape but may have a square cross section.  The square cross section is rare due to the difficulty of maintaining even heating throughout the cross section.  The ends may be open to the atmosphere or they may be hermetically closed. Hermetically closed furnaces are typically used for treatments in specialized atmospheres while open ended furnaces are sometimes used for continuous heat treatment processes.

Zone: A region of a furnace that is independently controlled. Each zone consists of at least one thermocouple, one heating element, and one PID controller.  Increasing the number of zones in a furnace allows for a higher level of control of the temperature inside the furnace.