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Frequently asked questions

What is ‘shape memory’?
When a deformed shape memory (SMA) wire is heated, it reverts to its ‘memorised’ hot shape.  Thus, the hot shape is memorised.  This process of deforming in the cold state and reversion to the hot, memorised shape can be repeated many times.   This shape restoration process is the result of a reversible phase transformation from austenite, which is the phase in the hot state, to martensite, which is the phase in the cold state.

What are transformation temperatures?
Austenite transforms to martensite during cooling; this transformation starts at Ms (martensite start temperature) and finishes at Mf (martensite finish temperature).
Martensite transforms to austenite during heating; this transformation starts at As (austenite start temperature) and finishes at Af (austenite finish temperature).  
The transformation temperatures are influenced by many parameters such as composition, heat treatments, and rate of cold work.  Nitinol alloys with a high Ni content have transformation temperatures below room temperature; such alloys are called superelastic alloys.  Nitinol alloys with a low Ni content have transformation temperatures above room temperature; such alloys are called shape memory alloys.

What is ‘superelasticity’?
A superelastic wire can be deformed about 10 times more than spring steel, and will still revert to its original shape, when the load is removed.  The transformation temperatures of a superelastic alloy are below room temperature.  When a component made of a superelastic alloy is deformed, the martensitic phase is stress induced and high deformations are the result. When the load is removed and since the transformation temperatures are below room temperature, the martensitic phase becomes unstable and the material regains its original shape. 

What is ‘as drawn’?
Wire is drawn from a large diameter to the final diameter in many steps. ‘As drawn’ means directly coming off the wire drawing machine.  Such wire is in general curved and does not exhibit shape memory or superelasticity.  ‘As drawn’ wire must be heat treated to set the hot shape (or shape-set) and to induce shape memory or superelasticity.  ‘As drawn’ wire is only suitable material for customers which have the expertise to apply the requested heat treatments. 

What is ‘straight annealed’?
The hot, memorised shape is set by fixing the shape memory component followed by annealing, which is a heat treatment at 400°C to 550°C.  Straight annealing means that the hot shape is set to a straight wire.  For most customers, wire in the ‘straight annealed’ state  is recommended. 

What is ‘fully annealed’?
After some wire drawing steps the wire becomes too hard for further wire drawing.  The wire is then softened by a heat treatment at 600°C and higher.  This treatment is called full annealing.  Wire in the fully annealed state is very unstabile.  Only customers with extended expertise on shape memory alloys should consider to buy such wire. 

What is ‘training’?
Training is a treatment to stabilise the cyclic behaviour of shape memory wires.  Standard, straight annealed wire will show some permanent deformation, after repeated cycles of deformation followed by heating to recover the shape.  By training, this permanent deformation is minimised. 
Trained wire is recommended for applications requesting a high cyclic stability, such as in robotics, model train applications, actuators, etc.  
Flexmet has developed a set-up for spool-to-spool training of shape memory wires.  Our trained wires have a better stability and a lower price than products from competitors (e.g. Flexinol, Muscle wires, etc).