The term ‘transition temperature’ is a bit of a misnomer as there are several critical temperatures in the shape memory cycle. The transformation and reverse transformation generally require a few °C from start to finish—giving us the austenite start and finish (As and Af) temperatures as well as the martensite start and finish (Ms and Mf) temperatures. Also of interest are the peak temperatures (Mp and Ap) when the transformation is occurring the fastest. If you are dealing with low hysteresis SMA, then the equilibrium temperature (M0) comes into play. This is the temperature that must be passed in order for a transformation to occur—even if the start temperature has already been passed. Lastly, when dealing with superelastic nitinol, the martensite difficult temperature (Md) may play a role. At temperatures above Md, the stress induced martensitic transformation generally does not occur, causing nitinol to behave more similarly to most engineering materials.
In standard, binary nitinol, the critical temperatures can be listed, from lowest to highest, as: Mf, Mp, Ms, M0, As, Ap, Af, Md. However, in low hysteresis nitinol, the ordering from lowest to highest can be: Mf, As, Ap, M0, Mp, Ms, Af, Md. Additionally, in wide hysteresis nitinol, the ordering from lowest to highest can be: Mf, Mp, Ms, M0, Md, As, Ap, Af.
A relatively simple at-home method of measuring transition temperature begins with placing the nitinol in the freezer to ensure that it has been chilled below Mf. Carefully bend or deform the wire and then place it in cold water. Gradually warm the water , using a digital thermometer to keep track of the temperature. When the wire begins to move, you have determined As. When the movement stops, you have determined Af. This is a modified form of ASTM-F2082, Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery.