Martensite start temperature (Ms): the temperature at which the transformation from austenite to martensite begins on cooling.
Martensite finish temperature (Mf): the temperature at which the transformation from austenite to martensite finishes on cooling.
Austenite start temperature (As): the temperature at which the transformation from martensite to austenite begins on heating.
Austenite finish temperature (Af): the temperature at which the transformation from martensite to austenite finishes on heating.
The definitions of these temperatures are illustrated in the figure above. Nitinol material specifications are generally defined by one of these transformation temperatures (most commonly As or Af) in the fully annealed condition (see ASTM F2063) while transformation temperature range (TTR) is a generic term used to describe the range of these temperatures.
Measuring transformation temperatures
Transformation temperatures for alloys are typically determined by Differential Scanning Calorimetry (DSC) which measures the heat flow between the Nitinol specimen and the environment as a function of temperature (ASTM F2004).
This figure illustrates a typical DSC curve and the measurement of transformation temperatures of a fully annealed nitinol alloy.
Active transformation temperatures can also be determined by Bend and Free Recovery (BFR) tests which trace the shape recovery as a function of temperature (ASTM F2082). The figure above illustrates an example of a BFR test and the determination of As and Af temperatures.
For actuator or fastener applications, transformation temperatures may be measured by Constant Load Dilatometry (CLD) to evaluate the effects of applied stress on the transformation. The figure above shows an example of a CLD test and the determination of transformation temperatures, Ms, Mf, As and Af on the curve.
How stress affects the transformation temperature
The presence of stress typically raises the transformation temperature in a linear fashion as shown in the figure below.
The typical hysteresis for nitinol alloys
For binary nitinol alloys, the hysteresis is typically 30 to 40° C thermally and 30 to 50 Ksi (200 to 350 MPa) mechanically. Hysteresis can be manipulated by alloying additions. For example, the addition of Copper to nitinol can reduce the thermal hysteresis to as low as 15°C while Niobium in a ternary NiTiNb alloy (Alloy X) will increase it to as high as 120°C.
Factors that influence transformation temperatures and mechanical characteristics
Material composition, amount of cold-work and heat treatment together with application-related parameters such as external stress all affect transformation temperatures and mechanical characteristics.
Active transformation temperature
Contrary to intrinsic alloy transformation temperatures which usually depict the material transformation temperatures in the fully annealed condition at the ingot level, the active transformation temperatures characterize the material's transformation at the final product level.