Transformation temperature hysteresis in Nitinol alloys
Upon heating or cooling, NiTi alloys do not completely undergo their phase transformation at one particular temperature. Instead, the transformation begins at one temperature (known as the start temperature) and is completed at another temperature (known as the finish temperature). Further, there is a difference in the transformation temperatures upon heating from martensite to austenite and cooling from austenite to martensite, resulting in a delay or "lag" in the transformation. This difference, known as the transformation temperature hysteresis, is generally defined as the difference between the temperatures at which the material is 50% transformed to austenite upon heating and 50% transformed to martensite upon cooling. This value can be approximated by the difference between Ap and Mp on a DSC curve. Typical values for binary NiTi alloys are about 25 to 50 deg.C.
In addition to the hysteresis, the overall span of the transformation may be important. If the device being designed requires complete transformation upon both heating and cooling, then the difference between Af and Mf (the finish temperatures of the transformations to austenite and martensite, respectively) must be considered. Typical values for the overall transformation temperature span are about 40 to 70 deg.C.
Both the hysteresis and the overall transformation temperature span are slightly different for different NiTi alloys. Further, alloying can greatly affect the transformation hysteresis. Copper additions have been shown to reduce the hysteresis to about 10 to 15 deg.C and Niobium (Columbium) additions can expand the hysteresis to over 100 deg.C. Cold working and heat treatment have less dramatic, but still measurable effects on the transformation hysteresis. The table below shows the differences in hysteresis and overall temperature span for some different binary alloys.
Example Nitinol transformation temperature values
|Mf||Mp||Ms||As||Ap||Af||Hysterisis (Ap-Mp)||Overall temp. span (Af-Mf)|
These numbers should provide some assistance when designing a device which utilises the shape memory effect. For example, if one were designing a device to activate at boiling water temperature (100 deg.C) that also must be fully re-transformed to martensite at room temperature (20 to 25 deg.C), there is a narrow set of binary alloys which meet the criteria. From the above table, one can estimate that one should consider alloys with As of approximately 60 to 80 deg.C to satisfy both criteria. Similarly, an alloy designed to be completely transformed by body temperature upon heating (Af < 37 deg.C) would require cooling to about -10 deg.C to fully re-transform to martensite.