NiTi alloys change from austenite to martensite upon cooling; "Mf" is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating "As" and "Af" are the temperatures at which the transformation from martensite to austenite starts and finishes. Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material). The maximum temperature at which SMAs can no longer be stress induced is called "Md", where the SMAs are permanently deformed.