The phase present depends on thermodynamics. The transformation between austenitic and martensite phase occur over a range of temperatures so we need to define the temperatures at which the different transformations take place.

As the temperature increases the material remains in the martensitic phase untill it reaches the austenitic transformation temperature, A_s

At A_s the martensitic phase begins to transform into the austenitic phase

At A_f the martensitic phase has fully transformed into the austenitic phase (although it should be noted that 'full transformation' may actually correspond to only a certain proportion (<100%) of the material being transformed)

At M_s the martensitic phase begins to grow inside the austenitic phase

At M_f the austenitic phase has fully transformed back into the martensitic phase