Thermal memory fading by heating to a lower temperature: Experimental data on polycrystalline NiFeGa ribbons and 2D statistical model predictions

Abstract Shape memory alloys are known to memorise one -or several- temperatures at which the martensite-austenite transformation was stopped before completion in the past, the memory manifesting as specific dips in subsequent calorimetric scans. Previous studies have shown that this memory can be erased by heating to higher temperatures than the ones previously recorded. In this paper, we study a distinct memory fading effect which takes place by heating to a lower temperature. This effect is reported in NiFeGa as polycrystalline ribbons, the alloy being initially studied as bulk for which the thermal memory effect was not found. If, after an initial incomplete heating up to T 1 one performs a second incomplete heating up to T 2 T 1 , a new calorimetric dip appears at T 2 , as expected, while less expected was that the dip corresponding to T 1 reduces in amplitude or even vanishes (if the arrest at T 2 is repeated). The memory fading effect is more clear for small differences T 1 − T 2 and less obvious or absent for large ones. The second part of the paper employs a statistical 2D model, which associates the memorized temperatures with a depletion of certain martensite plates sizes, and also supports the memory fading effect.