Phase transformation in NiTi studied by isothermal mechanical spectrometry

Abstract

NiTi samples were studied by internal friction measurements and differential scanning calorimetry (DSC). DSC experiments were performed using a Mettler TA30 DSC with a controlled cooling/heating rate of 1 K/min, to determine the transformation heats ({Delta}H) and the characteristic transformation temperatures. It was necessary to expect a very long time (about one day) after each temperature change to obtain a stabilized spectrum even for a small temperature step (10 K). This fact shows that the sample microstructure evolution occurring during this temperature change is in fact very slow. For higher frequency experiments the martensite-austenite transition corresponds to an internal friction decrease. This evolution exhibits in fact two stages: a first Q{sup {minus}1} diminution at 320 K and a second above 360 K corresponding to the A{sub S} temperature. Measurements performed at temperature higher than A{sub f} showed also a low frequency damping increasing. But in this case no thermally actived process was observed, experimental curves being exactly superimposed. The increasing internal friction at very low frequency was never observed before these experiments. As shown here, it disappears during the transition between the martensitic to austenitic phases and corresponds to mechanisms taking place in the martensite. This relaxation effect is thermallymore » actived. These relaxation effects observed in the martensitic phase can be attributed either to the motion of dislocations present in the plates or plate-boundaries or to the motion of the interface between martensite plate. So further experiments will be necessary to conclude about the origin of this thermally activated relaxation mechanism.« less