Atomic Migration Variation as an Effect of Thermomechanical Cycling

Thermal actuators, for temperature control in hydraulic oil systems, were manufactured from shape memory alloys (SMAs) trained in bending, up to 500 cycles. The structural effects of training were evaluated by differential scanning calorimetry (DSC), optical (OM), scanning electron (SEM) and atomic force microscopy (AFM), as well as diagrams of specimen’s free end displacement vs. temperature. An experimental hydraulic installation, equipped with active SM elements trained in bending to develop two way effect (2WE), enabled monitoring oil-temperature variations in time. The effects of training cycles numbers, corroborated with heating interruption, were analyzed by DSC, OM and AFM. Heating interruption, followed by decreasing-rate air cooling in the previous thermal cycle, enabled to reveal new calorimetric and structural aspects of temperature memory effect (TME). Different fragments of martensitic SMA were subjected to different heating rate (HR) values performed by DSC. The results revealed an obvious tendency of critical temperatures of martensite reversion to linearly increase with HR, the influence of which was corroborated with morphological changes of martensite plates, obtained after post-heating holding and cooling. As an effect of complex thermal cycling performed on DSC, meant to reproduce actual functioning conditions of a SM electrical actuator with uncontrolled air-cooling, the progressive decay of martensite reversion was associated with the reciprocal blocking of differentially oriented needles of stabilized lath-martensite. The reproducibility of martensite reversion was studied on different fragments of martensitic SMA subjected to complex thermal cycling comprising series of heating– cooling cycles intercalated with 37 days-natural ageing periods. DSC thermograms revealed a gradual diminution of martensite reversion associated with ‘amnesia’ occurrence, which was ascribed to the changes in martensite plate morphology and with an increase of chemical fluctuations, as an effect of atomic migration intensification.