METHOD TO INCREASE ENERGY STORAGE EFFICIENCY IN A Cu-Zn-Al-Fe SMA

摘要

The pseudoelastic effect or pseudoelasticity (PSE) is characterized by any nonlinearity occurring on the unloading portion of the tensile curves and is directly related to mechanical memory, observed both in austenitic and martensitic conditions. Among PSE parameters, energy storage efficiency represents the ratio between unloading released energy and the total energy consumed upon loading. Since the latter factor includes both unloading released energy and internal friction (proportional to the surface area between loading and unloading portions of the tensile curve) it follows that the most effective method to increase energy storage efficiency is to reduce mechanical hysteresis. In the case of Shape Memory Alloys (SMAs) with martensitic PSE, the efficiency is increased by minimizing the energy that is irreversibly transformed into heat due to internal friction, accompanying the crystallographic reorientation of martensite plate variants. The paper describes an experimental method to increase energy storage efficiency, by means of tempering, in the case of a quenched (martensitic) Cu-Zn-Al-Fe SMA. By means of tensile loading-unloading tests, both internal friction and energy storage efficiency have been determined for which continuous tendencies to decrease and to increase, respectively, have been emphasized with increasing tempering temperature. By means of structural analysis, comprising optical metallographic microscopy and X-ray diffraction, the phase structure has been determined. The gradual decrease of internal friction with increasing tempering temperature has been ascribed to the decrease of the thermally-induced martensite amount. This phenomenon was revealed by the disappearance of wedge-like martensite plates on optical micrographs and the intensity decrease of the diffraction maxima.