Fatigue Crack Propagation Micromechanisms in a Cu-Zn-Al Alloy with Pseudoelastic Effect

Cu-Zn-Al alloys are characterized by good shape memory properties due to a bcc disordered structure, stable at high temperature, called β-phase. These grades are able to change their microstructure by means of a reversible transition from β-phase to a B2 cell (after appropriate cooling process), and by means of a reversible transition from B2 secondary to DO3 order (after other cooling processes). In β-Cu-Zn-Al shape memory alloys, the martensitic transformation is not in equilibrium at room temperature. Therefore, a thermal heat treatment at high temperature followed by quenching is often necessary in order to obtain the martensitic structure. The martensitic phases can be obtained either by means of thermally-induced spontaneous transformation, or by means of other mechanisms (stress-induced, or temperature decrease). Direct quenching from high temperature to the martensite phase is the most effective process because of the non-diffusive character of the transformation. The martensite inherits the atomic order from the β-phase.In this work, an artificial Cu-Zn-Al SMA alloy has been microstructurally characterized by X-ray diffraction and Light Optical Microscope (LOM) observation. These analyses have been performed in“as cast alloy” conditions, and under load conditions in order to characterize the alloy behavior. Furthermore, a fatigue crack propagation test and fracture surface scanning electron microscope (SEM) observations have been performed in order to evaluate the main crack micromechanisms.