Fatigue crack behavior on a Cu-Zn-Al SMA

In recent years, mechanical property of many SMA has improved in order to introduce these alloys
in specific field of industry. Main examples of these alloys are the NiTi, Cu-Zn-Al and Cu-Al-Ni which are used
in many fields of engineering such as aerospace or mechanical systems. Cu-Zn-Al alloys are characterized by
good shape memory properties due to a bcc disordered structure stable at high temperature called ?-phase,
which is able to change by means of a reversible transition to a B2 structure after appropriate cooling, and
reversible transition from B2 secondary to DO3 order, under other types of cooling. In ?-Cu-Zn-Al shape
memory alloys, the martensitic transformation is not in equilibrium at room temperature. It is therefore often
necessary to obtain the martensitic structure, using a thermal treatment at high temperature followed by
quenching. The martensitic phases can be either thermally-induced spontaneous transformation, or stressinduced,
or cooling, or stressing the ?- phase. Direct quenching from high temperatures to the martensite phase
is the most effective because of the non-diffusive character of the transformation. The martensite inherits the
atomic order from the ?-phase. Precipitation of many kinds of intermetallic phases is the main problem of
treatment on cu-based shape memory alloy. For instance, a precipitation of ?-phase occurs in many low
aluminum copper based SMA alloy and presence of ?-phase implies a strong degradation of shape recovery.
However, Cu-Zn-Al SMA alloys characterized by aluminum contents less than 5% cover a good cold machining
and cost is lower than traditional NiTi SMA alloys. In order to improve the SMA performance, it is always
necessary to identify the microstructural changing in mechanical and thermal conditions, using X-Ray analyses.
In this work a Cu-Zn-Al SMA alloy obtained in laboratory has been microstructurally and metallographically
characterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. Furthermore
a fatigue crack propagation and fracture surface scanning electron microscope (SEM) observations have been
performed in order to evaluate the crack path and the main crack micromechanisms.