A technical and economic appraisal of shape memory alloys for aerospace applications

The adaptive serrated nozzle is one of the most promising concepts to help reduce the noise level generated by aero-engines. The serrations protrude into the jet stream at the nozzle exit and force mixing with the surrounding ambient air. One design solution for such nozzles is to employ shape memory alloy (SMA) as the actuation mechanism. Unlike conventional SMA designs, large sections of SMA are required for such serrations. This paper shows that there are two major technological/economic challenges in the implementation of SMA adaptive serrated nozzles. These are the needs for high temperature SMAs and large section sizes. On the technical front, the paper also shows that joining of NiTi SMAs to dissimilar alloys is the major risk factor in the development of the SMA adaptive serrated nozzle. The problem is that the shape memory performance is severely affected by temperature yet it is virtually impossible to achieve a high strength joint without using joining techniques with operating temperatures approaching or beyond the melting point of these alloys. A preliminary assessment of laser welding of thin sections of NiTi SMA is presented and has highlighted the challenges that need further investigation.