Mechanical properties of Ti_(50)Ni_(50-x)Cu_x(x=20, 25) shape memory alloy strips produced by melt overflow

摘要

Ti-Ni-Cu alloys have been known to be attractive shape memory alloys for applications as actuators because of large transformation elongation and small transformation hysteresis. However, Cu addition exceeding 10at% makes the alloys brittle and spoils the formability. Arc melt overflow enables Ti-Ni-Cu alloys with high Cu-content to be fabricated into thin strips. Then, the shape memory alloy strips of Ti_(50)Ni_(30)Cu_(20) and Ti_(50)Ni_(25)Cu_(25) have been fabricated by arc melt overflow technique. Their microstructures and shape memory characteristics were investigated by means of XRD, optical microscopy and DSC. The microstructure of as-cast strips exhibited columnar grains normal to the strip surface. X-ray diffraction analysis showed that the martensitic transformation of B2-B19 occurred in the alloy strips. During thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with the B2-B19 transformation were observed to be 4.9°C and 1.8% in Ti_(50)Ni_(30)Cu_(20) alloy strip and 3.5°C and 1.7% in Ti_(50)Ni_(25)Cu_(25) alloy strip. The as-cast strip of Ti_(50)Ni_(25)Cu_(25) alloy also showed a perfect superelasticity and its stress hysteresis was as small as 14 MPa. These mechanical properties and shape memory characteristics of the alloy strips were ascribed to B2-B19 transformation and the controlled microstructures produced by rapid solidification of the arc melt overflow process.