Reliability of resistance/deformation during electrically-driven thermal cycles under stress on Ni₂₅Ti₅₀Cu₂₅ melt spun ribbons

Abstract
The use of shape memory alloys for sensing/actuating strictly relies on the possibility to easily relate the Electrical Resistance (ER) to strain (E). Previous measurements of ER vs ε on Ni₂₅Ti₅₀Cu₂₅ ribbons detected during thermal cycling under a stress state have clearly shown that a linear relationship between ER and ε can be obtained for an applied stress lower than 50 MPa. ER and ε have been here investigated during repeated electrically driven thermal cycles under an applied stress either 50 or 100 MPa on Ni₂₅Ti₅₀Cu₂₅ melt spun ribbons up to 8000 electrically-driven cycles. Small plastic deformation, with a typical figure of ε_p 5.10^-4 for 50 MPa and ε_p 1.10^-3 for 100 MPa, is built in during cycling and levels after few thousand cycles. Under a stress of 100 MPa specimens break after 2000-4000 cycles. No broken specimen has been found even after 8000 cycles under 50 MPa applied stress. The hysteresis width of the ER-ε curves modifies with cycling for both applied stresses.