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 E on Ni2 5 Ti 5 oCu25 ribbons detected during thermal cycling under a stress state have clearly shown that a linear relationship between ER and e can be obtained for an applied stress lower than SO MPa.ER and £ have been here investigated during repeated electrically driven thermal cycles under an applied stress either 50 or 100 MPa on Ni 2 5Ti5oCu25 melt spun ribbons up to 8000 electrically-driven cycles.Small plastic deformation, with a typical figure of s P ~ 5 10" 4 for 50 MPa and e P ~ 110" 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-s curves modifies with cycling for both applied stresses.