Transition in superelasticity for Ni _55-xCo _xFe ₁₈Ga ₂₇ alloys due to strain glass transition

Here we report a transition in superelastic hysteresis loop from sharp with plateau to smooth without plateau for a Ni _55-xCo _xFe ₁₈Ga ₂₇ (x=0-12) alloy system with increasing Co substituting for Ni up to 10 at.%. With the Co content reaching 10 at.%, the alloy exhibits the obvious characteristic of the strain glass transition, i.e., the frequency-dependent shift in temperature of the internal friction peak and frequency-dependent dip temperature, T _g, of the storage modulus following the Vogel-Fulcher relationship. The high-energy X-ray diffraction provides the direct evidence that the smooth hysteresis loops stem from a finite avalanche martensite transformation mode, inducing a long-range inhomogeneous stress field in the remained parent phase during deformation. © 2012 Europhysics Letters Association.