Effect of Aging on Super-Elastic Response of a Polycrystalline FeNiCoAlNbB Shape Memory Alloy

This study reports the effect of aging duration on the super-elastic response of Fe-30%Ni-18%Co-10.5%Al-2%Nb-0.15%B (at.%) poly-crystals in compression. The aging temperature was 600°C and the aging durations were 20h, 45h, 60h and 72h, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were used in the work. The results show that with prolonging the aging duration, the super-elastic strain rises firstly and then descends. The super-elastic strain reaches the maximum 10.5% when the aging duration is 60h. The crisis stress for stress-induced martensite (σM) has no obvious changes, being about 250MPa when the aging duration is between 20h and 60h. But σM increases markedly when the aging duration prolongs to 72h. The hardness of the specimens changes in the same way as the superelastic strain, and reaches the maximum of 497HV10 when the aging duration is 60h. During the aging process, two factors react. One is the decomposing and reducing in size of the undissolved phase (σ). The other is the formation of the precipitation phase (γ'). Nb can dissolve into the matrix phase (γ) adequately and promote the formation of γ'. The combination of the two factors improves the strength and superelasticity of the specimens till the over-aging arises corresponding to the 72h aging duration.