Two-way shape memory effect in [001]B2-oriented Co-Ni-Al single crystals

Abstract

In the present study, the necessary conditions for the two-way shape memory effect (TWSME) and its stress-free thermal cycling stability in the Co35Ni35Al30 single crystals, oriented along the [001]B2 direction, were investigated. The TWSME is attributed to the internal stress fields created through the generation of stable dislocation structures and retained martensite in two ways. First, the training involved 100 loading/unloading cycles at room temperature T=295 К with the maximum given strain of 6.0% in compression. Second, the ageing at T=423К for 1.0 h under a stress of 500 MPa was applied to the martensitic state along the [001]B2||[110]L10 direction. It was experimentally shown that the aged crystals demonstrated the best properties of the TWSME response when compared with the trained crystals: i.e., a higher operating temperature range (250-297K), the high cyclic stability of the reversible strain εTWSME=(3.1±0.3)% during thermal cycles and the low value of the temperature hysteresis (ΔT=24 К). In contrast, in the trained crystals the operating temperature range of the TWSME is shifted towards a low temperature region (220-275 K), after 100 thermal cycles the reversible strain εTWSME is significant decreased from 3.0 to 2.0% and the temperature hysteresis ΔT is increased from 33 to 50K.