The influence of loading during cold shape formation on the amount of shape recovery in NiTi shape memory alloy

The long-term stability of the shape memory effect is critical for ensuring repeatability in memory alloy (SMA) based actuation devices. It is known that the degradation in shape originates mainly from the generation of a permanent strain and its accumulation over repeated cyclical operations. Such strains should be minimized and the strain maximized. This requires a detailed understanding of the deformation behaviour of SMAs under loading, heating and cooling. This study investigates how the application of loading under different representative heating and cooling paths between martensite and austenite (loading paths) influences the deformation behaviour and the amount of shape memory achievable in a NiTi SMA. The deformation behaviour differs, depending on the magnitude of the applied load and especially the material phase of SMA under which the external load is applied. During the formation of martensite upon cooling deformation occurs with great ease and much larger recoverable strains at the same level of external loading are achieved than in full martensite and austenite, while during the heating process for reverse transformation, even a small deformation arising from the formation of stress induced martensite is liable to introduce a permanent strain. A detailed analysis and an interpretation of these phenomena are presented.