Abstract
In situ neutron diffraction is used to study the strain relaxation on a single crystal and other powdered Cu-Al-Ni shape memory alloys (SMAs) around martensitic transformation temperatures. This work is focused on the analysis of the strain evolution along the temperature memory effect appearing in these alloys after partial thermal transformations. A careful study of the influence of partial cycling on the neutron diffraction spectra in the martensitic phase is presented. Two different effects are observed, the d-spacing position shift and the narrowing of various diffraction peaks, along uncompleted transformation cycles during the thermal reverse martensitic transformation. These changes are associated with the relaxation of the mechanical stresses elastically stored around the martensitic variants, due to the different self-accommodating conditions after uncompleted transformations. The evolution of the stresses is measured through the strain relaxation, which is accessible by neutron diffraction. The observed effects and the measured strain relaxations are in agreement with the predictions of the model proposed to explain this behavior in previous calorimetric studies. In addition, the thermal expansion coefficients of both martensite and austenite phases were measured. The neutron experiments have allowed a complete description of the strains during martensitic transformation, and the obtained conclusions can be extrapolated to other SMA systems.
Highlights
Shape memory alloys (SMAs) are functional materials characterized by their specific properties of shape memory and superelastic effects, which are based on a reversible first order diffusionless structural phase transition, called martensitic transformation (MT), taking place between the high temperature phase, austenite, and the low temperature phase, martensite, via an atomic lattice shearing responsible for the change of shape.[1,2,3]
We propose the following approach: the local stresses and their evolution during MT could be evaluated through the corresponding elastic local lattice strains, which should be accessible by neutron diffraction
In situ neutron diffraction experiments were performed on two different Cu-Al-Ni samples of slightly different composition and microstructure around the MT, with special emphasis on the temperature memory effects, TME, and hammer” effect (HE), taking place during reiterative cycling over a partial thermal reverse MT
Summary
Shape memory alloys (SMAs) are functional materials characterized by their specific properties of shape memory and superelastic effects, which are based on a reversible first order diffusionless structural phase transition, called martensitic transformation (MT), taking place between the high temperature phase, austenite, and the low temperature phase, martensite, via an atomic lattice shearing responsible for the change of shape.[1,2,3] the main strain associated with the MT is a shearing, it could involve shuffling, distortions, and expansions of the parent phase lattice and important stresses appear in between the two lattices of austenite and martensite. In the absence of any plastic deformation or dissipative processes, the creation of the martensite interfaces and the stored elastic energy are responsible for the hysteresis scitation.org/journal/jap associated with the MT, as well as for the broadening of the transformation temperature range.[4,5] The important point is that the elastic energy stored during the forward MT on cooling constitutes the driving force for the reverse MT on heating, and in the case that such stored elastic energy would be released, the transformation will be delayed to a higher temperature range If such evolution occurs, it would constitute an important issue for the reliability of SMA behavior, because the thermal shape memory effect and the recovery of the superelastic effect could be compromised due to the stabilization of the martensite. The reader is referred to a previous work[26] for a detailed description of such thermal effects
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