A variety of constitutive models have been proposed in the literature that describe many of the important features of the variant reorientation process of martensitic magnetic shape memory alloys such as the nonlinear strain response and the nonlinear variation to the magnetization. In order to improve the accuracy and applicability of these models it is necessary to understand how the evolution of the magnetic domain arrangement influences the reorientation process and the corresponding macroscopic material response. Some researchers argue that the domain structure may be neglected entirely, while others deem it essential in describing the motion of the twin boundary separating martensitic variants.
To gain further inside into this matter experimental results of micro scale observations using magnetic force microscopy are presented. Different material systems, e.g. NiMnGa and CoNiAl are used in the analysis. The focus of the investigation lies on the arrangement of magnetic domains in different stages of the reorientation process, i.e. different levels of externally applied magnetic fields under constant stress bias. The experimental information obtained on the evolution of the microstructure is used in the formulation of a thermodynamically consistent phenomenological constitutive model which incorporates the evolution of martensite variants as well as the magnetic domain structure. The gained knowledge proves especially useful in the postulation of a free energy function, the evolution equations for the internal variables and functions governing the onset and termination of the reorientation process.
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