The Influence of Aging on Critical Transformation Stress Levels and Martensite Start Temperatures in NiTi: Part I—Aged Microstructure and Micro-Mechanical Modeling

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Abstract
Transmission electron microscopy is used to determine the microstructures of a Ti-50.8 at% Ni alloy given different aging treatments. Two different peak-aging treatments are shown to result in disk shaped semi-coherent Ti3 Ni4 precipitates with a diameter ranging from 50 nm to 200 nm depending on the aging temperature. In the peak-aged materials, strong strain fields are clearly visible on TEM micrographs. An Eshelby based model is used to predict the local stress fields due to the differences in the lattice parameters of the precipitates and surrounding matrix. The position dependent local stress fields are then resolved onto the 24 different martensite correspondence variant pairs (CVP’s). It is further demonstrated that due to the unique orientation relationship that exists between the precipitate variants and the martensite CVP’s, the local resolved shear stresses are extremely large on some CVP’s and negligible on others. When the Ni rich NiTi is over-aged, it is found that the precipitates coarsen to approximately 1000nm, they become in-coherent, and the local stress fields disappear. It is also determined that after over-aging the average composition of the matrix drops from 50.8 at% Ni to approximately 50.4 at% Ni. In a subsequent paper (part II) the results here are used to explain the dependence of the critical transformation stress levels and martensite start temperatures on the aging treatment.