Abstract
After solution annealing and subsequently aging at temperatures between300 and 550 degrees C lenticular coherent Ni4Ti3-particIes precipitate.The presence of these precipitates strongly affects the martensitictransformations in Ni-rich NiTi alloys. In this work aging treatmentswere performed at 400 degrees C from 1 to 640 hours. Subsequently DSC,TEM, and neutron diffraction (ND) analysis were performed. TEM resultsshow that short and intermediate aging times result in a heterogeneousmicrostructure with Ni4Ti3 precipitates on grain boundaries andprecipitate-free regions in grain interiors. Correspondingly, theneutron diffraction indicates the presence of all four phases, 132, R,B19’, and Ni4Ti3 in the same sample after the first peak on cooling.Heterogeneous microstructures lead to complex transformation behaviourwhere regions with precipitates do not transform in the same way asprecipitate free regions.
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