Abstract
In the present work multi step martensitic transformations of Ni51Ti49 single crystal has been investigated by differential scanning calorimetric (DSC). The sample with the mass of 77 mg after solution annealing treatment was aged at 400˚C for different aging times between 10 minutes and 50 hours. DSC experiments have been performed on the sample after each aging treatment. Results show that for low aging times (10 to 45 minutes) martensitic transformations occur in two steps, for middle aging times (75 minutes to 4 hours) in three steps and higher aging times (more than 8 hours) martensitic transformations occur in two steps again. Indeed, the multi step martensitic transformations in single crystals of NiTi alloys indicate 2-3-2 behavior in DSC curves with aging times like polycrystals. In the present investigation proposed mechanism related to the multi step martensitic transformation in single crystals and polycrystals such as heterogeneity of composition in both microscopic and macroscopic scale and existence of stress fields around the Ni4Ti3 particles have been discussed.
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