The Study of the Mechanism of the All-Round Shape Memory Effect in TiNi Alloy

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Ni rich TiNi alloys (Ti-51-52at%Ni) exhibit the all-Round Shape Memory Effect by aging under constrain. The aim of this research project is to clarify the mechanism of this All-Round Shape Memory Effect. Shape memory effects of Ti-51 and 52at%Ni specimens were investigasted under the various conditions of constrained aging by means of optical microscope and eleotron microscope. The results are as follows.1. The TTT diagrams of precipitates in this alloy were made between 300-600゜C. Precipitates were <Ti_3> <Ni_4> (precisely <Ti_11> <Ni_14> ), <Ti_2> <Ni_3> and Ti <Ni_3> . Ti <Ni_3> is stable and the others are metastable phases.2. Fine and dispersed precipitates of <Ti_3> <Ni_4> with coherent grain boundary cause the All-Round Shape Memory Effect. This effect does not appear in the case of bigger precipitates grown up.3. The crystal structure of <Ti_3> <Ni_4> is rhombohedral structure (a=6.72 <。!A> , <alpha> =113.9) with ordered atomic arrangement.4. The lenticular precipitate <Ti_3> <Ni_4> has the anisotropy in both of inner (compression stress) side and outer (tension stress) side.5. Against the stress axis, the semimajor axis of presipitate <Ti_3> <Ni_4> is perpendecular in the compression side and parallel in the tension side, respectively.6. The growth direction of intermediate (R) phase and martensite (M) phase which generate in specimen during cooling is the best convenient direction which accomodates the internal strain between parent phase and precipitates, especially the strain in the direction of semiminor axis of precipitate. Therefore, after the growth of R phase with single variant around precipitate, martensite phase appears. As the result ofthe difference of lattice constant the compression stress side elongates and the tension stress side shrinks in bending constrained specimen during cooling. It is concluded that the shape of the specimen changes spontaneously and the All-Round Shape Memory Effect appears during cooling.