Structure of the R-phase in NiTi Shape Memory Alloys

H. Morawiec, T. Goryczka

University of Silesia, Katowice, Poland

The reversible martensitic transformation in near equiatomic NiTi alloys continues attract a good deal of interest due to the fact that it is associated with the shape memory effect. Prior to the formation of martensite the B2 - parent phase undergoes a rhombohedral distortion along the <111> direction, producing the so - called premartensitic R-phase. The R-phase is not a precursory phase but rather a separate electronically activated structural phase. It has been recently confirmed that the R-phase transition is of martensite type. After heating the deformed R-phase, recovery of the previous shape can be observed with a small thermal histeresis.

While the R-phase has found its applications, crystal structure has not been resolved yet.

Shapiro at al [1] using single crystals of the R-phase of NiTiFe alloy studied by x-ray diffraction stated that the symmetry of the phase is trigonal and descripted it as belonging to the P31m space group. Goo and Sinclair [2] confirmed this trigonal symmetry using the convergent beam electron diffraction pattern (CBED), and determined the parameters of the hexagonal lattice.. The structure of the R-phase was first studied by Vatanayon and Heheman [3] who considered the similarity of the R-phase to the z'₂ Au-Cd martensite with the P31m symmetry. Ohba et al [4] determined the crystal structure of the z'₂ martensite as belonging to the P3 space group. Hara et al [5] basing on electron diffraction studies compared the observed reflection intensities with the kinematically calculated model for four different space group. All results of comparison support the P3 model rather than any other P31 m/models. However the dynamical scattering treatment by multislice calculation has not shown the complete agreement to the observation for the P3 symmetry. Futher effort undertaken by Hara et al [6] to determine the crystal structure of the R-phase was based on powder diffraction studies using the Pawley method for lattice constant determination and Rietveld method for structure parameters refinements. The obtained reliability factors (R-factors), are better for the P3 model than for the P31m.

The presented results were obtained in experiment with wire specimen with a fiber texture which cause some limitation in the reliability factors in Rietveld method. For this reason we will also present our own results obtained from comparison of the experimental diffraction pattern with the calculated by Rietveld method for different space group model. The obtained results confirm the P3 space group for the R-phase.

As a conclusion: the R-phase in NiTi alloys belongs to a separate not martensitic precursory phase with a trigonal symmetry of P3 space group and hexagonal lattice with approximate parameters: a = 7.332 Ä and c = 5.280 Ä which changes with the chemical composition of those alloys.

1. S.M. Shapiro, Y. Noda, Y. Fuji, Y. Yamada - Phys.Review B30(1984)4314
2. E. Goo, R. Sinclair - Acta metall. 33(1985)1717
3. S. Vatanayon, R.F. Heheman - Shape Memory Effects in Alloys ed. by J. Perkins, New York 1975 p. 115
4. T. Ohba, S.M. Shapiro, S. Aoki, K.Otsuka - Jpn.J.Appl. Phys.33(1994)L1631
5. T. Hara, T. Ohba, S. Nenno, K. Otsuka - Advanced Materials'93, V/B Shape Memory Materials and Hybrides, ed. by K. Otsuka et al. 1994 Elsevier Science B.V. p. 1069
6. T. Hara, T.Ohba, K. Otsuka - J.de Phys. IV suppl. III 5(1995)C8-641