PHASE TRANSFORMATIONS IN SHAPE MEMORY ALLOYS

P. Sedlák^1,3, M. Landa¹, H. Seiner^1,3, V. Novák² , P. Šittner²

¹Institute of Thermomechanics, Academy of Sciences of the Czech Republic, Dolejskova 5,182 00 Prague 8, Czech Republic.

²Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance,182 00 Prague 8, Czech Republic.

³CTU Faculty of Nuclear Sciences and Physical Engineering, Trojanova 13, 120 00 Prague 2, Czech Republic.

Shape memory alloys (SMA) exhibit unique thermomechanical behaviors due to thermoelastic martensitic transformations (MT) driven by external stresses or temperature. Physical and structural understanding of MT processes enable to improve the unique properties such as superelasticity, superplasticity and shape memory.

The most important applications has Nitinol - approximately equiatomic Ti-Ni alloy (commercially called Nitinol). The high temperature parent phase of NiTi alloys has B2 type ordered cubic structure that transforms to B19’ monoclinic martensite phase by cooling or applied stress. Following suitable termomechanical treatments, the MT in NiTi may proceed as two step B2-R-B19’. The intermediate R-phase has rhombohedral structure differing only slightly from the B2 by rhombohedral distortion. The B2-R transformation exhibits very small transformation strain and hysteresis compared to the B2-B19’ transformation.

The activity of B2-R transformation, wich is rather difficult to recognize in a simple mechanical test on NiTi wires, were detected by in-situ ultrasonic and electric resistance measurement based on anomalous behavior of speed and attenuation of ultrasonic waves related with R-phase deformation/transformation processes [1]. Structure sensitive information were obtained by in-situ neutron diffraction.

Another important group of SMA - Cu based alloys were also investigated. A detailed analysis of the changes in elastic properties associated with b₁ austenite - g’₁(2H) martensite transformation in CuAlNi were performed [2,3].

[1] Landa, M. - Sedlák, P. - Maršík, F. - Šittner, P. – Novák, V.: Ultrasonics Characterization of Phase Transformation in NiTi wire during thermomechanical loading, XXI International Congress of Thoretical nd Applied Mechanics, Varšava, Polsko, 15. – 21. 8. 2004

[2] SEDLÁK, P. - SEINER, H. - LANDA, M. - NOVÁK, V. - ŠITTNER, P. - MAŇOSA, LL.: Elastic constants of bcc austenite and 2H orthorhombic martensite in CuAlNi shape memory alloy, Acta Materialia (2005) in print

[3] LANDA, M. - NOVÁK, V. - SEDLÁK, P. - ŠITTNER, P.: Ultrasonics Characterization of Cu-Al-Ni Single Crystals Lattice Stability in the Vicinity of the Phase Transition, Ultrasonics, Vol.42, 2004, pp. 519-526.