The intermediate phase of the shape-memory alloy NiTi

Abstract
The structural properties of equiatomic NiTi have been investigated by single-crystal neutron elastic scattering. The premartensitic phase transition of NiTi is characterised by a second-order displacive transition which is driven by a condensation of the T2A(110) phonon mode at Tc=75 degrees C. The lattice displacement amplitude behaves like a Landau order parameter with a critical exponent beta =1/2. Upon cooling, the martensitic transformation occurs at TM=35 degrees C by nucleation. The intermediate phase has a distorted CsCl structure of monoclinic symmetry (P2/m), providing 12 equidistributed structural domains.

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