Reorientation Processes in CuAlNi Single Crystals Studied by Neutron Diffraction Technique

P. Molnár^{1, 2}, P. Šittner¹, V. Novák¹ and P. Lukáš ³

¹ Institute of Physics, Na Slovance 2, CZ - 182 21 Prague 8

² Department of Electronic Structures, Charles University, CZ - 121 16 Prague 2

³ Nuclear Physics Institute, CZ - 250 68

molnar@fzu.cz

The microstructure of martensites created by thermally induced martensitic transformation from austenite single crystal consists of multiple habit plane variants arranged in a self-accommodated manner - i.e. the martensite sample containing large amount of internal interfaces is not a martensite single crystal. However, when such a crystal is deformed in martensite state (e.g. by successive compression deformations applied to different faces of a prism shaped sample [1]), the internal interfaces can be driven out of the sample and a true single crystal of the martensite phase may thus be prepared. Before such a relatively large martensite single-crystal is further utilized for e.g. measurement of martensite elastic constants by ultrasonic methods [1], its quality (whether the sample is really free of twin interfaces) needs to be checked.

One can inspect the polished faces of the martensite prisms for traces of interfaces, compare the measured shape of the martensite prism with a theoretical prediction [1], or take the linear compression stress-strain response as indirect evidences for the singlecrystalinity of the sample. Neutron diffraction yielding bulk structural information from a sample fully immersed in the neutron beam, on the other hand, may be used to provide direct evidence.

In this work, we describe a neutron single crystal diffraction approach recently developed for inspecting the quality of 2H martensite single crystals of CuAlNi alloy prepared from originally cuboid shape austenite single crystal samples. The experiments were carried out on a single crystal diffractometer at NPI Rez using monochromatic neutron radiation (wavelength 1.44Ǻ) equipped with a ³He counter, 2D imaging plate detector and miniature screw driven deformation rig placed in a 3-axis goniometer. Neutron diffraction results show that it is in fact quite difficult to prepare sufficiently good martensite single crystals, since the martensite twinning processes leading ultimately to single martensitic variant are often not completed, even if the indirect methods seem to suggest that the sample already exists in the desired martensite single crystal form.

1. P Sedlák, H. Seiner, M.Landa, V. Novák , P. Šittner, Ll. Mañosa, Acta Materialia, 53, 2005, 3643-3661.