The martensitic transformation from the hexagonal close-packed structure to the body-centered cubic structure and the reverse transformation in solid helium-4 were observed by SR X-ray topography. Because of very small X-ray scattering cross section of a helium atom, microstructures in a bulk crystal of the thickness of 10 mm can easily be observed. When a large crystal with the hcp structure transforms into the bcc structure, the new phase has many parallel subboundaries at intervals of about 0.5 mm, and there is no sign of internal stress by the transformation. However, the reverse transformation causes large internal stress if it takes place in a crystal with many subboundaries. These observations strongly suggest that the transformation is accompanied by macroscopic strains and also plastic deformation to relax the transformation stress as in metals.
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