In Situ Observation of Texture Changes During Phase Transformations

In one of the first user experiments with the new general purpose TOF neutron diffractometer HIPPO at Los Alamos, phase transformations during heating of bulk samples of quartzite and zirconium were investigated with a vacuum furnace. Both have geological relevance, for the crust and the inner core, respectively. A cylindrical sample (9mm diameter, 12mm high) of mylonitic quartzite was measured at room temperature, then at 500C, 625C, then cooled to 500C and heated again to 625C and finally cooled to 500C again to investigate memory effects during thermal cycling through the alpha-beta transition. A cubic (5mm) sample of zirconium was measured at 750C, 850C, 950C, 750C, 950C, 750C to investigate the hcp - bcc phase transformation. 30 detector banks at 150, 90 and 40 degree diffraction angles record differently oriented crystals. 180 spectra (in 6 sample orientations at each temperature) were then refined simultaneously with the Rietveld method MAUD to determine texture and crystal structure. The results indicate that in both materials there is a selection of orientation variants when samples are cooled from the disordered high temperature structure. It is speculated that a memory is imposed by the microstructure and topology of neighboring grains.