CDAC graduate student Nick Cunningham (University of Alabama-Birmingham) has shown that Pr metal at high pressure undergoes dramatic grain growth as the metal passes through the fcc-to-alpha uranium structural phase transition (Cunningham, Velisavljevic and Vohra, Phys. Rev. B, in press). At 16.0 GPa smooth diffraction rings indicate a highly crystalline material with small particle sizes for the hR24 (fcc) phase of Pr. The metal transforms to the fcc phase from the ambent pressure dhcp phase at 4 GPa. Diffraction data were obtained at HPCAT Sector 16-ID-B of the Advanced Photon Source, Argonne National Laboratory.
At 18.9 GPa, the diffraction pattern shows significant grain growth as the fcc-to-alpha uranium phase transition is completed, as evidenced by large and distinct spots in the diffraction rings. The volume collapse across the transition is 9.1%. There is, however, no evidence of any particular orientation in the grain growth, as might be expected with standard texturing effects.
Within about a 4 GPa increase in pressure, the grain growth begins to be supressed, and at 28.5 GPa, the randomized, homogeneous and polycrystalline metal is clearly restored. The grain growth appears to be a general characteristic of the formation of the alpha uranium phase in different materials as opposed to an effect associated with the large volume collapse.