Stability of the two-phase (α/ω) microstructure of shocked zirconium

The composite microstructure of two-phase (alpha/omega) shocked zirconium was studied in situ during heating (constant heating rate and isothermally) with high-energy X-ray diffraction techniques. The volume fraction of the metastable omega phase was monitored as the reverse phase transformation occurred: the start and finish temperatures being 470 and 550 K, respectively, during heating at 3 K min(-1). Moreover, isothermal transformation was observed when the shocked material was held at fixed temperatures from 420 to 525 K. Phase strains in each phase were monitored and separated in terms of thermal expansion and mechanical strains due to local phase constraints. Stresses in the alpha Zr were estimated to be a superposition of a hydrostatic component (of order -50 MPa) and uniaxial component (of order 600 MPa) along the c-axis. These stresses were relaxed during the reverse transformation. A high dislocation density was observed in both the a and omega phases in the as-shocked state. The dislocation density of the omega phase decreased preceding the reverse transformation, suggesting that it is the presence of the high concentration of defects in the omega phase which retarded the reverse transformation to the stable a phase and prevented the system from approaching equilibrium after the completion of the shock. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.