The method of loading by spherical converging shock waves is the most
perspective method for the investigation of the formation and
evolution of the ASBs and other instabilities of plastic flow because
it allows one to obtain extremely high stresses, substantially
exceeding dynamic yield strength, in rather large volumes of material
and growth of the strain rate at the front of spherical converging
shock wave with the decrease of ball radius. Although a number of
investigation have been made concerning the nature of the ASBs, the
understanding of the ASBs formation as mode of deformation is still
insufficiently. The purpose of the present work is to study the
formation and evolution of the ASBs in zirconium and its alloys with 1
and 2.5wt% niobium under loading by spherical converging stress waves
of the different intensity. Six balls (two zirconium balls 24 mm in
radii, two balls of the Zr-1wt% Nb alloy 35 and 32 mm in radii and two
balls of the Zr-2.5wt% Nb alloy of the same sizes) were subjected to
the loading by spherical converging detonation waves of the different
intensity. The initial pressure on the external surface of the balls
was over 30 GPa. In the central areas of the balls, pressures and
temperatures, sufficient for the melting of zirconium and its alloys
with niobium directly at the shock-wave front, were reached at
converging waves. The detailed analysis of the structure was performed
using the X-ray diffraction analysis, the optical (including optical
microscopy with a polarized light) and transmission electron
microscopy. As the result of spherical converging detonation waves
loading, the great amount of the ASBs was observed in the balls. It
was found that the amount of the ASBs and their distribution depends
both on the loading conditions and on the niobium content in the
alloy, but the loading intensity being essential factor than the
niobium content. In the case of low-intense loading, the ASBs
formation occurs in deep layers, predominantly near the hollow of the
plastic failure formed in the centre of each ball. The amount of the
ASBs increases with increasing the loading intensity, and the region
of the ASBs formation expands and is displaced to the extent surface
of the balls. In the most cases, the ASBs propagate curvilinearly. The
ASBs are sometimes dendrite-like by branching into several finer
bands. The branching of the ASBs is most often observed in zirconium
after high-intense loading. The structural defects such as grain
boundaries, twins, dislocations, cracks, pores do not prevent the ASBs
propagation. It was shown that ASBs have the form of extended ribbons
with different thickness and width. It is observed the formation of
several melting areas and pores within the ASBs spaced at some
distance apart. The pores of different size form in the melting areas
under action of tensile stresses. The appearance of the melting areas
within the ASBs is connected with different motion of shock adiabat
within and outside the ASBs. Pressure increases slower and temperature
increases faster within the ASBs then these parameters outside the
ASBs.
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