Direct observations of vacancies and vacancy-type defects in molybdenum following uniaxial shock-wave compression

Molybdenum sheet and small diameter wire samples were simultaneously shock-loaded to pressures of 150 and 250 kbar at a constant shock-pulse duration of 2 μs. Thin foils and emission end-forms prepared from annealed as well as the shock-loaded samples were simultaneously examined by transmission electron and field-ion microscopy respectively. Dislocation loop densities were observed to increase from an annealed value of 5 x 109 to 4 x 1014 cm-3 and 7 x 1014 cm-3 at 150 and 250 kbar, respectively, and the corresponding average loop diameters were measured to be 57, 104 and 152 A, respectively. A full determination of the relevant diffraction conditions and Burgers vectors of the loops in the electron microscope showed that 75 percent of the loops at 150 kbar and 80 percent of the loops at 250 kbar were vacancy type. By comparison, direct observations of vacancies and small vacancy aggregates in the field-ion microscope by systematic field-evaporation of atom layers showed a vacancy concentration of 1.2 percent in the annealed wires as compared with 2.2 and 6.0 percent vacancies in the 150 and 250 kbar shock-loaded wire samples respectively. Since the corresponding increase in dislocation density was determined to be 1.0 x 1010-1.2 x 1010 cm-2 between 150 and 250 kbar for a concomitant increase in average residual microhardness from 514 to 770 kg/mm2 in the same pressure range, it is shown that shock induced vacancies contribute significantly to residual shock hardening