Emissary dislocations associated with {1122} twins in zirconium

Abstract

The literature reports that emissary dislocations can be associated with deformation twins in several metals (1-3). Etch-pit observations on zinc (4,5) and microtwin results on zirconium (6) also suggest the occurrence of such dislocations with {lOI2} twins in the h.c.p. structure, although this evidence is not conclusive. In fact, in this structure, emissary dislocations should be more probable with n 122} (1123) twinning since {lll2} (}123) is also a known slip system. The present paper describes experimental evidence for emissary dislocations obtained during a room-temperature study of n Il2} twinning in zirconium of approximately 99.9% purity, containing 100-200 (weight) ppln oxygen as the major interstitial impurity. Rectangular parallelepiped samples of minimum size, 30 mm X 2.5 mm X 2.5 mm, consisting of one to six crystals were employed. The specimens were deformed in threepoint bending. Shock-loading, ~ccomplished by dropping an extra weight on the loading platform of the bending jig, was often used. Metallographic examination was carried out on an optical microscope with polarizing attachment. Twins were identified by two-surface analyses. An etch-pit technique (7) which reveals sub-boundaries as well as dislocation movement after the formation of a surface film was occasionally employed. Attempts to reveal emissary dislocations by etch-pitting 'were inconclusive since profuse etching of the newly twinned or detwinned area often made observation of the exact location of the twin boundary difficult. When samples containing previously formed {lll2} twins were shock-loaded, sheared but apparently untwinned regions