The direct observation of etch-pits at dislocations in molybdenum

Abstract

The correlation between etch-pits and dislocations in molybdenum has been examined by optical microscopy and replication techniques on bulk single crystals, and transmission electron microscopy of etched thin foils. Dilute Murakami chemical etch gives a one-to-one correspondence between etch-pits and dislocations on {421} planes; however, for other orientations and/or solutions, the one-to-one correspondence no longer exists. Detailed examination of the {421} plane shows that: (a) dislocation decoration by impurities is not required for etch-pitting; (b) annnealing of as-grown crystals in hydrogen at ∼ 1900°c reduces the dislocation density from ∼6 × 10⁷ cm⁻² to ∼ 10⁶ cm⁻², with an accompanying re-arrangement of dislocations into sub-grain boundaries; (c) standard strength Murakami etch produces etch-pits at dislocations and at other sites in the matrix; (d) the shape and orientation of the etch-pits is independent of the Burgers vector or angle of inclination of the dislocation to the free crystal surface. The etching behaviour is considered in the light of proposed mechanisms and an attempt is made to explain the discrepancies observed.