Observations on prismatic and grown-in dislocations in zone-melted molybdenum

Abstract

Abstract Dislocation configurations in undeformed electron beam zone-melted single crystal molybdenum have been examined by thin-film electron transmission microscopy. In addition to a relatively uniform distribution of dislocations of density ∼5 × 106 cm−2, certain of the foils are characterized by (i) long grown-in dislocations stabilized by precipitates, (ii) rows of prismatic dislocation loops parallel to <111> emanating from inclusions in the matrix. The loops are believed to be interstitial in nature, with a spacing consistent with the theory of Bullough and Newman (1960) and a critical resolved shear stress to move a loop ∼ 10−4 μ. Since the critical resolved shear stress of molybdenum at room temperature is 9 × 10−4 μ, it is concluded that the loops form, during cooling, at temperatures > 900°c. Observations made on crystals deformed in the temperature range 4•2—300°k show that the inclusions and immobile grown-in dislocation networks act as barriers to dislocation motion, and are the sites at which dislocation tangles originate. The available evidence indicates that the inclusions are Mo2C, the occurrence of which is dependent on the distribution and level of impurities in the starting material. The calculated differential expansion at the inclusion-matrix interface, based on the number of loops emitted, is consistent with growth of the carbides during cooling.