Structure of molybdenum bombarded with low-energy hydrogen and helium ions during creep tests

Abstract

To find the causes of the increase in the creep resistance as a result of ion bombardment, methods of high-voltage and analytic transmission electronmicroscopy were used to make a detailed study of the three-dimensional structure of specimens of single-crystal molybdenum, deformed without irradiation and under bombardment with low-energy ions from a glow discharge. Foils for transmission electron microscopy (TEM) were prepared from the axial testsections of the molybdenum specimens, which had been earlier subjected to creep tests [2]. Since bombardment with ions from a glow discharge causes bulk changes in the structure of the metal by acting on its surface, it seemed natural to expect the most significant and informative (in relation to an understanding of the phenomenon as a whole) structural changes near the surface. Accordingly, we first made a detailed study of the defect structure of the surface layers in the specimens subjected to bombardment without a mechanical load. For this purpose a series of disk-shaped (diameter 3 mm) single-crystal specimens of molybdenum were prepared [3] and were bombarded with helium and hydrogen ions from a glow discharge (T = 1500~ T = 1 h), after which the well-known methods of [4, 5] were used to prepare foils for TEM and their surface structure was studied with the aid of tomography. Most of the foils were examined in JEM-1000 and EM-301 G microscopes at an accelerating voltage of 1 }fV and i00 kV, respectively. The three-dimensional dislocation structures of the specimens bombarded in a.discharge of H2 or a mixture of He + H2 under strain differ considerably from the structures Of specimens tested in a vacuum. Figure 1 presents photomicrographs of low-angle boundaries in molybdenum specimens after creep testing without (a, b) and with (c, d) irradiation from a mediumof pure hydrogen. Even though the average size (~i00 ~m) and shape of the blocks in both specimens are roughly the same, the dislocation structures of the interblock boundaries and the form of the dislocations themselves differ markedly, as can be seen well from the photomicrographs. In the specimen tested under bombardment the dislocations are highly curved, while in block boundaries they are intertwined, often forming pileups that cannot be resolved by TEM. Near block boundaries are dislocation loops of a size of 30-80 n m, whose density varies within wide limits. Extremely diverse dislocation configurations are formed in specimens that have been bombarded under strain with ions from a glow discharge in an He + H~ mixture (Fig. 2). The photomicrographs in Fig. 2a, b show a large-cell hexagonal dislocation wall and a wall with elements of tetragonal cells formed in the bombarded specimen under strain. In practically all of the dislocation lines of such walls, including purely screw dislocation lines, a fine serrated structure was detected at higher magnifications. Even larger-celled dislocation walls with a less regular structure are encountered (Fig. 2c) and so are individual phase precipitates of