Hydride Plates Research Articles

The nucleation of hydrides in a Zr-2.5 wt% Nb alloy

The precipitation of hydride plates in a Zr-2.5 wt% Nb alloy is shown to be sensitive to the prior heat treatment of the alloy. For heat treatments that lead to a faceted α/β two phase microstructure, the α-β interface is characterized by an epitaxial dislocation array and steps with associated strain fields. For this structure, the nucleation of hydrides is controlled by the defect structure of the interface. On annealing the α-β structures below the monotectoid temperature, the retained β phase is unstable, decomposing first to β r and ω and ultimately to α and β Nb . At the intermediate stage, i.e. with a β r/ω two phase structure, hydride precipitation takes the form of stacks of small hydride plates confined to channels in the β lattice between the ω particles. For both microstructural conditions, the preferred site for hydride nucleation appears to be that which can most effectively accommodate the strains associated with hydride precipitation. Similarities in the precipitation behaviour in α-β Zr alloys and Ti alloys are discussed.

Hydride precipitation in α/β zirconium alloys

The precipitation of δ-hydride (ZrH 1.5) in fine-grained α/β zirconium alloys has been studied. Both interphase and transgranular hydrides are observed, the mechanism of growth being dependent on the nature of the α/β interface. Macroscopic hydrides observed in the optical microscope are always composed of stacks of smaller hydride plates with habit planes close to the basal plane. The apparent habit plane of the macroscopic hydride plate is determined by the geometry of the stack. The stability of different hydride stacks is considered and the role of the texture of the alloy and of a stress (external or internal) acting on the basal planes in determining the orientation of the stacks is explained.

Hydrides in zirconium-2.5 wt. % niobium alloy pressure tubing

The pressure tubes for CANDU-PHW reactors are made from cold-worked Zr−2.5 wt. % Nb. As this alloy is susceptible to delayed hydrogen cracking, the crystal structure and morphology of the hydride was examined to determine its effect on the delayed hydrogen cracking process. Here we report on our thin foil and replica TEM (transmission electron microscope) examination of a number of pressure tube samples containing hydrides formed both under stress and in as fabricated material. The main conclusions are: 1. (1) the fcc δ-hydride is the predominant hydride in all materials; 2. (2) the hydride platelets formed in the as-fabricated material are circumferential (radial normals) and are bounded by the β-phase; 3. (3) the β-phase has no effect on the precipitation of stress-reoriented hydride; 4. (4) the hydride cracks run through the hydride rather than at the hydride/α-Zr interface; and 5. (5) the orientation relationship between the δ-hydride and the α-Zr matrix facilitates formation of radial hydride plates (circumferential normals) in the pressure tubes.

Experimental studies of the crystallography of the precipitation of β vanadium hydride

The structure of the interface between β vanadium hydride plates and the parent b.c.c. solid solution of hydrogen in vanadium has been studied by transmission electron microscopy, and measurements have been made of the shape strain and orientation relationship. The results are generally in good agreement with the predictions of the phenomenological theory of martensitic transformations. The prediction that the interface dislocations generating the lattice invariant shear should be resolvable has been confirmed, and direct confirmation obtained of the hypothesis that the complementary shear system is {112} 〈 11 1〉. For the identity correspondence and specific shear system (112) B [ 11 1] R the predictions of the theory that the habit plane is near (2̄27) B and the displacement direction near [22̄7] B have been confirmed by the electron microscopy and the shape strain measurements. The measured magnitudes of both shape and complementary strains agree well with predicted values. Orientation determinations from Laue and Kossel patterns indicate that the [001] direction of the hydride is inclined to the (001) plane and are compatible with monoclinic hydride structures in which either χ or β is greater than 90°. However, measurements of the orientation relationship are generally in better agreement with predictions of the theory for structures with β 〉 90°.

The crystallography of the precipitation of β vanadium hydride

The phenomenological theory of martensitic transformations is applied to the precipitation of β vanadium hydride from b.c.c. solutions of hydrogen in vanadium. The predictions of the theory are examined for a body centred tetragonal hydride structure and for a range of monoclinic structures compatible with the reported mode of hydrogen ordering. This reaction, which involves substantial diffusion of hydrogen, exhibits martensitic relief effects and the habit plane near {221} is distinctly different from the {h0k} habit planes exhibited by a number of transformations involving similar changes in structure. It is shown that the habit plane and the reported orientation relationship are predicted, within experimental error, when the complementary shear is assumed to occur on the (112) B[1̄1̄1] B system. These predictions are obtained using the initial and final lattice parameters; correct predictions are not obtained if it is assumed that transformation occurs in regions of high hydrogen concentration. Since the complementary shear plane is not a plane of symmetry, (112) twins cannot form with equivalent correspondences and the hydride plates are not expected to be twinned. The interface should therefore contain dislocations and the small magnitude calculated for the complementary shear implies that these should be resolvable in the electron microscope.

Spontaneous hydride formation in β-phase niobium zirconium alloys

A spontaneous transformation has been observed in thin foils of metastable β-phase (b.c.c.) niobium 16–60% zirconium alloys during electro-polishing. The product has a f.c.c. crystal structure and a plate-like morphology. The transformation is governed by the concentration of both hydrogen, which has to exceed a critical value and zirconium. Consequently the spontaneously formed plates are only stable at room temperature in foils where the zirconium concentration exceeds 40%. The hydride plates have a habit plane close to {133} β and contain either slip dislocations, stacking faults or fine twins on {111} h planes or mixtures of these. The orientation relationship between the hydride plates and the β-matrix corresponds closely with the Kurdjumov-Sachs relationship. Complex interactions between plates result in a further transformation which has been interpreted as a reverse transformation (f.c.c. → b.c.c.). The results have been considered in terms of the phenomenological crystallographic theory for martensite formation. Satisfactory agreement between theory and experiment has been established.

Dislocation structure in dehydrogenated niobium

Dislocation structures and mechanical properties of dehydrogenated niobium specimens were investigated by means of transmission electron microscopy and tensile testing. Dehydrogenated specimens were prepared by vacuum annealing of a Nb-30 at.% H alloy. The traces of hydride plates were formed with tangled dislocations in a specimen dehydrogenated at lower temperatures, and the flow stress of such a specimen was very high. It is concluded that the high flow stress of the dehydrogenated specimen is due to the high density of dislocations and their heterogeneous distribution. The probable mechanism of the hydride formation is proposed. (auth)