Precipitate evolution in the Zircaloy-4 oxide layer

Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

Hydrogen-assisted processing of materials

Mechanical and thermal behaviour of cordierite-zirconia composites

The crystal structure of high cordierite and that of high cordierite solid solutions containing either Mn or Ga or Ge were refined. The structural changes caused by substitutions and the characteristic features of deformations of each T1O4, T2O4 tetrahedron and MO6 octahedron were clarified. Thorough crystal chemical considerations led to a conclusion that the structure of Mn-bearing solid solution at room temperature may be regarded as a model structure of high cordierite at an elevated temperature. On the basis of the thermal expansion mechanism derived from this consideration, the thermal expansion behavior of the solid solutions is presented.

Crystal Structures and Mechanism of Thermal Expansion of High Cordierite and Its Solid Solutions

The Al-Zr phase diagram was reviewed in this section [1993Oka1], as shown by solid lines in Fig. 1. Dashed lines in Fig. 1 show the Al-Zr phase diagram obtained with thermodynamic assessment by Wang et al. [2001Wan]. The diagrams of Okamoto [1993Oka1] and Wang et al. [2001Wan] are significantly different around the melting point of Al2Zr and along the liquidus and solidus of ( Zr). Because the liquidus of Al2Zr in the diagram of Okamoto [1993Oka1] is too asymmetric according to a criterion of Okamoto and Massalski [1993Oka2], the diagram of Wang et al. [2001Wan] may be more realistic. The significant difference in regard to the liquidus and solidus of ( Zr) must be resolved experimentally. Until now there have been no data that corroborate either form. Figures 2 and 3 show the detail of the Al and Zr rich end of Fig. 1, as reported by Wang et al. [2001Wan].

Al-Zr (aluminum-zirconium)

From a literature survey, the hydrogen-induced amorphization behaviour in RM2 (R=rare earth, M = Ni, Fe, Co) Laves phases is classified into two groups according to whether a crystalline hydride phase appears during the transition to an amorphous state or not. The major factor that determines the presence or non-presence of the crystalline metal hydrides is related to the stability of the compound, which can be estimated by the heat of formation (δHf) or the decomposition temperature of the compound itself. The crystalline metal hydrides are found only in the compounds with high stability. It is proposed that the mechanism for the two types of amorphization is related to the degree of lattice expansion with hydrogen absorption, which is a function of the elastic modulus of the compounds. To explain the dependence of the elastic modulus of the compound on the two types of amorphization behaviour, the concept of a nucleation barrier for amorphization is adopted. Compounds with a large lattice expan...

General features of hydrogen-induced amorphization in RM2 (R = rare earth, M = transition element) Laves phases

The mechanism of hydrogen-induced amorphization in intermetallic compounds

Structural study of a K-substituted synthetic cordierite

Hydrogen absorption into the ordered Zr 3 Al intermetallic compound (Cu 3 Au type, L1 2 structure) leads to an amorphization at below 550 K. As the reaction temperature increases, the hydrogenated Zr 3 Al decomposes into the equilibrium phases ( ZrH 2 + Zr 2 Al ). The amorphous samples are studied by X-ray diffraction, differential scanning calorimetry, optical microscopy, scanning electron microscopy and high-resolution transmission electron microscopy to clarify the transformation mechanism. During DSC scanning, ZrH 2 phase is crystallized from amorphous matrix at first. The evolution of hydrogen from amorphous phase begins at low temperature at about 313 K, and continues until the full crystallization of amorphous phase into ZrH 2 + Zr 2 Al . It is observed by optical microcopy that the formation of amorphous phase takes place from the free surface of the sample with a discrete a/c (amorphous/crystal) boundary layer. From SEM observation, many cracks are found at the front of the advancing amorphous phase, which are thought to be caused by the hydrogen absorption. This result clearly indicates the accumulation of the elastic strain by hydrogen absorption. High-resolution TEM on the partially hydrogenated samples show that the spatially continuous amorphous phase and the unreacted crystalline islands is coexisted with a sharp a/c phase boundary, and the mode of the nucleation of amorphous phase is heterogeneous. Electron and X-ray diffraction patterns in this region reveal that the ordered crystalline structure is maintained with a lattice expansion of about 2 vol.%, and there is no indication of the chemical disordering. This volume expansion is considered to be a critical limit for the maintenance of the crystalline structure. Based on the observed experimental results, it is suggested that the amorphization is mainly caused by the elastic strain during hydrogenation.

Transformation characteristics of hydrogen-induced amorphization of the ordered zr3al phase

A structural study of a potassium-substituted cordierite with K0.5Mg2Al4.5Si4.5O18 formula was performed between 20 and 825° C using a time-of-flight powder neutron diffraction technique, on sintered samples obtained by glass crystallization. The thermal evolution of this compound is essentially of the same nature as for pure hexagonal cordierite, but is very dependent on the presence of potassium atoms within the channels. The very low value of the thermal expansion coefficient (
 
$$\bar \alpha = 0.4 x 10^{ - 6} K^{ - 1} $$

 between 20 and 825° C) results from an opposite evolution of thea andc lattice parameters which may be explained in terms of a reorganization and a regularization of (Al, Si)O4 and MgO6 polyhedra. It strongly depends on the Al/Si ordering degree.

Yong-Gyoo Kim

Papers

General features of hydrogen-induced amorphization in RM2 (R = rare earth, M = transition element) Laves phases

The mechanism of hydrogen-induced amorphization in intermetallic compounds

Structural study of a K-substituted synthetic cordierite

Transformation characteristics of hydrogen-induced amorphization of the ordered zr3al phase

Powder neutron diffraction study of the thermal expansion of a K-substituted cordierite