Abstract
The effect of microstructure and crystal direction on the extent of phase transformation (EPT) of Ni into β-NiH by cathodic charging with H has been investigated by X-ray diffraction, EPT is controlled by the crystal direction in the case of heat-treated specimens. In the case of electrodeposited specimens, the imperfections of which are commensurate with those of cold-worked metals, EPT is controlled by both the crystal direction and the “dislocation-induced” anisotropy at the same time. The decomposition of β-NiH follows the Kolmogorov–Avrami–Johnson–Mehl kinetics. The microstructural anisotropy induces anisotropy in the rate of decomposition of β-NiH.
Highlights
The results we have presented unambiguously show that the decomposition process both for annealed and unannealed specimens are controlled by the Avrami, Johnson-Mehl kinetics
Our studies indicate that the anisotropy of extent of phase transformation (EPT) in polycrystals, consisting of crystallites with lattice imperfections, is controlled by two factors: (1) crystallographic direction, i.e. orientation of the crystallites versus surface of charging with H
In the case of polycrystals, consisting of "perfect" crystallites which have grown during the annealing process, the anisotropy of the phase transformation is controlled by one main factor--the crystallographic direction
Summary
It is well known that as a result of the cathodic charging of nickel with H in the presence of inhibitors, an instable at room temperature nickel hydride phase is produced (Janko 1960, Boniszewski et al 1961, Baranowski 1964, Calbe et al 1964, Borbe et al 1980, Pielaszek 1985). The kinetic analysis of the decomposition of fl-NiH was done with the help of volumetric (Baranowski 1959) and X-ray diffraction (Borbe et al 1980, Pielaszek 1985, Janko 1962, Pielaszek 1972, Rashkov et al 1982) methods Most of these indicated the fl-NiH decomposition is expressed, as a rule, by a first-order reaction (Borbe et al 1980, Pielaszek 1985, Baranowski 1959, Janko 1962, Pielaszek 1972), but there is a case presented (Rashkov et al 1982) in which the fl-NiH decomposition is described by another function. These results (Borbe et al 1980, Pielaszek 1985; Baranowski 1959, Janko 1962, Pielaszek 1972) are irreconcilable with the regularities in the kinetics which have been observed in the formation and decomposition of metal hydrides (Larsen, Livesay 1980, Douglas, Northwood 1983)
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