Hydrogen-induced metal atom rearrangements in pd 3+χmn 1−χ alloys with χ⩾0 using transmission electron microscopy, electrical resistivities and H 2 solubilities

Abstract

In this research we have characterized the products of the hydrogen-induced ordering of disordered and of L1 2−χ ordered Pd 3+χMn 1−χ alloys with χ⩾0. The long-period superstructure (L1 2−s) form is produced by a heat treatment at a temperature of about 700 K in vacuo. An L1 2 ordered form can be prepared by exposing Pd-Mn alloys to hydrogen (1 MPa or above) at temperatures of 500 K or above; this ordering is induced by hydrogen under conditions where ordering does not occur in its absence. After the alloys have transformed to their L1 2 form, they remain in this form if the hydrogen is removed at moderate temperatures. Large dislocation densities appear from the hydrogen-induced ordering of the disordered alloys; the density of these dislocations appears to decrease with increase in χ. Dislocations are also introduced by the disorder → L1 2−s transformation which occurs in vacuo. The mechanism of the hydrogen-induced ordering is unknown but it must be related to the lattice expansion caused by the hydrogen. It should be noted that not very much dissolved hydrogen is needed to cause these transitions, i.e. a hydride phase does not form.