The Influence of Phase Changes upon Permeability

It has been found that in the region of concentrations where two hydrogen-palladium alloys coexist (the perpendicular sections of the curves of Fig. 40) it is not possible to trace out the same isobaric curve on sorption and desorption. There is a hysteresis effect illustrated by Fig. 40. It is interesting to find in the study by Lombard, Eichner and Albert (96) that the permeability-temperature curve follows in an inverse manner the absorption isobar, there being a great increase in the permeability in the region 180-200° C. It may be that this rapid alteration in permeability marks the change from )ff-phase to a-phase alloy. 

The same type of hysteresis loop which was noted in the absorption of hydrogen in palladium, where a- and yS-phases co-exist, is observed also in the absorption of hydrogen by iron (31), where now the two phases are produced by allotropy in the metal. The permeability-temperature curve shows a break at this point (Fig. 63) (5i). Indeed, wherever a phase change occurs one may look for a variation in the permeability, so that the property of permeability may be used to determine transition points. The change in permeability of nickel towards hydrogen has similarly been used to characterise the Curie point in nickel (62). 

When the phase changes are brought about by alloying two metals, one would anticipate that in the same way the permeability would vary with the composition of the alloy, and discontinuously so wherever a new phase is formed. Evidence upon these points is scanty, but the data of Baukloh and Kayser(97) indicate that discontinuities do exist in the permeability-composition curve of alloys of nickel with copper and with iron (Fig. 64). 

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