K. Takao

Diffusivity and solubility of hydrogen in Pd-Ag and Pd-Au alloys

Abstract The temperature dependences of the diffusion coefficient and the solubility of hydrogen in Pd-Ag and Pd-Au alloys with silver and gold contents in the range 0–50 at.% were investigated using the electrochemical method.

Order-disorder transitions of the Pd7Dy(Y) phase in Pd-Dy(Y) alloys

Abstract Electron diffraction and electrical resistance measurements of Pd-Dy(Y) alloys in the composition range between 8.0 and 17.5 at.% Dy(Y) indicate the presence of an ordered phase in the form of a Pd 7 Dy(Y) superlattice. This is similar to the order found in Pd 7 Ce and Pd 7 Gd, which is isomorphous with Pt 7 Cu. The order-disorder transition of the Pd 7 Dy(Y) phase in the alloys between about 10.5 at.% Dy(Y) and the palladium-rich L 1 2 -type Pd 3 Dy(Y) is accompanied with a peritectoid reaction: α - Pd + L 1 2 -type Pd 3 Dy(Y)⇄Pd 7 Dy(Y). The peritectoid temperature ( T c ) is equal to 767 ± 10 K for Pd 7 Dy, and 777 ± 10 K for Pd 7 Y. The order-disorder transition temperatures for the Pd-8.0at.%Dy(Y) and Pd-10.0at.%Dy(Y) alloys are lower than the peritectoid temperature and the transition temperatures correspond to the solid solubility limit of Dy(Y) in the α-Pd solid solution. The palladium-rich portion of the phase diagram of the Pd-Dy(Y) alloys associated with these new ordered Pd 7 Dy(Y) phases is presented.

Evidence for an ordered Pd7Gd phase in PdGd alloys

Abstract The PdGd alloys in the composition range 8.0–21.0 at.% Gd were studied by X-ray diffraction, electron microscopic observations, electrical resistance measurements and metallographic examinations. The results for annealed alloys (with the exception of the 21.0 at.% Gd alloy) indicate the presence of long-range order in the form of a Pd 7 Gd superlattice. This is similar to the order found in Pd 7 Ce and Pd 7 Y, which are isomorphous with Pt 7 Cu. The order-disorder transition of the ordered Pd 7 Gd phase in the Pd-Gd alloys in the composition range 11.5–17.5 at.% Gd is accompanied by the peritectoid reaction, Pd 7 Gd  α - Pd + L 1 2 -type Pd 3 Gd , at T c = 810 ± 10 K. Evidence was found for the existence of the same ordered phase in the Pd-8.0at.%Gd and Pd-10.0at.%Gd alloys from electron diffraction and electrical resistance measurements. The palladium-rich portion of the phase diagram of the PdGd system is presented in a revised form taking into account the ordered Pd 7 Gd phase.

The phase transitions in the palladium-rich PdCe alloy system

Abstract The PdCe alloys in the composition range 10.8–21.0 at.% Ce have been studied by metallographic examination, X-ray diffraction, electron microscopy and electrical resistance measurements. The solid solubility of cerium in α-Pd is 10.8 at.% Ce at 800 ± 15 K and 11.7 at.% Ce at 918 ± 10 K. At temperatures below the solid solubility limit the ordered Pd 7 Ce phase forms in the α-Pd phase. The phase transition of the ordered Pd 7 Ce in the alloys between about Pd-12.3 at.% Ce and Pd-16.6(LPd 5 Ce)at.%Ce is accompanied by a peritectoid reaction: α-Pd + L-Pd 5 Ce ⇌ Pd 7 Ce at 982 ± 5 K. The phase transition L-Pd 5 Ce ⇌ H-Pd 5 Ce in the alloys between about Pd-12.5at.%Ce and Pd-24at.%Ce occurs at 1074 ± 10 K. The phase diagram of the palladium-rich portion of the Pd-Ce system is presented in a revised form taking into account the phase transitions.

Evidence for an ordered Pd7Sm phase in Pd-Sm alloys

Abstract Pd-Sm alloys were studied between 5.0 and 15.0 at.% Sm by metallographic observations, X-ray diffraction, electron diffraction and electrical resistance measurements. The presence of an ordered phase, having a Pd 7 Sm superlattice was observed in the annealed alloys. The superlattice is isomorphous with that of the previously reported Pd 7 R (R ≡; Ce, Gd, Dy, Y). The order-disorder transition of the Pd 7 Sm phase in the alloys between about 9 at.% Sm and Pd 5 Sm at 16.6 at.% Sm is accompanied with a peritectoid reaction: α - Pd + Pd 5 Sm ⇄ Pd 7 Sm at T c = 820 ± 10 K. The order-disorder transition temperatures for the alloys with less than about 9 at.% Sm are lower than that for the stoichiometric alloy Pd 7 Sm, and the transition temperatures correspond to the solid solubility limit of samarium in the α-Pd phase. The phase diagram of the palladium-rich Pd-Sm alloys is presented in a revised form taking into account the new data for the ordered Pd 7 Sm phase.

Observations of ordered Pd7RE (RE Tb, Tm, Yb, Lu) phases in PdRE alloys

Abstract The presence of ordered Pd7RE phases has been observed in palladium-rare earth (RE) alloys with RETb, Tm, Yb and Lu. The order-disorder transition is accompanied by the peritectoid reaction α-Pd + Ll2-type Pd3RE⇄Pd7RE, where the transition temperature decreases in the sequence Tb > Tm > Yb > Lu.

Hydrogen-induced suppression of the peritectoid reaction of alpha -Pd+Pd3Gd (or H-Pd5RE (RE identical to Sm or Eu)) to Pd7RE (RE identical to Gd, Sm or Eu)

The peritectoid reaction of alpha -Pd+Pd3Gd (or H-Pd5RE (RE identical to Sm or Eu)) to Pd7RE (RE identical to Gd, Sm or Eu) in Pd-Gd, Pd-Sm and Pd-Eu alloys is found to be suppressed after the slow cooling to room temperature of alloys in the presence of hydrogen (about 30 bar) which has been introduced above the peritectoid reaction temperature and then maintained during slow cooling. The suppression of the ordering to the Pd7Gd ordered structure in Pd-Gd alloys is believed to be due to the retardation of nucleation and growth of the Pd7Gd ordered domain during cooling as a consequence of a weakening of Pd-Gd bonding by the dissolved hydrogen in the alpha -Pd phase. In the cases of Pd-Sm and Pd-Eu alloys, the suppression is related to the transition of H-Pd5Sm(Eu) phases which initially coexist with alpha -Pd phases to L12-type Pd3Sm(Eu) phases under hydrogen exposure. The initially short-range-ordered Pd7Gd (Sm, Eu) phases coexist with the alpha -Pd, Pd3Gd or H-Pd5Sm(Eu) phases below the peritectoid reaction temperatures, but relatively at high temperatures they also have a tendency to disorder upon hydrogen exposure. Similarly, for the low-temperature hydrogen treatments, the initially coexisting H-Pd5Sm(Eu) phases with the Pd7Sm(Eu) phases also transform to the L12-type Pd3Sm(Eu) phases. The Pd3Sm(Eu) phases which have been transformed by hydrogen are reverse transformed into the alpha -Pd and H-Pd5Sm(Eu) phases by heating above the peritectoid reaction temperatures via the formation of Pd7Sm(Eu) phases.

Hydrogen-induced suppression of ordering to Pd7M (M=Sm,Gd,Li) (Pt7Cu-type superlattice)

It has been observed that the phase transition of alpha -Pd solid solution to the corresponding ordered Pd7M (M=Sm,Gd,Li) phase in Pd-6.5 and 8.0 at.% Sm, Pd-10.0 at.% Gd and Pd-7.2 at.% Li alloys is suppressed by exposure to hydrogen pressures of >20 bar. According to published phase diagrams the short-range ordered Pd7M phase coexists with the alpha -Pd solid solution phase below the solid solubility limits of the solute metals, but at relatively high temperatures within the two-phase field the ordered phase has a tendency to disorder upon exposure to a hydrogen atmosphere (p(H2)>20 bar) and subsequent slow cooling to room temperature with the pressure maintained. The suppression effect of ordering to Pd7M structure may be due to the retardation of nucleation and growth of the Pd7M ordered domain during cooling as a consequence of a weakening of Pd-M bonding by the dissolved hydrogen.

Hydrogen-induced suppression of phase transitions in Pdrare earth alloys

Abstract The phase transition of α-Pd solid solution to the corresponding ordered Pd7RE (RE ≡ Gd,Sm,Eu) phase in Pd-10.0at.% Gd, Pd-8.0at.%Sm and Pd-10.0at.%Eu alloys and the peritectoid reaction of αPd + Pd3Gd to Pd7Gd in the Pd-14.0at.%Gd alloy are suppressed during slow cooling to room temperature of these alloys in the presence of hydrogen (about 30 bar) which has been introduced above the solid solubility limit of the α-Pd phase and above the peritectoid reaction temperature.

Investigations of ordered structures in boron-containing Pd3Mn alloys

Abstract The order transitions in Pd 3 MnB y with y = 0-0.2 were studied by X-ray, electron diffraction and electrical resistance measurements. The order transition behaviour in low boron content alloys with up to about y = 0.1 is substantially the same as that of B-free Pd 3 Mn, except that the long-range order ⇄ short-range order transition of the L1 2 − s structure shifts to a higher temperature with increasing boron content. For high boron content alloys with y = 0.125−0.2 quenched from about 1173 K, however, the electron diffraction patterns in some cases exhibited strong superlattice reflections due to the L1 2 structure; in other cases the patterns showed weak and diffuse reflections due to superimposed L1 2 − s and L1 2 superlattice reflections. For annealed alloys with high B content, obtained by slow cooling from about 1153 K, the diffraction patterns exhibited mainly two-variant reflections of L1 2 − s accompanied by the formation of twinned structures.