Activity and diffusivity of oxygen in tantalum and niobium base-alloys
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I.
and
INTRODUCTION
THE interactions of interstitial atoms with ternary inter-
Nb-NbO/ThO2-Y:O3/Nb-X-O solid solution
stitial or substitutional solute atoms in solid solutions can be studied by measurements of the activity and of transport phenomena. The group Va metals Ta and Nb are excellent model substances for this type of investigation since they exhibit large solution ranges for interstitials like N or O and for many substitutional transition metal additions. Papers have been published on activity measurements in ternary N-Systems, ~'2'3 and O-Systems, 4'5 on thermo and electrotransport, 6 on diffusion of O, 7'8'9 and on internal friction experiments. 10,11,12 The use of solid ThO2-Y203 electrolytes provides a versatile and precise technique for the investigation of O activity changes and diffusion processes and, therefore, they have been used in this study to reveal also relatively small changes of activity coefficients and diffusion constants in some ternary transition metal systems. Experiments were performed in the systems Ta-O, Ta-N-O, Ta-W-O, Ta-Re-O, Nb-O, Nb-Mo-O, and Nb-V-O. In the concentration and temperature range chosen the samples were single phase solid solutions and, thus, the effects observed can be attributed to solute-solute interactions between O atoms and ternary additions in the Ta or Nb matrix. In most experiments two phase Ta/Ta205 or Nb/NbO samples were used as reference electrodes. In some runs where small effects had to be confirmed, a new differential method was applied with a binary Ta-O or Nb-O solid solution with the same O concentration as in the ternary alloy as reference electrode. By this technique activity differences between binary and temary alloys could be determined directly.
X is the ternary addition to the Nb-O and Ta-O-alloys. The ThO2-Y203 electrolyte permits activity experiments down to a < 10 -30 bar 0214'15'16 pressure. By tests with the electrolytes used in the experiments it could be demonstrated that small deviations from the theoretical behavior of emf vs t curves in the low O-activity range of diffusion experiments are due to the electronic conductivity of the electrolytes. 7'17
reference electrode me ]I
electro- sample sink lyte me I
tl ,t2 tl ,t2
.o_
EXPERIMENTAL
emf cells: The emf technique for O activity measure-
to t
(b)
4-/
U 0 u
I
meI
to ,t 1 ,t2 -t 1
(c)
[1]
E. ALBERT, formerly Ph. D. Student at the Max-Planck-lnstitut, is now with M e s s e r s c h m i t t - B 6 1 k o w - B l o h m G m b H , D 7312 K i r c h e i m u. Teck/Nabem. E. FROMM and R. KIRCHHEIM are Research Scientists at the Max-Planck-lnstitut fiir Metallforschung, Institut fiir Werkstoffwissenschaften, Seestra[3e 92, D 7000 Stuttgart 1, Germany. Manuscript submitted February 7, 1983. METALLURGICALTRANSACTIONS A
mem
t 0, tl , t2
ments with ThO2-Y:O3 solid electrolytes is well established. In this study the following ceils are used most frequently (see Figure l(a)):
Ta-Ta2Os/ThO2-Y203/Ta-X-O solid solution
(a)
meI
0
me
II.
[2]
distance Fig. 1 - emf cells and
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