- Email: [email protected]
The NbH solvus: A discussion
Scripta
METALLURGICA
Vol. Printed
i0, p. 75, 1976 in the U n i t e d S t a t e s
Pergamon
Press,
Inc.
THE Nb-H SOLVUS: A DISCUSSION
D. G. Westlake Materials Science Division Argonne National Laboratory Argonne, 111inols 60439
(Received October 15, 1975) (Revised October 29, 1975) Precipitation of NbH in Nb has been studied by Whitton et.al. (i) (WMSW) using a novel technique, the channeling of 1 MeV He ions. They report a solvus significantly different from one obtained by us (2,3) using resistometry. In private communication, Dr. Whitton has assured me that they are aware of supercooling, hysteresis and stress effects in the Nb-H system and will publish their information regarding these later. Meanwhile, it is important for the reader to be aware that the differences between the solubility limits reported in Reference 1 and 2 are explainable in terms of these effects and are not due to the sensitivity inherent in the channeling technique at low hydrogen concentrations. The crltical temperatures T c for precipitation upon cooling Nb-H alloys are roughly 20 K lower in Reference 1 than in Reference 2. T c is highly dependent on thermal history as shown by the following examples from References 2 and 3. i. In Nb-l.15 at. pct. H, T c was 224 K for the first cooling. After 20 cycles between 77 and 300 K, T_ was 246 K for very slow cooling. The values used for our solvus (2) were obtained after tberma~ cycling, whereas those of WMSW were obtained during the first cooling. 2. For a sample of Nb-0.14 at. pct. deuterium (3) after thermal cycling, T c = 152 K for a coollng rate roughly 1 K/min. When the cooling rate was decreased about 200 times, T c ffi 174.5 K. We used the slower cooling rate to determine our solvus, whereas, WMSW used 1 K/mln. We do not claim our solvus is unequivocal, because we know that the nucleation and growth of ulobiumhydride in a nioblummatrix are influenced by mechanical constraint (2). Unfortunately, the channeling technique does not alleviate that problem. In fact, it is the lattice distortion accompanying precipitation of niobium hydride that makes the channeling technique effective (i). Regarding precision, possible errors in the resistometric investigation (2) were discussed. The orlglnal results (data available upon request) were plotted as points in Fig. 1, Reference 2 and Fig. 3, Reference 3. The scatter of points appears considerably greater in the channeling study (Fig. 3, Reference 1). Thus, while we fully agree that channeling He ions offers a new and useful means of detecting the onset of phase changes, we cannot agree with WMSW that the evidence presented in Reference 1 shows clearly that the new technique gives greater accuracy or precision than resistometry for the study of the Nb-H system.
References l°
J. L. Whitton, J. B. Mitchell, T. Schober, and H. Wenzl, Scripta Met. 9 851 (1975).
2.
D. G. Westlake, Trans. TMS-AIME 245, 287 (1969).
3.
D. G. W e s t l a k e a n d S. T. O c k e r s ,
Met. Trans.
6_~A, 399 ( 1 9 7 5 ) .
Work supported by the U. S. Energy Research and Development Administration. 75
METALLURGICA
Vol. Printed
i0, p. 75, 1976 in the U n i t e d S t a t e s
Pergamon
Press,
Inc.
THE Nb-H SOLVUS: A DISCUSSION
D. G. Westlake Materials Science Division Argonne National Laboratory Argonne, 111inols 60439
(Received October 15, 1975) (Revised October 29, 1975) Precipitation of NbH in Nb has been studied by Whitton et.al. (i) (WMSW) using a novel technique, the channeling of 1 MeV He ions. They report a solvus significantly different from one obtained by us (2,3) using resistometry. In private communication, Dr. Whitton has assured me that they are aware of supercooling, hysteresis and stress effects in the Nb-H system and will publish their information regarding these later. Meanwhile, it is important for the reader to be aware that the differences between the solubility limits reported in Reference 1 and 2 are explainable in terms of these effects and are not due to the sensitivity inherent in the channeling technique at low hydrogen concentrations. The crltical temperatures T c for precipitation upon cooling Nb-H alloys are roughly 20 K lower in Reference 1 than in Reference 2. T c is highly dependent on thermal history as shown by the following examples from References 2 and 3. i. In Nb-l.15 at. pct. H, T c was 224 K for the first cooling. After 20 cycles between 77 and 300 K, T_ was 246 K for very slow cooling. The values used for our solvus (2) were obtained after tberma~ cycling, whereas those of WMSW were obtained during the first cooling. 2. For a sample of Nb-0.14 at. pct. deuterium (3) after thermal cycling, T c = 152 K for a coollng rate roughly 1 K/min. When the cooling rate was decreased about 200 times, T c ffi 174.5 K. We used the slower cooling rate to determine our solvus, whereas, WMSW used 1 K/mln. We do not claim our solvus is unequivocal, because we know that the nucleation and growth of ulobiumhydride in a nioblummatrix are influenced by mechanical constraint (2). Unfortunately, the channeling technique does not alleviate that problem. In fact, it is the lattice distortion accompanying precipitation of niobium hydride that makes the channeling technique effective (i). Regarding precision, possible errors in the resistometric investigation (2) were discussed. The orlglnal results (data available upon request) were plotted as points in Fig. 1, Reference 2 and Fig. 3, Reference 3. The scatter of points appears considerably greater in the channeling study (Fig. 3, Reference 1). Thus, while we fully agree that channeling He ions offers a new and useful means of detecting the onset of phase changes, we cannot agree with WMSW that the evidence presented in Reference 1 shows clearly that the new technique gives greater accuracy or precision than resistometry for the study of the Nb-H system.
References l°
J. L. Whitton, J. B. Mitchell, T. Schober, and H. Wenzl, Scripta Met. 9 851 (1975).
2.
D. G. Westlake, Trans. TMS-AIME 245, 287 (1969).
3.
D. G. W e s t l a k e a n d S. T. O c k e r s ,
Met. Trans.
6_~A, 399 ( 1 9 7 5 ) .
Work supported by the U. S. Energy Research and Development Administration. 75