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Crack formation and fracture of LaNi5 during hydrogenation
Mat. Res. Bullo Vol. Ii, pp. Z81-Z84, 1976. Printed in the United States.
Pergamon
Press, Inc.
CRACK FORMATIONAND FRACTURE OF LaNis DURING HYDROGENATION* R. Wang Battelle Northwest Laboratories Richland, Washington 99352
(Received D e c e m b e r 30, 1975; Refereed) ABSTRACT The formation of microcracks and fracture surfaces of LaNis during hydrogenation were examined by scanning electron microscopy. There is no indication that hydrogen attacks preferably the grain boundaries and the surface defects. I n i t i a l hydrogenation produces equally spaced microcracks and wavy cracks due to volumetric expansion of the hydride. Extensive hydrogenation produces fine powders through b r i t t l e fracture and cleavage. The fracture surface contains fine cleavage facets which possibly resulted from the slip system of {lOTO} typ~ This study indicates that hydrogenation of LaNis near room temperature is a bulk diffusion process enhanced by stress corrosion. Introduction Hexagonal RENis intermetallic compounds, where RE denotes the rare earths La, Ce and Misch-metals, have been shown to possess interesting hydrogenation characteristics useful for energy storage applications (I-4). In the case of LaNis, a large amount of hydrogen can be absorbed to form LaNisH6 7 at nearly room temperature (1). The calculated density of absorbed hydrogen is more than twice as high as for liquid hydrogen. The absorption may be attributed to the availability of two kin~s of i n t e r § t i t i a l voids which can accommodate atoms with diameters of 0.35 A and 0.32 A, respectively, in the LaNis structure. The lattice dimensions of LaNis were a = 5.017 A and o c = 3.982 A. After saturation with hydrogen, i t expanded to a = 5.440 A and c = 4.310 A which represents a volumetric expansion of nearly 25% (3). This paper described microstructural observation of the crack formation and fracture features during the volumetric expansion of LaNis hydrides. I t is intended to determine the role of grain boundaries and surface defects in the hydrogen-material interaction. *This work is supported by the Division of Physical Research, U.S. Energy Research and Development Administration. Z81
Z8Z
R. W A N G
Volo II, No. 3
Experimental Nearly 20 grams of LaNi5 alloy were prepared by arc melting under argon atmosphere and homogenized 24 hours at llOO°C. The alloy was then sliced into 5 mm cubes. Hydrogenation was performed inside a small brass reactor 1.3 cm diameter and 5 cm long. The temperature of the reactor was controlled by a water bath. The LaNis cubes were f i r s t mounted in thermal setting resin and mechanically polished as in normal metallurgical sample preparation. The polished cubes were placed inside the reactor and were reacted with hydrogen at 150O psi for five minutes at room temperature in order to obtain i n i t i a l hydrogenation at the surface. Fine powders were obtained by placing cubes into the reactor for 30 minutes at 90°C. Specimens were studied by scanning electron microscopy (SEM). Results and Discussions A comparision of the surfaces for as-polished and hydrogenated LaNi5 alloys is given in Figure la and lb. Voids and polishing striations are the common features on the as-polished surface (Figure la). After hydrogenation, welldefined long cracks are clearly seen (Figure Ib). The pre-existing surface voids and cracks seem to not serve as the nucleation sites for the long cracks since many of them are free from the i n i t i a l cracks, and most of the cracks contain no original voids.
(a) Scanning Electron Micrograph
(b)
of a Polish Surface of LaNi5
The polished Surface of LaNi5 after Exposed at 1500 psi H2 for 5 min. at Room Temperature.
Figure l Detail observations on these cracks are shown in Figures 2a to 2c. In Figure 2a several crack facets were found and fine bands were associated with the facets. An intersection of two cracks is shown in Figure 2b. At the lower right corner of the micrograph parallel microcracks were initiated. I t is interesting to note that scratches formed during polishing (shown particularly in Figure 2b) neither initiated any cracks nor interferred with the formation of these cracks.
Volo ii, No. 3
HYDROGENATION
OF
LaNi 5
Z83
J
(a) The Microstructure of a Crack
(b)
The Junction of Two Cracks
(c) Equally Spaced Micro Cracks Near a Major Wavy Crack
Figure 2 Another interesting observation is given in Figure 2c at I0500X magnification. A large number of planar microcracks and some wavy microcracks were associated with a major crack having wavy character. The spacing between the planar microcracks were found unique as e i t h e r 1 ~m or 0.5 ~m, The gradually faded i n t e n s i t i e s of these microcracks from the major crack apparently indicated that formation of these microcracks was i n i t i a t e d from the stress f i e l d s produced by the major crack.
284
R.
WANG
Volo 11, No. 3
From the observations shown in Figures 1 to 2 , i t is reasonable to conclude that the hydrogenation of LaNi S is a bulk diffusion process which l a t e r is enhanced by stress corrosion. I n i t i a l l y , the surface uniformly absorbed hydrogen and generated stress by the l a t t i c e expansion. The stress could then have easily exceeded the e l a s t i c l i m i t of this b r i t t l e compound and originated microcracks by s l i p bands. As the hydrogenation proceeded f u r t h e r stress buildup deepened the microcracks into large major cracks having a wavy character. The results of extensively hydrogenated LaNis powders are shown in Figures 3a and 3b where large amounts of major cracks and fracture surfaces are common features. Uniform p a r t i c l e d i s t r i b u t i o n around 20 ~m is shown in Figure 3a Both b r i t t l e fracture and cleavage characteristics were i d e n t i f i e d (Figures 3b) The fracture surface is rather smooth. On the cleavaged surface a large number of fine-cleavage facets, having ~ 120° angle were formed. These
(a) Fine Powder Produced by Hydrogenation
(b)
Fracture Surface of LaNi
5
Containing Fine Cleavage
of LaNi 5 at 1500 psi Hydrogen for
Facets
30 min. at 90°C. Figure 3
cleavage facets may be produced by the s l i p system {I01-0} type as commonly observed f o r hcp elements with c/a r a t i o lower than the 1.633. References .
.
J.H.N. vanVucht, F.A. Kuijpers and H.C.A.M. Bruing, Philips Res. RPTS, 25 (1970) 133. R.H. Wiswall, Jr. and J.J. R e i l l y , BNL-16889 (1972).
3.
K.H.J. Buschow and H.H. vanMal, J. of Less Common Metals, 29 (1972) 203.
4.
C.E. Lundin, The Use of LaNi s as a Hydrogen Source for Fueling a Nonp o l l u t i n g Internal Combustion Engine, Proceedings of the Tenth Rare Earth Conference, Carefree, Arizona, May (1973).
Pergamon
Press, Inc.
CRACK FORMATIONAND FRACTURE OF LaNis DURING HYDROGENATION* R. Wang Battelle Northwest Laboratories Richland, Washington 99352
(Received D e c e m b e r 30, 1975; Refereed) ABSTRACT The formation of microcracks and fracture surfaces of LaNis during hydrogenation were examined by scanning electron microscopy. There is no indication that hydrogen attacks preferably the grain boundaries and the surface defects. I n i t i a l hydrogenation produces equally spaced microcracks and wavy cracks due to volumetric expansion of the hydride. Extensive hydrogenation produces fine powders through b r i t t l e fracture and cleavage. The fracture surface contains fine cleavage facets which possibly resulted from the slip system of {lOTO}
Z8Z
R. W A N G
Volo II, No. 3
Experimental Nearly 20 grams of LaNi5 alloy were prepared by arc melting under argon atmosphere and homogenized 24 hours at llOO°C. The alloy was then sliced into 5 mm cubes. Hydrogenation was performed inside a small brass reactor 1.3 cm diameter and 5 cm long. The temperature of the reactor was controlled by a water bath. The LaNis cubes were f i r s t mounted in thermal setting resin and mechanically polished as in normal metallurgical sample preparation. The polished cubes were placed inside the reactor and were reacted with hydrogen at 150O psi for five minutes at room temperature in order to obtain i n i t i a l hydrogenation at the surface. Fine powders were obtained by placing cubes into the reactor for 30 minutes at 90°C. Specimens were studied by scanning electron microscopy (SEM). Results and Discussions A comparision of the surfaces for as-polished and hydrogenated LaNi5 alloys is given in Figure la and lb. Voids and polishing striations are the common features on the as-polished surface (Figure la). After hydrogenation, welldefined long cracks are clearly seen (Figure Ib). The pre-existing surface voids and cracks seem to not serve as the nucleation sites for the long cracks since many of them are free from the i n i t i a l cracks, and most of the cracks contain no original voids.
(a) Scanning Electron Micrograph
(b)
of a Polish Surface of LaNi5
The polished Surface of LaNi5 after Exposed at 1500 psi H2 for 5 min. at Room Temperature.
Figure l Detail observations on these cracks are shown in Figures 2a to 2c. In Figure 2a several crack facets were found and fine bands were associated with the facets. An intersection of two cracks is shown in Figure 2b. At the lower right corner of the micrograph parallel microcracks were initiated. I t is interesting to note that scratches formed during polishing (shown particularly in Figure 2b) neither initiated any cracks nor interferred with the formation of these cracks.
Volo ii, No. 3
HYDROGENATION
OF
LaNi 5
Z83
J
(a) The Microstructure of a Crack
(b)
The Junction of Two Cracks
(c) Equally Spaced Micro Cracks Near a Major Wavy Crack
Figure 2 Another interesting observation is given in Figure 2c at I0500X magnification. A large number of planar microcracks and some wavy microcracks were associated with a major crack having wavy character. The spacing between the planar microcracks were found unique as e i t h e r 1 ~m or 0.5 ~m, The gradually faded i n t e n s i t i e s of these microcracks from the major crack apparently indicated that formation of these microcracks was i n i t i a t e d from the stress f i e l d s produced by the major crack.
284
R.
WANG
Volo 11, No. 3
From the observations shown in Figures 1 to 2 , i t is reasonable to conclude that the hydrogenation of LaNi S is a bulk diffusion process which l a t e r is enhanced by stress corrosion. I n i t i a l l y , the surface uniformly absorbed hydrogen and generated stress by the l a t t i c e expansion. The stress could then have easily exceeded the e l a s t i c l i m i t of this b r i t t l e compound and originated microcracks by s l i p bands. As the hydrogenation proceeded f u r t h e r stress buildup deepened the microcracks into large major cracks having a wavy character. The results of extensively hydrogenated LaNis powders are shown in Figures 3a and 3b where large amounts of major cracks and fracture surfaces are common features. Uniform p a r t i c l e d i s t r i b u t i o n around 20 ~m is shown in Figure 3a Both b r i t t l e fracture and cleavage characteristics were i d e n t i f i e d (Figures 3b) The fracture surface is rather smooth. On the cleavaged surface a large number of fine-cleavage facets, having ~ 120° angle were formed. These
(a) Fine Powder Produced by Hydrogenation
(b)
Fracture Surface of LaNi
5
Containing Fine Cleavage
of LaNi 5 at 1500 psi Hydrogen for
Facets
30 min. at 90°C. Figure 3
cleavage facets may be produced by the s l i p system {I01-0}
.
J.H.N. vanVucht, F.A. Kuijpers and H.C.A.M. Bruing, Philips Res. RPTS, 25 (1970) 133. R.H. Wiswall, Jr. and J.J. R e i l l y , BNL-16889 (1972).
3.
K.H.J. Buschow and H.H. vanMal, J. of Less Common Metals, 29 (1972) 203.
4.
C.E. Lundin, The Use of LaNi s as a Hydrogen Source for Fueling a Nonp o l l u t i n g Internal Combustion Engine, Proceedings of the Tenth Rare Earth Conference, Carefree, Arizona, May (1973).