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Hydrogen embrittlement of some Fe-base amorphous alloys
Materials Science and Engineering, 23 (1976) 257 - 259 © Elsevier Sequoia S.A., L a u s a n n e - Printed in the Netherlands
257
Hydrogen Embrittlement of Some Fe-Base Amorphous Alloys*
M. NAGUMO and T. TAKAHASHI
Fundamental Research Laboratories, Nippon Steel Corporation 1618, Ida, Nakahara-ku, Kawasaki-City (Japan211)
INTRODUCTION
One of the promising properties of amorphous metallic alloys is their tensile strengths attaining as high as 300 kg/mm 2, at which strength levels ordinary steels are inevitably sensitive to hydrogen embrittlement (HE). Chromium bearing Fe-base amorphous alloys have been shown to have excellent corrosion resistance [ 1 ], b u t local cell formation might cause the permeation of hydrogen into the specimens. The examination of HE in amorphous metallic alloys is thus stimulating from a practical point of view. Embrittlement of amorphous materials results from various origins, such as compositions, improper quenching rate and aging treatment. If HE t o o k place in amorphous materials, comparison of its features with those of other embrittlements may be of use to reveal the flow or fracture mechanism of amorphous materials.
EXPERIMENTAL P R O C E D U R E A m o r p h o u s F e s o P l s C 7 and Fe70Cr10P13C 7 alloys were prepared into ribbon filaments by the modified Pond and Maddin method. The thickness and width of ribbons were a b o u t 0.02 mm and 0.3 mm respectively. These samples were cut into 150 mm lengths and hydrogen was introduced by electrolytic charging. Cathodic charging of the specimens was c o n d u c t e d at a current density of 1 mA/ c m 2 in aqueous solution of 3 wt.% NaC1 or at 10 m A / c m 2 in a 5 wt.% H 2 S O 4 and 4 mgfl Na2HAsO4 solution for 15 rain.
*Paper presented at the Second International Conference on Rapidly Quenched Metals, held at the Massachuset~ Institute of Technology, Cambridge, Mass., November 17 - 19, 1975.
The cathodically charged specimens were subjected to tensile or bend tests at room temperature after aging at 25 °, 0 ° and --25 °C for varying periods of up to 24 hours. Ductility of the specimens was evaluated in terms of the critical radius of curvature of bending, and tensile tests were conducted mainly to examine the fractographic features at a strain rate of 2 × 10 - 4 sec -1. Tensile tests in the course of cathodic charging were also conducted in a 3 wt.% NaC1 solution at a strain rate of I × 10-5 sec- 1.
RESULTS AND DISCUSSIONS
The as-splat-quenched specimens could be bent 180 ° to close contact. Hydrogen charging in NaC1 solution did not result in the loss of ductility in so far as it was evaluated b y bend tests. However, an increase in the a m o u n t of charged hydrogen caused a decrease in ductility. Figure 1 shows the decrease and recovery of ductility in terms of the inverse bend radii of curvature as degassing proceeded by aging. The charging was in a 5 wt.% H2SO4 and 4 mg/1 Na2HAsO4 solution at a current density of 10 and 50 m A / c m 2 for FeToCrloP13C 7 and FesoPlsC7 alloys respectively. It should be noted that accompanying the charging and degassing of hydrogen, the ribbon filament specimens curled up and straightened in a reversible manner. It indicates that hydrogen within amorphous matrix induces large elastic strains, b u t n o t internal crackings or shear. Hydrogen embrittlement of amorphous metallic alloys revealed above is distinct from that of ordinary steels in the following aspects. The first is that the high corrosion resistance of chromium bearing amorphous alloys minimizes the permeation of hydrogen under natural environment. The second is the re-
258
FeP13C7
I
£
~id II Q
ii
~. 0s
FeR~C,Cr,o
~,\[
0.2 3~
o-25°C
O.
i
3 Aging t i m e
10
i
4 (hr)
I
5
(a)
Fig. 2. Temperature dependence of diffusion constants
FeP,~Cr m
z o
.~.~
T
E E v
35 z~0 I/TX10 3 ( ' K I )
8 C-~ = ~--2 exp (--t/T)
(3)
r = 4h2/~2D,
(4)
1 /
o.~ ,, 25°C
0.2 0,1
0 0 "C -25°C I
1
I
2 3 Aging time
J
I
4 5 (hr)
(b)
Fig. 1. Critical radii of curvature of bending during degassing by aging after cathodic charging at a current density of 10 and 50 m A / c m 2 for Fe70CrloP13C 7 and Fes0P13C 7 alloys, respectively, in solutions of 5 wt.% H2SO 4 and 4 mg]l Na2HAsO 4.
versibility of large strains induced by absorbed hydrogen, the amount of which is reported to be much more than in steels [2]. The third is a slow diffusion rate of hydrogen compared with that in iron or steels. It is quantitatively analyzed in the following way. The tensile strain at the outer surface of a bent specimen is given as e
=
h/r,
(1)
where h is measured from the neutral plane to the surface and is set to be half the specimen thickness, and r is the radius of curvature of bending. Here an assumption will be made that the critical strain to fracture is inversely proportional to hydrogen c o n t e n t CH. Then it is written as c . = Xro/h,
(2)
where k is the proportional constant and r c is the critical radius. As a diffusion problem in a finite system, a standard formula of
is applicable, where C and Co are mean and initial hydrogen contents, respectively, and D is the diffusion constant of hydrogen. Figure 2 shows the calculated diffusion constants at three temperatures. From the temperature dependence of D, the activation energies of diffusion of hydrogen are estimated to be 7.8 and 4.2 kcal mo1-1 for Fes0P13C7 and Fe7oCrl0PlaCv alloys respectively. Diffusion constants thus obtained are much smaller than those in iron or steels and are rather close to those in covalent crystals. An example of the latter is the diffusion constant of H2 in SiO2, D = (13.7 to 35) X 10 -6 exp ( - - I O , I O 0 / R T ) [3], and an extrapolation gives D - 1.0 X 10 -12 cm 2 sec -1 at 300 °K. A hard position should be avoided at present because of the assumptions employed to derive eqn. (2), b u t the situation likely reflects the role of covalent bonding between metal-metalloid and metalloid-metalloid atoms. Fracture plane orientation in tensile tests is 45 ° to the tensile axis for as splat-quenched specimens and changes to 90 ° for hydrogen charged ones. Fractographic features of as splat-quenched specimens are characterized by vein markings, while those of hydrogen charged ones are of a cellular pattern with large ridges. A SEM view is shown in Fig. 3. These features are also reversible as ductility recovers by degassing. Tensile tests during cathodic charging in 3 wt.% NaC1 solution resulted in an embrittled fracture surface although bend tests after charging did n o t show a loss of ductility.
259
features are also reported in amorphous metallic alloys w i t h o u t metalloid elements [4]. In most cases the embrittlement is accompanied by the increase in hardness, b u t preliminary measurements of the change of hardness by hydrogen charging were within the scatter range. Whether we take the viscous shear or dislocation model as the flow mechanism of amorphous materials, a cellular fracture pattern is an indication of heterogeneous nucleation of local shear. In the case of HE, hydrogen may form small cavities within the matrix, and the fracture front may propagate, linking these cavities.
REFERENCES Fig. 3. SEM view of hydrogen embrittled fracture surface in a tensile test of a FesoP13C7 specimen in a 3 wt.% NaC! solution with cathodic charging at a current density of 1 m A / c m 2.
Fractographic features associated with HE are similar to those associated with other origins of embrittlement. Such fractographic
I M. Naka, K. Hashimoto and T. Masumoto, Nippon Kinzoku Gakkaishi, 38 (1974) 835. 2 K. Hashimoto et al., to be published. 3 W. Jost, Diffusion in Solids, Liquids and Gases, Academic Press, New York, 1960. 4 J. M. Vitek and N. J. Grant, Met. Trans., 6A (1975) 1472.
257
Hydrogen Embrittlement of Some Fe-Base Amorphous Alloys*
M. NAGUMO and T. TAKAHASHI
Fundamental Research Laboratories, Nippon Steel Corporation 1618, Ida, Nakahara-ku, Kawasaki-City (Japan211)
INTRODUCTION
One of the promising properties of amorphous metallic alloys is their tensile strengths attaining as high as 300 kg/mm 2, at which strength levels ordinary steels are inevitably sensitive to hydrogen embrittlement (HE). Chromium bearing Fe-base amorphous alloys have been shown to have excellent corrosion resistance [ 1 ], b u t local cell formation might cause the permeation of hydrogen into the specimens. The examination of HE in amorphous metallic alloys is thus stimulating from a practical point of view. Embrittlement of amorphous materials results from various origins, such as compositions, improper quenching rate and aging treatment. If HE t o o k place in amorphous materials, comparison of its features with those of other embrittlements may be of use to reveal the flow or fracture mechanism of amorphous materials.
EXPERIMENTAL P R O C E D U R E A m o r p h o u s F e s o P l s C 7 and Fe70Cr10P13C 7 alloys were prepared into ribbon filaments by the modified Pond and Maddin method. The thickness and width of ribbons were a b o u t 0.02 mm and 0.3 mm respectively. These samples were cut into 150 mm lengths and hydrogen was introduced by electrolytic charging. Cathodic charging of the specimens was c o n d u c t e d at a current density of 1 mA/ c m 2 in aqueous solution of 3 wt.% NaC1 or at 10 m A / c m 2 in a 5 wt.% H 2 S O 4 and 4 mgfl Na2HAsO4 solution for 15 rain.
*Paper presented at the Second International Conference on Rapidly Quenched Metals, held at the Massachuset~ Institute of Technology, Cambridge, Mass., November 17 - 19, 1975.
The cathodically charged specimens were subjected to tensile or bend tests at room temperature after aging at 25 °, 0 ° and --25 °C for varying periods of up to 24 hours. Ductility of the specimens was evaluated in terms of the critical radius of curvature of bending, and tensile tests were conducted mainly to examine the fractographic features at a strain rate of 2 × 10 - 4 sec -1. Tensile tests in the course of cathodic charging were also conducted in a 3 wt.% NaC1 solution at a strain rate of I × 10-5 sec- 1.
RESULTS AND DISCUSSIONS
The as-splat-quenched specimens could be bent 180 ° to close contact. Hydrogen charging in NaC1 solution did not result in the loss of ductility in so far as it was evaluated b y bend tests. However, an increase in the a m o u n t of charged hydrogen caused a decrease in ductility. Figure 1 shows the decrease and recovery of ductility in terms of the inverse bend radii of curvature as degassing proceeded by aging. The charging was in a 5 wt.% H2SO4 and 4 mg/1 Na2HAsO4 solution at a current density of 10 and 50 m A / c m 2 for FeToCrloP13C 7 and FesoPlsC7 alloys respectively. It should be noted that accompanying the charging and degassing of hydrogen, the ribbon filament specimens curled up and straightened in a reversible manner. It indicates that hydrogen within amorphous matrix induces large elastic strains, b u t n o t internal crackings or shear. Hydrogen embrittlement of amorphous metallic alloys revealed above is distinct from that of ordinary steels in the following aspects. The first is that the high corrosion resistance of chromium bearing amorphous alloys minimizes the permeation of hydrogen under natural environment. The second is the re-
258
FeP13C7
I
£
~id II Q
ii
~. 0s
FeR~C,Cr,o
~,\[
0.2 3~
o-25°C
O.
i
3 Aging t i m e
10
i
4 (hr)
I
5
(a)
Fig. 2. Temperature dependence of diffusion constants
FeP,~Cr m
z o
.~.~
T
E E v
35 z~0 I/TX10 3 ( ' K I )
8 C-~ = ~--2 exp (--t/T)
(3)
r = 4h2/~2D,
(4)
1 /
o.~ ,, 25°C
0.2 0,1
0 0 "C -25°C I
1
I
2 3 Aging time
J
I
4 5 (hr)
(b)
Fig. 1. Critical radii of curvature of bending during degassing by aging after cathodic charging at a current density of 10 and 50 m A / c m 2 for Fe70CrloP13C 7 and Fes0P13C 7 alloys, respectively, in solutions of 5 wt.% H2SO 4 and 4 mg]l Na2HAsO 4.
versibility of large strains induced by absorbed hydrogen, the amount of which is reported to be much more than in steels [2]. The third is a slow diffusion rate of hydrogen compared with that in iron or steels. It is quantitatively analyzed in the following way. The tensile strain at the outer surface of a bent specimen is given as e
=
h/r,
(1)
where h is measured from the neutral plane to the surface and is set to be half the specimen thickness, and r is the radius of curvature of bending. Here an assumption will be made that the critical strain to fracture is inversely proportional to hydrogen c o n t e n t CH. Then it is written as c . = Xro/h,
(2)
where k is the proportional constant and r c is the critical radius. As a diffusion problem in a finite system, a standard formula of
is applicable, where C and Co are mean and initial hydrogen contents, respectively, and D is the diffusion constant of hydrogen. Figure 2 shows the calculated diffusion constants at three temperatures. From the temperature dependence of D, the activation energies of diffusion of hydrogen are estimated to be 7.8 and 4.2 kcal mo1-1 for Fes0P13C7 and Fe7oCrl0PlaCv alloys respectively. Diffusion constants thus obtained are much smaller than those in iron or steels and are rather close to those in covalent crystals. An example of the latter is the diffusion constant of H2 in SiO2, D = (13.7 to 35) X 10 -6 exp ( - - I O , I O 0 / R T ) [3], and an extrapolation gives D - 1.0 X 10 -12 cm 2 sec -1 at 300 °K. A hard position should be avoided at present because of the assumptions employed to derive eqn. (2), b u t the situation likely reflects the role of covalent bonding between metal-metalloid and metalloid-metalloid atoms. Fracture plane orientation in tensile tests is 45 ° to the tensile axis for as splat-quenched specimens and changes to 90 ° for hydrogen charged ones. Fractographic features of as splat-quenched specimens are characterized by vein markings, while those of hydrogen charged ones are of a cellular pattern with large ridges. A SEM view is shown in Fig. 3. These features are also reversible as ductility recovers by degassing. Tensile tests during cathodic charging in 3 wt.% NaC1 solution resulted in an embrittled fracture surface although bend tests after charging did n o t show a loss of ductility.
259
features are also reported in amorphous metallic alloys w i t h o u t metalloid elements [4]. In most cases the embrittlement is accompanied by the increase in hardness, b u t preliminary measurements of the change of hardness by hydrogen charging were within the scatter range. Whether we take the viscous shear or dislocation model as the flow mechanism of amorphous materials, a cellular fracture pattern is an indication of heterogeneous nucleation of local shear. In the case of HE, hydrogen may form small cavities within the matrix, and the fracture front may propagate, linking these cavities.
REFERENCES Fig. 3. SEM view of hydrogen embrittled fracture surface in a tensile test of a FesoP13C7 specimen in a 3 wt.% NaC! solution with cathodic charging at a current density of 1 m A / c m 2.
Fractographic features associated with HE are similar to those associated with other origins of embrittlement. Such fractographic
I M. Naka, K. Hashimoto and T. Masumoto, Nippon Kinzoku Gakkaishi, 38 (1974) 835. 2 K. Hashimoto et al., to be published. 3 W. Jost, Diffusion in Solids, Liquids and Gases, Academic Press, New York, 1960. 4 J. M. Vitek and N. J. Grant, Met. Trans., 6A (1975) 1472.