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Influence of liquid mercury on the structure of nanocrystalline nickel
NanaStructured Materials. Vol. 5. No. 1, pp. 63-69.1995
Pergamon
Copyright Q 1995 Elsevier Science Ltd Printed in the USA.
All rights reserved
096%9773,95
$9.50 + .OO
0%5-9773(95)00010-0
INFLUENCE
OF LIQUID MERCURY OF NANOCRYSTALLINE
ON THE STRUCTURE NICKEL
*L.I. Trusov, **VA Mel’nikova, *T.P. Khvostantseva *Scientific-Research Enterprise “Ultram”, 103030, Krasnoproletarskaya ul. 32, Moscow, Russia ** Institute of Materials Science Problems, 252142, Kiev, Ukraine
(Accepted December 1994)
Abstract- The data on the degradation of nanocrystalline nickel (nc-Ni) in the presence of liquid mercury are presented. Mercury etches nc-Ni on grain boundaries and simultaneously a new phase Ni-Hg appears. Crack nucleation takes place in an amalgam area.
INTRODUCTION An investigation of the localized corrosion of stainless steel in hydrochloric acid (HCl) has shown that the breakdown potential of the sputtered 304 type stainless steel film with the grain size of 25 nm is approximately 850 mV higher than that of the conventional bulk material of the same composition (1). The improved corrosion resistance of the sputtered films is attributed to the nanocrystalline grain size and to homogeneity of the metal surface. The corrosion behavior of nanocrystalline 99.9% Ni with the average grain size of 32 nm in sulfuric acid media has been investigated by potentiodynamic and potentiostatic test methods (2). The results obtained in (2) indicate that nanocrystalline processing of Ni may catalyze hydrogen reduction processes, reduce the kinetics of passivation and compromise the passive film stability. It is possible that some liquid metals may exert an even stronger corrosion (chemical) effect on metals. On the macroscopic level, for conventional metals with a micrometer grain size, embrittlement by liquid metals causes a decreased lifetime, strength and plasticity of metals (3). While investigating embrittlement of nanocrystalline nickel (nc-Ni) by mercury (4,5), a factor of lo2 decrease in fracture toughness, in comparison with conventional polycrystalline material, was observed. At the same time, the rate of subcritical crack growth was lower by a factor of 102-ld than that in polycrystals, and therefore the resistance to the corrosive cracking might be higher. In the present investigation, peculiarities of nc-Ni attack in the presence of mercury are discussed. 63
64
LI TRUSOV,VA MEL’NIKOVAANO TP KHVOSTANTSEVA
Figure 1. (a) The initial (polished and treated with HCl) surface of nc-Ni, and (b) a view of surface of nc-Ni after contacting with Hg. PROCEDURE Samples of nc-Ni with the mean grain size of 60 nm and the density 94-95% were produced from an ultrafine nickel powder by sintering under pressure (6). Samples with diameter 9 mm and 4 mm thick were subjected to bending deformation at a room temperature in the presence of mercury. A mechanically polished surface of each sample was treated with hydrochloric acid before deformation, and then a part of the stretched surface was contacted with a drop of mercury. After a crack appeared, the sample was kept under a constant load for 102 sec. If fracture did not occur, the sample was unloaded and deformed again up to its disruption. For details of the experiment see ref. (43. The surface structures and the edges of the cracks in nc-Ni were investigated by a transmission electron microscopy (TEM) and a scanning electron microscopy (SEM) on replicas and on thin foil treated by ion bombardment. RESULTS The Surface Structure of nc-Ni Before and After its Contact with Mercury.
The initial (polished and treated with hydrochloric acid) surface of a nc-Ni sample is shown in Figure la. In contrast to the shiny initial surface, the surface structure of nc-Ni has a dull appearance after interaction with mercury. Small holes from etching are evident on the replica (Figure lb). As it has been noted in ref. (43, after the removal of mercury the surface of nc-Ni, originally plain and without any structure, appears as a faceted surface of an ordinary polycrystal with grain boundaries etched away. The mean size of the facets (“clusters”) is of 8- 10 pm (Figure 2a). In its turn, the clusters consist of grains, the sizes of which correspond to the size of the particles of the initial Ni powder (7). The structure of the cluster is shown in Figure 2b. The cluster structure shown in Figure 2(a,b) has beenpredictedin ourearlierpaper (8). According to the cluster model, two types of boundaries occur in the nanocrystalline ensemble. These are boundaries
INFLUENCE OF LICIUID MERCURY ON NANOCRYSTALLINE NICKEL
65
Figure 2. (a) A view of nc-Ni surface after long-term contact with mercury (Ids) at 355K (ref. 4,5), and (b) a view of structure of nc-Ni inside a cluster. between nanoparticles [inside the cluster] (type 1) similar to usual grain boundaries: and clusters boundaries (type 2). This model was effectively used to explain optical, electrical (9) and corrosion properties (45) of nanocrystalline materials. On the basis of the electron microscopy observation, the following conclusions may be made: the influence of mercury on the surface of nc-Ni leads to different structural changes. The etching occurs along both boundaries of the clusters and along the boundary of the grain inside the clusters. The formation of a new phase takes place simultaneously with the etching. Electron diffraction patterns of the areas where a new phase (further: a Ni-Hg phase) exists in an embryonic state show very bright reflections of Ni and very weak reflections of nickel oxide. But extended areas of the phase Ni-Hg (up to 0.5 l.trn)(Figure 3a) are found in the surface layer, which is evident from the spot diffraction patterns with diffuse arc-like reflections. That proves the disorientation of blocks in a monocrystal phase (Figure 3b). The spot diffraction patterns cannot give a complete set of interplanar distances of this particular phase to identify it. A set of interplanar distances (d) were obtained: d (nm): 0.321,0.227,0.187,0.161,0.143,0.131,0.107. It is evident that the phase formed on the surface of Ni in the presence of mercury is a thin film with large grains. There are many small holes in the film. What is their origin? Most likely a film is etched under the influence of the ions beam by ion bombardment during the samples preparation for TEM; and the fact that amalgamation of surface layers keeps the material from oxidation proves our conclusion. This effect is observed in the samples under investigation and, consequently, a protective film of the phase Hg-Ni must be continuous with minimum defects. That will be proved below. What is the stoichiometry of this phase? It is known that nickel is very poorly dissolved in mercury-only 0.000 1 weight % at 20°C (10). However, mercury is not poorly dissolved in nickel forming amalgams in the form of liquid and solid solutions, compounds, colloid solutions and suspensions. But neither SEM nor Auger spectroscopy permit determination of the chemical
Figure 3. (a) The structure of a phase Ni-Hg formed on a layer surface of nc-Ni in contact with Hg and (b) a diffraction pattern of phase Ni-Hg.
Figure 4. The structure of an area of nc-Ni where a crack nucleated. The edge of the sample is marked by an arrow. composition of this phase, because of its small amount, and besides it exists on the nickel background. Structure
of a Surface Fracture
In spite of the fact that ref. (4,5) notes that the nucleation of a crack in the sample is not connected with mercury influence, the structural investigations show that the edge of the crack is presented by the phase Ni-Hg. The structure of an area where a crack nucleation (its edge) occurs is shown in Figure 4. The fracture surface here is perpendicular of the picture plane. An edge of a crack consists of a phase Ni-Hg formed as a result of amalgamation. The crack is nucleated in a place of defects accumulation or impurities embrittlement. No wonder that the crack appears in an amalgamation area. It is evident that its strength is less than
INFLUENCE OF LIOUIOMERCURYON NAN~CRYSTALUNE NICKEL
67
Figure 5. An SEM image of the structure of a surface fracture. A,C-a hole fracture, Ba surface of jump-wise lengthening of a crack, D-a strip of a brittle fracture.
the strength of a sintered nc-Ni. The structure of the fracture surface is shown in Figure 5. They illustrate the propagation of the crack from a place where the surface has contacted with mercury into the depth of the Ni sample where it propagates steplike by sliding grooves. Periodic accumulation of such defects in near-surface layers influences their creation. In our experiments such behavior is the result of mercury diffusion from the grain boundaries to their depth and, thus, of Ni amalgamation. A structural image (Figure 5) shows successive steps of fracture of polycrystalline nickel. A plastic deformation, creation of small holes, follow the crack origin. Figure 5a shows a hole fracture (area A). Then the crackbecomes long in a jump-wise way up to 15 pm, and stops, having lost its energy for new hole fractures (area B). Another hole fracture slowly appears (area C) and again there is a new strip of brittle fracture (area D). The deeper the crack is the less is the role of plastic deformation in energy dissipation. The hole fracture disappears and grooves become separated by crowns of break. Replicas prove that disruption takes place along the grain boundaries in the area of a brittle fracture (Figure 6). The experiments show that nc-Ni surface is oxidized actively and as a result, a great number of extricated particles of oxide are found in the replica (Figure la), though replicas are pure, without oxide particles, on the stage of crack nucleation and at the beginning of its development, that is in the areas which are under the influence of mercury. So, amalgamation protects Ni from oxidation.
68
LI
TRUSOV, VA
MEL’NIKOVAAND TP KHVOSTANTSEVA
Figure 6. A TEM image by replica of the structure of a surface fracture.
Figure 7. The structure of surface fractured nc-Ni at final disruption of a sample.
Structure of a Sluface Fracture at the Final Disruption of the Sample
A fracture during the final disruption of nc-Ni is always the same according to intragranular embrittlement propagation of cracks. Mercury has no influence in that area and the surface of fracture is oxidized, particles of nickel oxide are found on the replica (Figure 7). CONCLUSION The structural investigation given in this paper shows that: (i) mercury selectively etches nc-Ni along the grainboundaries with simultaneous appearance of anew phase of Ni-Hg; (ii) crack nucleation takes place in an amalgam area, whose strength is less than the strength of nc-Ni; (iii) a crack propagates steplike, forming sliding grooves; and (iv) it is noticeable that amalgamation protects nc-Ni from oxidation. REFERENCE 1. 2. 3. 4. 5. 6. 7.
R.B. Inturi and 2. Szlclarska-Smialowska, Corrosion 48,398 (1992). R. Rofagha, R. Langer, A.M. El-She& U. Erb, G. Palumbo and K.T. Aust, Scripta Met. et Mater. 2,2867,1991. W. Rostoker, J.M. MacCaughey, M. Markus, Embrittlement by Liquid Metals, Van NostrandReinhold, New York (1960), p. 162. V.I. Igoshev, L.G. Rogova, L.I. Trusov, TX Khvostantseva, Nanostruct. Mater. 2, 189 (1993). V.I. Igoshev, L.G. Rogova, L.I. Trusov, T.P. Khvostantseva, J. Mat. Sci. 29, 1569 (1994). L.G. Khvostantsev, L.F. Vereshagin, A.P. Novikov, H. Tern.- H.Press. & 637 (1977). L.I. Trusov, V.I. Novikov, T.P. Khvostantseva, Proc. of the 1994 Powder Met. World Cong. and Exhib., Paris, France 3,1789 (1994).
INFLUENCEOF LIQUID MERCURYON NANOCRYSTALLINE NICKEL
8. 9. 10.
69
L.I. Trusov, V.I. Novikov, V.G. Gryamov, Growth of Crystals,Consultants Bureau, New York and London (1991), p. 55. L.I. Trusov, N.G. Askuntovich, R.P. Borovikova, Phys. Let. Am, 306 (1992). M. Hansen, Constitutionof BinaryAlfoys, McGraw-Hill, New York, (1958), p. 1315.
Pergamon
Copyright Q 1995 Elsevier Science Ltd Printed in the USA.
All rights reserved
096%9773,95
$9.50 + .OO
0%5-9773(95)00010-0
INFLUENCE
OF LIQUID MERCURY OF NANOCRYSTALLINE
ON THE STRUCTURE NICKEL
*L.I. Trusov, **VA Mel’nikova, *T.P. Khvostantseva *Scientific-Research Enterprise “Ultram”, 103030, Krasnoproletarskaya ul. 32, Moscow, Russia ** Institute of Materials Science Problems, 252142, Kiev, Ukraine
(Accepted December 1994)
Abstract- The data on the degradation of nanocrystalline nickel (nc-Ni) in the presence of liquid mercury are presented. Mercury etches nc-Ni on grain boundaries and simultaneously a new phase Ni-Hg appears. Crack nucleation takes place in an amalgam area.
INTRODUCTION An investigation of the localized corrosion of stainless steel in hydrochloric acid (HCl) has shown that the breakdown potential of the sputtered 304 type stainless steel film with the grain size of 25 nm is approximately 850 mV higher than that of the conventional bulk material of the same composition (1). The improved corrosion resistance of the sputtered films is attributed to the nanocrystalline grain size and to homogeneity of the metal surface. The corrosion behavior of nanocrystalline 99.9% Ni with the average grain size of 32 nm in sulfuric acid media has been investigated by potentiodynamic and potentiostatic test methods (2). The results obtained in (2) indicate that nanocrystalline processing of Ni may catalyze hydrogen reduction processes, reduce the kinetics of passivation and compromise the passive film stability. It is possible that some liquid metals may exert an even stronger corrosion (chemical) effect on metals. On the macroscopic level, for conventional metals with a micrometer grain size, embrittlement by liquid metals causes a decreased lifetime, strength and plasticity of metals (3). While investigating embrittlement of nanocrystalline nickel (nc-Ni) by mercury (4,5), a factor of lo2 decrease in fracture toughness, in comparison with conventional polycrystalline material, was observed. At the same time, the rate of subcritical crack growth was lower by a factor of 102-ld than that in polycrystals, and therefore the resistance to the corrosive cracking might be higher. In the present investigation, peculiarities of nc-Ni attack in the presence of mercury are discussed. 63
64
LI TRUSOV,VA MEL’NIKOVAANO TP KHVOSTANTSEVA
Figure 1. (a) The initial (polished and treated with HCl) surface of nc-Ni, and (b) a view of surface of nc-Ni after contacting with Hg. PROCEDURE Samples of nc-Ni with the mean grain size of 60 nm and the density 94-95% were produced from an ultrafine nickel powder by sintering under pressure (6). Samples with diameter 9 mm and 4 mm thick were subjected to bending deformation at a room temperature in the presence of mercury. A mechanically polished surface of each sample was treated with hydrochloric acid before deformation, and then a part of the stretched surface was contacted with a drop of mercury. After a crack appeared, the sample was kept under a constant load for 102 sec. If fracture did not occur, the sample was unloaded and deformed again up to its disruption. For details of the experiment see ref. (43. The surface structures and the edges of the cracks in nc-Ni were investigated by a transmission electron microscopy (TEM) and a scanning electron microscopy (SEM) on replicas and on thin foil treated by ion bombardment. RESULTS The Surface Structure of nc-Ni Before and After its Contact with Mercury.
The initial (polished and treated with hydrochloric acid) surface of a nc-Ni sample is shown in Figure la. In contrast to the shiny initial surface, the surface structure of nc-Ni has a dull appearance after interaction with mercury. Small holes from etching are evident on the replica (Figure lb). As it has been noted in ref. (43, after the removal of mercury the surface of nc-Ni, originally plain and without any structure, appears as a faceted surface of an ordinary polycrystal with grain boundaries etched away. The mean size of the facets (“clusters”) is of 8- 10 pm (Figure 2a). In its turn, the clusters consist of grains, the sizes of which correspond to the size of the particles of the initial Ni powder (7). The structure of the cluster is shown in Figure 2b. The cluster structure shown in Figure 2(a,b) has beenpredictedin ourearlierpaper (8). According to the cluster model, two types of boundaries occur in the nanocrystalline ensemble. These are boundaries
INFLUENCE OF LICIUID MERCURY ON NANOCRYSTALLINE NICKEL
65
Figure 2. (a) A view of nc-Ni surface after long-term contact with mercury (Ids) at 355K (ref. 4,5), and (b) a view of structure of nc-Ni inside a cluster. between nanoparticles [inside the cluster] (type 1) similar to usual grain boundaries: and clusters boundaries (type 2). This model was effectively used to explain optical, electrical (9) and corrosion properties (45) of nanocrystalline materials. On the basis of the electron microscopy observation, the following conclusions may be made: the influence of mercury on the surface of nc-Ni leads to different structural changes. The etching occurs along both boundaries of the clusters and along the boundary of the grain inside the clusters. The formation of a new phase takes place simultaneously with the etching. Electron diffraction patterns of the areas where a new phase (further: a Ni-Hg phase) exists in an embryonic state show very bright reflections of Ni and very weak reflections of nickel oxide. But extended areas of the phase Ni-Hg (up to 0.5 l.trn)(Figure 3a) are found in the surface layer, which is evident from the spot diffraction patterns with diffuse arc-like reflections. That proves the disorientation of blocks in a monocrystal phase (Figure 3b). The spot diffraction patterns cannot give a complete set of interplanar distances of this particular phase to identify it. A set of interplanar distances (d) were obtained: d (nm): 0.321,0.227,0.187,0.161,0.143,0.131,0.107. It is evident that the phase formed on the surface of Ni in the presence of mercury is a thin film with large grains. There are many small holes in the film. What is their origin? Most likely a film is etched under the influence of the ions beam by ion bombardment during the samples preparation for TEM; and the fact that amalgamation of surface layers keeps the material from oxidation proves our conclusion. This effect is observed in the samples under investigation and, consequently, a protective film of the phase Hg-Ni must be continuous with minimum defects. That will be proved below. What is the stoichiometry of this phase? It is known that nickel is very poorly dissolved in mercury-only 0.000 1 weight % at 20°C (10). However, mercury is not poorly dissolved in nickel forming amalgams in the form of liquid and solid solutions, compounds, colloid solutions and suspensions. But neither SEM nor Auger spectroscopy permit determination of the chemical
Figure 3. (a) The structure of a phase Ni-Hg formed on a layer surface of nc-Ni in contact with Hg and (b) a diffraction pattern of phase Ni-Hg.
Figure 4. The structure of an area of nc-Ni where a crack nucleated. The edge of the sample is marked by an arrow. composition of this phase, because of its small amount, and besides it exists on the nickel background. Structure
of a Surface Fracture
In spite of the fact that ref. (4,5) notes that the nucleation of a crack in the sample is not connected with mercury influence, the structural investigations show that the edge of the crack is presented by the phase Ni-Hg. The structure of an area where a crack nucleation (its edge) occurs is shown in Figure 4. The fracture surface here is perpendicular of the picture plane. An edge of a crack consists of a phase Ni-Hg formed as a result of amalgamation. The crack is nucleated in a place of defects accumulation or impurities embrittlement. No wonder that the crack appears in an amalgamation area. It is evident that its strength is less than
INFLUENCE OF LIOUIOMERCURYON NAN~CRYSTALUNE NICKEL
67
Figure 5. An SEM image of the structure of a surface fracture. A,C-a hole fracture, Ba surface of jump-wise lengthening of a crack, D-a strip of a brittle fracture.
the strength of a sintered nc-Ni. The structure of the fracture surface is shown in Figure 5. They illustrate the propagation of the crack from a place where the surface has contacted with mercury into the depth of the Ni sample where it propagates steplike by sliding grooves. Periodic accumulation of such defects in near-surface layers influences their creation. In our experiments such behavior is the result of mercury diffusion from the grain boundaries to their depth and, thus, of Ni amalgamation. A structural image (Figure 5) shows successive steps of fracture of polycrystalline nickel. A plastic deformation, creation of small holes, follow the crack origin. Figure 5a shows a hole fracture (area A). Then the crackbecomes long in a jump-wise way up to 15 pm, and stops, having lost its energy for new hole fractures (area B). Another hole fracture slowly appears (area C) and again there is a new strip of brittle fracture (area D). The deeper the crack is the less is the role of plastic deformation in energy dissipation. The hole fracture disappears and grooves become separated by crowns of break. Replicas prove that disruption takes place along the grain boundaries in the area of a brittle fracture (Figure 6). The experiments show that nc-Ni surface is oxidized actively and as a result, a great number of extricated particles of oxide are found in the replica (Figure la), though replicas are pure, without oxide particles, on the stage of crack nucleation and at the beginning of its development, that is in the areas which are under the influence of mercury. So, amalgamation protects Ni from oxidation.
68
LI
TRUSOV, VA
MEL’NIKOVAAND TP KHVOSTANTSEVA
Figure 6. A TEM image by replica of the structure of a surface fracture.
Figure 7. The structure of surface fractured nc-Ni at final disruption of a sample.
Structure of a Sluface Fracture at the Final Disruption of the Sample
A fracture during the final disruption of nc-Ni is always the same according to intragranular embrittlement propagation of cracks. Mercury has no influence in that area and the surface of fracture is oxidized, particles of nickel oxide are found on the replica (Figure 7). CONCLUSION The structural investigation given in this paper shows that: (i) mercury selectively etches nc-Ni along the grainboundaries with simultaneous appearance of anew phase of Ni-Hg; (ii) crack nucleation takes place in an amalgam area, whose strength is less than the strength of nc-Ni; (iii) a crack propagates steplike, forming sliding grooves; and (iv) it is noticeable that amalgamation protects nc-Ni from oxidation. REFERENCE 1. 2. 3. 4. 5. 6. 7.
R.B. Inturi and 2. Szlclarska-Smialowska, Corrosion 48,398 (1992). R. Rofagha, R. Langer, A.M. El-She& U. Erb, G. Palumbo and K.T. Aust, Scripta Met. et Mater. 2,2867,1991. W. Rostoker, J.M. MacCaughey, M. Markus, Embrittlement by Liquid Metals, Van NostrandReinhold, New York (1960), p. 162. V.I. Igoshev, L.G. Rogova, L.I. Trusov, TX Khvostantseva, Nanostruct. Mater. 2, 189 (1993). V.I. Igoshev, L.G. Rogova, L.I. Trusov, T.P. Khvostantseva, J. Mat. Sci. 29, 1569 (1994). L.G. Khvostantsev, L.F. Vereshagin, A.P. Novikov, H. Tern.- H.Press. & 637 (1977). L.I. Trusov, V.I. Novikov, T.P. Khvostantseva, Proc. of the 1994 Powder Met. World Cong. and Exhib., Paris, France 3,1789 (1994).
INFLUENCEOF LIQUID MERCURYON NANOCRYSTALLINE NICKEL
8. 9. 10.
69
L.I. Trusov, V.I. Novikov, V.G. Gryamov, Growth of Crystals,Consultants Bureau, New York and London (1991), p. 55. L.I. Trusov, N.G. Askuntovich, R.P. Borovikova, Phys. Let. Am, 306 (1992). M. Hansen, Constitutionof BinaryAlfoys, McGraw-Hill, New York, (1958), p. 1315.