EVOLUTION OF A GRAIN-BOUNDARY ENSEMBLE OF SUBMICROCRYSTALLINE MOLYBDENUM ANNEALED UNDER NICKEL DIFFUSION ALONG GRAIN BOUNDARIES
I. P. Mishin and G. P. Grabovetskaya UDC 539.219.3: 546.77
The effect of annealing, including that performed under impurity diffusion from an external medium (coating), on an evolution of the structure and grain-boundary ensemble of submicrocrystalline molybdenum produced by severe plastic deformation is investigated. The distribution of grain boundaries over misorientations is shown to depend on the degree of strain experienced by the material during severe plastic deformation. Grain-boundary diffusion fluxes of nickel atoms from the surface of the specimen are found to increase the fraction of special grain boundaries in the grain-boundary ensemble of submicrocrystalline molybdenum. Presumably, this is related to nucleation of new grains induced by the grain-boundary diffusion fluxes of nickel.
An efficient way of developing materials with an optimum combination of properties under different service conditions is formation of a nano- or a submicrocrystalline (SMC) state (the crystallite size d is ~1 µm). A special feature of the materials in question is a large number of grain boundaries. The latter are classified as small- and large-angle, special and general. A combination of grain boundaries of different types in the materials makes up an ensemble that largely determines an evolution pattern of deformation and diffusion developing there. A fundamental characteristic of the grain-boundary ensemble is the grain-boundary distribution over misorientations. Of great importance, in this connection, is to study the evolution of the grain-boundary ensemble during recrystallization and interaction of grain boundaries with impurities diffusing along grain boundaries.
In the present work, a study is made on the effect of annealings, including those performed in the case of impurity diffusion from an external medium (coating) along grain boundaries, on the evolution of the structure and grain-boundary ensemble formed by severe plastic deformation (SPD). The test material is SMC molybdenum used as an example.
TEST MATERIAL AND INVESTIGATION TECHNIQUES
The investigations employed SMC molybdenum and a Mo(Ni) system (hereinafter the bracketed element implies a diffusant). SMC molybdenum structure was formed using torsion by 5 revolutions at a pressure of 6 GPa and a temperature of 673 K. The specimens were discs 10 mm in diameter and 0.3 mm in thickness. SMC molybdenum was annealed in a vacuum of 5·10–3 Pa at 673–1173 K in a working chamber.
Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences,e-mail: [email protected]; [email protected]. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika,No. 1, pp. 81–87, January 2012. Original article submitted July 17, 2011.
A 7–8 µm thick layer of the Ni diffusant was deposited on the prepolished surface of SMC molybdenum specimens using an electrolytic technique. Nickel is a horophilic impurity in molybdenum hardly soluble in the bulk of molybdenum grains at an annealing temperature of 1023 K for the Mo(Ni) system [1]. The annealing temperature used corresponds to the unsteady-state diffusion regime C (criterion [2]) in nickel along the molybdenum grain boundary. This eliminates a volume diffusion effect on possible changes in the grain-boundary ensemble. Another feature of the Mo(Ni) system is a tendency of molybdenum to intercrystalline failure. This makes it possible to determine the nickel penetration depth along molybdenum grain boundaries for transverse fracture of a specimen subjected to diffusion annealing.
Thin-foil electron microscopy was performed in an EM-125K transmission electron microscope. Grain-boundary misorientation angles for molybdenum were determined by electron backscattering diffraction (EBSD), using a Quanta 200 3D scanning electron microscope equipped with a device for an EBSD analysis at an accelerating voltage of 30 kV and a scan step of 0.1 µm. The scanning area was 20×20 µm2. The grain-boundary misorientation angles were determined relative to the [100] axis. The misorientation detection sensitivity was 1°.
The distribution of the nickel concentration (CNi) in grain boundaries with depth in molybdenum subjected to diffusion annealing was determined on the transverse fracture surface of the specimens using a Shkhuna system. An Auger analyzer with an energy resolution of 0.7% was placed in an ultrahigh vacuum chamber (10–7 Pa). The electron beam diameter was 1–1.5 µm. The inaccuracy in determining the nickel concentration in grain boundaries was no more than 20%.
RESULTS AND DISCUSSION
Thin-foil electron microscopy has shown that SPD of molybdenum produced by high-pressure torsion gives rise to a structure of submicron element size. The electron diffraction patterns of the structure obtained from an area of 1.4 µm2 reveal a large number of reflections uniformly distributed over a circle. This is evidence of numerous structure elements per unit volume and their significant misorientation. A detailed microscopic examination has shown that the average size of the structure elements found from dark-field images reduces with increase in the distance from the center of the specimen to its edge. In the central part of the specimen (~1 mm from the center), the average size of the grain-subgrain structure elements is ~0.45 µm (Fig. 1). A major part of the material is occupied by structure elements of size 0.2–0.6 µm. At the periphery of the specimen (the distance from the central part is ~3.5 mm), the average size of the grain-subgrain structure elements is 0.26 µm (Fig. 2), and it hardly varies with specimen thickness.
Variations of the average size of the grain-subgrain structure elements along the specimen radius appear to be due to the method used to form the SMC structure. The size of the SMC structure elements formed by SPD is known to depend on the degree of strain experienced by the material. In the case of high-pressure torsion, the degree of strain can be calculated by the following formula [3]:
2 1/ 20ln[1 ( / ) ] ln /r p p pε = + ϕ ⋅ + (1)
where φ is the angle of rotation in radians, r is the disk radius, and p0 and p are the initial and the final specimen thickness, respectively.
Hence it follows that the larger is r, the higher is ε, and the smaller is the average size Dav of the SMC structure elements formed. The calculations by formula (1) show that in the specimens studied the true degree of strain ε at a distance of 1 mm from the central part of the specimen is about 5, whereas at a distance of ~3.5 mm, ε is ~6.5.
Using the EBSD technique, the total range of grain-boundary misorientations in the SMC structure of molybdenum subjected to SPD can be determined only at the specimen center (Fig. 1c). This is due to the fact that high internal stresses make it impossible to obtain Kikuchi bands at distances larger than 1 mm from the central part of the specimen. As evident from Fig. 1c, the grain-boundary misorientations in the central part of the specimen exhibit one maximum around small-angle boundaries with θ < 4°. The total fraction of the small-angle boundaries (θ < 15°) for
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the misorientations observed is ~40%. The grain-boundary distribution over large misorientations is virtually uniform (Fig. 1c).
We have studied the annealing effect on the size of the structure elements in the temperature interval 293–1373 K for an annealing time of 1–3 h. It has been found experimentally that marked changes in the SMC structure of molybdenum take place only at an annealing temperature of 973 K and above [4]. However, at an annealing
0.2 0.4 0.6 0.8 1.0 0
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d av = 0.45 μm
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Fig. 1. SMC molybdenum: electron micrograph (a), histogram of grain size distribution (b), and grain-boundary misorientation angles in the central part of the specimen prior to SPD.
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Fig. 2. SMC molybdenum: electron micrograph (a) and histogram of grain size distribution (b) at the periphery of the specimen prior to SPD.
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temperature as low as 673 K for an exposure time of 1 h, the total range of grain-boundary misorientation angles in SMC molybdenum can be determined by the EBSD technique both at the specimen center and at its periphery (Fig. 3).
Figure 3a shows that in the central part of the specimen (ε ≈ 5), the range of grain-boundary misorientation angles in the SMC molybdenum specimen subjected to annealing at 673 K for 1 h is hardly different from that seen in the uncoated material (cf. Fig. 1c). The fraction of small-angle grain boundaries in SMC molybdenum decreases with distance from the specimen center, and at a distance of ~3–3.5 mm (ε = 6–6.5) from the center, it is ~20% (Fig. 3b). The grain-boundary distribution over misorientations at the periphery of the specimen exhibits a clearly defined bimodal pattern. One peak lies for misorientations θ < 4°, whereas another diffused peak is in the range θ = 30–60°. Notably, the grain-boundary misorientation angles obtained from longitudinal and transverse sections of the specimen are in close agreement within the misorientations angle measurement error. Further discussion of the evolution of the grain-boundary ensemble in SMC molybdenum subjected to annealing will deal with a structure corresponding to a distance of 3.5 mm (ε = 6–6.5) from the specimen center.
Figure 4 shows the structure and grain-boundary misorientations in SMC molybdenum subjected to annealing at 973 K for 1 h. In as-annealed SMC specimens with grain-subgrain structure elements of invariable size, the dislocation density in the grains is seen to decrease. In addition, we notice grain boundaries with a banded contrast characteristic of equilibrium grain boundaries (Fig. 4a). This is evidence of reduced internal stresses and transformation of part of grain boundaries to an equilibrium state. However, the grain-boundary misorientations in as-annealed SMC molybdenum specimens at 973 K for 1 h (Fig. 4b) are nearly the same as those for the specimens annealed at 673 K for 1 h.
An increase in the annealing temperature up to 1023 K and a concurrent increase in the exposure time up to 3 h cause further decrease in the dislocation density and increase in the number of grain boundaries with a banded contrast in the SMC molybdenum structure. The average size of the grain-subgrain structure elements and the grain-boundary misorientations are hardly affected in this case. On annealing at 1223 K for 1 h, the SMC molybdenum structure remains unchanged, whereas the average size of the grain-subgrain structure elements increases up to 0.59 µm (Fig. 4c). Figure 4d shows that for grain-boundary misorientations characteristic of this structure, the fraction of small-angled grain boundaries decreases by nearly threefold in the boundaries with θ < 4° as compared to the original structure, and for 4 < θ < 15°, the fraction of boundaries of this type increases. The total fraction of small-angle boundaries decreases but slightly for an annealing temperature of 1173 K and an exposure time of 1 h. The number of large-angle boundaries increases for misorientations of 55–60°.
Grain-boundary misorientations for SMC molybdenum subjected to annealing at 1023 K for 3 h under nickel diffusion from the specimen surface along grain boundaries are presented in Fig. 5. The misorientations were determined for structures corresponding to a distance of ~20, 80, and 150 µm from the nickel-coated surface. Grain-boundary misorientations for structures lying at a distance of 80 and 150 µm from the nickel-coated surface are statistically the same and agree closely with misorientations for SMC molybdenum subjected to uncoated Mo annealed
20 40 600
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a b
N/N 0 ,%
Misorientation angle, deg
N/N0,%
Misorientation angle, deg
Fig. 3. Grain-boundary misorientations in SMC molybdenum subjected to annealing at 673 K for 1 h: the central part (a) and the periphery of the specimen (b).
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at 973 K for 1 h (Figs. 5a and 5b). However, for a distance of 20 µm from the surface, the fraction of grain boundaries with misorientation angles θ ~ 28–30° is nearly doubled (Figs. 5c and 5d), whereas the fraction of small-angle boundaries varies but slightly.
Auger electron spectroscopy of SMC molybdenum has shown that in the material subjected to annealing at 1023 K for 3h, nickel penetrates into molybdenum along grain boundaries to a depth of ~60 µm. Consequently, the foregoing variations of grain-boundary misorientations in as-annealed molybdenum at a distance of ~20 µm from the nickel-coated surface are attributable to Ni diffusion along grain boundaries.
Under certain conditions, grain-boundary diffusion fluxes of impurity atoms along grain boundaries initiate grain-boundary migration and grain growth in annealed metallic polycrystals and nucleation of new grains at the migrating grain boundaries and triple points [5–7]. A plausible mechanism of nucleation of new grains in the conditions studied is grain-boundary splitting to form low- and high-mobility boundaries [5]. Low-mobility boundaries of newly formed grains are considered to be special boundaries of the Σ3 type. High-mobility boundaries that provide further growth of the newly formed grains are of general type.
In the cubic lattice, a misorientation angle of 28.07° with respect to the [100] axis corresponds to a grain boundary of special type with Σ17a [8]. Thus an increase in the number of grain-boundaries of SMC molybdenum subjected to diffusion annealing with a misorientation angle of 28–30° coinciding with a misorientation angle of 28.07° within the measurement error is assumed to be due to nucleation of new grains because of splitting the boundaries of original grains.
A comparative microscopic examination of the SMC molybdenum structure at a depth of ~20 and 150 µm shows (Figs. 5a and 5c) that at the penetration depth of nickel into molybdenum, newly formed grains smaller than the initial ones are seen at certain triple points and boundaries (Fig. 5c). However, transmission electron microscopy fails to determine misorientations of these boundaries because of small grain size.
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Fig. 4. SMC molybdenum annealed at 973 (a and b) and 1223 K for 1 h (c and d): electron micrographs (a and c) and grain-boundary misorientations angles (b and d).
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Standard software supplied together with the Quanta 200 3D microscope enabled us to reveal grain boundaries on grain-boundary orientation maps with any given misorientation angles. Figure 6 depicts SMC molybdenum grain-boundary orientation maps with boundaries shown in bold type characterized by a misorientation angle of 28.07°. It is
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Fig. 5. SMC molybdenum subjected to annealing at 1023 K for 3 h under nickel diffusion from the specimen surface: electron micrographs (a and c) and grain-boundary misorientation angles (b and d) at a distance of ~150 (a and b) and ~20 µm from the nickel-coated surface of the specimen (c and d). Arrows show nuclei of new grains at triple points.
a b
2 μm 2 μm
Fig. 6. SMC molybdenum: grain-boundary orientation maps corresponding to the structure lying at a distance of ~20 (a) and 150 µm (b) from the nickel-coated surface. Grain boundaries characterized by a misorientation angle of 28.07° are shown in bold type.
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evident from the grain-boundary orientation map corresponding to the SMC molybdenum structure that there are much more boundaries with a misorientation angle of 28.07° at a distance of ~20 µm from the nickel-coated surface (Fig. 6a) than on the map for the molybdenum structure at a distance of ~150 µm from the surface (Fig. 6b). These boundaries generally belong to small grains lying at triple junctions or boundaries of grains of larger size.
CONCLUSIONS
The grain-boundary distribution over misorientations and the ratio between the number of small- and large-angle boundaries in a grain-boundary ensemble of submicrocrystalline (SMC) molybdenum are essentially dependent on the degree of strain experienced by the material during severe plastic deformation. Transformation of the grain boundaries in the as-annealed polycrystals to an equilibrium state in the absence of grain growth leaves the distribution pattern of grain boundaries over misorientation angles unchanged. Heating the SMC molybdenum structure causes the grain size and, in consequence, the share of the large-angle boundaries with a misorientation angle of θ ~ 60° in the grain-boundary ensemble to increase.
Grain-boundary diffusion fluxes of nickel atoms from the surface of the specimen are responsible for an increase in the share of large-angle grain boundaries approaching a misorientation angle θ = 28.07° of boundaries of special type in the grain boundary ensemble of SMC molybdenum. The effect is assumed to be due to nucleation of new grains attributable to the grain-boundary diffusion fluxes of nickel atoms induced by splitting of grain-boundaries into low-mobility boundaries of special type with θ = 28.07° and high-mobility boundaries.
The work was supported in part by Russian Foundation of Basic Research (Grant № 10-01-00034-a). The investigations were performed with the use of instrumentation provided by Tomsk Collective Materials Science Equipment Sharing Center.
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