The microstructure and hardness of binary Al–Zn and Al–Mg alloys were studied both in the as-cast state and after high-pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) ...grains and of reinforcing second-phase precipitates decreases drastically after HPT and reaches the nanometre range. During HPT, the Zn-rich supersaturated (Al) solid solution decomposes and closely approaches the equilibrium state corresponding to room temperature. The decomposition of Mg-rich supersaturated (Al) solid solution is less pronounced. In the as-cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to solid-solution hardening. However, after HPT the work hardening and Hall–Petch hardening due to the decreasing grain size compete with softening due to the decomposition of a supersaturated solid solution. As a net effect, severe plastic deformation results in softening of the binary Al–Zn and Al–Mg alloys. Softening is more pronounced in the Al–Zn alloys where HPT leads to almost full decomposition of the supersaturated solid solution.
Nanograined (grain size 6–15 nm) ZnO films with various Fe content (between 0 and 40 at.%) were synthesized by the novel liquid ceramics method. The films with 0, 0.1, 5 and 10 at.% Fe contain only ...ZnO-based solid solution with wurtzite structure. The films with 20 at.% Fe contain mainly amorphous phase. The peaks of the second phase (ZnFe
2
O
4
with cubic lattice) become visible in the X-ray diffraction spectra at 30 at.% Fe. Therefore, the overall solubility of Fe in nanograined ZnO films at 550 °C is about 20 at.% Fe. The solubility limit in the bulk is about 1.5 at.% Fe. The recently published papers on the structure and magnetic behaviour of Fe-doped ZnO allowed us to obtain the dependence of Fe solubility in ZnO on the grain size. The overall Fe solubility drastically increases with the decreasing grain size. The quantitative estimation shows that, close to the bulk solubility limit, the thickness of a Fe-enriched layer in grain boundaries is that of several monolayers.
High pressure torsion (HPT) has been used for the severe plastic deformation (SPD) treatment of molten Fe–12.3at% Nd–7.6 at% B alloy (5GPa, 1rpm, 5rot, room temperature). After HPT the microstructure ...contained the nanograins of the ferromagnetic Nd2Fe14B phase embedded in the amorphous matrix with uniform composition. It is different to the commercial multicomponent FeNdB-based alloy where two different amorphous phases appeared after HPT (B.B. Straumal, A.R. Kilmametov, A.A. Mazilkin, S.G. Protasova, K.I. Kolesnikova, P.B. Straumal, B. Baretzky, Mater. Lett., 2015, 145, pp. 63–66). The SPD-treatment at room temperature TSPD=30°C is frequently equivalent to the heat treatment at a certain elevated temperature Teff>30°C. The composition of phases in the studied NdFeB-based alloy after HPT corresponds to the state at Teff ~1140°C.
•The NdFeB-based alloy amorphises under the action of severe plastic deformation (SPD).•The amorphous phase with embedded Nd2Fe14B nanocrystals is formed.•SPD at room temperature is equivalent to the annealing at Teff ~1140°C.•Teff depends on the composition of an alloy.
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•Heavy tungsten-based alloys contain W grains and intergranular binder phase.•The W/W grain boundaries (GBs) are completely or partially wetted by the Ni-rich phase.•Some of W/W GBs ...are pseudopartially wetted and not “dry”.•They contain the ∼3nm uniformly thin Ni-rich layer.
The excellent properties of heavy tungsten-based alloys are based on the combination of hardness of W grains, as well on the toughness and ductility of the binder with low melting temperature (usually containing nickel, iron, or copper). The topology (and resulting properties) of binder network are controlled by the complete and incomplete wetting of W/W grain boundaries (GBs) and GB triple junctions (TJ). We observed for the first time that pseudoincomplete (or pseudopartial, or frustrated complete) GB wetting by Ni layers is also present in W–Ni alloys. Namely, the channel of a Ni-rich solid solution in GB TJ forms the non-zero dihedral contact angle not only with “dry” W/W GBs (incomplete GB wetting), but also with W/W GBs containing the uniformly thin (3nm) Ni-rich layer (pseudoincomplete GB wetting).
The high pressure torsion (HPT) has been used for the severe plastic deformation (SPD) treatment of liquid-phase sintered hard magnetic NdFeB-based alloy (5GPa, 1rpm, 5 torsions, ambient ...temperature). The amorphization of the crystalline alloy under the action of HPT has been observed. HPT permitted to obtain for the first time the mixture of two different amorphous phases with embedded grains of the ferromagnetic Nd2Fe14B phase. The SPD-treatment at ambient temperature TSPD=300K is frequently equivalent to the heat treatment at a certain elevated temperature Teff>300K. The composition of phases in the studied NdFeB-based alloy after HPT corresponds to the state at Teff~1170°C.
•The NdFeB-based alloy amorphises under the action of severe plastic deformation.•Such amorphisation has been observed for the first time.•Mixture of two different amorphous phases with embedded nanocrystals is formed.•One amorphous phase is Fe-rich, another is Nd-rich.
Some of the authors of this article recently demonstrated that severe plastic deformation permits the production of metallic alloys containing two coexisting amorphous phases from the crystalline ...multiphase Ni
60
Nb
18
Y
22
alloy. The aim herein is to provide a detailed description of the microstructure of this system of coexisting amorphous phases by transmission electron microscopy (TEM) analysis. As-cast Ni
50
Nb
20
Y
30
alloy was coarse grained and contained mainly NiY phase (grain size 25 μm) as well as NbNi
3
, Ni
2
Y, Ni
7
Y
2
, and Ni
3
Y phases (grain size 3–5 μm). High-pressure torsion (4 GPa, 10 torsions) completely changed the structure. After severe plastic deformation (SPD), the sample contained two glassy phases and two other nanocrystalline NiY and Nb
15
Ni
2
phases (grain size about 20 nm). Bright-field TEM micrographs showed fine, 5–10 nm, round bubbles of bright Y-rich amorphous phase embedded in darker Nb-rich “grains.” In turn, the dark Nb-rich grains were separated by layers of bright Y-rich amorphous phase a few nanometers thick. This stricture permits one to speak about mutual wetting of “grain boundaries” in both amorphous phases.
Pure ZnO thin films were obtained by the wet chemistry (“liquid ceramics”) method from the butanoate precursors. Films consist of dense equiaxial nanograins and reveal ferromagnetic behaviour. The ...structure of the ZnO films was studied by the high-resolution transmission electron microscopy. The intergranular regions in the nanograined ZnO films obtained by the “liquid ceramics” method are amorphous. It looks like fine areas of the second amorphous phase which wets (covers) some of the ZnO/ZnO grain boundaries. Most probably these amorphous intergranular regions contain the defects which are responsible for the ferromagnetic behaviour.
The microstructure of binary Al–10 at% Zn and Al–15 at% Zn alloys after long anneals (800–4000 h) was studied between 190 and 258 °C. The contact angles between (Zn) particles and (Al)/(Al) grain ...boundaries (GBs) were measured. They decrease with decreasing temperature. First (Al)/(Al) GBs completely wetted by the second solid phase (Zn) appear below
T
wsAl0%
= 205 ± 5 °C. Above
T
wsAl0%
= 205 ± 5 °C all (Al)/(Al) GBs are incompletely wetted by (Zn) solid phase. The extrapolation of the maximal contact angle θ to zero permits to obtain the
T
wsAl100%
= 125 ± 10 °C. Below this line all (Al)/(Al) GBs has to be completely wetted by (Zn) solid phase.
The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such ...alloys are controlled by the thin layers of a Nd-rich phase in Nd2Fe14B/Nd2Fe14B GBs. These GB layers ensure the magnetic isolation of Nd2Fe14B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd2O3 at the triple junctions between Nd2Fe14B grains. The goal of this work was to check whether the Nd2Fe14B/Nd2Fe14B GBs could also contain the thin layers of Nd2O3 oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd2Fe14B/Nd2Fe14B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd2O3. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd2Fe14B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd2Fe14B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets.
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•It has been observed that Nd2Fe14B/Nd2Fe14B GBs in the NdFeB-based alloys for permanent magnets can contain Nd2O3 oxide layers.•It has been also shown that Nd2O3 oxide GB layers, similar to GBs with metallic Nd or intermetallic phases, can effectively ensure the magnetic isolation of Nd2Fe14B grains from each other.•Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets.
The behavior of an NdFeB-based multicomponent alloy subjected to high-pressure torsion has been studied. The high-pressure torsion results in the partial amorphization of the alloy, where the ...fractions of the amorphous and crystal phases vary in the process of torsion. The torque increases smoothly with the angle of rotation of anvils, and self-sustained oscillations of the torque appear beginning with a certain torsion (~1000°). The torque varies from 500 to 600 N m with a period of about 1.5 s. The torsion of the alloy in the regime of self-sustained oscillations is accompanied by intense acoustic emission at a frequency of ~1–2 s
–1
. This phenomenon can be explained by the periodic change in the mechanism of the high-pressure torsion from the dislocation one (characteristic of the crystal phase) to the dislocation-free mechanism (typical of the amorphous state).
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