Severe plastic deformation not only leads to grain refinement but also accelerates mass-transfer and drives phase transformations in these Cu-based alloys. This review is devoted to the dynamic ...equilibrium between decomposition of (supersaturated) solid solution and dissolution of precipitates during high pressure torsion (HPT) of diluted Cu-based Cu–X alloys. The precipitation of second phase particles from a solid solution and their dissolution take place simultaneously and compete with each other. During HPT, a certain steady-state concentration Cs of a second component in a solid solution is reached, as if a sample would be annealed at a certain effective temperature Teff. We found that Teff linearly increases with increase of activation enthalpy of bulk tracer diffusion HD. The correlation between activation enthalpy of bulk tracer diffusion HD and melting temperature Tm of diffusing alloying component has been found for the first time. As a result, Teff linearly increases with increase of melting temperature Tm of diffusing alloying component as well. The observed correlations allow one for the first time to predict the behaviour and phase transitions in the Cu-based alloys under high pressure torsion.
•Severe plastic deformation (SPD) drives phase transformations in the materials.•The competition between formation and dissolution of precipitates during SPD was studied.•A certain steady-state concentration Cs of a second component in a solid solution establishes.•Cs is as if a sample would be annealed at a certain effective temperature Teff.•Teff linearly increases with increase of melting temperature Tm of alloying component.
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 microstructure of binary Co–13.6 wt% Cu and Cu–4.9 wt% Co alloys after long anneals (930–2,100 h) was studied between 880 and 1,085 °C. The contact angles between (Co) particles and (Cu)/(Cu) ...grain boundaries (GBs) in the Cu–4.9 wt% Co alloy are between 50° and 70°. In the Co–13.6 wt% Cu alloy, the transition from incomplete to complete wetting (coverage) of (Co)/(Co) GBs by the second solid phase (Cu) has been observed. The portion of completely wetted (Co)/(Co) GBs increases with increasing temperature beginning from
T
wss
= 970 ± 10 °C and reaches a maximum of 15% at 1,040 °C. This temperature is very close to the Curie point in the Co–Cu alloys (1,050 °C). Above 1,040 °C, the amount of completely wetted (Co)/(Co) GBs decreases with increasing temperature and reaches zero at
T
wsf
= 1,075 ± 5 °C. Such reversible transition from incomplete to complete wetting (coverage) of a GB by a second solid phase is observed for the first time.
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.
The high-pressure torsion (HPT) of binary copper alloys with 3, 5, 8, 10 wt. % Ag and 14 wt. % Sn has been studied at room temperature THPT. Before HPT, the Cu–Ag alloys have been annealed at 12 ...different temperatures between 320 and 800 °C and Cu–14 wt. % Sn has been annealed at 9 different temperatures between 310 and 500 °C. Thus, before HPT the Cu–Ag alloys consisted of Ag-particles in the Cu-based matrix with silver content cinit from almost zero to 8 wt.%. The Cu–14 wt. % Sn samples had Cu-based matrix with tin concentration cinit from almost zero to 14 wt.% Sn and precipitates of ε or δ Hume-Rothery intermetallic phases. After about 1.5 plunger rotations a certain steady-state concentration css of the alloying element is reached in the matrix. The measured css values were 5.5±0.1 wt. % Ag and 13.1±0.1 wt. % Sn. If the initial concentration cinit in Cu matrix was below css (cinit < css), it increased towards css during HPT. If cinit > css it decreased towards css. We observed that css did not depend on cinit in broad interval of cinit and was, therefore, equifinal. The equifinal css values corresponded to the certain equilibrium solubilities of silver and tin in Cu matrix and allowed to estimate the (elevated) effective temperature as Teff (Ag) = 700±10 °C and Teff (Sn) = 400±10 °C, respectively. The observed phenomena are discussed using the ideas of non-equilibrium thermodynamics of open systems. During HPT the decomposition of a solid solution competed with dissolution of precipitates. As a result, a dynamic equilibrium established between precipitation and dissolution at steady-state deformation stage. In this dynamic equilibrium a certain steady-state concentration css of the alloying element is reached in the matrix. In Cu-based alloys, the obtained Teff is always higher than THPT and correlates with activation enthalpy of dopant diffusion in Cu. Other HPT-driven phenomena such as accelerated mass transfer, intermetallic phase formation, grain boundary faceting and grain boundary segregation are taken into account to evaluate the effective temperature Teff.
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Grain boundary complexions and pseudopartial wetting Straumal, B.B.; Mazilkin, A.A.; Baretzky, B.
Current opinion in solid state & materials science,
October 2016, 2016-10-00, 20161001, Letnik:
20, Številka:
5
Journal Article
Recenzirano
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•Grain boundary (GB) complexions are connected with wetting phenomena.•The pseudopartial GB wetting can be observed between complete and partial ones.•In this case the GB multilayer ...complexion coexists with non-zero GB contact angle.•The pseudopartial GB wetting is observed in technologically important alloys.
The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.
In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting ...by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.
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.
In this review, the phenomenon of grain boundary (GB) wetting by melt is analyzed for multicomponent alloys without principal components (also called high-entropy alloys or HEAs) containing titanium. ...GB wetting can be complete or partial. In the former case, the liquid phase forms the continuous layers between solid grains and completely separates them. In the latter case of partial GB wetting, the melt forms the chain of droplets in GBs, with certain non-zero contact angles. The GB wetting phenomenon can be observed in HEAs produced by all solidification-based technologies. GB leads to the appearance of novel GB tie lines Twmin and Twmax in the multicomponent HEA phase diagrams. The so-called grain-boundary engineering of HEAs permits the use of GB wetting to improve the HEAs’ properties or, alternatively, its exclusion if the GB layers of a second phase are detrimental.
During severe plastic deformation (SPD), the processes of lattice defect formation as well as their relaxation (annihilation) compete with each other. As a result, a dynamic equilibrium is ...established, and a steady state is reached after a certain strain value. Simultaneously, other kinetic processes act in opposite directions and also compete with each other during SPD, such as grain refinement/growth, mechanical strengthening/softening, formation/decomposition of solid solution, etc. These competing processes also lead to dynamic equilibrium and result in a steady state (saturation), albeit after different strains. Among these steady-state phenomena, particle fragmentation during the second phase of SPD has received little attention. Available data indicate that precipitate fragmentation slows down with increasing strain, though saturation is achieved at higher strains than in the case of hardness or grain size. Moreover, one can consider the SPD-driven nanocrystallization in the amorphous phase as a process that is opposite to the fragmentation of precipitates. The size of these crystalline nanoprecipitates also saturates after a certain strain. The fragmentation of precipitates during SPD is the topic of this review.
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