Dynamic balance between grain refinement and grain growth during high-pressure torsion of Cu powders

High-pressure torsion (HPT) was applied to unmilled coarse-grained (CG) Cu powders with low initial dislocation density and cryomilled nanocrystalline (nc) Cu powders with high initial dislocation density, with identical processing parameters. HPT of unmilled CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between deformation-induced dislocation accumulation and dislocation annihilation. The equilibrium structures are governed by these two dynamic balances.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

New insights on tensile plasticity of ultrafine-grained metallic materials

ABSTRACT

The tensile properties of TiNi_43.5Fe_6.5 alloy samples having different grain sizes (0.16, 0.35, 1.7, 2.3, and 3.9?μm) and fabricated by severe plastic deformation and annealing were investigated. It was observed that both the strength and the elongation of the alloy increase with a decrease in the grain size until the average size reaches 1.7?μm. However, for average grain sizes smaller than 1.7?μm, the elongation decreases continuously with further grain refinement. On the other hand, the strain-hardening rate does not decrease with the decrease in plasticity but instead increases slightly. The poor ductility of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is accompanied by a high degree of strain hardening. This newly observed ductility behaviour of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is elucidated by characterising the intragranular and grain boundaries.     

Stress-enhanced grain growth in a nanostructured aluminium alloy during spark plasma sintering

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration.     

Superconductivity in Ca-intercalated bilayer graphene

Recent observation of proximity effect [H.B. Heersche, P. Jarillo-Herrero, J.B. Oostinga, L.M.K. Vandersypen, and A.F. Morpurgo, Nature, bf 446 (2007) p. 05555.] has ignited interest in superconductivity in graphene and its derivatives. We consider Ca-intercalated graphene bilayer and argue that it is a superconductor, and likely with a sizeable T _c . We find substantial and suggestive similarities between Ca-intercalated bilayer (C₆CaC₆), and CaC₆, an established superconductor with T _c = 11.5 K. In particular, the nearly free electron band, proven to be instrumental for superconductivity in intercalated graphites, does cross the chemical potential in (C₆CaC₆), despite the twice smaller doping level, satisfying the so-called “Cambridge criterion”. Calculated properties of zone-center phonons are very similar to those of CaC₆. This suggests that the critical temperature would probably be on the same scale as in CaC₆.     

Nanoindentation response of anisotropic piezoelectric materials

A three-dimensional finite element model is developed to accurately capture the force–depth and charge–depth nanoindentation response of several classes of anisotropic piezoelectric materials such as relaxor ferroelectrics for which analytical models are at present unavailable. Upon validating the finite element model for transversely isotropic materials, it is demonstrated that the nanoindentation response of anisotropic piezoelectric materials displays a strong dependence on the nature of the indenter geometry and relatively weak dependence on the indenter conductivity. Furthermore, by recourse to “longitudinal” and “transverse” indentations, the nanoindentation method can also be used to identify the poling directions in piezoelectric materials as well.     

Effect of crystallization on the surface area of porous Ni-based metallic glass foams

The change of the specific surface area in porous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass (MG) upon partial crystallization was investigated. The observed increase in the surface area of the annealed Ni-based MG foams is due to the formation of homogeneously distributed Ni₁₀(Zr,Ti)₇ rod-shape intermetallic phases with nominal diameters around 250?nm and ～800?nm length on the surface of MG struts during the crystallization. For longer annealing, the specific surface area decreases again due to a change of the morphology of the crystals from rod-like to disc-like appearance, thus suggesting an optimum regime for increasing the specific surface area upon isothermal annealing at a given temperature.     

High-pressure electrical resistivity behaviour of nanocrystalline perovskite (La,Sr)(Mn,Fe)O3

We report here the electrical resistivity of nanocrystalline perovskite-structured La–Sr manganites as a function of pressures up to 8?GPa, at room temperature. The nanocrystalline perovskite manganites were prepared by the sol–gel technique and found to have crystallite sizes of 12–18?nm. The pressure dependence of the electrical resistivity shows a first-order phase transition at 0.66(2)?GPa and a subtle phase transition between 3.5 and 3.8?GPa. The first-order transition at 0.66?GPa can be related to the transition from localized-electron to band magnetism.     

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni₅₀Al₅₀ amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10^?2 m/s. The heat of the intermixing with the formation of Ni₅₀Al₅₀ amorphous alloy can be estimated at approximately ?0.34 eV/at. Next, the crystallization of the Ni₅₀Al₅₀ amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately ?0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.     

Microstructure evolution and hardness variation of pack-rolled nanocrystalline nickel

The microstructure evolution and hardness of nanocrystalline nickel during pack rolling at room temperature have been investigated. It was found that the roll-bonding side (R) and non-roll-bonding side (NR) behaved quite differently. The hardness of side R is higher than that of side NR. No obvious work softening was observed in either side R or side NR until the strain reached ~ 0.611. Quantitative X-ray diffraction analysis indicated that the grain size in side NR increases faster than that in side R, a result confirmed by transmission electron microscopy. Texture analysis showed that (2 0 0) preferred orientation first strengthens but then weakens in both sides NR and R, while a strong (2 2 0) preferred orientation emerges, particularly in side R. Further texture analysis suggests that dislocation slip is responsible for the texture discrepancy between side NR and side R. The dislocation activity, grain rotation and grain growth are discussed based on the experimental results.