Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACT

Intriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ～ 25?nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ～ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.     

Experimental investigations of TB2 alloy by pack boriding with rare-earth oxides

ABSTRACT

An orthogonal test with four factors, namely temperature, time, type of rare-earth oxides (REOs) and REO content, was performed to obtain the optimised boriding parameters of TB2 alloy by pack boriding with REOs. It is found that temperature has the strongest effect on the boride layer thickness, while time has the strongest effect on the surface hardness and coefficient of friction. The optimum parameters for pack boriding of TB2 alloy with REO are a temperature of 1373?K, a time of 20?h, La₂O₃ as the REO with a content of 4?wt.%.     

Highly inhomogeneous compressive plasticity in nanocrystal-toughened Zr–Cu–Ni–Al bulk metallic glass

A Zr₆₂Cu_15.5Al₁₀Ni_12.5 bulk metallic glass with a large supercooled liquid region of 90 K, produced by copper-mould casting, exhibits a high strength of 1730 MPa and superior but highly inhomogeneous plasticity under uniaxial compression at ambient temperature. Micro-X-ray diffraction shows that compressive loading facilitates crystallization in the monolithic glassy alloy, resulting in room-temperature plasticity. The plastic deformation of the Zr₆₂Cu_15.5Al₁₀Ni_12.5 BMG may be attributed to in situ precipitation of nanocrystals during compression in heavily deformed areas.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

The inherent tensile strength of iron

The cohesive energy of Fe as a function of structure, strain and magnetic state has been computed using the full potential linearized augmented-plane-wave method within the framework of density functional theory and the generalized gradient approximation. Calculations corresponding to uniaxial stress in the <100> direction reveal that the ideal tensile strength of bcc Fe is about 14.2GPa and is determined by instability with respect to transformation into an unstable ferromagnetic fcc structure. The low-energy fcc phase is a modulatedantiferromagnetic fcc structure that is connected to the bcc phase via a first-order magnetic transformation and does not compromise its ideal strength.     

Recent advances in electron-beam-induced damage models in yttria fully stabilized zirconia single crystals

Dislocation loop formation in yttria fully stabilized zirconia single crystals observed in a transmission electron microscope has been reported in the literature. To account for the loop formation, two different explanations have been proposed. Here we present original experiments focusing on a comparison of the two models. We conclude that the origin of the loops is precipitation of a second phase, enhanced by heating in the electron beam.     

Ab initio study of Heusler alloys Co2XY (X = Cr,Mn, Fe; Y = Al,Ga) under high pressure

The structural, electronic, and magnetic properties of ferromagnetic Heusler alloys Co₂XY (X?=?Cr, Mn, Fe; Y?=?Al, Ga) have been investigated by means of the all-electron full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange and correlation potential. The main focus of this study is to elaborate the changes brought about in the electronic and the magnetic properties by applied pressure. The calculated total spin magnetic moments of all the compounds are found to be in good agreement with experiments. Out of these compounds, Co₂CrAl is found to be perfectly half-metallic (HM) and the other compounds are found to be nearly HM. Thus the HM to metal transition was observed at 75?GPa pressure for Co₂CrAl and the nearly half-metal to half-metal transition was observed at 40?GPa and 18?GPa for Co₂CrGa and Co₂MnAl, respectively, while no transition is observed in other compounds under investigation. It can be clearly seen that pressure affects the minority spin states rather than the majority states, leading to a slight reconstruction of the minority spin states with a shift in the Fermi level driving the above-mentioned transition.     

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.     

Nanoporous gold obtained from a metallic glass precursor used as substrate for surface-enhanced Raman scattering

Nanoporous gold (NPG) has been synthesized by electrochemical de-alloying a new precursor, amorphous Au₃₀Cu₃₈Ag₇Pd₅Si₂₀ (at.%), starting from melt-spun ribbons. Ligaments ranging from 75 to 210 nm depending on the de-alloying time were obtained. Analytical and electrochemical evidence showed the ligaments contain residual Cu, Ag and Pd. Surface-enhanced Raman scattering from the NPG was investigated using pyridine and 4,4′-bi-pyridine as probe molecules. It was found that the activity is at maximum when the ribbon is fully de-alloyed although the ligaments then have a larger size. The enhancement is attributed to the small size of crystals in the ligaments, to their morphology and to trapped atoms.     

Iron in solution with aluminum matrix after non-equilibrium processing: an atom probe tomography study

In this paper, we report on the influence of rapid solidification and severe plastic deformation on the solid solubility of Fe in Al. Atom probe tomography, for the first time, was performed on fine (3–4 μm diameter) and coarse (~100 μm) as-atomised Al-5 at.% Fe powder and cryomilled Al-5 at.% Fe powder. The atomised powders exhibited negligible Fe in solution with Al, whereas the cryomilled powder contained ~2 at.% Fe in solution. Moreover, our results suggest that severe plastic deformation is preferable to atomisation/rapid solidification for increasing the non-equilibrium solid solubility of Fe in Al.