Novel fabrication of bulk Al with gradient grain size distributions via powder metallurgy

We describe a novel approach to synthesize gradient microstructures, defined hereafter as containing a broad but continuous distribution of grain sizes. These microstructures extend the concept of a bimodal grain size distribution and the ability to design with multiple length scales. We demonstrate the proposed approach via experiments involving cryogenic ball milling of Al–4.5Mg–0.4Mn–0.05Fe and Al–50Mg powder followed by subsequent consolidation. Our results reveal that the grains in the consolidated powder present a gradient size distribution ranging from <100 nm to >3 μm. Moreover, phase composition analysis revealed a unique “interfingered” structure where the two starting phases were intermixed in a complex three-dimensional mesh. Hardness studies of this gradient microstructure show average Vickers hardness values of 200 ± 2.6, 204 ± 4.3 and 266 ± 50 for macrohardness, microhardness and nanoindentation, respectively. The standard deviation values highlight that the gradient microstructure is disordered locally, but homogenous macroscopically.     

Observation of Cu nanometre scale clusters formed in Fe85Si2B8P4Cu1 nanocrystalline soft magnetic alloy by a spherical aberration-corrected TEM/STEM

Microstructure of a nanocrystalline soft magnetic Fe₈₅Si₂B₈P₄Cu₁ alloy (NANOMET^®) was investigated by the state of the art spherical aberration-corrected TEM/STEM. Observation by TEM shows that the microstructure of NANOMET^® heat treated at 738 K for 600 s which exhibits the optimum soft magnetic properties has homogeneously distributed bcc-Fe nanocrystallites with the average grain size of 30 nm embedded in an amorphous matrix. Elemental mappings indicate that P is excluded from bcc-Fe grains and enriched outside the grains, which causes to retard the grain growth of bcc-Fe crystallites. The aberration-corrected STEM-EDS analysis with the ultrafine electron probe successfully proved that Cu atoms form nanometre scale clusters inside and/or outside the bcc-Fe nanocrystallites.     

Influence of cryomilling on the microstructural features in HVOF-sprayed NiCrAlY bond coats for thermal barrier coatings: Creation of a homogeneous distribution of nanoscale dispersoids

Previous research has revealed that thermal barrier coatings with cryomilled bond coats exhibit improved thermal cycling lifetime by growing a continuous and uniform oxide layer at a slower rate; yet the mechanism controlling the ultimate failure remains unclear. In an effort to provide a foundation for understanding the improved behavior, the influence of cryomilling on the microstructure of the NiCrAlY bond coat material is investigated in this article. Rather than focusing on the alumina scale formation, the microstructural features and their evolution within the high-velocity oxy-fuel (HVOF)-sprayed NiCrAlY bond coats themselves, prepared from conventional powder and cryomilled powder, were carefully compared through extensive scanning electron microscope/energy-dispersive X-ray spectroscopy characterization. In addition, the as-cryomilled NiCrAlY powder is characterized to provide evidence of the direct influence of cryomilling and to exclude the impact from the HVOF spraying. It is found that the essential change in microstructural features resulting from the cryomilling is the creation of a homogeneous distribution of ultrafine (nanoscale) Al-rich oxide/nitride dispersoids, which remain thermally stable even after exposure at 1100°C for 100 h. The TEM study on the as-cryomilled powder, prior to the HVOF spraying, indicates that some Al and Y-rich oxides are already present within the material as a direct result of the cryomilling process.     

On the enhancement of wear resistance of tool steels by cryogenic treatment

The present article aims to resolve the debate on the degree of benefit of processing tool or die steels by cryogenic treatment. This has been done by measuring transition loads for quenched and tempered die steel with and without cryogenic treatment. The wide range of reported degrees of improvement of wear resistance by cryotreatment is explained by the operative modes and mechanisms of sliding wear considering the wear rate ratio of conventionally treated to cryotreated steels on a two-dimensional map.     

On the deformation mechanism of twinning-induced plasticity steel

The microstructures of an Fe–20Mn–2Si–2Al Twinning-Induced Plasticity (TWIP) steel deformed at different temperature were characterized by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Based on microstructural features revealed at various characteristic temperature regions of deformation, the necessary and sufficient conditions of TWIP effect are proposed. Likewise, the competitive characteristic deformation mechanism occurring in this TWIP steel is presented and discussed qualitatively in terms of phase stability and stacking fault energy of the austenitic matrix.     

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.     

Reactive solid-state dewetting

High-temperature annealing in air of thick crystalline silver films deposited on high-purity nickel foils promotes solid-state dewetting, whereas no hole through the film is produced when annealing under high purity argon. Scanning electron microscopy observations of the film surface and of cross-sections reveal that dewetting occurs only if a nickel oxide layer forms at the silver-nickel interface, as a result of oxygen diffusion through the silver film. The main dewetting mechanism over short times (1h 1120K in air) is observed to be the condensation, at the silver-nickel oxide interface, of vacancies into voids which grow towards the free surface of the silver film.     

Investigations of the optical spectra and local structure for nickel(II) ions in XF2 (X = Mg,Zn) single crystals

A complete diagonalization energy matrix method (CDM) is proposed for 3d ⁸ ions in orthorhombic site (D _{2 h} ) symmetry. As an application, the optical spectra of XF₂ : Ni²⁺ (X = Mg, Zn) is well explained on the basis of both the CDM and the semi-self-consistent field d-orbital (semi-SCF d-orbital) theory. In addition, by establishing the relationship between the optical spectra and local structure, it has been possible to calculate the crystal structure parameters for XF₂:Ni²⁺ from optical measurements on Ni²⁺ ions. The theoretical results are in good agreement with experimental findings.     

Primary dendrite arm spacing during transient directional solidification of Al alloys with low redistribution coefficients

Particular analytical expressions based on a curve fitting on numerical results for primary dendritic growth have been proposed in the literature to encompass binary alloy systems whose redistribution coefficient (k ₀) approaches zero. To date, these expressions have not been checked against experimental results of transient solidification. The present study developed a comparative analysis between the theoretical predictions furnished by such expressions and results from transient solidification experiments with low k ₀ aluminum alloys. The analysis has shown that the proposed approach generally describes the primary dendritic spacing during solidification of Al–Sn alloys which are characterized by very low k ₀ values as well as by very large solidification ranges.