Materials Engineering

ABSTRACT A new quaternary phase with the approximate composition U–18.6at%Fe–29.2at%Al–32.6at%Si was observed in U–Fe–Al–Si system. The crystal structure of this phase was investigated by electron diffraction and X-ray powder diffraction techniques. It has an orthorhombic unit cell with lattice parameters a=12.241Å, b=18.362Å and c=4.066Å and can be described by the Immm space group.

ABSTRACT A new quaternary phase with the approximate composition U–18.6at%Fe–29.2at%Al–32.6at%Si was observed in U–Fe–Al–Si system. The crystal structure of this phase was investigated by electron diffraction and X-ray powder diffraction techniques. It has an orthorhombic unit cell with lattice parameters a=12.241Å, b=18.362Å and c=4.066Å and can be described by the Immm space group.

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Abstract Despite several decades' long study, the identity of the precipitating phases responsible for the strengthening of maraging steels is still not clear. In the current work, this issue was extensively investigated using experimental and theoretical approaches. First, in-depth characterization of the precipitates in C250 steel and the precipitation order were performed through a combination of various Transmission Electron Microscopy (TEM) methods. In parallel, thermodynamic calculations were used for the prediction of the phase content at equilibrium. Then, in order to isolate the effects of the different precipitates, model alloys were cast and aged. It was shown that the phases responsible for the strengthening during the initial stages of aging are Ni3Mo and Ni3Ti. In the over-aged (close to equilibrium) condition, the steel consists of martensite, reverted austenite, Ni3Ti, and Fe–Mo phases. This conclusion was found to be in perfect agreement with thermodynamic calcu...

ABSTRACT The unique combination of high strength and high fracture toughness in maraging steels is attributed to the precipitation of mainly two phases, Ni3Mo and Ni3Ti, in the soft bcc martensitic matrix. Experimentally, it is difficult to distinguish between these phases; moreover, the effect of each precipitating phase on the properties of the steel is ambiguous. In the present work, we tackle these questions by analyzing the elastic fields and elastic energy associated with each precipitating phase and their role in the determination of the shape and relative arrangement. Using a semi-analytic approach based on the solution of the equations of elasticity by Fourier transform, the elastic fields and elastic energy associated with each precipitate were calculated. The calculations show that the minimum self-energy of both precipitates is accomplished when the smallest crystallographic misfit lies parallel to the longest geometric axis. The combination of elastic energy and surface energy provides an explanation to the observed morphologies of the precipitates, each type having an elongated shape but with a different aspect ratio. According to the calculations, the minimum self-elastic energy associated with Ni3Ti coherent precipitate is 2.8-fold larger than the energy of Ni3Mo. This implies for a larger strengthening effect of the former, as was confirmed experimentally in model alloys, one without Mo and the other without Ti. The elastic model also predicts that elongated shape and the anisotropic misfit strains favor several vertical arrangements of both precipitates, as was also verified experimentally by transmission electron microscopy of aged model alloys.

ABSTRACT The sensitivity of thermo-electric power (TEP) measurement to detect the undesired α–α′ phase separation occurring in Cr-rich oxide dispersion strengthened steels was investigated. TEP values were found to increase with time of exposure to 475 °C—the maximum temperature of the miscibility gap in Fe–Cr alloys. Exposure to 650 °C did not induce any changes in the TEP value. By examining the bend contours in dark-field TEM images, α′ precipitates were evidenced in aged PM2000. Using the Fe–XCr–YAl model alloys and X-ray photoelectron spectroscopy, the changes in TEP were traced back to the depletion of Cr from the matrix, caused by the formation of the Cr-rich α′ phase. By quantifying the effect of Cr content on the TEP value of model alloys, it was estimated that following 1000 h of aging, the Cr concentration in the α matrix of PM2000 alloy was reduced from 20 to ~13.5 at.%.

Partial 1100, 1000, 900 and 700°C isothermal sections of the Al–Rh–Ru phase diagram were determined. The isostructural binary AlRu and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. The Al9Rh2 and C-Al5Rh2 dissolve up to 4.5 and 13at.% Ru, while Al13Ru4 and Al2Ru dissolve up to 14.5 and 8at.% Rh, respectively. A ternary orthorhombic structure (Pbma,

Friction and wear of copper rubbed with lubrication in wide range of loads and sliding velocities were studied. The results of friction and wear experiments are presented as the Stribeck curve where the boundary lubrication (BL), mixed (ML) and elasto-hydrodynamic lubrication (EHL) regions are considered. The structural state of subsurface layers in different lubricant regions is studied by X-ray photoelectron

ABSTRACT Owing to the individuality of intermetallic compounds, they are regarded as a special class of materials. As such, there is a need to develop a step-by-step methodology for solution of their structure. The current paper adapts the methodology of structure solution from precession electron diffraction (PED) zonal data for intermetallics. The optimization of PED parameters for structure determination was achieved through the development of the atomic model of a well known Mg17Al12 (β) intermetallic phase. It was concluded that the PED acquisition parameters, the number of unique reflections and the quality of the merging process are the most important parameters that directly influence the correctness of a structure solution. The proposed methodology was applied to the structure solution of a highly complex new Mg48Al36Ag16 phase, which was recently revealed in the Mg–Al–Ag system. The final atomic model consisted of 152 atoms in the unit cell, distributed over 23 unique atomic positions. The correctness of the atomic model was verified by the reasonability of the interatomic distances and coordination polyhedra formed. It was found that the experimental model of Φ-Al17.1Mg53.4Zn29.5 can be assigned as a structure type for the Mg48Al36Ag16 phase. The Δ value, which measures the similarity between two structures, was calculated as 0.040.