Correlating microstrain and activated slip systems with mechanical properties within rotary swaged WNiCo pseudoalloy

Due to their superb mechanical properties and high specific mass, tungsten heavy alloys are used in demanding applications, such as kinetic penetrators, gyroscope rotors, or radiation shielding. However, their structure, consisting of hard tungsten particles embedded in a soft matrix, makes the deformation processing a challenging task. This study focused on the characterization of deformation behavior during thermomechanical processing of a WNiCo tungsten heavy alloy (THA) via the method of rotary swaging at various temperatures. Emphasis is given to microstrain development and determination of the activated slip systems and dislocation density via neutron diffraction. The analyses showed that the grains of the NiCo2W matrix refined significantly after the deformation treatments. The microstrain was higher in the cold swaged sample (44.2 x 10(-4)). Both the samples swaged at 20 degrees C and 900 degrees C exhibited the activation of edge dislocations with {110} or {111} slip systems, and/or screw dislocations with slip system in the NiCo2W matrix. Dislocation densities were determined and the results were correlated with the final mechanical properties of the swaged bars.Web of Science131art. no. 20

Microstructural stability of Co-Re-Cr-Ta-C alloy strengthened by TaC precipitates

It is becoming increasingly clear that new materials that can operate at substantially higher temperatures than Ni-base superalloys are needed for future gas turbines. High melting Co-Re-Cr based alloy strengthened by carbides, particularly the MC type carbide, shows promise [1]. A fine dispersion of globular TaC precipitate is exploited for this purpose. Additionally Cr, which is mainly added to improve oxidation resistance, also stabilizes lamellar M23C6 type Cr carbide. The microstructure of a Co-Re-Cr-Ta-C alloy with the two types of carbides is seen in Fig. 1. Please click Additional Files below to see the full abstract

Misfit in Inconel-Type Superalloy

An important parameter for the characterization of microstructural changes in nickel base superalloys is the misfit - the relative difference between lattice parameters of γ matrix and γ′ precipitates. The misfit in IN738LC superalloy was examined at POLDI time-of-flight (TOF) neutron diffractometer both at room temperature and in situ at elevated temperatures using a high-temperature furnace. A careful out-of-furnace measurement yielded the lattice parameters of both γ and γ′ phase at room temperature (aγ=3.58611(10) Å, aγ′=3.58857(17) Å) as well as the misfit (equal to 6.9(6)×10-4). The in situ measurement at elevated temperatures provided the temperature dependence of the lattice parameters of γ (up to 1120°C) and γ′ (up to 1000°C). Using these data, the evolution of the misfit with temperature was calculated. The misfit decreases with increasing temperature until it reaches zero value at a temperature around 800°C. Above 800°C, it becomes negative

Neutron diffraction as a precise and reliable method for obtaining structural properties of bulk quantities of graphene

Graphene based carbon materials have attracted a great deal of attention in the last decade; nowadays tons of graphene are produced yearly. However, there is lack of precise and reliable techniques for the determination of structural properties of graphene on the bulk scale. The analytical methods being routinely applied for graphene characterization, including TEM and AFM, can be only used for the study of scant amounts of graphene samples and do not give general information on the average number of layers and the structure of the prepared graphenes. On the other hand, diffraction methods can be advantageously used to obtain information on the average thickness of the produced graphene as well as on the average sheets lateral dimensions, without the necessity of sample dispersion in solvents. We present a study of the structural properties of graphene prepared by chemical and thermal reduction of graphite oxide, comparing SEM, STEM, AFM, Raman spectroscopy, BET, X-ray and neutron diffraction methods. Our study brings new deep insights into the basic structural properties of graphene in a bulk form. Given the importance of a suitable characterization technique on the bulk materials, we wish to highlight the importance of these diffraction techniques for accurate determination of the graphene thickness and lateral parameters.Published versio

Monotónní a cyklické vlastnosti TiAl slitin legovaných Nb, Mo a C

Six grades of TiAl alloys doped with 7 at.% Nb and variable content of C and/or Mo were studied. The as-received microstructure and phase composition of the alloys was characterized using neutron diffraction experiments and electron microscopy. They are substantially different in individual alloys. In Mo containing alloys, ordered beta0 phase is present while carbon suppresses the occurrence of this phase. Alternating lamellae of gamma and alpha2 phases are characteristic for all specimens. The specimens have been subjected to monotonic testing (tensile and compression) and to low cycle fatigue testing at ambient and at elevated temperatures. The differences in microstructure result in scatter of mechanical behaviour of the alloys, e.g. the yield stress at 750C varies from 445 to 626 MPa. The mechanical properties of the materials are discussed in relation to the initial microstructure and phase composition.Bylo studováno šest typů TiAl slitin legovaných 7 at.% Nb a různým množstvím C a/nebo Mo. Charakterizace mikrostruktury „as-received“ stavu a složení fází byla provedena pomocí neutronové difrakce a elektronové mikroskopie. Vzorky byly podrobeny monotónním testům (tah a tlak) a také nízkocylkové únavě jak za pokojových tak za zvýšených teplot. Rozdíly v mikrostruktuře vedou k různým mechanickým vlastnostem jednotlivých typů slitin

Texture and differential stress development in W/Ni-Co composite after rotary swaging

Knowledge of texture and residual stresses in tungsten heavy pseudoalloys is substantial for the microstructure optimization. These characteristics were determined in cold and warm rotary swaged W/NiCo composite with help of neutron diffraction. The results were discussed in view of the observed microstructure and mechanical properties. The investigated bars consisted of tungsten agglomerates (bcc lattice) surrounded by NiCo-based matrix (fcc lattice). No preferential crystallographic orientation was found in the as-sintered bar. A strong texture was formed in both the tungsten agglomerates ( fiber texture parallel to the swaging axis) and in the NiCo-based matrix ( fiber texture) after rotary swaging. Although usually of double-fiber texture, the fiber of the fcc structures was nearly missing in the matrix. Further, the cold-swaged bar exhibited substantially stronger texture for both phases which corresponds to the higher measured ultimate tensile strength. The residual stress differences were employed for characterization of the stress state of the bars. The largest residual stress difference (approximate to 400 MPa) was found at the center of the bar deformed at room temperature. The hoop stresses were non-symmetrical with respect to the swaging axis, which was likely caused by the elliptical cross section of the as-sintered bar.Web of Science1312art. no. 286

Coexistence of Two Cubic-Lattice Co Matrices at High Temperatures in Co-Re-Cr-Ni Alloy Studied by Neutron Diffraction

In situ neutron diffraction measurements were performed during heating to high temperature and cooling for a Co-17Re-23Cr-25Ni alloy. The allotropic transformation of the Co matrix and the evolution of the low-temperature hexagonal and high-temperature cubic Co phases were studied. A surprising observation was the splitting of the face-centred cubic (fcc) Co phase peaks at high temperature during heating as well as cooling. The phase evolution was monitored, and an appearance of the secondary fcc phase could be linked to the formation of σ phase (Cr2Re3 type) associated with a compositional change in the matrix due to diffusion processes at high temperature