An addition of boron largely increases the ductility in polycrystalline high-temperature Co–Re alloys. Therefore, the effect of boron on the alloy structural characteristics is of high importance for ...the stability of the matrix at operational temperatures. Volume fractions of ε (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr
2
Re
3
type) phases were measured at ambient and high temperatures (up to 1500 °C) for a boron-containing Co–17Re–23Cr alloy using neutron diffraction. The matrix phase undergoes an allotropic transformation from ε to γ structure at high temperatures, similar to pure cobalt and to the previously investigated, more complex Co–17Re–23Cr–1.2Ta–2.6C alloy. It was determined in this study that the transformation temperature depends on the boron content (0–1000 wt. ppm). Nevertheless, the transformation temperature did not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm of boron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which binds hcp-stabilizing elements (Cr and Re). Moreover, borides were identified in alloys with high boron content.
Neutron diffraction measurement was performed in-situ at high temperatures on Co-Re-Ta-C alloys with and without Cr addition. This included alloys containing different C content with the C/Ta ratio ...varying between 0.5 and 1.0. The Co-Re-solid solution matrix of the experimental alloys is polymorphic (like in pure cobalt) and transformed from low temperature hexagonal ɛ phase to high temperature cubic γ phase on heating. This transformation is reversible and show hysteresis. The main alloying addition, Re, stabilizes the ɛ Co-phase and increases the transformation temperature to above 1273 K. The onset of the
transformation during heating and cooling was found to differ depending on the alloy composition. In alloys without Cr addition the transformation was not completed on cooling and the high temperature γ phase was partly retained at room temperature in metastable state with the amount depending on the cooling rate from high temperature. The diffraction and microstructural results showed that Cr is ɛ stabilizer (similar as Re) but the role of Ta is not clear. The C/Ta ratio has no direct effect on the matrix phase transformation. Nevertheless, it influences indirectly by determining the amount of Ta which is freely available in the matrix.
Boron largely increases the ductility of polycrystalline high-temperature Co–Re–Cr alloys. Therefore, the effect of boron addition on the alloy structural characteristics is of large importance for ...the stability of the alloy at operational temperatures. Along with the Co-solid solution matrix phase transformation from hcp to fcc structure, additional structural effects were observed in situ at very high temperatures (up to 1500 °C) using neutron diffraction (ND) in boron-containing Co–17Re–23Cr alloys. Increasing boron content up to 1000 wt. ppm lowers the temperature at which sublimation of Co and Cr from the matrix occurs. As a result, the composition of the matrix in the surface region is changed leading to the formation of a second and a third matrix hcp phases at high temperatures. The consideration on the lattice parameter dependence on composition was used to identify the new phases appearing at high temperatures. Energy-dispersive spectroscopy and ND results were used to estimate the amount of Co and Cr which sublimated from the surface region of the high-boron sample. In the sense of alloy development, the sublimation of Co and Cr is not critical as the temperature range where it is observed (≥1430 °C) is significantly above the foreseen operation temperature of the alloys (1200 °C).
Co-Re-Cr alloys are being developed for high-temperature application in gas turbines. In these alloys, the Cr2Re3-based σ phase is stable when the Cr content is higher than 20 atomic %. The addition ...of Ni is being studied to partially substitute Cr, which aims to suppress σ formation without sacrificing the benefit of Cr in the oxidation resistance of the alloy. The microstructure of the alloys with varying Cr (18–23%) and Ni (8–25%) was investigated by electron microscopy in the present study, primarily to look into the stability of the σ phase and its influence on the Co matrix phase transformation. The σ phase is mainly found in two morphologies in these alloys, where at high temperature only blocky σ phase is present at grain boundaries but cellular σ is formed through a discontinuous precipitation within the grains at lower heat treatment temperatures. The presence of fine cellular σ phase influences the alloy hardness. Moreover, the σ precipitation, which depletes the matrix in Re, also influences the allotropic transformation of the Co matrix.
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.
Three single-crystalline Ni-base superalloys with varying content of the γ′ phase are investigated with respect to their microstructure and creep behaviour at 1000 °C. It is shown by convergent beam ...electron diffraction, X-ray diffraction and energy-dispersive X-ray spectroscopy that the composition of the matrix and precipitates as well as the lattice misfit between both phases are similar for all three model alloys despite a variation of the γ′ content from about 50 to about 65 %. This allows to study the influence of the precipitate volume fraction on high-temperature behaviour in separation from other microstructural parameters. Highest creep strength is found for the alloy with intermediate particle content (about 58 %), which is significantly below the particle volume fraction of contemporary single-crystal superalloys such as CMSX-4.
The deformation behaviour of the single crystal superalloy SC16 has been studied under low cycle fatigue (LCF) loading at 1223 K, 1123 K and 298 K with a strain amplitude range of 0.4% to 1.0% and a ...constant strain rate of 10
s
. Microstructural investigation by transmission electron microscopy of the deformed specimens revealed that the deformation mechanism depends on strain amplitude as well as on temperature of deformation. At large strain amplitudes at elevated temperatures, the γ′ precipitates are sheared by the matrix dislocations leading to stacking fault (SF) formation within the precipitates, while at small strain amplitudes deformation is confined within the γ matrix. At room temperature, the planar faults in γ′ precipitates are complex. The deformation under cyclic loading is compared with that under monotonic loading. The threshold stresses for shearing of γ′ precipitates at different temperatures are estimated.
•Increasing Ni additions improve the oxidation behaviour of Co-17Re-18Cr alloys due to enhanced Cr activity.•Higher Ni contents accelerate the formation of the σ phase at elevated temperatures ...deteriorating the oxidation behaviour.•A Si addition of 2 at.% shows a positive effect on the oxidation behaviour of the Co-17Re-18Cr-15Ni alloy at 900 °C–1100 °C.•Si improves the oxidation resistance by enhancing the Cr activity but also influences the alloy microstructure.
The present study deals with the oxidation behaviour of three Co-17Re-18Cr-XNi (X = 8, 15, 25 at.%) alloys and one Co-17Re-18Cr-15Ni-2Si alloy in the temperature range 800 °C–1100 °C. The experimental results reveal improved oxidation resistance with increasing Ni concentration. It seems that Ni enhances the diffusion of Cr. However, higher Ni concentrations accelerate the formation of the Cr-rich σ phase detracting significant amounts of Cr from the matrix and deteriorating the corrosion resistance. Results of the oxidation tests reveal a positive effect of Si on the oxidation behaviour at 900 °C–1100 °C. It was found that Si enhances the Cr activity, though, facilitating the formation of the σ phase.
Core-shell type magnetic nanoparticles are finding attractive applications in biomedicine, from diagnostic to cancer therapy. Both for targeted drug delivery and hyperthermia, as well as a contrast ...agent used for external biomedical imaging systems, small (< 20 nm) superparamagnetic nanoparticles are desired. Some iron oxide nanoparticle formulations are already approved for human administration as contrast agent for magnetic resonance imaging. However, search continues for nanoparticles with higher saturation magnetisation. Metallic, bi-metallic and intermetallic magnetic nanoparticles are finding attention. Biocompatibility and optimal clearance are important criteria for the medical applications and therefore core-shell type particles are favored, where a biocompatible shell (e.g. polymer, Silica) can prevent inadvertent host reaction with the magnetic core. A recently developed novel synthesis method (electrochemical selective phase dissolution - ESPD), which can produce core-shell magnetic nanoparticles, is reviewed in this paper. ESPD, as the name suggests, uses electro-chemical separation of a phase from metallic alloys to synthesize nanoparticles. It is a versatile method and can be adopted to produce a wide range of nanostructures in addition to the core-shell magnetic nanoparticles.
We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain ...boundary sliding are supplanted by amorphization in a highly localized nanoscale band, which allows for massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are lack of dislocation slip systems, low thermal conduction, and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting-point elements: specifically, we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.
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