The effect of heat treatment on the microstructure and mechanical properties of Ni-base superalloy Haynes 282 was investigated. Applying a standard two-step ageing (1010 °C/2 h + 788 °C/8 h) to the as-received, mill-annealed, material resulted in a the presence of discrete grain boundary carbides and finely dispersed intragranular g´, with an average size of 43 nm. This condition showed excellent room temperature strength and ductility. The introduction of an additional solution treatment at 1120 °C resulted in grain growth, interconnected grain boundary carbides and coarse (100 nm) intragranular g´. The coarser g´ led to a significant reduction in the strength level, and the interconnected carbides resulted in quasi-brittle fracture with a 50 % reduction in ductility. Reducing the temperature of the stabilization step to 996 °C during ageing of the mill-annealed material produced a bi-modal g´ distribution, and grain boundaries decorated by discrete carbides accompanied by g´. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary g´ and a bi-modal intragranular g´ distribution can be used to tailor the mechanical properties to suit specific applications. The yield strength of all three conditions could be accurately predicted by a unified precipitation strengthening model.
The formation mechanism and properties of white layers created during broaching are not well investigated and understood to date. In the present study, multiple advanced characterization techniques with a nano-scale resolution, including transmission electron microscope (TEM), transmission Kikuchi diffraction (TKD), atom probe tomography (APT) as well as nanoindentation, have been used to systematically examine the microstructural evolution and corresponding mechanical properties of a surface white layer formed when broaching the nickel-based superalloy Inconel 718. The TEM observations showed that the broached white layer consists of nano-sized grains, mostly in the range of 20 nm to 50 nm. The crystallographic texture detected by TKD further revealed that the refined microstructure is primarily attributed to strong shear deformation. Colocated Al-rich and Nb-rich fine clusters have been identified by APT, which are most likely to be γ′ and γ′′ clusters in a form of co-precipitates, where the clusters showed elongated and aligned appearance associated with the severe shearing history. The microstructural characteristics and crystallography of the broached white layer suggest that it was essentially formed by adiabatic shear localization in which the dominant metallurgical process is rotational dynamic recrystallization based on mechanically-driven subgrain rotations. The grain 2 refinement within the white layer led to an increase of the surface nano-hardness by 14% and a reduction in elastic modulus by nearly 10% compared to that of the bulk material. This is primarily due to the greatly increased volume fraction of grain boundaries when the grain size was reduced down to the nanoscale.
The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crack propagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometer scale and had been termed “dynamic embrittlement.” More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 µm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grain-boundary character. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys
Self-assembly due to phase separation within a miscibility gap is important in numerous material systems and applications. A system of particular interest is the binary alloy system Fe-Cr, since it is both a suitable model material and the base system for the stainless steel alloy category, suffering from low-temperature embrittlement due to phase separation. Structural characterization of the minute nano-scale concentration fluctuations during early phase separation has for a long time been considered a major challenge within material characterization. However, recent developments present new opportunities in this field. Here, we present an overview of the current capabilities and limitations of different techniques. A set of Fe-Cr alloys were investigated using small-angle neutron scattering (SANS), atom probe tomography, and analytical transmission electron microscopy. The complementarity of the characterization techniques is clear, and combinatorial studies can provide complete quantitative structure information during phase separation in Fe-Cr alloys. Furthermore, we argue that SANS provides a unique in-situ access to the nanostructure, and that direct comparisons between SANS and phase-field modeling, solving the non-linear Cahn Hilliard equation with proper physical input, should be pursued.
Chip formation during metal cutting involves high strain rates and large deformations. Under many conditions, the deformation is concentrated in narrow bands due to shear localisation from adiabatic heating. In order to understand the localisation process, it is necessary to increase the knowledge regarding the microstructural evolution during deformation. However, the deformation that occurs during chip formation is hard to measure. Therefore, this study utilises top-hat specimens deformed at high strain rates in order to generate localised shear bands in the Ni-based superalloy Alloy 718, with defined measurable deformation. The resulting shear bands in the top-hat specimens are compared with those generated in metal cutting chips and studied in order to characterise the deformation occurring at a microstructural level. The resulting microstructures in the top-hat specimens and machining chips are found to be similar, with heavily localised deformation into narrow shear bands and homogeneously sheared microstructure adjacent to the bands. The centres of the shear bands are heavily deformed with ultra-fine grains, indicating dynamic recrystallisation during the deformation. The results indicate that the shear deformation produced by high strain rate testing of top-hat specimens can provide an excellent means of replicating the conditions for shear localisation during metal cutting. However, care should be taken to design the tests so that the local conditions are representative in terms of strains and strain rates.
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