Search

Pressure-induced polymorphism has recently been demonstrated in several high entropy alloys. This offers a new window into the much-debated issue of phase selection and stability in these systems. Here, we examine the effect of cryogenic temperatures on the pressure-induced transition from face centered cubic to hexagonal close-packed structures of the prototype CoCrFeMnNi (Cantor) alloy. We observe a reduction in the critical pressure for the onset of the polymorphic transition as the temperature decreases, confirming the progressive stabilization of the hexagonal phase with decreasing temperature previously predicted by ab initio calculations accounting for magnetic interactions. We argue that in situ high-pressure experiments at cryogenic temperatures, which suppress time-dependent transformation triggered at higher temperatures, present a unique opportunity to significantly improve our understanding of these complex alloys. [ABSTRACT FROM AUTHOR]

As the dimensions of interconnects in integrated circuits continue to shrink, an urgent need arises to understand the physical mechanism associated with electromigration. Using x-ray nanodiffraction, we analyzed the stresses in Blech-structured pure Cu lines subjected to different electromigration conditions. The results suggest that the measured residual stresses in the early stages of electromigration are related to relaxation of stresses caused by thermal expansion mismatch, while a developing current-induced stress leads to reductions in the residual stress after longer test times. These findings not only validate the feasibility of measuring stress in copper lines using nanodiffraction but also highlight the need for a further understanding, particularly through in situ electromigration experiments with x-ray nanodiffraction analysis. [ABSTRACT FROM AUTHOR]

Forging on an industrial scale often involves slow, size-limited cooling rates or high temperature hold times between, or after, deformation. This enables the dynamic recrystallization (DRX) initiated during forging to further progress under static conditions, a phenomenon called meta-dynamic recrystallization (mDRX). As mDRX will influence the final grain size, and thus properties, it is critical to understand and control it during processing. Here, we study the mDRX evolution in Ni-based superalloy Haynes 282 during post-deformation hold times of up to 120 s at 1080 °C after partial DRX. We find that mDRX is the dominating mechanisms responsible for the microstructure evolution the hold time. The very rapid mDRX kinetics in the initial stages suggest that quench delays (the time between the end of the deformation and the onset of the quenching intended to arrest the microstructure evolution) must be kept well below 1 s in order to allow reliable conclusions to be drawn from post-deformation microstructure investigations. A larger prior strain (larger DRX fraction) leads to faster mDRX kinetics and a larger final grain size. Larger strains leads to earlier impingement of the growing grains, which, in combination with smaller remaining deformed regions into which the grains can grow, limits the maximum size of the mDRX grains. We also note a close correlation between static recovery and stress relaxation during the hold time, whereas no such correlation between mDRX and stress relaxation can be observed. [ABSTRACT FROM AUTHOR]

Mo(Si,Al)2 with different yttrium (Y) additions (up to 2 at.%) was synthesised by dry powder mixing followed by compaction and sintering. In as-sintered materials, Y was present as yttrium aluminium garnet. The materials were exposed in air at 1500 °C for up to 250 h to study the effect of Y on oxidation behaviour. The oxides formed were investigated using scanning electron microscopy (SEM)-based techniques and X-ray diffraction. While the Y-free Mo(Si,Al)2 formed a scale consisting of Al2O3 and a small amount of mullite, the Y-containing samples formed oxides containing both yttrium silicate and larger fractions of mullite, in addition to Al2O3. Oxidation rate, scale spallation, as well as the evaporation of Mo, all increased with Y addition. [ABSTRACT FROM AUTHOR]

Cooling-induced precipitation of the strengthening γ' phase is commonly investigated in Ni-base superalloys with a high γ' volume fraction, where it is used to control the final microstructure and properties. Less is known about the phase separation in low-volume-fraction alloys during cooling, although the microstructural state after cooling from solution treatment is known to affect subsequent heat-treatment steps. We use atomic-scale characterisation of Ni-base superalloy Haynes 282 (equilibrium γ' volume fraction around 20%) to show that air cooling after solution or carbide stabilisation results in precipitation of nm-sized γ' particles, whereas precipitation was suppressed during water quenching. The solution treatment has a significant effect on the hardness and γ' precipitation during air cooling from the subsequent carbide stabilisation temperature. Also, the carbide-stabilisation treatment itself affects the γ' precipitation during subsequent air cooling. [ABSTRACT FROM AUTHOR]

Discontinuous dynamic recrystallization can occur during dynamic tensile extrusion of copper, which is subjected to uniaxial tensile strains of ~5 and strain rates up to 10 s in the extruded section. Through high-resolution transmission Kikuchi diffraction, we show that nucleation occurs through subgrain rotation and grain boundary bulging at boundaries between 〈001〉 and 〈111〉 oriented grains. The observed nuclei consist of subgrains with a size of approximately 200 to 400 nm. [ABSTRACT FROM AUTHOR]

Thermomechanical processes, such as forging, are important steps during manufacturing of superalloy components. The microstructural development during processing, which controls the final component properties, is complex and depends on e.g., applied strain, strain rate and temperature. In this study, we investigate the effect of process parameters on the dynamic and post-dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. Specifically, we address the effect of deformation below the grain boundary carbide solvus temperature. During deformation, discontinuous and continuous dynamic recrystallization was observed at the grain boundaries, and particle-stimulated nucleation occurred at primary carbides. Strain rate was determined to be the governing factor controlling the recrystallization fraction for strain rates up to 0.5 s−1 above which adiabatic heating became the dominating factor. Careful examination of the temperature development during deformation showed that the response of the closed-loop temperature control system to adiabatic heating can have important effects on the interpretation of the observed behavior. During a 90 s post-deformation hold, grain growth and an increasing fraction of twin boundaries significantly changed the deformation-induced microstructure and texture. The microstructure developed during post-dynamic recrystallization was mainly controlled by the temperature and only weakly coupled to the prior deformation step. [ABSTRACT FROM AUTHOR]

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 γ′, 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 γ′. The coarser γ′ 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 γ′ distribution, and grain boundaries decorated by discrete carbides accompanied by γ′. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary γ′ and a bi-modal intragranular γ′ 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]