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/2h +788°C/8h) 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 43nm. 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 (100nm) 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.
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•Load distribution in a γ′ strengthened superalloy is measure by in-situ neutron diffraction.•The effect of particle size on Load distribution at cryogenic temperatures is ...studied.•Co-deformation of phases leads to intergranular load transfer between in matrix.•Coarse γ′ microstructure experience both intergranular and intragranular load transfer.•Coarse particles initially behave elastically but yield at higher stresses.
In-situ neutron diffraction was performed during tensile deformation of a Ni-based superalloy, Haynes 282, at 20, 100 and 300 K. Two distinct uni-modal microstructures with fine (20 nm) and coarse (200 nm) γ′ particles were investigated. On the macro-scale yield strength increased and ductility decreased with decreasing temperature, although the most significant decrease in ductility occurred between 100 and 20 K. The work hardening differed between the two microstructures, but was independent of temperature for each microstructure. On the micro-scale intergranular elastic interactions mainly lead to a transfer of the load to grains with the 200 orientation parallel to the tensile axis. No further load re-distribution between matrix and particles occurred in the microstructure with fine γ′, where shearing of precipitates lead to co-deformation at all temperatures. In the coarse γ′ microstructure, the load was transferred intragranularly from matrix to particles, in addition to the intergranular load transfer. The particles initially behaved elastically while the matrix deformed plastically, but at higher stresses a change in load partitioning indicated that also the γ′ phase underwent plastic deformation as a result of the stress build-up from the load partitioning. The tendency for, and effect of, plastic deformation of γ′ increased with decreasing temperature.
Whereas microstructure evolution in adiabatic shear bands have been thoroughly studied, reports on the stability of hardening precipitates during shear localisation are scarce. We report an atomic ...scale investigation of solute distribution in adiabatic shear bands in a precipitation strengthened Ni-Fe-based superalloy, showing that the hardening particles are completely dissolved. Temperature estimations indicate that peak temperatures in the shear band above the solvus limits of the precipitates are not unrealistic, and thus diffusion-assisted transformations during the severe plastic deformation cannot be ruled out.
Anisotropic 1-site and 2-site self-consistent models are developed to describe the elastic-viscoplastic behavior of polycrystalline materials deformed to finite strains on the basis of rate-dependent ...crystallographic slip and a generalized Hill-Hutchinson self-consistent approach. The choice of rate-dependent constitutive law at single crystal level implemented in the models is discussed through fitting experimental data and calibrating viscous parameters. It is found that drag-stress type Norton law works well for the 1-site elastic-viscoplastic self-consistent (EVPSC) model while threshold stress type Norton law is suitable for the 2-site EVPSC model to assure that the viscoplastic inter-granular interaction is realistic. Both models have been verified by thoroughly fitting experimental data in literatures. For the 1-site EVPSC model, selected experimental data covers both macroscopic and microscopic mechanical responses of steels during deformation with a large range of strain rate from the quasi-static (10−4s−1) to the dynamic (~104s−1). For the 2-site EVPSC model, in situ neutron diffraction data of nickel-based superalloys with various microstructures was fitted. Both models generally fit the experimental data well. A comparison between the EVPSC and elastic-plastic self-consistent (EPSC) models on the prediction of lattice strains has also been made for both the 1-site and 2-site cases, which verifies the predictability on lattice strains of the newly developed EVPSC models. A validation of the homogenization approach for the EVPSC modeling has been performed, which confirms that the proposed EVPSC models are applicable for cubic structure materials with finite deformations. Our formulation of EVPSC modeling developed in this work shines a spotlight on the way of developing a multi-functional self-consistent model to predict both macroscopic and microscopic deformation behaviors of various polycrystalline materials under different loading rates of 10−4s−1~104s−1.
•General finite strain elastic-viscoplastic self-consistent models are developed.•Predictions of the 1-site model fit the dynamic and static tests of steels well.•Predictions of the 2-site model adequately fit the experimental data of superalloys.•Simulated transverse lattice strains are sensitive to the time dependent response.•The homogenization approach for the EVPSC modeling is validated.
Electron beam freeform fabrication is a wire feed direct energy deposition additive manufacturing process, where the vacuum condition ensures excellent shielding against the atmosphere and enables ...processing of highly reactive materials. In this work, this technique is applied for the α + β-titanium alloy Ti-6Al-4V to determine suitable process parameter for robust building. The correlation between dimensions and the dilution of single beads based on selected process parameters, leads to an overlapping distance in the range of 70%-75% of the bead width, resulting in a multi-bead layer with a uniform height and with a linear build-up rate. Moreover, the stacking of layers with different numbers of tracks using an alternating symmetric welding sequence allows the manufacturing of simple structures like walls and blocks. Microscopy investigations reveal that the primary structure consists of epitaxial grown columnar prior β-grains, with some randomly scattered macro and micropores. The developed microstructure consists of a mixture of martensitic and finer α-lamellar structure with a moderate and uniform hardness of 334 HV, an ultimate tensile strength of 953 MPa and rather low fracture elongation of 4.5%. A subsequent stress relief heat treatment leads to a uniform hardness distribution and an extended fracture elongation of 9.5%, with a decrease of the ultimate strength to 881 MPa due to the fine α-lamellar structure produced during the heat treatment. Residual stresses measured by energy dispersive X-ray diffraction shows after deposition 200-450 MPa in tension in the longitudinal direction, while the stresses reach almost zero when the stress relief treatment is carried out.
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.
Understanding the microstructural behaviour of materials during thermomechanical processing is a vital step towards optimizing the mechanical properties. One important aspect during forming ...processes, such as forging, is dynamic recrystallization (DRX), which sets the starting microstructure for the subsequent manufacturing steps. Here we investigated the DRX behaviour of Ni-base superalloy Haynes 282 during hot compression with a strain rate of 0.05 s−1 at 1080 °C, with care taken to minimize the effects of meta-dynamic recrystallization (mDRX) and adiabatic heating. Small DRX grains could be observed already at ε = 0.1, i.e. before the peak strain εp = 0.15. The DRX process accelerated significantly above ε = 0.2, and the material was fully recrystallized at ε = 1.5. Up to ε = 0.8 DRX occurred through continuous nucleation of new grains, whereas above ε = 0.8 the number density of DRX grains decreased and the increase in recrystallized fraction was due to growth of existing grains. Contrary to common assumptions of DRX nuclei being essentially dislocation free, many of the DRX grains contained pronounced dislocation substructures, even at small strains where they are not expected to have undergone deformation.
•Dynamic recrystallization in a Ni-base superalloy was investigated.•Recrystallized grains existed at strains below the maximum stress.•The microstructure is fully recrystallized at a strain of 1.5.•Kinetics derived from microstructure and stress response agree well.
This study presents a unique melting strategy in electron beam-powder bed fusion of Alloy 718 to tailor the grain morphology from the typical columnar to equiaxed morphology. For this transition, a ...specific combination of certain process parameters, including low scanning speeds (400–800 mm/s), wide line offsets (300–500 μm) and a high number of line order (#10) was selected to control local solidification conditions in each melt pool during the process. In addition, secondary melting of each layer with a 90° rotation with respect to primary melting induced more vigorous motions within the melt pools and extensive changes in thermal gradient direction, facilitating grain morphology tailoring. Four different types of microstructures were classified according to the produced grain morphology depending on the overlap zone between two adjacent melt pools, i.e., fully-columnar (overlap above 40 %), fully-equiaxed (overlap below 15 %), mixed columnar-equiaxed grains, and hemispherical melt pools containing mixed columnar-equiaxed grains (overlap ~20–25 %). The typical texture was ; however, the texture was reduced significantly through the transition from the columnar to equiaxed grain morphology. Along with all four different microstructures, shrinkage defects and cracks were also identified which amount of them reduced by a reduction in areal energy input. The hardness was increased through the transition, which was linked to the growth of the γʺ precipitates and high grain boundary density in the fully-equiaxed grain morphology.
The use of hard X‐ray transmission nano‐ and microdiffraction to perform in situ stress and strain measurements during deformation has recently been demonstrated and used to investigate many thin ...film systems. Here a newly commissioned sample environment based on a commercially available nanoindenter is presented, which is available at the NanoMAX beamline at the MAX IV synchrotron. Using X‐ray nanoprobes of around 60–70 nm at 14–16 keV and a scanning step size of 100 nm, we map the strains, stresses, plastic deformation and fracture during nanoindentation of industrial coatings with thicknesses in the range of several micrometres, relatively strong texture and large grains. The successful measurements of such challenging samples illustrate broad applicability. The sample environment is openly accessible for NanoMAX beamline users through the MAX IV sample environment pool, and its capability can be further extended for specific purposes through additional available modules.
Scanning nanodiffraction mapping was used to map stresses, plastic deformation and cracking of hard coatings during nanoindentation.
A 2-site elastic-plastic self-consistent (EPSC) model is developed and implemented in order to account for crystallographic texture development and grain morphology evolution under strong ...correlations between neighbor grains of different phases, both in space and orientation. Predictions of the model adequately fit the published in situ neutron diffraction data for nickel-based superalloys at ambient and elevated temperatures, in which γ and γ' phases exhibit exact cube-cube orientation relationship. Comparison with 2-site model (small strain algorithm, non-rotation scheme) and 1-site model (finite strain algorithm, co-rotation scheme) has been made, and the result shows that the present 2-site model (finite strain algorithm, rotation scheme) leads to better predictions in lattice strain evolution where both rotation of crystal lattice and correlation between inclusions are accounted for, especially when the applied strain is larger than 0.02 for transverse direction and 0.05∼0.18 for axial direction for the materials studied in this work. Based on a systematic study on the effects of grain-grain interaction and total grain number on the homogenized results, we found that transverse lattice strains of γ (200) and/or γ' (100) are sensitive to the interplay between γ-γ' interaction and evolution of grain orientation distribution with deformation, while that of γ (220) and γ' (110) are sensitive to the initial crystallographic texture.
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