GND Density Research Articles

Understanding tool cutting-edge microstructure and deformation mechanism induced by adhesive wear in the turning of nickel-based superalloys

Nickel-based superalloy has great potential in the aerospace industry as key components. However, difficult-to-machine characteristics have further limited its application. Thermal-barrier coatings of the titanium-based family on carbide tools offer exceptional performance in cutting superalloys, combining high-temperature stability and remarkable toughness. This work primarily concentrates on understanding cutting-edge microstructure and deformation induced by tool adhesive wear in the turning of nickel-based superalloys with experiment and modelling. The dominant tool wear mechanisms are revealed to be adhesive wear and abrasive wear by SEM/EDS. The microstructure of the cutting-edge interface is qualitatively and quantitatively investigated by SEM/EDS and EBSD. The adhesive layer thickness at cutting edge is about 10–30 μm. The GND density of WC grains at cutting edge is 15-20 × 1014/m. The nanomechanics properties of the tool wear interface were quantitatively evaluated by nanoindentation. The average hardness of WC and Ni-superalloy at cutting-edge interface is evaluated to be 15–19 GPa and 6.2–6.5 GPa, respectively. Further, the underlying deformation mechanisms induced by tool wear behaviours are revealed through the transient heat conduction model and cutting-edge stress distribution model. This research offers a framework and mechanism for the cutting tool wear surface/interface characteristics targeted to the difficult-to-cut superalloy materials.

Multi-pass butt welding of thick TA5 titanium-alloy plates by MIG: Microstructure and properties

A 35 mm-thick TA5 titanium alloy plate was successfully joined by metal inert-gas (MIG) welding. The welded joints were analyzed in a more integrated and comprehensive manner. The joint microstructure was analyzed by OM, SEM, EBSD, and TEM. Mechanical properties were analyzed by tensile, impact, and microhardness tests. The corrosion resistance of the joints was determined by FeCl3 immersion corrosion. Experimental results showed that the mechanical properties and corrosion resistance of the fusion zone (FZ) were poorer than those of the base material (BM). The mechanical properties of the welded joints were related to the acicular grains in the FZ. The 260 A joints showed relatively good mechanical properties. With increased welding current, the proportion and size of the acicular grains increased, and the strength of the joint increased, but the plastic toughness decreased. When the welding current was too small, anisotropy occurred in the FZmiddle, which affected the mechanical properties of the joints. The corrosion resistance of the FZ was lower than that of the BM, and the corrosion resistance of the three groups of joints decreased in the order 300 A > 230 A > 260 A.The welding current affected the corrosion resistance of the FZ by influencing the grain size of the FZ, the frequency of the HAGBs, and the GND density.

Microstructure evolution and mechanical properties of new Co-free maraging steel produced by wire arc additive manufacturing

In this work, P-MIG is used as additive process, a new low-cost Co-free maraging steel wire is used as raw material to additive manufacturing block component, and the microstructure and mechanical properties of the component is investigated. The microstructure of the component is composed of martensite and austenite, and the closer to the bottom, the more serious the elemental segregation and the more austenite content. From the top to the bottom the high-angle grain boundaries increase and the grain size decrease, the difference of the heat dissipation condition and the thermal cycle are the reasons for this phenomenon. The texture of austenite is mainly Cube texture, and the texture of martensite is Rotated Cube texture and Goss texture. The tensile strength of XD, YD, ZD three directions is 1172 ± 15 MPa, 1021 ± 7 MPa, 1139 ± 16 MPa, respectively, impact toughness is 64.7 ± 1.6 J/cm2, 51.1 ± 5.6 J/cm2, 51.4 ± 3.1 J/cm2, respectively, TRIP effect occurs in the process of tensile, the main mode of fracture in tensile and impact is toughness fracture. The microhardness decreases gradually from bottom to top, which is due to the short-lived aging strengthening by thermal cycle, the change of grain size and GND density. The results of the work reveal the successful application of the new low-cost Co-free maraging steel wire in WAAM and provide an important reference for the adjustment of additive process, subsequent heat treatment and the adjustment of wire composition.

Investigation of size effect on flow stress of Cu-3wt.%Ag-0.5 wt%Zr thin sheets at room and cryogenic temperature based on geometrically necessary dislocation evolution behavior

The influence of the grain sizes ranging from 23.4 to 69.2 μm on mechanical properties of Cu-3wt.%Ag-0.5 wt%Zr thin sheets was investigated by conducting uniaxial tensile experiments at room temperature (RT 298 K) and cryogenic temperature (CT 77 K). At RT, the strength and ductility of the sheet decreased as the grain size increased, and the size effect on flow stress was observed when the ratio of thickness to grain size (t/d) was less than 4.5. At CT, the sheet exhibited excellent strength and ducility, accompanied by a reduction in the size effect on flow stress. GND distribution maps showed that the average GND density of grains in the sample surface (surface grains) was lower than that of grains in the core of the sample (core grains). As the distance from the grain boundary increased, the GND density decreased. A constitutive model was developed for Cu-3wt.%Ag-0.5 wt%Zr thin sheets based on the dislocation accumulation and recovery process, which was able to predict the effects of grain size and deformation temperature on the flow stress and GND evolution in Cu-3wt.%Ag-0.5 wt%Zr thin sheets. This constitutive model indicated that CT not only inhibited the GND recovery process but likewise promoted the GND accumulation process. The established constitutive model revealed that the ratio of SSD density to GND density increased with increasing strain. The total dislocation density and GND density of samples deformed at CT was higher than those of samples deformed at RT.

Microstructurally-sensitive fatigue crack nucleation in a Zircaloy-4 alloy

Microstructurally-sensitive fatigue crack nucleation and subsequently short crack growth were observed at an edge-notch in a textured Zircaloy-4 sample with the c-axis aligned perpendicular to the viewing surface. To understand the competition between the main crack and the secondary crack nucleation at the edge-notch root, a combined experimental and computational investigation was conducted to analyse various micro-scale mechanical quantities, including local strain, local stress, GND density, and stored energy density. A computational crystal plasticity finite element simulation that incorporated grain morphology, crystallographic orientation, and loading conditions was found to have good agreement with experimental results from the three-point bend fatigue test. The results showed that both the main crack and secondary crack nucleation sites had the highest magnitudes of local stored energy density, indicating that this factor was both necessary and sufficient for crack nucleation.

Effects of added tungsten on the microstructure and mechanical properties of laser-directed energy deposited Inconel 625 alloys

To satisfy the extreme operational requirements of Inconel 625 alloys, the development of novel approaches for improving their strength and compensating for the corresponding loss in toughness/plasticity is critical. This study employed laser-directed energy deposition to fabricate Inconel 625 alloys containing different amounts of tungsten (W) (6.0–16.5 wt%) and investigated the effects of added W (based on the quantity) on the microstructure and mechanical properties of the samples. The moderate addition of W reduced the size of γ-Ni grains and increased the GND density, which can improve the sample strength. Adding W can increase the density of nano-stacking faults (nano-SFs) and promote the Lomer-Cottrell (L-C) locks production, thus improving the work-hardening capacity and balancing the strength and plasticity of the alloys. As the amount of W increased from 0.0 to 16.5 wt%, the elongation of the samples increased and then decreased, whereas the strength increased monotonously. In the prepared samples, the one containing 6.0 wt% W exhibited the highest elongation (51.8 ± 3.2%), 1.38 times higher than the sample without W. Furthermore, the sample containing 16.5 wt% W exhibited the highest strength: 1167.2 ± 16.5 MPa, 289 MPa greater than that without W. The theoretical yield strengths of all samples were also calculated considering the strengthening mechanisms of the grain boundary, dislocation, solid solution, and precipitation; the results agreed with the experimental data.

Electropulsing treatment-induced the enhanced recrystallization of deformed Al0.1CoCrFeNi high-entropy alloy at lower temperature

It is urgent to explore an efficient treatment strategy to obtain characteristic microstructures for better mechanical properties of alloys in an era of energy shortage. Here, we investigated whether electropulsing treatment (EP-treatment) could efficiently manipulate the microstructural evolution of pre-deformed Al0.1CoCrFeNi HEA. Under the condition of EP-treatment, the temperature required for complete static recrystallization of Al0.1CoCrFeNi HEA is reached earlier at 590 °C. This temperature is significantly lower than the recrystallization temperature induced by conventional heat treatment. The preferred accumulation of dislocations at grain boundaries in EP-treated Al0.1CoCrFeNi HEA provides favorable conditions for the nucleation of static recrystallization. This is due to the increased vacancy concentration and defect mobility, which are activated by the additional driving force of the electromigration effect. Moreover, the enhanced dislocation mobility leads to an increased nucleation rate within a very short electrical pulse time, which consequently facilitates accelerated recrystallization at a lower temperature. In addition, the static recrystallization process was accompanied by the accelerated annihilation of dislocations and the decreased dislocation density, which is consistent with the results of the global GND density decreasing from 2.0 × 1013 m−2 to 1.6 × 1013 m−2.

Microstructure, deformation behaviors and GND density evolution of Ti-Al laminated composites under the incremental compression test

It is extremely difficult to study the deformation behaviors through the direct experimental observations. In this work, GND density was used to study the deformation process of composites under the incremental compression test. The SEM and EBSD techniques were used to investigate the microstructure, phase formation process and deformation process of Ti-Al laminated composites in detail. Results show that: during the deformation process, the GND density of the Al and Ti layers gradually increased, while the Al 3 Ti layer decreased first and then increased. And the Al 3 Ti layer was a coordinated deformation under the constraint of layered structure and then plastic deformation occurred. GND was uniformly distributed in the Al layer first and then concentrated at the Al/Al 3 Ti interface, while it was not uniformly distributed in the Ti layer. The formation process of the Al 3 Ti layer and the fracture surface morphology have also been studied.

An efficient way to induce recrystallization of deformed Al0.1CoCrFeNi high-entropy alloy

Direct-current treatment (DC-treatment) was employed to tune the microstructural evolution of deformed Al0.1CoCrFeNi high-entropy alloy (HEA) in this paper. The time that partial recrystallization required during the DC-treatment is far less than that of conventional annealing at similar temperatures. The observation of dislocations configuration and stacking faults confirmed that the accelerated effect of DC-treatment on the promoted recrystallization of deformed Al0.1CoCrFeNi HEA, especially the distinct role of the athermal effect. Besides, results of EBSD and GND density also evidence the promoted recrystallization and the corresponding dislocations distribution under DC-treatment.

Investigation of creep degradation in a Super304H austenitic steel and its nonlinear ultrasonic assessment method

ABSTRACT This paper reports the degradation behaviour of Super304H during the creep process. The failure mechanism of this Super304H is the typical intergranular damage, the creep voids and micro-crack are mainly concentrated on the GBs and triple point GBs. Meanwhile, there is a lot of precipitates inside void chain, those particles are the M23C6 phase, which indicate that the coarsening and growth of M23C6 phase are detrimental to the microstructural property of Super304H. Besides, the GND density of Super304H increased remarkably at the creep acceleration period, and the dislocation structure also changed from the pattern of straight-line to the zigzag. In addition, the nonlinear ultrasonic measurement results showed that there is a general upward trend of nonlinear parameter β’ of Super304H during creep. The increase of the nonlinearity effect in Super304H could be due to several factors such as the precipitates precipitation, dislocation density/structure change, and the microdefects formation.

Process and feedstock driven microstructure for laser powder bed fusion of 316L stainless steel

In the pursuit of improving additively manufactured (AM) component quality and reliability, fine-tuning critical process parameters such as laser power and scan speed is a great first step toward limiting defect formation and optimizing the microstructure. However, the synergistic effects between these process parameters, layer thickness, and feedstock attributes (e.g. powder size distribution) on part characteristics such as microstructure, density, hardness, and surface roughness are not as well-studied. In this work, we investigate 316L stainless steel density cubes built via laser powder bed fusion (L-PBF), emphasizing the significant microstructural changes that occur due to altering the volumetric energy density (VED) via laser power, scan speed, and layer thickness changes, coupled with different starting powder size distributions. This study demonstrates that there is not one ideal process set and powder size distribution for each machine. Instead, there are several combinations or feedstock/process parameter ‘recipes’ to achieve similar goals. This study also establishes that for equivalent VEDs, changing powder size can significantly alter part density, GND density, and hardness. Through proper parameter and feedstock control, part attributes such as density, grain size, texture, dislocation density, hardness, and surface roughness can be customized, thereby creating multiple high-performance regions in the AM process space.

Twin boundary fatigue crack nucleation in a polycrystalline Nickel superalloy containing non-metallic inclusions

Non-metallic inclusions and twin boundaries are preferred fatigue crack nucleation locations in polycrystalline Ni-based superalloys. A Heaviside HR-DIC, EBSD and multi-scale modelling strategy (CPFE-DDP) was used to assess experimental observations of fatigue crack nucleation near agglomerated inclusions in RR1000 at elevated temperature. Inclusion fracture and decohesion were observed within the first cycle of loading. Fatigue crack nucleation at the non-metallic inclusion coincided with that in an adjacent coarse grain containing a twin boundary. DDP modelling of the twin boundary showed the establishment of slip activation parallel as well as oblique to the interface, as observed in DIC characterisation. The DDP results showed pile-up driven generation of local GND density at the interface, in turn driving high local stored energy density and fatigue crack nucleation. These conditions were shown to result from the anisotropic elastic constraint at the twin boundary. In addition, the complex stress field arising from the agglomerate drives plasticity near the twin boundary contributing to the necessary conditions for fatigue crack nucleation.

Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

With decreasing system sizes, the mechanical properties and dominant deformation mechanisms of metals change. For larger scales, bulk behavior is observed that is characterized by a preservation and significant increase of dislocation content during deformation whereas at the submicron scale very localized dislocation activity as well as dislocation starvation is observed. In the transition regime it is not clear how the dislocation content is built up. This dislocation storage regime and its underlying physical mechanisms are still an open field of research. In this paper, the microstructure evolution of single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed under compression loading. Experimental in situ HR-EBSD measurements as well as 3d continuum dislocation dynamics simulations are presented. The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities. Results are presented that show, how the plastic deformation of the material takes place and how the different slip systems are involved. A central finding is, that an increasing amount of GND density is stored in the system during loading that is located dominantly on the slip systems that are not mainly responsible for the production of plastic slip. This might be a characteristic feature of the considered size regime that has direct impact on further dislocation network formation and the corresponding contribution to plastic hardening.

Assessment of the degradation in mild structural steel using electron backscatter diffraction (EBSD) technique

This paper reports the degradation assessment of mild steel during the plastic tensile process. The electron backscatter diffraction (EBSD) technique was adopted in this study. The orientation maps showed that with the increase of tensile strain, the grain surface become wrinkled, and the deviation level of intragranular orientation also increased. Meanwhile, the parameters based on the image quality of the Kikuchi bands (i.e. BC and MAD) as well as the crystallographic orientation (i.e. LAGBs content, GND density, GOS, and GROD) can be used to evaluate the degradation degree of the mild steel. The results showed that the change of BC and MAD was significant at the end of plastic stage, but was not sufficiently distinctive at the early stage; Meanwhile, the LAGBs content and GND density increased evidently during the plastic tensile. Compared with the former, the GND density exhibited stronger regularity and better evaluation effect; Besides, a general upward trend of GOS and GROD was observed at this tensile process. However, the GROD changed less at the certain plastic stage. Compared with GROD, the GOS exhibited a relatively better evaluation effect; To sum up, the GND density and GOS are the better indicators for evaluating the degradation degree of mild steel.

Influence of microstructural morphology on the continuous/discontinuous yielding behavior in a medium manganese steel

The continuous and discontinuous yielding behaviors are observed in the medium Mn steels with granular and lath-typed microstructures, respectively. The continuous yielding behavior is attributed to the unique lath-typed microstructure, in which the sub-division of unit cells by laths leads to a more homogeneous distribution of GNDs and thus a higher GND density.

Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy René88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

Creep degradation assessment of a Super304H austenitic steel by using misorientation‐derived parameters (electron backscatter diffraction)

AbstractThis paper reports the degradation assessment of Super304H during the creep by using misorientation‐derived parameters (EBSD). The results manifested that with increasing creep time, the change of misorientation parameters (i.e., Kernel Average Misorientation [KAM], grain reference orientation deviation [GROD], Grain Orientation Spread [GOS], and geometrically necessary dislocation [GND] density) of Super304H was significant. Meanwhile, the above parameters presented a non‐monotonic variation trend, which decreased slightly at first and then increased at the later. In addition, the increase amplitude of GROD and GOS was larger than the KAM and GND density, which manifest that the GROD and GOS are more suited to the creep degradation assessment of Super304H.

Effect of sample thinning on strains and lattice rotations measured from Transmission Kikuchi diffraction in the SEM

Cross correlation based high angular resolution EBSD (or HR-EBSD) has been developed for measurement of elastic strains, lattice rotations (and estimating GND density). Recent advances in Transmission Kikuchi diffraction (TKD), especially the on-axis geometry allows the possibility of acquiring patterns at higher spatial resolution. However, some controversy remains as to whether stresses/strains measured after the sample thinning process are still representative of the bulk sample. In this paper, we explore a way of applying the HR-EBSD method to study strains and lattice rotations in an initially bulk sample, that is then progressively thinned down until a similar analysis can be performed on thin (and electron transparent) samples. Thus, HR-TKD will be compared as a possible alternative to HR-EBSD, in scenarios when it is not always possible to perform EBSD on the surface of the sample. An estimate of strain relaxation in the sample as a result of sample thinning is presented.

Quantifying geometrically necessary dislocation density during hot deformation in AA6082 Al alloy

Physically based constitutive equations incorporating the key microstructural mechanisms e.g. dislocations, grain size, etc, have been used widely to predict the stress-strain behaviour of alloys at plastic and viscoplastic conditions. This enables an accurate prediction of the formed geometry as well as the final underlying microstructures. However, these physically based constitutive equations have not been practically validated due to the lack of systematic experimental data at microscopic scale. This leads to a large number of unknown constants required to be determined through various optimization algorithms. The aim of this paper is to provide direct and systematic experimental data by revealing the dislocation (geometrically necessary one) density and grain size evolution of AA6082, which is a widely used high-strength aluminium alloy for automobile structural panels, as functions of strain, strain rate and temperature, and is the first-time using Electron Back Scattering Diffraction (EBSD) technique to visualize the microstructures during the hot deformation. The evolution of the dislocation accumulation during the hot tensile deformation at 300, 450, 530 °C using various strain rates (i.e. 1/s, 0.1/s, 0.01/s) was achieved. EBSD maps were analysed on samples submitted to a true strain level of ~0.1 and ~0.3 under each condition. These maps cover >3000 grains and enable to capture the statistical nature of the geometrically necessary dislocation densities during hot deformation. Despite the rapidly plateaued flow stress curves at high temperatures, a continuously increased average GND density was observed in AA6082 with the imposed true strain levels under all conditions. Dislocation channel structures were observed in the hot deformed samples. Dynamic recrystallization was also observed, which coupled with the GNDs and affected the hardening behaviour of the flow stress-strain curves. This work is the first study, using EBSD, to visualize the high temperature and high strain rate induced dislocation distributions over a relatively large area, providing valuable data that may be used for subsequently improving and calibrating the physically based material models.

FFT-based simulations of slip and kink bands formation in 3D polycrystals: Influence of strain gradient crystal plasticity

The ability of softening strain gradient plasticity to simulate intragranular plastic slip localization modes called slip and kink bands, at is investigated at incipient plasticity. A strain gradient crystal plasticity model based on a quadratic energy contribution of the Nye tensor has been implemented within the massively parallel FFT-based solver AMITEX_FFTP. It is shown that a classical evaluation of the double curl operator does not properly account for the backstress induced by GNDs in slip and kink bands. Consequently, a better suited differentiation operator is proposed for this purpose. Besides, four practical implementations of interface conditions at grain boundaries are implemented and discussed. It is demonstrated that these conditions have a strong influence on GND induced hardening but weakly affect the formation of slip localization patterns.A theoretical and numerical study of an idealized kink band shows that this modeling framework, contrary to classical crystal plasticity models, is able to capture their essential physical features. A quantitative analysis of high resolution 3D polycrystalline simulations, based on imaged processing and a peak detection on plastic slip profiles, is applied to show that gradient effects strongly influence the mechanisms of selection between slip and kink banding, to the favor of the first. In particular, some kink bands are shown to decompose into an homogeneous distribution of parallel slip bands of finite length. Finally, the competition between structural effects and energetic cost of lattice curvature in the formation of localized slip bands networks in polycrystals is discussed. The present results demonstrate the relevance of softening crystal plasticity modeling including an energetic contribution of GND density to accurately capture slip localization in polycrystals.