Fraction Of Grain Boundaries Research Articles

Solute segregation induced stabilizing and strengthening effects on Ni Σ3 [110](111) symmetrical tilt grain boundary in nickel-based superalloys

Grain boundary segregation engineering (GBSE) is a promising approach for accurately manipulating chemical composition, structure and properties of grain boundaries (GBs). With the aim of attenuating or even removing disadvantages of GBs in nickel-based superalloys, the GB segregation behaviors of 3d (Ti–Co), 4d (Zr–Rh) and 5d (Hf–Ir) transition-metal (TM) solutes X and their effects on the thermodynamic stability and fracture strength of Ni Σ3 [110](111) symmetrical tilt grain boundary were systematically investigated, using first-principles total-energy calculations based on density functional theory. We found that all TM solutes considered herein can segregate towards the Ni Σ3 [110](111) GB. For TM elements from same row of the periodic table, their segregation energies show a concave-up parabolic-like dependency on the atomic number of TM elements. The GB segregations of all considered solutes can lead to a reduction in GB energy and render the Ni GB more stable, and the GB energies of X-segregated GBs vary linearly with the GB segregation energies of solutes X. Except elements Zr, Hf and Rh, the GB segregations of the other 15 TM alloying elements increase the GB fracture strength. The Mn and Re segregations improve the GB fracture strength and thermodynamic stability to a maximum extent, respectively. The underlying mechanisms for segregation-induced GB stabilizing and strengthening effects were further revealed at the electronic level by analyzing the crystal orbital Hamiltonian populations and deformation charge densities. Our work could provide new insights into designing Ni-based superalloys with desired performances based on the concept of GBSE.

Analysis of the twin variant selection in polycrystalline cobalt

The samples of polycrystalline cobalt were strained in tension in the second stage of work hardening in order to activate twinning. The identification of twins and their orientation was analyzed using electron backscatter diffraction. The study of twin variant selection was carried out based on two parameters, namely the Schmid factor (SF) and the geometric compatibility factor (m'). The results indicate that the grains contained the $$\left\{ {10\overline{1}2} \right\}$$ extension twins in most cases while the $$\left\{ {11\overline{2}1} \right\}$$ extension twin mode was present in few cases, as well. In the grains containing only one twin variant, the SF analysis correctly predicted the activated twins in most cases. In the grains with variant pairs in para-position, nearly half of the studied twins had the highest SF of all possible variants, which means that they followed the Schmid law. On the other hand, for twin pairs in meta- or ortho-position, a different behavior was observed. Twins in these configurations were selected rather by the strain compatibility criterion than by the Schmid law, as SF values of the activated twins were significantly lower than those of the twins in para-position. Of these two configurations, the ortho-position one is highly preferred. The combined analysis using SF and m' parameter, predicted the occurrence of specific twin modes and variants only partially. A local analysis considering the effect of neighboring grains should be developed to account for the cases where the two parameters fail to explain the occurrence of particular twins, which seems to be related to the presence of a high fraction of high-angle grain boundaries.

Study on Fracture Mechanism and Microstructural Evolution in 780 MPa Grade FB Steel with a High Hole Expanding Ratio

FB780 steel with a high hole-expanding ratio, as ferrite–bainite dual-phase steel, is an indispensable material to manufacture different automobile parts. In the present study, uniaxial tension tests and metallographic analyses were conducted to reveal the fracture mechanism in FB780 steel. TiN particles ruptured and formed micron-sized crack sources due to stress concentrations, and voids were generated by dislocation pile-ups in Fe3C particles. Intragranular dislocations mainly accumulated around second-phase particles and grain boundaries. And these dislocations started to manifest intertwining characteristics with the prolonged deformation and formed serious stress concentrations, thus finally, turned into the crack sources. Hence, the aggregation and growth of crack sources and voids resulted in ductile fracture. In addition, the fraction of low-angle grain boundaries in ferrite grains increased with the prolonged deformation. Moreover, the γ fiber texture and the {001} texture in the tensile deformation area facilitated the propagation of cracks. Small-sized newborn grains appearing in the tensile fracture mainly accumulated at low-angle grain boundaries and facilitated center separation during fracture.

Unveiling the role of hydrogen on the creep behaviors of nanograined α-Fe via molecular dynamics simulations

Hydrogen embrittlement (HE) substantially deteriorates the mechanical properties of metals. The HE behavior of nanograined (NG) materials with a high fraction of grain boundaries (GBs) may significantly differ from those of their coarse-grained counterparts. Herein, molecular dynamics (MD) simulations were performed to investigate the HE behavior and mechanism of NG α-Fe under creep loading. The effects of temperature, sustained stress, and grain size on the creep mechanism was examined based on the Mukherjee-Bird-Dorn (MBD) equation. The deformation mechanisms were found to be highly dependent on temperature, applied stress, and grain size. Hydrogen charging was found to have an inhibitory effect on the GB-related deformation mechanism. As the grain size increased, the HE mechanism transitioned from H-induced inhibition of GB-related deformation to H-enhanced GB decohesion. The current results might provide theoretical guidance for designing NG structural materials with low HE sensitivity and better mechanical performance.

Superplasticity of the fine-grained friction stir processed Mg–3Gd–1Zn sheets

Friction stir processing (FSP) was conducted on the hot-rolled Mg–3Gd–1Zn (GZ31) sheets for 1 and 4 passes to create a fine-grained structure. The superplastic behavior of the upper surface layer in the center of the stirred zone (SZ) was evaluated by the localized shear punch test (SPT) in the temperature range of 673–748 K. Microstructural analysis showed that FSP transformed the coarse-grained structure of the hot-rolled alloy with an average grain size of 49.5 μm to the fine and equiaxed grains of 4.6 and 2.7 μm after 1 and 4 passes, respectively. The maximum strain rate sensitivity (SRS) indices (m-values) of 0.23, 0.31, and 0.48 were achieved for the hot-rolled, 1 pass, and 4 passes FSPed conditions at 698 K, respectively. The corresponding activation energies were found to be 321, 212, and 119 kJ mol−1 for these conditions. The shear stress-strain rate behavior of the alloy after 4 FSP passes conformed to regions I, II and III, which is characteristic of superplastic behavior. This was manifested by the high m-value of 0.48 and the activation energy of 119 kJ mol−1, which are indicative of superplastic behavior, caused by the grain boundary sliding (GBS) of the fine equiaxed grains having a large fraction of high angle grain boundaries (HAGBs) of about 87%.

Improving mechanical anisotropy and corrosion resistance of extruded AA7075 alloy by warm cross rolling and annealing

The anisotropy of mechanical properties and poor corrosion resistance are important problems for high strength aluminum plate. In this study, the combined process of hot extrusion and warm cross rolling was conducted on AA7075 alloy, and both the extruded and rolled plates were annealed at various temperatures. The effects of warm cross rolling and annealing on the microstructure, mechanical properties, and corrosion behavior were studied. The results indicated that both cross rolling and annealing weakened the intensity of main textures of brass, Dillamore, and S. Compared to the extruded plate, the cross rolled ones exhibited higher hardness, higher conductivity, and lower anisotropy of tensile strength. Both cross rolling and annealing made the tensile strength of a plate along different directions more uniform. Moreover, annealing greatly enhanced the corrosion resistance by consuming the distortion energy, reducing the number of crystalline defects and increasing the fraction of low-Σ coincident site lattice grain boundaries. After annealing, the rolled AA7075 plates exhibited better corrosion resistance than the extruded ones, since they possessed lower fractions of brass texture, finer grains, and higher fraction of low-Σ coincident site lattice grain boundaries.

Optimization of the Post-Process Heat Treatment of Inconel 718 Superalloy Fabricated by Laser Powder Bed Fusion Process

In the present study, multi-objective optimization is employed to develop the optimum heat treatments that can achieve both high-mechanical performance and non-distinctive crystallographic texture of 3D printed Inconel 718 (IN718) fabricated by laser powder bed fusion (LPBF). Heat treatments including homogenization at different soaking times (2, 2.5, 3, 3.5 and 4 h) at 1080 °C, followed by a 1 h solution treatment at 980 °C and the standard aging have been employed. 2.5 h is found to be the homogenization treatment threshold after which there is a depletion of hardening precipitate constituents (Nb and Ti) from the γ-matrix. However, a significant number of columnar grains with a high fraction (37.8%) of low-angle grain boundaries (LAGBs) have still been retained after the 2.5 h homogenization treatment. After a 4 h homogenization treatment, a fully recrystallized IN718 with a high fraction of annealing twins (87.1%) is obtained. 2.5 and 4 h homogenization treatments result in tensile properties exceeding those of the wrought IN718 at both RT and 650 °C. However, considering the texture requirements, it is found that the 4 h homogenization treatment offers the optimum treatment, which can be used to produce IN718 components offering a balanced combination of high mechanical properties and adequate microstructural isotropy.

Effect of Surface Ultrasonic Rolling on Evolution of Surface Microstructure of EA4T Axle Steel

The effect of surface ultrasonic rolling (SUR) on the evolution of surface microstructure of EA4T axle steel is studied by using several microanalysis techniques. Fatigue life of samples after SUR is measured by means of rotary bending fatigue test. After SUR, the surface roughness of the sample is reduced, and the grain size in surface microstructure of the sample is obviously refined. With the increase in SUR passes, the fraction of high-angle grain boundary is increased and the size of ferrite grains is decreased. After SUR with 50 passes, the size of ferrite grains at sample surface is about 100 nm. The refinement mechanism of ferrite grains includes four steps: First, a large amount of dislocations is formed in ferrite. With the increase in strain, the tangled dislocations change into dislocation cells. As the strain continued to increase, the dislocation cells transform into low-angle grain boundary. Finally, the low-angle grain boundary gradually changes to high grain boundary to contribute to the refinement of ferrite grains. The fatigue life of the sample tested at 500 MPa after SUR with one pass is about 23 times higher than that of the sample after turning. However, after SUR with 10, 30 and 50 passes, the excessive plastic deformation reaches ductility limit to cause the surface damage during SUR. The surface damage, as the fatigue cracks initiation sites, can reduce fatigue life.

Analysis of Strain Partitioning in Intercritically Deformed Microstructures via Interrupted Tensile Tests

Intercritically deformed steels present combinations of different types of ferrite, such as deformed ferrite (DF) and non-deformed ferrite (NDF) grains, which are transformed during the final deformation passes and final cooling step. Recently, a grain identification and correlation technique based on EBSD has been employed together with a discretization methodology, enabling a distinction to be drawn between different ferrite populations (NDF and DF grains). This paper presents a combination of interrupted tensile tests with crystallographic characterization performed by means of Electron Backscatter Diffraction (EBSD), by analyzing the evolution of an intercritically deformed micro-alloyed steel. In addition to this, and using the nanoindentation technique, both ferrite families were characterized micromechanically and the nanohardness was quantified for each population. NDF grains are softer than DF ones, which is related to the presence of a lower fraction of low-angle grain boundaries. The interrupted tensile tests show the different behavior of low- and high-angle grain boundary evolution as well as the strain partitioning in each ferrite family. NDF population accommodates most of the deformation at initial strain intervals, since strain reaches 10%. For higher strains, NDF and DF grains behave similarly to the strain applied.

Microstructure and mechanical properties in core of a carburizing 20CrNi2MoV bearing steel subjected to cryogenic treatment

Microstructure and mechanical properties in core of a carburizing 20CrNi2MoV bearing steel subjected to cryogenic treatment were investigated. Conventional treatment sample was quenched and tempered at 180 °C for 2 h. Cryogenic treatment samples were quenched, cryogenically treated at − 80 and − 196 °C for 4 h, slowly returned to room temperature and thereafter tempered at 180 °C for 2 h, and finally tempered at 180 °C for 2 h. The scanning electron microscope, electron backscattering diffraction, X-ray diffraction and transmission electron microscope were adopted for microstructure characterization. The results show that cryogenic treatment increases the fraction of high-angle grain boundaries and the precipitation of finely dispersed carbides in the matrix, decreases the volume fraction of inter-lath retained austenite, and hence improves the strength and hardness. Compared with the conventional treatment, the hardness, yield strength and ultimate tensile strength of the steel after cryogenic treatment are increased by 11.7%, 12.6% and 18.3%, respectively, while the impact energy is decreased by 9.8%.

Material flow behavior and microstructural evolution during refill friction stir spot welding of alclad 2A12-T4 aluminum alloy

In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of al-clad 2A12-T4 aluminum alloy during refill friction stir spot welding. There are two material flow components, i.e., the inward- or outward-directed spiral flow on the horizontal plane and the upward- or downward-directed flow on the vertical plane. In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer. In the refill stage, this is process reversed. As such, there is no obvious vertical plasticized metal flow between adjacent layers. Welding leads to the coarsening of S (Al2CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone. Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.

Factors Affecting Toughness and Corrosion Resistance of AA6082 Forgings

AA6082 extruded bars were annealed and then hot-deformed at 693 K, cold-deformed at room temperature (RF), or treated at sub-zero temperatures (CF). After the solution and the aging treatment, the mechanical properties of the samples were tested. The experimental results demonstrate that the hot-deformed samples were stronger but less elongated than the RF and CF samples. All hot-deformed (HF), RF, and CF samples had mechanical properties that exceeded UTS > 310, YS > 260 MPa and an elongation > 10%. The RF and CF samples greatly enhance elongation (up to 16%) and toughness in addition to moderate strength (UTS ≥ 320 and YS ≥ 295 MPa). Affected by having high fractions of high misorientation angle grain boundaries (MAGBs) per unit area in the matrix, the RF and CF samples deteriorated their corrosion resistance by degrading Ecorr values. The surface of corroded HF50 was recorded to confirm that the particles were attacked by chloride and oxidized. Increasing the total lengths of high-MAGBs and particle counts per unit area accelerated corrosion to lift up Icorr values.

EBSD studies on cryorolled LM6 Al alloy

In present work, EBSD and mechanical properties were studied for cryorolled LM6 Al alloy. Solution treated (ST) alloy was cryorolled for dissimilar thickness reduction upto 50%, 75% and 90%. To analyze the effect of CR on microstructure of LM6 alloy, EBSD and TEM microscopy techniques were made in practiced and its mechanical properties are analyzed by using tensile test and hardness. The CR LM6 Al alloy manifest an enhancement in mechanical properties on comparison with ST LM6 Al alloy. Results revealed that fraction of low angle grain boundaries and kernel average misorientation increases with increase in percentage thickness reduction. CR processed LM6 Al alloy shows the improved mechanical properties among all sample conditions substantiating the effect of grain refinement.

Microstructure Characteristics and Corrosion Resistance of Friction Stir Welded 2205 Duplex Stainless Steel

In this study, 2205 duplex stainless steel was friction‐stir‐welded at different welding speeds. The microstructural characteristics such as grain sizes, grain boundary misorientation angles, and phase contents in the stir zones of the joints were detected. The potentiodynamic polarization and electrochemical impedance spectroscopy were also measured to evaluate the corrosion property of the stir zones. The effects of the microstructures on the corrosion property for friction‐stir‐welded 2205 duplex stainless steel were analyzed. The results indicated that the FSW process refined the grains and increased the ferrite contents in the stir zones. With increasing the welding speed from 30 mm/min to 50 mm/min, the grain size in the stir zone decreased from 1.64 μm to 0.96 μm, the ferrite content decreased from 59% to 54.4%, and the fractions of low angle grain boundaries for austenite and ferrite phases decreased from 63.9% and 92.6% to 18.0% and 41.1%, respectively. In this study, the effect of grain size on the corrosion resistance was the most significant and it was followed by the phase composition and the grain boundary misorientation angle. The stir zone obtained at 50 mm/min has the highest corrosion resistance, and it is followed by the base material and the stir zone obtained at 30 mm/min.

Exploring the Strain Hardening Mechanisms of Ultrafine Grained Nickel Processed by Spark Plasma Sintering

Ultrafine grained (UFG) materials in the bigger grain size range (0.5–1) µm display a good combination of strength and ductility, unlike smaller size UFG and nanostructured metals, which usually exhibit high strength but low ductility. Such difference can be attributed to a change in plasticity mechanisms that modifies their strain hardening capability. The purpose of this work is to investigate the work hardening mechanisms of UFG nickel considering samples with grain sizes ranging from 0.82 to 25 µm. Specimens processed combining ball milling and spark plasma sintering were subjected to monotonous tensile testing up to fracture. Then, microstructural observations of the deformed state of the samples were carried out by electron backscattered diffraction and transmission electron microscopy. A lower strain hardening capability is observed with decreasing grain size. Samples in the submicrometric range display the three characteristic stages of strain hardening with a short second stage and the third stage beginning soon after yielding. Microstructural observations display a low fraction of low angle grain boundaries and dislocation density for the sample with d = 0.82 µm, suggesting changes in plasticity mechanisms early in the UFG range.

Crystal cracking of grain-gradient aluminum by a combined CPFEM-CZM method

Gradient nanostructured metals attract extensive attention due to their excellent mechanical properties, while they are prone to microcracks during forming and servicing process and eventually develop into a sudden fracture. Actually, experimental studies show that microcracks occurred in both the grain boundary (GB) and the intragrain, but most of the current reports only focus on the GB cracking, very limited work has studied the intragranular cracking. In this paper, the crystal plasticity finite element method (CPFEM) and cohesive zone model (CZM) are combined to study the cracking mechanism of polycrystalline aluminum (Al) with different grain-gradient structures under tensile load, where molecular dynamics (MD) method is used to determine the cohesive parameters of the intragrain and GB. The results not only show the crack initiation and propagation process of polycrystalline Al with different grain-gradient structures, but also reveal the mechanism of intragrain fracture, GB fracture and crack transgranular. The grain-gradient distribution with optimal comprehensive performance is obtained. Moreover, it is also found that the initial microcracks with different positions, numbers and angles have a great influence on the cracking mechanism and effective properties of the whole material. This study provides a solid theoretical basis for improving the quality and operation life of metal parts.

The effect of cold drawing process on the microstructure evolution of Ti-4.5Al-2.5Fe-0.15O titanium alloy

The novel Ti-4.5Al-2.5Fe-0.15O alloy was subjected to a cold drawing process. The initial microstructure shows a fully equiaxed microstructure with the domination of α and minor intergranular β phase that could sustain a deformation up to 51% area reduction. Microstructure observation showed a deformed structure of all cold-drawn specimens by the presence of a high fraction of low-angle grain boundaries and kernel average misorientation angle. Higher dislocation density was discovered in the β phase than in the α phase, with steadily decreasing discrepancies as higher drawing reduction was applied. This study systematically observed the microstructure changes especially in the behavior of α and β phase after cold drawing, which consequently could affect the properties of Ti-4.5Al-2.5Fe-0.15O alloy.

Effects of γʹ size and carbide distribution on fatigue crack growth mechanisms at 650 °C in an advanced Ni-based superalloy

Two distinct and different unimodal γʹ size distributions have been produced in a disc alloy RR1000 to understand the combined effects of γʹ size (linked to slip character), grain boundary (GB) precipitates/carbides and dwell time on fatigue crack growth (FCG) mechanisms at elevated temperature. The FCG behaviour has been investigated on single edge notched bend samples of both γʹ variants at 650 °C under trapezoidal waveform loading with dwell times of 1 s and 90 s at maximum load. The fine γʹ variant has better FCG resistance for both dwell times, and the difference in FCG rate becomes even more marked with longer dwell time. The coarse γʹ variant exhibits faster and more time dependent FCG behaviour, with more evident intergranular fracture modes and rougher fracture surface than the fine γʹ variant. Intergranular failure modes become dominant with the 90 s dwell time in both variants. A more continuous distribution of carbides is seen on the GB in the coarse γʹ variant, which is likely to influence oxidation behaviour on GB and intergranular fracture behaviour. The effects of dwell time and consequent oxidation damage at the crack tip on fatigue behaviour are further investigated by transitioning between cyclic and time dependent regimes to assess the damage zones formed. The results show that an oxidation damage zone contributes to enhanced FCG after switching frequency from cyclic to time dependent regimes while a tortuous crack path is related to crack wake (shielding due to prior crack path tortuosity), resulting in improved FCG resistance.

Improved Predictability of Microstructure Evolution during Hot Deformation of Titanium Alloys.

Two different mesoscale models based on dislocation reactions are developed and applied to predict both the flow stress and the microstructure evolution during the hot deformation of titanium alloys. Three distinct populations of dislocations, named mobile, immobile, and wall dislocations, describe the microstructure, together with the crystal misorientation and the densities of boundaries. A simple model consisting of production and recovery terms for the evolution of dislocations is compared with a comprehensive model that describes the reactions between different type of dislocations. Constitutive equations connect the microstructure evolution with the flow stresses. Both models consider the formation of a high angle grain boundary by continuous dynamic recrystallization due to progressive lattice rotation. The wall dislocation density evolution is calculated as a result of the subgrain size and boundary misorientation distribution evolutions. The developed models are applied to two near-β titanium alloys, Ti-5553 and Ti-17, and validated for use in hot compression experiments. The differences in the predictability between the developed models are discussed for the flow stress, dislocation densities and microstructure evolutions. Only the comprehensive model can predict the different reactions and their contributions to the evolution of mobile and immobile dislocation densities. The comprehensive model also allows for correlating the elastic strain rate with the softening and hardening kinetics. Despite those differences, the selection of the model used has a small influence on the overall prediction of the subgrain size and the fraction of high angle grain boundaries.

Additive manufacturing of isotropic-grained, high-strength and high-ductility copper alloys

This work reports additive manufacturing of nickel-aluminum-bronze alloys by electron beam powder bed fusion (EB-PBF), with a relatively homogeneous microstructure at the micro- to meso-scale and exhibiting an exceptional combination of tensile strength and ductility — well surpassing that of conventionally wrought counterparts. The attainment of both high yield and ultimate tensile strength are attributed to collective effects of uniformly distributed precipitates, near-equi-axed grains, and a high fraction of high-angle grain boundaries. High tensile ductility originates from a sustainable work-hardening ability via dislocation-dislocation and dislocation-precipitate interactions, enhanced by moderate twinning, stacking faults, and back-stress hardening. Significantly, the EB-PBF alloys exhibit near-isotropic tensile properties in directions parallel and vertical to the build direction due to high-density precipitates enabled by equi-axed grains.