Fraction Of Grain Boundaries Research Articles

Tensile Test Coupled with an EBSD Study of a GH4169 Ring Rolled Product.

An in situ tensile test of the ring-rolled GH4169 alloy is performed to investigate the plastic deformation behavior at the micro level. Slip system activations are identified by slip traces captured by a scanning electron microscope and lattice orientation data acquired by electron backscattered diffraction. Our results demonstrated that the fraction of low-angle grain boundaries gradually increased upon tensile deformation, and the misorientation evolution in the grain interior was severely inhomogeneous. The Schmid factors at the grains of interest are calculated for comparison with the actual activated slip systems. Most of the slip system activation coincides with the Schmid law, as opposed to the initiation of other potential slip systems at some grains.

Effect of electrodeposition temperature on texture, strain, grain boundary constitution and corrosion behavior of Ni-P coatings with low phosphorous content

Nickel-phosphorus (Ni-P) alloy coatings were electrodeposited over mild steel substrate from a constant current DC power source. Effect of deposition bath temperature (15 ˚C, 20 ˚C, 25 ˚C, 35 ˚C) on the coating corrosion behavior in 3.5 wt.% NaCl solution was investigated. Corrosion analysis was conducted using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The 15 ˚C coating showed lowest polarization resistance (Rp = 2818.52 Ω.cm2) value indicating degraded corrosion resistance, whereas the 25 ˚C coating showed highest polarization resistance (Rp = 6992.285 Ω.cm2) value implying highest corrosion resistance performance amongst all coatings. This was also confirmed by the potentiodynamic polarization measurement where the 15 °C coatings and 25 °C coating showed respectively the highest and lowest corrosion current density (Icorr) values. Icorr values of 15 °C and 25 °C coating was 7.79 μA.cm−2 and 1.534 μA.cm−2, respectively. Analysis of the coating texture, strain and grain boundary constitution by the electron back scatter diffraction (EBSD) technique showed that the predominance of (101) high energy texture, lower fraction of low angle grain boundaries (LAGBs), and higher coating strain yielded high vulnerability to the corrosive attack for 15 ˚C coating, while (001) low energy growth texture with a relatively higher fraction of LAGBs and lower coating strain led to improved corrosion resistance in 25 °C coating.

The Nonlinear Diffusion Model of Recrystallization

Abstract The diffusion equation with a jump change in diffusion coefficient depending on the diffusant concentration is considered. The phase transition problem with moving boundary to describe the features of activated recrystallization is formulated. An analytical description of the motion of the activated recrystallization front in the presence of a thin coating, which causes changes in the microstructure and physical properties of polycrystalline metals, is derived. The nonlinear equation, the solution of which describes the motion of activated recrystallization front, is found. It is shown that the dependence of the depth of the recrystallized layer is determined by such structural factors as the average size of recrystallized grains, the fraction of stationary grain boundaries, and jump in the average concentration of impurities in the zone of the front of activated recrystallization. The physical interpretation of coefficient of the Stefan condition at the moving boundary is given.

Creep deformation phenomena in near-surface carburised layers of 316H stainless steels

The local creep-induced phenomena occurring within the near-surface carburised layer in two pre-carburised type 316H austenitic stainless steels, containing 0.98 or 1.52 wt% Mn, was assessed at 550 °C. The pre-carburisation exposed both steel grades at either 550 °C (8000 h) or 600 °C (3000 h) to simulated CO2-rich gas environment of UK Advanced Gas-cooled Reactors. The local properties within the carburised layer causes a reduction in minimum creep rate as compared to the non-carburised condition. In both steels carburised at 600 °C, ferrite is observed to form in the most heavily carburised regions, including slip bands, during the initial plastic deformation to the target applied stress. The ferrite area fraction further increases with creep strain. Within the ferritic areas, new fine grains are observed in the creep tested specimens and the fraction of high angle grain boundaries is >60%. Those fine grains have a lower degree of lattice strain compared to the surrounding ferrite area and especially to the austenite matrix. The lattice strain localises within the carburised layer, and the formation of ferrite from the strained austenite, and potentially a dynamic recrystallization within the ferrite regions, pinpoints to a dominant strain relief-driven mechanism. Lattice strain localises primarily in the pre-carburised austenite grains and its carbide-decorated boundaries, and induces the continuous formation and refinement of ferrite grains, and the development of intergranular cracks that evolve further into the bulk structure. These results emphasise the necessity to account for environmental-creep interactions at elevated temperatures in structural assessment procedures, in the current gas-cooled fission reactor fleet and beyond into (very) high-temperature Gen IV and advanced modular reactor designs.

Micro-mechanical investigation about degradation of grain boundary cohesive strength of neutron-irradiated stainless steels

ABSTRACT Degradation of grain boundary (GB) cohesive strength due to neutron irradiation is a possible factor causing IASCC. In order to clarify the dose dependence and degradation factors of GB cohesive strength of neutron-irradiated stainless steels, micro-tensile testing and microstructural analysis by transmission electron microscopy were conducted on individual GBs of neutron-irradiated stainless steels that were harvested from flux thimble tubes (FTTs) irradiated up to 73 dpa and a baffle former bolt (BFB) irradiated to 19 dpa. The micro-tensile specimens fabricated from the FTTs that included a GB showed higher intergranular fracture susceptibility at higher doses, while the change in susceptibility was small at intermediate doses. On the other hand, the micro-tensile specimen that was fabricated from the BFB had less intergranular fracture susceptibility than the FTT specimen of a similar dose. Fine cavities were confirmed both in matrix and on GBs of the FTTs, whereas they were not found on GBs of the BFB. Considering the present results and those from a previous study investigating effects of helium on GB fracture, it was concluded that the GB cavities degraded GB cohesive strength. However, other factors were possibly promoting intergranular fracture of neutron-irradiated stainless steels such as GB segregation.

Effect of Al–5Ti–1B grain refiner on microstructure and properties of arc-additive-manufactured Al–Mg alloy

In this study, a strip-shaped Al–5Ti–1B grain refiner was employed to produce Al–Mg alloy using cold metal transfer arc additive manufacturing. The microstructure and mechanical properties of the straight-wall samples (SWSs) were analyzed. The results indicated that the Al3Ti phase occurred in SWS with Al–5Ti–1B, and acted as heterogeneous nucleation sites during solidification. Formation of Al3Ti particles led to the transformation of the columnar grains between the layers into equiaxed grains and refined the grains at the center of the layers. Furthermore, the Al3Ti particles increased the fraction of low-angle grain boundaries by 8.2%. In addition, the anisotropy in mechanical properties (i.e. tensile strength and elongation) was reduced significantly. Also, the low hardness zone between the layers was eliminated. Thereby, improving the mechanical properties of the additive-manufactured Al–Mg alloy samples.

Effects of V–N Microalloying on Microstructure and Property in the Welding Heat Affected Zone of Constructional Steel

Shielded metal arc welding and welding thermal simulation experiment were carried out for constructional steel containing 0% V and 0.10% V, and the microstructure, precipitation feature, microhardness HV0.2, and −20 °C impact value in the welding heat affected zone (HAZ) were investigated. The results showed that in the coarse-grained heat affected zone (CGHAZ), V and N were completely dissolved in the matrix of steel containing 0.10% V to promote the growth of prior austenite grains, meanwhile the fraction of high angle grain boundaries (HAGBs) decreased, thereby leading to the mean −20 °C impact value decreases from 87 J to 18 J. In the grain refined heat affected zone (GRHAZ), V(C, N) precipitates experience re-dissolution and re-precipitation at grain boundaries, V–N microalloying changes the microstructure from lath bainite + granular bainite + small amount of polygonal ferrite to polygonal ferrite + pearlite + martensite, thereby leading to the mean microhardness decreases from 335 HV0.2 to 207 HV0.2, and the mean −20 °C impact value decreased from 117 J to 103 J. In the intercritical heat affected zone (ICHAZ), V(C, N) precipitates experience re-dissolution, re-precipitation, and growth, causing the formation of micro-sized V(C, N) precipitates, thereby leading to the mean −20 °C impact value decreases from 93 J to 62 J.

Effect of microstructure on the tensile properties and impact toughness of TC21G titanium alloy

Based on TC21 alloy, the modified TC21G alloy with excellent comprehensive mechanical properties was designed. The influences of the double annealing process on microstructure and mechanical properties of TC21G alloy were investigated. The quantitative relationship between microstructure parameters, strength and impact toughness was established. Microstructural evolution with different double annealing conditions was characterized using SEM, TEM and EBSD. Results show that with reducing the cooling rate of first annealing, the size and content of the equiaxed and lamellar α phase both increased, and the width of the secondary α phase was increased, whereas the content of the fine acicular secondary α phase was decreased. Meanwhile, the fraction of high angle grain boundaries increased with reducing the cooling rate. The fine acicular secondary α phase was beneficial to improve the strength of TC21G alloy. Microstructural parameters of the equiaxed and lamellar α phase play a dominant role in improving ductility with reducing the cooling rate. Moreover, the crisscross lamellar and equiaxed α phase, the fraction of high angle grain boundaries are the main factors affecting impact toughness.

Selective laser melting of bulk immiscible alloy with enhanced strength: Heterogeneous microstructure and deformation mechanisms

To overcome the dimension limits of immiscible alloys produced by traditional techniques and enhance their mechanical properties, bulk Cu-Fe-based immiscible alloy with abundant nanotwins and stacking faults was successfully produced by selective laser melting (SLM). The SLM-produced bulk immiscible alloy displays a heterogeneous microstructure characterized by micro-scaled γ-Fe particles dispersed in fine ε-Cu matrix with a high fraction (∼92%) of high-angle grain boundaries. Interestingly, abundant nanotwins and stacking faults are generated in the interior of nano-scaled γ-Fe particles embedded within ε-Cu matrix. The heterogeneous interface of soft domains (ε-Cu) and hard domains (γ-Fe) not only induces the geometrically necessary dislocations (GNDs) but also affects the dislocation propagation during plastic deformation. Therefore, the bimodal heterogeneous interface, and the resistance of nanotwins and stacking faults to the propagation of partial dislocation make the bulk immiscible alloy exhibit an enhanced strength of ∼590 MPa and a good ductility of ∼8.9%.

Microstructure and mechanical properties of Al-Mg-Si alloy during solution heat treatment and forging integrated forming process

The integrated forming process of an Al-Mg-Si alloy was designed, which combined solution heat treatment (SHT) and hot forging into one operation followed by artificial aging. The effect of deformation strains on the microstructure and mechanical properties was investigated by electron backscatter diffraction (EBSD), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. The results show that the tensile strength and yield strength of the compressed specimens were increased by at least 12.9% and 13.8%, respectively, when compared to the undeformed T6 specimen. {001}〈110〉 and {011}〈110〉 texture components were produced at low strains and gradually vanished as strain increased, in contrasting to the T6 specimen, which produced strong recrystallized texture components. As a result of the dynamic recovery (DRV) mechanism, intense {112}〈110〉 and {111}〈110〉 texture components were obtained in the integrally processed specimens at high strains along with a high fraction of low angle grain boundaries (LAGBs). The dispersed Al12(Fe,Mn)3Si phases aided in the formation of cellular structures and facilitated in the development of the DRV. The dislocations significantly accelerated the precipitation of β'' phases, which conversely pined the movement of dislocations and improved the mechanical properties of the Al-Mg-Si alloy.

Zirconium-enhanced segregation tendency of solutes X and Zr-X co-segregation induced synergistic/antagonistic effects on Ni Σ5 [001](210) grain boundary

Manipulating grain boundary (GB) segregation of alloying elements has been established as an important pathway to optimize structural materials, due to the GB segregation induced changes in material properties, whereas solute interaction effects on GB segregation and properties remain far less developed. Here, the effects of solute interactions of Zr with X (X = Zr, Re, Ta and Cu) on the segregation behavior of solutes X at the Σ5 [001](210) GB of nickel-based superalloys, GB excess energy and fracture strength were investigated detailedly from first-principles calculations. It was found that the pre-segregation of Zr at the Ni GB not only enhances the segregation ability of subsequent solutes Zr, Ta and Cu, but also induces a GB segregation of Re that prefers grain interiors originally. Zr-Zr/Ta, Zr-Re and Zr-Cu co-segregations have synergistic potentiation, antagonistic and synergistic additive effects on improving the considered GB properties, respectively. Relative to the unsegregated GB, the Zr-Zr/Ta/Re co-segregations could improve the GB stability and fracture strength simultaneously, while the Zr-Cu co-segregation could only improve the GB stability. The co-segregation induced GB stabilizing/destabilizing and strengthening/weakening mechanisms were probed. These results would provide theoretical supports for the GB segregation engineering and quantitative composition design of high-performance nickel-based superalloys.

Spray-formed commercial aluminum alloy anodes with suppressed self-corrosion for Al-Air batteries

Commercial 7050 Al alloys produced by spray forming technique and subsequent heat treatment have been investigated as anodes for Al-air batteries with greatly suppressed self-corrosion. The microstructures of the prepared Al alloy anode have been characterized and compared with those of the cast 7050 Al alloy. The spray formed Al anode possesses finer grains and lower volume fraction and discontinuous distribution of the second phases, leading to a smaller self-corrosion rate. After deformation and heat treatment, the spray formed Al alloy shows a further reduced volume fraction of the second phase and a high fraction of low angle grain boundaries. Therefore, the Al-air battery with the prepared Al anode displays a large specific capacity and a high working voltage. This study demonstrates that commercial Al alloys are likely to be used as anodes for Al-air batteries by the combination of spray forming and proper heat treatment.

Effect of remelting processes on the microstructure and mechanical behaviours of 18Ni-300 maraging steel manufactured by selective laser melting

Selective laser melting (SLM) is an established metal additive manufacturing technology extensively used in the automotive domain to manufacture metallic components with complex structures from 18Ni-300 maraging steel. However, achieving high-performance 18Ni-300 maraging steel using SLM still presents a challenge in terms of formulation of the processing parameters. The remelting process has the potential to address this challenge during SLM before post-treatments. This paper systematically investigated the effect of remelting on the microstructure and mechanical behaviours of the SLM-built 18Ni-300 maraging steel. The experimental results suggest that increases in the relative density of the as-built samples from 99.12% to 99.93% are achieved by a specific combination of remelting parameters (laser power 200 W, scan speed 750 mm/s, remelting rotation 90° and hatch spacing 0.11 mm) that eliminate large-sized pores. Compared with the as-built condition, remelting can slightly coarsen the average grain sizes and increased the fraction of low-angle grain boundaries (2°–15°). The tensile strength showed no remelting dependence, whereas both the ductility and microhardness increased. Elongation of the as-built sample increased from 10.5 ± 0.8% to 13 ± 3.5% after remelting under the #28 condition. These findings provide a fundamentally new understanding of how a combination of SLM and remelting can aid in the manufacture of high-performing 18Ni-300 maraging steel.

The Improvement of the Irradiation Resistance of Amorphous MoS2 Films by Thermal Annealing.

Among the structural materials used in fusion reactors, amorphous materials can effectively inhibit the accumulation and growth of radiation-induced defects, thereby improving irradiation resistance. However, the application of solid lubricating materials should also consider the changes in their lubricating properties after irradiation. This study shows that the ability to inhibit the deterioration of lubricating properties is not reflected in the amorphous MoS2 film. When the ion fluence reached 4.34 × 1014 ion/cm2, its wear life was reduced by two orders of magnitude, reaching 8.2 × 103 revolutions. After the amorphous MoS2 film is vacuum annealed, its structural stability and resistance to deterioration of lubricating properties are improved. When the ion fluence reaches 1.09 × 1015 ion/cm2, for instance, the wear life of the MoS2 film annealed at 300 °C remains at 8.4 × 104 revolutions. The higher irradiation tolerance of MoS2 films comes from the reduction in intrinsic defects by thermal annealing, which increases the internal grain size and volume fraction of grain boundaries, further providing an effective sink for irradiation defects.

On the Recycling of Water Atomized Powder and the Effects on Properties of L-PBF Processed 4130 Low-Alloy Steel.

The microstructure and mechanical properties of a 4130-grade steel processed by L-PBF using a feedstock of low-cost water atomized powder have been investigated considering the effects of powder recycling. Chemical analysis of the recycled powder showed a constant amount of alloying elements with a slight reduction in oxygen content. The as-built microstructure was mainly composed of a martensitic structure separated by a high fraction of low-angle grain boundaries, suggesting the application of a direct tempering treatment starting from the as-built condition as a cost-effective post-process thermal treatment rather than the conventional quench and tempering treatment. Moreover, the degree of anisotropy generated by L-PBF in as-built specimens could be reduced after performing either the direct tempering or the quench and tempering treatments. The possible degradation of powder properties on the steel performance was also investigated. After various powder recycling events, no significant deterioration in tensile properties was measured, indicating that the water atomized powder could be a sustainable feedstock candidate for L-PBF.

Mechanical properties and deformation dependent microstructural aspects of laser shock peened 7075-T6 aluminum alloy without coating

This work presents the mechanical properties and elaborately discusses microstructural aspects of LSPwC (laser shock peening without coating) peened 7075-T6 aluminum alloy using the results coming from various characterization techniques such as surface roughness, Vickers microhardness, residual stress analysis, tensile test, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). The possibilities and consequences of thermo-mechanical interaction, deformation behavior, grain refinement, fractional change of grain boundaries, compressive residual stress and nucleation of precipitates during LSPwC are discussed along with the underlying mechanisms. Microhardness increased by about 25% post LSPwC with a work hardened layer of more than 1000 μm. Maximum compressive residual stress of −227 MPa (in sub-surface region of peened specimens) was induced with effective depth (i.e. thickness of compressive layer) of 900 μm. The defect (dislocation) concentration, volume fraction of precipitates, area fraction of fine grains (of below 50 μm) and number fraction of low angle grain boundaries (especially of 2°–5° misorientation) increased significantly after LSPwC. Besides, the effects of observed improvements (by LSPwC) in tensile behavior were also investigated. The near surface strengthening or modifications observed in this investigation are collectively attributed to LSPwC induced plastic deformation, compressive residual stress, additionally induced precipitates, grain refinement and increased fractions of LAGBs. • The compressive and work hardened layer of ~1000 μm is obtained with LSPwC. • Grains are refined from 197 μm to 32 μm post LSPwC. • It is possible to induce precipitates by LSPwC. • Increased fraction of LAGBs assisted in nucleation of precipitates.

Stress corrosion cracking resistant nanostructured Al-Mg alloy with low angle grain boundaries

For decades, high susceptibility to intergranular corrosion and stress corrosion cracking is the Achilles's heel for the 5xxx series Al alloys with high Mg (> 3 wt%) owing to precipitation of electrochemically active Mg-rich β phase at grain boundaries (GBs) at moderately elevated temperatures. Herein, nanostructured Al-5Mg alloy with a high proportion of low angle grain boundary equiaxed grains were generated by dynamic plastic deformation and appropriate annealing treatment. Electron back-scattered diffraction discloses that the fraction of low angle boundaries reaches 70%, which has been demonstrated to greatly suppress the precipitation of β phase at GBs. The nanostructured Al-5Mg alloy exhibits excellent intergranular corrosion resistance and stress corrosion cracking resistance after being sensitized at 150 °C for 100 h. Moreover, its strength and ductility are much higher than the corresponding coarse-grained alloys, thus solving the classic problem that has existed for decades. The introduction of a high fraction of low angle grain boundaries by dynamic plastic deformation and appropriate annealing provides a novel strategy for the development of advanced nanostructured Al alloys.

Microstructural characterisation and high-temperature oxidation of laser powder bed fusion processed Inconel 625

Oxidation of Laser Powder Bed Fused (L-PBF) IN625 was studied in the as-built and heat-treated conditions. Following heat treatment, recrystallisation was observed with a large fraction of twin grain boundaries, and Nb-rich carbides decorated grain boundaries. Samples of L-PBF IN625 were exposed at 950 °C in laboratory air and pressure for up to 504 h. An external chromia scale was formed, alongside a near-continuous δ-phase layer at the scale/alloy boundary. The kinetics of oxidation were similar to wrought IN625, but more closely followed cubic behaviour. It is suggested that the cubic behaviour was caused by the formation of the δ-phase layer.

The impact of microstructural refinement on the tribological behavior of niobium processed by Indirect Extrusion Angular Pressing

This research, for the first time, conducted a comprehensive study into the effect of a modern Severe Plastic Deformation technique, “Indirect Extrusion Angular Pressing (IEAP)”, on the microstructural refinement of niobium and the subsequent impact on the tribological properties. The samples were processed for 4, 9, and 12 passes of IEAP and then exposed to Pin-on-disk wear test in dry sliding conditions. The results showed that the grain refinement occurred in niobium from an average grain size of 13 μm to 0.5 μm, along with an increase in hardness from the initial value of 79 HV to 180 HV after 12 passes. Results of wear tests revealed that IEAP processing reduced the wear width, volume loss, and specific wear rate by 14%, 38%, and 38%, respectively, and thereby enhancing the wear resistance of the material. Evaluation of the morphology of wear tracks demonstrated less spalling and less fatigue propagated cracks as well as finer detachment of wear debris on the surface of the processed samples, which led to producing a mild wear regime. The outcomes suggested that IEAP processing resulted in higher resistance in niobium against abrasion and against the development of fatigue propagated cracks which were respectively associated with higher hardness and larger fractions of high-angle grain boundaries (HAGBs).

The brittle-to-ductile transition of pure tungsten processed by closed dual equal channel angular pressing process

The deformation behavior of pure tungsten was investigated to understand the influence of microstructure changes on the brittle-to-ductile transition (DBT) during closed dual equal channel angular pressing (CDECAP). In the CDECAP process, four deformation regions and three processing routes were introduced and route B with high strain uniformity and low eccentric load was selected for pure tungsten. A fine-grained microstructure with high fraction of low-angle grain boundaries (LAGBs), more edge dislocations and near {011} <100> texture was developed during the microshear bands assisted continuous dynamic recrystallization (cDRX). And the hot hardness in a wide range of temperatures suggested that the brittle-to-ductile transition temperature (DBTT) reduced significantly from 276 °C to 165 °C after 3 passes. The results indicate grain boundary (GB), dislocation and texture are three important factors controlling the ductility of pure tungsten during CDECAP.