Large-angle Grain Research Articles

Corrosion and wear performance and mechanism study of ZrB2/AA6016

Abstract This study involved the fabrication of aluminum matrix composites reinforced with ZrB2/AA6016 particles using the KBF4-Al-K2ZrF6 reaction system， the composites were then subjected to T6 heat treatment. An investigation was conducted to examine the impact of varying friction speeds on the corrosion and wear characteristics of ZrB2/AA6016. An investigation was conducted to study the frictional wear behavior of ZrB2/AA6016 in the presence of 3.5wt. % NaCl, both before and after T6 heat treatment. The study also aimed to understand the underlying mechanism of this behavior. The results indicate that the T6 heat treatment mitigates the impact of thermal stresses and strains caused by thermal mismatch, hence enhancing the material's wear resistance. The coefficient of friction (COF) for heat-treated ZrB2/AA6016 is lower than that for unheated-treated ZrB2/AA6016. As friction increases, the pace at which the material wears down tends to decrease. At a friction wear velocity of 50 mm/s, the wear rate of the material is minimized both before and after heat treatment, measuring 0.23×10-2mm3/Nm and 0.22×10-2mm3/Nm, respectively. Through the utilization of XRD, SEM, EBSD, TEM, and XPS analytical techniques, it has been determined that the ZrB2 particles exhibit strong bonding with the Al matrix. Additionally, the particle diameters range from 50~150nm. Following the T6 heat treatment, the grain size measured 40.53μm, while the proportion of large-angle grain boundaries was found to be 66.4%. The accumulation of Cl- resulted in the formation of localized corrosion pits on the surface undergoing wear, hence hastening the deterioration of the material. The primary causes of wear failure are corrosive wear, abrasive wear, and oxidative wear.

Interfacial characterization and bonding mechanism of W/ODS-316 L steel multi-material structure fabricated by laser powder bed fusion

Interfacial characterization and bonding mechanism of W/ODS-316 L steel multi-material structure fabricated by laser powder bed fusion

Effect of tempering process on the mechanical properties and corrosion resistance of E690 marine steel

Abstract This study investigated the effect of the tempering process on the microstructure, mechanical properties, and corrosion resistance of E690 marine steel, aiming to replicate actual production conditions and reduce costs. Various techniques, including metallurgical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, impact experiments, tensile tests, and electrochemical corrosion tests, were employed to evaluate the material’s properties and performance. The results indicated that as the tempering temperature increased, the tempering degree in the tempered martensite structure improved, martensite strips coarsened, the size of precipitated carbides increased, and the proportion of large-angle grain boundaries decreased. Consequently, the tensile and yield strengths initially increased and then decreased, while the impact toughness and elongation gradually improved. At a tempering temperature of 600 °C, the steel exhibited the best overall mechanical properties, with a tensile strength of 729 MPa, a yield strength of 649 MPa, and an elongation of 18%. Furthermore, at a tempering temperature of 550 °C, the test steel showed optimal corrosion resistance, with a corrosion rate of 0.03233 mm/y and an open-circuit potential of -0.36 V.

Effects of intercritical annealing time on microstructure, tensile properties, and cryogenic toughness of newly designed ultra-low C low Ni Medium-Mn steel

An ultra-low carbon and low nickel medium manganese heavy steel plates was fabricated by quenching and intercritical annealing process. The effect of annealing time on the tensile properties and impact toughness were studied in relation to the formation of reversed transformation. The results show that excellent mechanical properties with yield strength of 680 MPa, tensile strength of 871 MPa, total elongation of 38.2% and high impact energy of 187 J at −60 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite (RA) together with a small amount of martensite after intercritical annealing. Increasing the intercritical annealing time resulted in coarser matrix microstructures, larger and more voluminous retained austenite, but with notably reduced stability. Highly stable RA enhances the work hardening capacity of the steel via sustained deformation-induced transformation (TRIP effect), significantly contributing to the toughening of the multiphase system. In addition, Austenite stability, rather than large-angle grain boundaries and the amount of austenite transformation, is the primary factor determining the low-temperature toughness of medium manganese steel.

Microstructure evolution and grain growth characteristics of IN625 in laser surface melting: Effects of laser power and scanning speed

This study investigated the effects of laser power and scanning speed on the microstructure evolution and grain growth characteristics of IN625 in laser surface melting. Laser powers of 400 W, 600 W and 800 W at scanning speeds from 200 mm/min to 800 mm/min were employed to melt the surface of as-cast IN625 using a continuous Yb-doped fibre laser. The microstructure from the surface to the melt pool boundary was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It was shown that a cellular structure was developed at low energy densities (≤ 240 J/mm2), since low energy densities resulted in more rapid solidification. A coarse grain region was found near the surface after melting and a columnar grain growth region was formed close to the melt pool boundary at increasing laser power. Large angle grain boundaries were eliminated and medium angle grain boundaries exhibited an area fraction of 90 % after laser surface melting. This was due to the fact that the twinned grains were fully melted in the melt pool and no twins were formed after solidification. An analytical approach was proposed to the estimate the melt pool depth and good agreement between experimental and calculated melt pool depth was obtained at laser power of 400 W and 600 W.

RESISTANCE OF FERRITE-PERLITE STEEL TUBING BRANCH PIPE METAL TO DESTRUCTION IN HYDROGEN SULFIDE MEDIUM

This paper discusses the problems of resistance of the metal of the tubing branch pipe (tubing) to the process of its destruction in a medium containing hydrogen sulfide and when exposed to external forces. A 73х73 tubing nozzle made of 38G2SFA steel was selected as the material for research. The tubing fragment was destroyed during production well operation. To establish the reasons for the failure of the tubing nozzle, a visual inspection was carried out, tests were performed to assess the hardness and mechanical properties, the microstructure and microtexture of the metal were analyzed using scanning electron microscopy (SEM) methods. To analyze the nature of the distribution of introduced defects, the mutual orientation of structural components and crystallographic texture, studies were carried out by the method electron backscatter diffraction (EBSD). Studies of the suspended branch pipe, which does not have chemical protection, showed that the cause of its destruction was hydrogen sulfide stress corrosion cracking (SSCC) as a result of hydrogen embrittlement and the action of external tensile forces exceeding the yield strength of the metal. It was established that as a result of the interaction of hydrogen sulfide of the distilled product with the metal of the branch pipe, hydrogen embrittlement of the material of the branch pipe occurred. The process of hydrogen embrittlement of the fragment was proved by detecting hydrogen blisters in the metal microstructure and establishing the process of coalescence of neighboring inclusions. In addition, the results of the test to measure the hardness of the metal along the wall thickness further supported the hypothesis of hydrogen embrittlement of the tubing. EBSD analysis showed that the threaded part of the branch pipe is characterized by a small size of structural elements and an increased density of introduced defects (geometrically necessary dislocations), which are a promising site for the deposition of molecular hydrogen. It is concluded that reaching the limit concentration of molecular hydrogen reduces the binding energy of metal atoms and this process initiates the formation of pores. It has been established that the development of cracks along large-angle grain boundaries is facilitated, and small-angle and coincidence site lattice (Σ3, Σ5, Σ11 and Σ13b) grain boundaries are active barriers to crack propagation. It has been shown that by optimizing the elemental composition of steel, controlling the microstructure, crystallographic texture, as well as the type and state of grain boundaries, new pipeline steels can be produced that are more resistant to SSCC.

The effects of quenching and partitioning on the microstructure and tensile properties of high strength suspension spring steel

The evolution of microstructure and mechanical properties were investigated by the quenching and partitioning heat treatment for a kind of micro-alloyed 51CrMnV spring steel. The results show that the microstructure of the test steel treated by quenching and partitioning heat treatment is mainly composed of martensite, retained austenite and carbides. Based on the quenching treatment, the optimal mechanical properties of test steel were obtained by the process of partitioning treatment at 350°C for 20 minutes. The tensile strength and elongation were 1812 MPa, 11.5 %, respectively. This is largely due to the microstructure of test steel having a good combination of martensite, filmy retained austenite, and fine carbides. As the partitioning temperature increases, the fraction of retained austenite decreases from 13.1 % at 250°C to 3.11 % at 350°C. Furthermore, the retained austenite almost disappears when the partitioning temperature increases to 400°C. In addition, as the partitioning temperature increases, the tensile strength remains above 1600 MPa, and the fracture behavior changes from brittle fracture to ductile fracture. At the same time, the grain size becomes larger, the proportion of large-angle grain boundaries increases, and the geometric dislocation densities become smaller.

Improvement of the fatigue life of an electron-beam welded Ti2AlNb joint subjected to an electromagnetic coupling treatment

The local stress concentration created by the seam welding process often reduces the fatigue life of the material along and around the seam. Post-welding heat treatment is usually used to improve the weld performance, but the improvement effect on fatigue life may not be obvious. In this study, a new treatment method of electromagnetic coupling was applied to regulate the Ti2AlNb electron beam weld after heat treatment, so as to improve the fatigue properties of the welded joint. The results show that after electromagnetic coupling treatment, the fatigue limit of the welded joint is increased by 10.4 %, the residual compressive stress is increased, and the fatigue crack propagation rate is decreased. The microstructure analysis shows that after electromagnetic coupling treatment, the fatigue crack source starts in the subsurface layer, the substructural crystals in the fusion zone and the heat affected zone decrease, the recrystallization content and the large angle grain boundary increase, and the dislocation density increases and tends to be homogenized. An increase of at least 2.3 % in hardness of the welded joints also indicates the increase in the dislocation density. Local high density dislocation is found in the grain, which causes dislocation entanglement and hinders crack initiation and propagation and this provides an important basis for improving fatigue properties. This study provides a new method for improving the fatigue properties of Ti2AlNb electron beam welds which can also be used to study the properties of other welded joints.

EFFECT OF ULTRASONIC PROCESSING ON THE PROPERTIES OF ULTRAFINE-GRAINED BRASS

This article establishes the relationship between the parameters of an ultrasonic action on the microstructure and the complex properties of ultrafine-grained brass. Ultrasonic action on brass after radial-shear rolling at certain amplitudes of mechanical stresses helps to relax the nonequilibrium structure of grain boundaries and relieve internal stresses. In this case, the relaxation effect of the structure depends on the amplitude of the ultrasound. It has been shown for the first time that ultrasonic processing is an effective way to relax the structure of highly deformed materials, which leads to an increase in the proportion of large-angle grain boundaries and triple grain joints, without leading to a significant grain growth.

Study on the microstructure evolution and effect on mechanical properties of P92 steel during long term service

P92 heat-resistant steel is widely utilized in ultra-supercritical units due to its exceptional properties. This study examines the microstructure evolution and mechanical properties change of P92 during service spanning 0 to 93,000 h, while also exploring the correlation between microstructure changes and property variations. The results indicate that the matrix of P92 steel retains its characteristic tempered martensite structure, and the large-angle and small-angle grain boundaries in the steel remain nearly unchanged throughout extended service periods. However, the deterioration of P92 steel properties can mainly be attributed to alterations in precipitate size and martensitic lath width. The size of M23C6 and Laves particles increased from their initial dimensions of 150 nm and 0 nm to 260 nm and 370 nm, respectively. Moreover, the width of the martensitic lath increased from 350 nm to 700 nm. The martensitic lath contributes over 60 % of the theoretical tensile strength, playing a pivotal role in the exceptional strength of P92 steel during service. The presence of coarse Laves particles at the interface leads to a rapid reduction in impact energy, decreasing from approximately 130 J to 50 J, negatively affecting the impact energy of P92 steel.

The nucleation and growth of fine austenitic grain at interface of the L415QS/N08825 bimetallic composite pipe by explosive welding

The nucleation and growth of fine austenitic grain at interface of L415QS/N08825 bimetallic composite pipe achieved by explosive welding is systematically studied. Results show that the L415QS/N08825 interface have heat affected zone (HAZ) that is similar with GH2132 superalloy (Fe-25Ni-15Cr). Two kinds of austenitic grains: columnar grains and ultra-fine equiaxed grain with large angle grain boundaries were identified at HAZ that nucleated from both L415QS and N08825 base alloy in the melts zone, and the growth of the nucleus is based on the size of melts zone. Room temperature nanoindetation test revealed that these fine equiaxed and columnar austenitic grains exhibit enhanced hardness compared with those of the base metals.

Atomistic to continuum mechanics description of crystal defects with dislocation density fields: Application to dislocations and grain boundaries

The atomic structure of crystal defects such as dislocations, grain or phase boundaries, control these defects’ properties: their mobility, ability to cross-slip, or solute segregation. These crystal defects can be conveniently studied by atomistic simulations and one then needs to transfer relevant information at the upper scale to model microstructures containing a large number of defects, e.g., a polycrystal. Here, we propose an atomistic to continuum mechanics crossover method that (i) represents the atomic structure of dislocations cores by an appropriate Nye dislocation density tensor field and (ii), captures quantitatively the short and long range mechanical fields of defects. For (i), we propose a modified and improved interpolation method based on the original work by Hartley and Mishin. For (ii), we use a field dislocation mechanics framework that rigorously calculates/evaluates the mechanical fields associated with any Nye dislocation density distribution. The transfer method relies on molecular static calculations using two energetic models — ab-initio for screw dislocation core simulations in tungsten, and EAM potential for low and large angle grain boundaries in copper. Our findings demonstrate the effectiveness of the proposed approach in reconstructing the Burgers vector, and continuous strain and rotation fields. The framework is further applied to analyze the elastic interactions between extrinsic edge dislocations and a low angle grain boundary in copper.

Crack characteristics analysis and mechanisms in GH3536 alloy manufactured by laser powder bed fusion

The presence of cracks poses a significant challenge in the application of laser powder bed fusion (LPBF) for manufacturing high Ni-base superalloys. The crack characteristics of LPBF-fabricated GH3536 superalloy are investigated in this paper, with a focus on analyzing the generation and expansion mechanism of micro-cracks through an investigation into the evolution of the molten pool, thermal stress and micro-characteristics. The findings suggest that thermal stress serves as the primary driving force for both crack initiation and propagation, with a predominant concentration of thermal stress observed in the heat-affected zone located at the periphery of the molten pool. Grain disorientation is another crucial factor influencing both the initiation and propagation of cracks. The difference in orientation between adjacent grains at the crack exceeds 40°, with the crack primarily originating from large angle grain boundaries (LAGBs) and propagating along them. Furthermore, there exists inconsistency in terms of elastic modulus and deformation coordination between carbides (Mo2C, (Ni, Cr) Mo3C6) particles, and matrix. This inconsistency results in two normal partial stresses acting in different directions, leading to dislocation aggregation and crack nucleation.

Crack generation and propagation mechanism of Mo[sbnd]14Re alloy laser welding

This paper investigates the mechanism of crack generation and propagation in laser-welded joints of Mo14Re alloy plates. The study utilized optical microscopy (OM) and scanning electron microscopy (SEM) to examine the microstructure and morphology of the crack area in the Mo14Re laser-welded joint. Additionally, energy dispersive spectroscopy (EDS) was employed to characterize the elements at the crack defect, while electron backscatter diffraction (EBSD) was utilized to analyze the grain boundary and crystal orientation at the crack defect.The analysis of crack formation and propagation considered various factors, including grain boundary segregation, characteristics of grain boundaries and crystal orientation, and the Schmidt factor. The findings revealed that cracks formed in the weld area exhibited distinct characteristics of brittle fracture along the grain. Notably, these cracks displayed evident carbon and oxygen segregation attributed to the presence of a low melting point eutectic liquid film. The segregation, along with high-temperature compounds on the fracture surface, emerged as the primary factors contributing to crack formation.In the proximity of the crack, there exists a concentration of strain, inducing plastic deformation and thereby facilitating crack initiation. Cracks exhibit a tendency to originate along large-angle grain boundaries and deflect along these boundaries. Owing to the difference in atomic slip characteristics on both sides of the crack, the direction of crack propagation is more inclined to deflect towards grains with a smaller Schmidt factor.

Microstructure analysis of 7050 aluminum alloy joint fabricated by linear friction weld

The microstructure of a linear friction welded joint of 7050 aluminum alloy was investigated through optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The analysis focused on grain boundary types, microstructure, and mechanical properties of the joint. The results reveal that fine equiaxed crystals are generated in the welded zone through dynamic recrystallization. The average grain size is 6.8 μm, with a volume fraction of large angle grain boundaries reaching 69.5%. The microstructure primarily consists of Cube {001}<100> texture, {111}<110> Brass recrystallization texture, and a small amount of copper {112}<111> deformation texture. The thermo-mechanically affected zone (TMAZ) presents a linear structure with an average grain size of 15.8 μm and a volume fraction of 36.1% at large grain boundary, resulting in a deformed Brass {011}<211> texture and a small amount of {236}<385> as well as {111}<110> Brass recrystallization texture. The welded joint exhibits a tensile strength of 492 MPa and a yield strength of 380 MPa, which represents 94.2% and 78.5% of the base material, respectively. Furthermore, the elongation of the joint is 10.2% and 98% of the base material, respectively. The fracture of the tensile sample is observed in the TMAZ, showing good mechanical properties with a mixed fracture mode with some degrees of ductility and brittleness.

Hot compression behavior and microstructure evolution of Mg-Gd-Y-Zn-Zr alloy

In this work, the hot compression behavior and microstructure evolution of Mg-8Gd-3Y-xZn-0.6Zr (x = 0, 1.5) alloys were investigated by Gleeble-1500D thermal simulation test machine, SEM, EBSD and other equipment. The results showed that, prior to hot deformation, the grain size of Mg-8Gd-3Y-1.5Zn-0.6Zr (1.5Zn) alloy was smaller than that of Mg-8Gd-3Y-0.6Zr (0Zn) alloy. Both alloys exhibited weak texture strength with predominantly large angle grain boundaries. The true stress-strain curve of the 1.5Zn alloy distinctly revealed dynamic recrystallization features. Notably, the 1.5Zn alloy exhibited higher peak stress and thermal activation energy compared to the 0Zn alloy. Furthermore, the 1.5Zn alloy demonstrated a lower critical strain for dynamic recrystallization, suggesting a heightened likelihood and increased degree of recrystallization. After hot deformation, the Mg-8Gd-3Y-xZn-0.6Zr alloy experienced a notable reduction in grain size, particularly evident in the 1.5Zn alloy, where the grains were smaller compared to the 0Zn alloy. Moreover, the texture strength of 1.5Zn alloy was obviously weaker than that of 0Zn alloy. Dynamic precipitation and dynamic recrystallization were more likely to occur at grain boundaries. Simultaneously, the constitutive model, hot processing map and recrystallization kinetics model were constructed for Mg-8Gd-3Y-xZn-0.6Zr alloy.

The Influence of the Second Phase on the Microstructure Evolution of the Welding Heat-Affected Zone of Q690 Steel with High Heat Input.

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150-300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite.

Investigating the microstructure and mechanical properties of 316L/TiB2 composites fabricated by laser cladding additive manufacturing

In this paper, the 316L stainless steel matrix composite strengthened by TiB2 was successfully fabricated using laser cladding additive manufacturing technology. The microstructures of 316L/TiB2 with the addition of TiB2 (mass fractions of 0, 3 and 6, respectively) were characterised. Hardness tests and tensile tests were conducted to evaluate the physical and mechanical properties of the samples. The results indicated that all prepared samples exhibit a complex cellular microstructure composed of sub-grains. The matrix phase was verified as γ austenite with a honeycomb-like reinforcement phases forming along the boundaries. Further analysis revealed the net-like reinforcements were (Fe,Cr)2B and M3B2 (M is Cr, Fe, Mo, Ni, Ti) phases. The Microhardness of the samples with 0 wt%,3 wt% and 6 wt% of TiB2 were 148 HV, 170 HV and 186 HV, respectively. The addition of 3 wt% and 6 wt% TiB2 composites increased the yield strength by 22 % and 29 %, respectively, compared to 316L composite. Also, the tensile strength and modulus of elasticity of the reinforcement composites were improved by 8 % and 13 %, respectively. However, the elongation of the reinforcement composites was reduced by 36 % and 47 %, respectively. The reinforcement with high strength and modulus is formed in 316L/TiB2 composite, which leads to a reduction in grain size while enhancing the density of large-angle grain boundaries. Multiple strengthening effects dominated by Orowan strengthening were simultaneously experienced during the preparation process, which improved the mechanical properties of the composites.

A novel study on microstructure and crystallographic characteristics of Cu-Cr-Zr alloy manufactured by laser powder bed fusion

This work systematically investigated and discussed the differences in microstructure and crystallographic characteristics between the horizontal and vertical directions of the Cu-Cr-Zr alloy prepared by laser powder bed fusion (LPBF) and analyzed the formation mechanism of the subgrains. A sample with a relative density close to full density (99.63 ± 0.1%) was prepared. Strong texture existed parallel to building direction of < 110 > , and large geometrically necessary dislocations (GNDs) density were distributed within the grains. The proportion of large-angle grain boundaries was about 12% higher than that in the building direction due to the larger number of fine grains in the horizontal direction, and the average grain size was reduced by twice. Further stable subgrains (∼368 ± 99 nm) were attributed to substructure induced by dislocation networks, which was mainly the combined effect of large residual stress, low stacking fault energy, and internal heat treatment effects.

Toughening mechanism and delayed fracture analysis of 4145H steel under intercritical quenching heat treatment process

Tuning thermal treatment schedule has been confirmed as an effective technical route for achieving optimum properties of steels. In the present work, the intercritical quenching tempering(IQT) process was applied to steel 4145H for oil drilling tools, and the performance enhancement and delayed fracture mechanism of the IQT process were comprehensively elucidated using multiscale characterization methods and various mechanical tests. Compared with the quenching and tempering(QT), the IQT process results in a separate island ferrite plus finely tempered sostenite organization, which reduces the dislocation density in the tempered organization and increases the proportion of recrystallized grains. Due to the grain refinement effect of multiple quenching, the pristine austenite grains were refined by 58 %. At the same time, the proportion of large-angle grain boundaries increased from 68.6 % to 80.3 %. The results of the mechanical property tests show that the intercritical 790 °C quenching treatment and tempering at 600 °C exhibits a tensile strength of 1087 MPa, a high elongation of 21.72 %, and a high impact work of 104 J compared to QT. The improved impact toughness is mainly attributed to the grain refinement effect of multiple quenching and the large-angle grain boundaries provided by the composite ferrite organization and M23C6 carbides. The impact of different ratios of large-angle grain boundaries on crack extension was also evolved by means of the crystal phase field, and the delayed fracture effect of large-angle grain boundaries was verified.