Large-angle Grain Research Articles

Rapid solidification structure and grain refinement mechanism of undercooled Cu60Ni40 alloys

Applying the composite technology of molten glass coating and superheating method, the molten Cu60Ni40 alloy has obtained different undercooling degrees. The microstructure of the alloy during rapid solidification was observed by metallographic microscope, and the evolution process of microstructure morphology was systematically studied. It is found that two significantly different grain refinement phenomena occur in the range of low undercooling and high undercooling. The characteristics of high proportion of large angle grain boundaries, random grain orientation and a large quantity of twin boundaries show that the grain refinement structure under high undercooling is due to recrystallisation. And the microhardness test results show that the average hardness of the grain refinement structure under large undercooling suddenly decreases sharply at the critical undercooling, which also confirms the recrystallisation mechanism of the refined grain under high undercooling.

Deformation microstructure and thermomechanical processing maps of homogenized AA2070 aluminum alloy

The hot deformation behavior of homogenized AA2070 Al–Li–Cu alloy was studied using isothermal compression in a temperature range of 300 °C–500 °C and a strain rate range of 0.001 s-1 to 1 s-1. Deformation microstructure was characterized using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) including weak beam dark field STEM imaging to correlate the microstructure evolution with hot deformation mechanism. TEM/STEM results show that a high density of fine T1 (Al2LiCu) phase forms in the microstructure through dynamic precipitation at compression temperatures lower than 400 °C, especially at low strain rates. Constitutive analysis shows that back stress due to dynamic precipitation of T1 phase decreases with increasing compression temperature and increasing strain rate. EBSD characterization indicates that dynamic recovery is the main softening mechanism at temperatures below 400 °C. At 400 °C and above, intermetallic phases including T1, Al20Cu2Mn3 and Al3Cu2 in the microstructure gradually dissolve, and dynamic recrystallization becomes the dominant softening mechanism, resulting in texture weakening and grain refinement. Three dynamic recrystallization mechanisms including discontinuous dynamic recrystallization (DDRX) at large angle grain boundaries, particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) were observed. Based on hot work efficiency, deformation instability and microstructure evaluation, processing maps were constructed using both conventional method and Garofalo sinh equation, providing optimum processing conditions (temperature >400 °C and strain rate <0.1 s-1) for industrial forging, extrusion and rolling processes.

Single and multiple track formation mechanism of laser powder bed fusion 316L/CuSn10 multi-material

This paper focuses on the influence mechanism of process parameters on the geometry morphology and microstructure characteristic of single-track and multi-track 316L/CuSn10 multiple materials manufactured by laser powder bed fusion (LPBF). The width of single-track increases with the increase of laser power and layer thickness, and decreases with the increase of scanning speed. Both variations in the copper content and wetting angle of the molten pool were investigated. In addition, the comparison of defect characteristics shows that the scanning speed and layer thickness have a greater influence on the formation of single cracks. The traces of Marangoni convection are observed directly in the centre of the molten pool due to the difference in microstructure between 316L and CuSn10 alloys. It is found that the copper penetration cracks appear on the steel side at the molten pool bottom. Moreover, fine grains appear in the copper-rich region, and the large-angle grain boundary distribution seems to be the reason for the concentration of dislocations. Four types of crack formation mechanisms are found in the cross section of the multi-track: crack formation inside the molten pool, passing through the track boundary, extending to the steel substrate, and copper penetration cracks.

One-dimensional metallic grain boundary in transition metal dichalcogenides

In contrast to their more often detrimental role, large-angle (60°) grain boundaries (GBs) provide opportunities to tailor the properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs), bringing new functionalities and exciting opportunities to form intrinsic electronic heterostructures within monolayer TMDCs. In domains facing the GBs, polarization vectors along armchair direction are inversed and thus polar discontinuity arises at the GBs, as a consequence of nonzero bulk polarization of TMDCs. In order to screen the polarization charge, which gives rise to an electric field, depending on the bonding nature at GBs free electrons or holes are driven to accumulate on the GBs leading to the dispersive boundary states of metallicity. In particularly, 60° GBs in 2D TMDCs can behave as one-dimensional (1D) metallic quantum wires, serving as 1D channels for free carriers or excitons.

The Influence of Ni on Bainite/Martensite Transformation and Mechanical Properties of Deposited Metals Obtained from Metal-Cored Wire

The multi-pass deposited metals were prepared by metal-cored wire with low (2.5 wt%) and high (4.0 wt%) Ni to research the effect of Ni on the bainite/martensite transformation. Results showed that deposited metals exhibited a multiphase structure comprised of bainite, martensite and residual austenite, which is not only explained from SEM/TEM, but also identified and quantified each phase from crystallographic structure through XRD and EBSD. With Ni content increasing, the fraction of martensite increases from 37% to 41%, and that of bainite decreases from 61% to 55% accordingly because 4% Ni element narrows the temperature range of the bainite transformation ~20 °C. The 7.8% residual austenite exhibited block and sheet in the deposited metal with low Ni, while the fraction of residual austenite was 3.26% as a film with high Ni, caused by different transformation mechanisms of bainite and martensite. The tensile strengths of deposited metals were 1042 ± 10 MPa (2.5% Ni) and 1040 ± 5 MPa (4% Ni), respectively. The yield strength of deposited metals with high Ni was 685 ± 18 MPa, which was higher than low Ni due to the high fraction of martensite. The impact values of deposited metals with high Ni content decreased because the volume fraction of bainite and residual austenite and area fraction of large-angle grain boundary were lower.

Mechanical behavior of BDD films after different heat treatments

The applications of diamond are limited by its brittleness although it has numerous remarkable properties. The in-situ hydrogen plasma treatment and vacuum annealing were investigated to improve the mechanical properties of boron-doped diamond (BDD) films. Before and after different heat treatments, the surface morphologies , phase composition, and microstructure of BDD films were characterized by SEM, Raman spectroscopy , and TEM, respectively. The results show that vacuum annealing can increase the defect density of BDD due to the formation of more twins. On the contrary, in-situ hydrogen plasma treatment is beneficial to reduce defects such as twins. The nano-indentation test results demonstrate plastic deformability of BDD film is significantly improved after vacuum annealing with a decrease of 38.3% in H 3 /E 2 and an increase of about 59.1% in plasticity index PI value. In contrast, the role of in-situ hydrogen plasma treatment is just the opposite, which leads to a high hardness due to lower defects. • The lattice distortion and defects of BDD films reduced after the H plasma treatment. • Many twins and large-angle grain boundaries occurred after the V annealing. • The change of mechanical properties is closely related to the defects of BDD films.

Failure behavior of electromagnetic driven nailing riveted joint for Al/steel structures subjected to fatigue loading

Electromagnetic high-speed nailing (E-HSN) has wide application prospect due to the advantage of high flexibility, unilateral operation and excellent performance. In this paper, the fatigue behavior of E-HSN joints with the aluminum alloy 5052 (Al)/high-strength steel DP590 (HSS) structure was systematically studied. The fatigue tests were conducted and Basquin equation was used to fit the fatigue data. Fatigue fracture were observed. The results showed that the joint had two typical fatigue failure modes. The cyclic stress of 253 MPa was the critical point of two failure modes. The rivet shank fractured under a higher cyclic stress (mode 1, over or equal to the critical point) and rivet head fractured under a lower cyclic stress (mode 2, below the critical point). Moreover, the fracture appearance was analyzed to reveal the failure mode and mechanism of the joints. The quasi-cleavage fracture in the fatigue fracture mode 1 was characterized by ductile and brittle mixed fracture. For the fatigue fracture mode 2, cleavage cracks appeared cleavage fan shape due to the large-angle grain boundary. Finally, based on the damage evolution model, the fatigue life prediction values of E-HSN joints were obtained. The predicted results were basically distributed within 2 times error band, which showed good accuracy and consistency.

Effects of post-production heat treatment on the mechanical and corrosion behaviour of CoCrMoW alloy manufactured through selective laser melting

In this work, the matrix of CoCrMoW alloy fabricated by selective laser melting is mainly γ-Co phase. After the aging treatment at 750 °C, lamellar ε-Co phase formed according to a Shoji-Nishiyama orientation relationship with γ-Co phase and nanometer-scaled Co3(Mo,W)2Si precipitation uniformly distributed on the matrix, the strength and corrosion resistance increased, but the ductility decreased. After the solution treatment at 1200 °C, recrystallization happened, the fine columnar sub-grains of γ-Co phase in the as SLMed alloy were replaced by larger grains mainly with large-angle grain boundaries, the ductility increased, but the tensile strength and corrosion resistance decreased. Results of this work can provide a perspective towards microstructure regulation of SLMed CoCrMoW alloy.

Effect of High Rotational-Speed Friction-Stir Welding on Microstructure and Properties of Welded Joints of 6061-T6 Al Alloy Ultrathin Plate

The butt joint of an Al alloy ultrathin plate with a thickness of 0.5 mm is realized by a high rotational-speed friction-stir welding process. It overcomes the welding difficulty that the ultrathin plate is often torn, and it cannot be formed by conventional friction-stir welding. The results show that the weld surface is well-formed at a high-rotational speed (more than 8000 rpm), and there are no obvious defects in each area of the joint section. The nugget zone (NZ) is a recovery recrystallization structure dominated by large-angle grain boundaries, with a grain size of about 4.9 μm. During grain growth, the texture is randomly and uniformly distributed, and the strength is balanced. The microhardness of the NZ increases significantly with the increase in rotational speed, and the fluctuation range of hardness value is small. The NZ β–Mg2Si is finer and significantly less than the base metal (BM). The heat dissipation of the thin plate is fast, so a Cu plate is used as the backing plate to slow down the steep temperature-drop process in the weld area. Compared with a low rotational speed, the precipitation amount of brittle phase Al–Cu–Mg–Cr and Al–Fe–Si–Mn is significantly reduced, which is conducive to improving the mechanical properties of the joint. At a high rotational speed, 12,000 rpm, the best tensile strength of the joint is 220 MPa, which is about 76% of the BM (290 MPa), and the highest elongation is 9.3%, which is about 77.5% of the BM (12%). The fracture mode of the joint is a typical plastic fracture.

Study on the Hot Cracking Law of Inconel 690/52M Welding Material on F304LN Base Metal by Multi-Layer Cladding

During the welding of 690 nickel-based alloy, solidification cracking (SFC) and ductility-dip cracking (DDC) easily forms, which has a negative effect on the quality of welded joints and service life. The present study examined the effects of welding heat input and cladding layers on the SFC and DDC, as well as their formation mechanism. The microstructure observation, elemental distribution, and Varestraint test were carried out. The results show that SFC and DDC were formed for the Inconel filler metal 52M, and SFC is more prone to form than DDC. The alloy elements such as Fe, Si, C, and P from base metal can expand the solidification temperature range, such that the SFC sensitivity increases. With the increase of welding heat input, the grain size of cladding metal is increased with a great SFC sensitivity. The increasing welding heat input also makes DDC possible due to the formation of a large angle grain boundary.

Creep behaviors of a Mg–Li based alloy at elevated temperatures

Improvement of the mechanical stability is a long challenge for the light and strong dual-phases MA21 (Mg-7.99Li-5.3Cd-4.57Al, wt.%) alloy with critical applications in aerospace and automotive industries. The present work experimentally investigated the relationship between the creep behaviors and microstructures of MA21 alloy at elevated temperatures. It was revealed that plate-like θ-phase precipitated in the α-phase and AlLi phase particles grew in the β-phase. These precipitate phase impeded the dislocation dynamics giving rise to the creep deformation. At low temperature and high stress, the creep behavior was dominated by dislocation climb accompanying Li atom diffusion, where the θ-phase was shear deformed. The AlLi particles were so hard to be penetrated by dislocation, so the dislocation slip can only propagate bypass it. In contrast, extensive atom diffusion of Al and Cd elements was revealed to support the dislocation slip mechanism with solute drag for the creep behaviors at high temperature and low stresses. The dislocation slip during creep resulted in grain reorientation with enhanced (0001) plane texture and transformation of large angle grain boundaries in the initial state into small angle grain boundaries. Our results shed new light on understanding of the time dependent deteriorated mechanical properties of Mg–Li commercial alloys and might give rise to a new strategy to stabilize the mechanical properties of MA21 alloy and relevant Mg–Li alloy system by engineering the precipitation phase in the α-Mg and β-Li phases.

Microstructure refinement mechanism of undercooled Cu55Ni45 alloys

Abstract Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the maximum undercooling degree reached 284 K. The microstructure of the alloy was observed by metallographic microscope, and the evolution of microstructure was studied systematically. There are two occasions of grain refinement in the solidification structure of the alloy: one occurs in the case of low undercooling, and the other occurs in the case of high undercooling. Electron backscatter diffraction (EBSD) technology was used to analyze the rapid solidification structure under high undercooling. The features of flat polygonal grain boundary, high proportion of twin boundary, and large proportion of large angle grain boundary indicate recrystallization. The change in microhardness of the alloy under different undercooling degrees was studied by microhardness tester. It was found that the average microhardness decreased sharply at high undercooling degrees, which further confirmed the recrystallization of the solidified structure at high undercooling degrees.

Dynamic recrystallization behavior of 6082 aluminum alloy during hot deformation

The dynamic recrystallization behaviors of 6082 aluminum alloy in the temperature range of 623–773 K and strain rate range of 0.01–5 s−1 were studied by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). According to the experimental results, dynamic recrystallization occurs during hot deformation of 6082 aluminum alloy, although the true stress-strain curve has no obvious single peak characteristic, and the degree of dynamic recrystallization is closely related to the Z parameter. Hot compression with lnZ = 24.9014 (723 K, 0.1 s−1) gives rise to the highest recrystallization fraction of 38.6%. The initial critical strain of dynamic recrystallization was determined by the work hardening rate. The quantitative relationship between the critical strain and Z parameters was established: [Formula: see text]. Based on the EBSD analysis and measurement results, dynamic recrystallization kinetics models of 6082 aluminum alloy during hot deformation were deduced. Microstructure analysis showed that the subgrain structure formed in the original grain is coarsened by grain boundary migration, and the orientation difference increases continuously until a large-angle grain boundary forms, resulting in dynamic recrystallization of grains. The likely mechanism is continuous dynamic recrystallization.

Phase-field crystal study of topological evolution during grain annihilation process in polycrystalline materials

The topological evolution and annihilation mechanism of the three-, four- and five-sided grains are studied by using the phase field crystal (PFC) model. Both the large-angle and small-angle grain boundaries are discussed during the grain annihilation process. The results show that for the large-angle grain boundaries, grain annihilation is achieved through grain boundaries migration and absorption by triple junction. For the small-angle grain boundaries, the grain annihilation is achieved by dislocation slip, reaction, and absorption by triple junction. However, when the number of sides of a grain increases to seven, the grains show obvious growth instead of annihilation. Two kinds of growth mechanisms for the seven-sided grain are obtained by simulating the growth process. The effects of the number of grain sides on the evolution of grain annihilation are explored. These results are consistent with the Von Neumann-Mullins model and Euler formula.

The Effect of Inclusions on a Medium Temperature Transformation Microstructure and Toughness of High-Strength HSLA Steel.

In this study, the evolution of high-strength HSLA steel microstructure was studied using high-temperature laser confocal microscopy and SEM, TEM, and EPMA techniques. The effect of precipitates on grain boundary migration of austenite during high-temperature heating and the effect of inclusions in undercooled austenite on AF phase transformation were studied. The effect of multiphase microstructure on impact toughness was studied by Gleeble thermal simulation at 550, 600, and 650 °C. The results show that the austenite grain is refined by TiN pinning at high temperatures, and a large number of NbC and VCN are precipitated in ferrite for precipitation strengthening. The (Ti-Mn-O) + (Al + Si + Mn-O) + MnS composite inclusions with smaller sizes have a greater promoting effect on the nucleation of acicular ferrite than single-phase MnS. With a decrease in isothermal temperature, the content of acicular ferrite increases. When the isothermal temperature is 550 °C, an increase in the maximum impact toughness of acicular ferrite with large-angle grain boundary is clearly observable.

Laser shock peening regulating aluminum alloy surface residual stresses for enhancing the mechanical properties: Roles of shock number and energy

The enhancement of the mechanical performance of materials is a continuous pursuit in engineering. Laser shock peening (LSP) is an advanced manufacturing method employing extreme conditions, which effectively improves the mechanical properties of the material surface. Herein, LSP was used to treat aircraft 5A06 aluminum alloy, leading to significant enhancement of properties. In the LSP process, the pulse shock wave caused severe plastic deformation of the metal surface, where the strain rate reached 107 s−1. The dislocations multiplied in large numbers and then evolved into large-angle grain boundaries and other structures to achieve grain refinement. Also, LSP significantly improved the residual stress of metals, causing a series of mechanical improvement effects. Under the impact of fine-grain strengthening, the resultant 5A06 aluminum alloy's micro-hardness was increased to 140 HV, a value 65% higher than that of bare aluminum. After the LSP process, the maximum plastic deformation layer depth (PDLD) reached 800 μm. On this basis, the ultimate tensile strength of aluminum alloy was increased by 27%, and the ultimate tensile displacement was also increased. The LSP treatment increased the strength of aluminum alloy without losing ductility. The resultant sample's friction coefficient was reduced by 34%, and the overall surface-displayed uniform wear. This work reveals the structure-activity relationships between the surface residual stress and metal mechanical properties and clarifies the LSP process's strengthening mechanism.

Microstructure evolution of as-extruded Zn–0.62Mn alloys during room temperature compression

The microstructure evolution of Zn–0.62Mn alloys during the room temperature compression with different strains was investigated by scanning electron microscope and X-ray diffraction. The results showed that with the increasing compression strains, the refined dynamic recrystallization grains increase gradually, and some grains rotate. It can also be found that the compression strain promotes the transformation from small-angle grain boundary to large-angle grain boundary. The highest basal textural can be obtained after the room-temperature compression with 80% strain. These results indicate that the microstructure evolution of as-extruded Zn–0.62Mn alloys during the room-temperature compression can be attributed to the grain refinement, grain boundary variation and texture changes.

Microstructure, texture, and fracture of pure magnesium adiabatic shear band under high strain rate compression

In the paper, the microstructure and texture of adiabatic shear band (ASB) in hat-shaped pure magnesium specimens under dynamic shear loading were investigated, and the formation and fracture mechanism of ASB were discussed as well. Results showed that, the straight ASB, with a width about 90 μm was formed upon shear loading, being consisted of the transition and ASB central regions. Equiaxed grains with large angle grain boundaries (LAGBs) were deposited in the ASB central region, and had a size distribution ranging from 0.3 to 3.0 μm. From ASB central to matrix regions, grains and strain both increased, and strain concentration occurred in matrix region. <0001> direction of the grains in matrix and transition regions was parallel to the loading direction (LD), while that of the grains in ASB central region was perpendicular to the shear band direction (SBD). The mechanically-driven subgrain rotation model provides a reasonable explanation for the texture in ASB. Dynamic recrystallized (DRX) grains slip and rotate under normal stress, resulting in (0001)//SBD texture with the maximum pole intensity of 16.73. Due to the high stress at the triple junction of grains boundaries, voids appear, and the formation and combination of voids lead to secondary and main cracks. Finally, the main crack propagates rapidly along the interface between central region and transition region, and then fracture occurs along ASB with a mixed fracture mode of brittle and ductile fracture.

Effect of Hot Rolling on Microstructural Evolution and Wear Behaviors of G20CrNi2MoA Bearing Steel

Hot rolling can improve the mechanical properties after heat treatment by improving the microstructure. The effect of hot rolling (HR) deformation on the microstructural transformation of G20CrNi2MoA bearing steel in the subsequent CQT (carburizing-quenching and tempering) and RQT (reheating-quenching and tempering) processes was studied. The results indicate that the austenite grain size decreased by 20% after 45% hot rolling reduction, and the number of large-angle grain boundaries increased due to the recovery and recrystallization induced by hot deformation. The refinement effect of hot deformation on austenite grains was retained after dual austenitizing, and the large-angle grain boundaries and massive dislocation in the grains caused by hot deformation promoted the diffusion of carbon atoms during carburization, resulting in a higher surface carbon concentration. The refined grains and higher carbon concentration affected the volume fraction and size of undissolved carbides in RQT specimens. When the initial hot rolling reduction reached 45%, the average particle size of carbides decreased by 40%, and the area volume fraction increased by 37%. The Vickers hardness increased, but the friction coefficient and wear rate were significantly reduced with the increase in the initial hot rolling reduction. The main reasons for the improved wear resistance were fine grains, superior carbide distribution and high hardness.

Effect of grain boundary engineering on corrosion behavior of nickel-based alloy 825 in sulfur environment

Grain boundary engineering (GBE) by appropriate deformation and heat treatment processes was applied to nickel-based alloy 825 due to its excellent corrosion resistance. The microstructure of nickel-based alloy was analyzed by electron backscatter diffraction (EBSD) and the effect of GBE on the corrosion resistance in high sulfur-containing environments was studied by electrochemical method. The results show that the ratio of low Σvalue coincidence site lattice (CSL) grain boundary is obviously improved from 47.1% to 65.5%. The special angle grain boundaries Σ3, Σ9, Σ27 and so on are randomly distributed on the network of the large angle grain boundaries, which destroy the connectivity of the original grain boundary network, and effectively block the corrosion cracking of the material along them. The corrosion resistance of nickel-based alloy 825 in high sulfur environments is enhanced verified by polarization curve and scanning electrochemical microscope, providing references for corrosion protection during the exploitation of high sulfur gas reservoirs.