Angle Grain Research Articles

Multiscale microstructural consideration of enhanced shear strength in TiAl intermetallic/K4169 alloy composite joints prepared by vacuum brazing with (Ti, Zr)-Ni-based amorphous filler metal

TiAl intermetallic could be used to replace Ni-based alloy in assemblies to generate excellent specific strength. A (Ti,Zr)-Ni-based amorphous filler metal Ti21.25Zr25Ni25Cu18.75 (at.%) was designed using a cluster-plus-glue-atom model to successfully vacuum braze K4169 and TiAl bimetallic assemblies. At various brazing temperatures and holding time, the quantitative relationships between lattice distortion, grain boundary, dislocation density, and hardness, elastic modulus, shear strength of the joints were investigated. Meanwhile, the fracture mechanism of the joints was revealed. The brazed seam mainly consisted of solid diffusion reaction layers (Zones I and III) and filler metal residue zone (Zone II). When the brazing temperature increased to 1030 °C, grain refinement occurred in the brazed seam. Zone I was primarily composed of (Ni)ss[0-11]+TiNi[011]/(Cr,Fe,Ni)ss[0-11]/(Ti,Zr)Ni[0-1-1]+(Cr,Fe,Ni)ss[0-11]. The (Ti,Zr)(Ni,Cu)[001] and (Ti,Zr)(Ni,Cu)[101] intermetallic compound-based solid solutions were formed in Zone II. And the lattice distortion of (Ti,Zr)(Ni,Cu)[101] and (Ti,Zr)(Ni,Cu)[001] was 32.05% and 14.82%, respectively. As a result, the proportion of low angle grain boundaries (LAGBs) and deformed grains in Zone II rose to 38.6% and 38.7%. In Zones I and III, the proportion of LAGBs reduced to 8% and 3.4%, respectively. As the holding time increased, the long-range diffusion of Al in Zone II caused the (Ti,Zr)(Ni,Cu)[001] with cubic structure to transform into (Ti,Zr)(Ni,Cu,Al)[00-1] with hexagonal crystal system structure, where the lattice distortion was 4.42% and 10.49% for a and c. At 1030 °C/10 min, the average geometrically necessary dislocation densities (GNDs) in Zones I, II and III were 9.87 × 1014 m−2, 8.55 × 1014 m−2 and 11.4 × 1014 m−2, respectively. Therefore, the shear strength of joints reached 322 MPa due to the lattice distortion, dislocation strengthening and fine grain strengthening. Meanwhile, the plastic and brittle hard phases were generated in Zone II and displayed a mechanical interlocking structure that contributed to the performance of the joint. Both (Ti,Zr)(Ni,Cu)[001] and (Ti,Zr)(Ni,Cu)[101] in Zone II formed along different low-index cleavage planes during transgranular fracture. The cracks initiated in this region extended to the interface between Zones I and II and exhibited bimodal grain characteristics.

Tribological properties and sulfuric acid corrosion resistance of laser clad CoCrFeNi high entropy alloy coatings with different types of TiC reinforcement

CoCrFeNi, directly added TiC (D-TiC) and in-situ synthesized TiC (I-TiC) reinforced CoCrFeNi high entropy alloy coatings (HEACs) were prepared on the surface of 316 stainless steel (SS) by laser cladding (LC). The HEACs all achieved a dense and uniform structure, the TiC distributed at the grain boundaries (GBs) reduced the dilution rate of the HEACs, and the I-TiC composite coating realised a fine and diffuse distribution of the reinforcing phase compared to the D-TiC HEAC. The introduction of TiC dramatically refined the grain size of HEA system and inhibited the formation of high angle grain boundaries (HAGBs), with the average grain size decreasing from 23.6 µm in the TiC-free HEAC to 12.3 µm in D-TiC HEAC and 9.3 µm in I-TiC HEAC, and the proportion of low angle grain boundaries (LAGBs) of D-TiC and I-TiC composite coatings was even as high as 64.8% and 70.4%. As a result, the density of geometrically necessary dislocations (ρGND) in TiC-containing coatings also increased greatly, and ρGND in CoCrFeNi, D-TiC and I-TiC HEACs were 0.55 × 1015, 1.08 × 1015 and 1.27 × 1015 m−2, respectively. The highest average microhardness values of 420.2 ± 6 HV0.2 was achieved with I-TiC HEAC, which relied on TiC precipitation in a nucleation-growth mode. In electrochemical corrosion tests, the introduction of TiC also provided additional channels for ion diffusion. The ordered and dense passivation film of the I-TiC composite coating impeded sulfuric acid attack effectively, with an Icorr value of only 2.28 × 10−2 ± 0.8 μA/cm2, which was less than that of the substrate as well as the other two HEACs. In both dry sliding friction and wear and corrosive wear experiments, abrasive wear was the dominant wear pattern of I-TiC HEAC due to the increase in local stresses caused by the "point-to-point" contact between the TiC particles and the friction pair. Thanks to the diffuse and homogeneous distribution of the TiC reinforcing phase, I-TiC HEAC exhibited the best tribological performance in both dry sliding friction and wear or corrosive wear experiments with specific wear rates as low as 0.05 mm3/Nm in air and 0.06 mm3/Nm in sulfuric acid.

“In-plane” site-specific FIB lamella extraction from deformed magnetite and the investigation of low angle grain boundaries under TEM

In this study a modus operandi to investigate site-specific nanostructures in thin films (lamellae) excavated “in-plane” across (sub)grain boundaries is presented. This is done by discussing the case of a magnetite grain hosted in a thin section of banded iron formation (Norway) that is prepared parallel to the kinematic reference frame (XZ section of the strain ellipsoid). SEM-EBSD analysis reveal that the magnetite grains do not develop a strong crystallographic preferred orientation, although individual grains are strained and show evidence of intracrystalline deformation in form of low angle grain boundaries (LAGB's). Two “in-plane” lamellae using focused ion beam (FIB) technique are excavated from a magnetite grain in the kinematic reference frame, and nanostructures are studied along three LAGB's using high resolution transmission electron microscopy imaging followed by Fourier transformation (FT), inverse FT and estimation of dislocation densities. Our data establish an empirical relationship for the studied LAGBs, namely, the smaller the angle between LAGB and X-direction, the larger are the shear strain and dislocation density. This relationship is validated from numerical simulations of viscoplastic deformation and dynamic recrystallisation of polycrystalline aggregates of halite, which is also a cubic mineral analogous to magnetite. In addition to the site-specific “in-plane” FIB lamella information, this study also shows that in a deformed mineral the different orientations of the LAGB compared to the principal strain axes show a different dislocation density. This approach of full tracking of the extension direction (X) from the macroscopic to the nano-scale could play an important role in forward modelling of microstructure evolution in future studies.

Plastic deformation behavior and strengthening mechanism of SLM 316L reinforced by micro-TiC particles

In this work, the integrated study on the microstructure evolution, plastic deformation behavior, and strengthening mechanism of Selective Laser Melting (SLM) 316L austenitic stainless steel (ASS) reinforced with micro-TiC particles was investigated. The results indicated that there were lots of small angle grain boundaries in SLM 316L ASS, and the high dislocation density and component segregation of the cellular dislocation structure resulted in a good combination of high strength and good elongation in performance. The tensile strength, yield strength, and elongation of SLM 316L ASS were 682 MPa, 571 MPa, and 49.9%, respectively. The addition of TiC reinforced particles induced microstructure evolution, including grain refinement, reduction of epitaxial columnar dendrites, reduction of low angle grain boundary (LAGB) ratio and increase of Σ3 (60°/<111>) twin boundaries. Through the effects of grain refinement strengthening, Orowan strengthening, and dislocation strengthening, SLM 316L-2TiC showed a higher tensile strength of 1021 MPa and higher yield strength of 809 MPa, while maintaining an elongation of 30.8% due to the suppression of plasticity by twinning induced plasticity (TWIP) effect. Although nano-reinforced particles were commonly used to improve mechanical properties, in our work, we chose to add micrometer-sized TiC particles, and the strength of the SLM 316L-2TiC metallic matrix composites (MMCs) exceeded the previous reports of adding nanoparticles. The reason was that the small contact angle between TiC and 316L provides good wettability between the particles and the melt, and the selection of laser processing parameters ensures good bonding between TiC particles and the substrate.

Investigation of microstructural evolution in a hybrid additively manufactured steel bead

Abstract Motivated by the beneficial effects of friction stir processing (FSP) for microstructural grain refinement, equiaxed grain production, and minimizing metallurgical defects, additive bead (AB) produced by the gas metal arc welding-wire arc additive manufacturing (GMAW-WAAM) technique was subjected to FSP. This was because deposited additive bead often develops defects such as shrinkage, voids, solidification cracking, during liquid to solid transformation. In this study, a low carbon steel double pass additive bead with 32 % lateral overlap was fabricated by the GMAW-WAAM technique followed by hybridization through FSP in the overlapped region (OR). The peak temperature estimation during bead deposition and FSP on bead was done through modeling by using ABAQUS. The microstructural analysis was carried out by using optical microscopy, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. The microstructure of OR of deposited additive bead is dominated by a combination of ferrite and bainite while that of hybrid additive bead (HAB) is dominated by a combination of ferrite and martensite. Further, the analysis revealed the effects of FSP on the OR in the form of grain refinement from 5.56 µm to 3.50 µm and a decrease in the low angle grain boundaries from 35.4 % to 10.6 %. The continuous dynamic recrystallization is observed since the bainitic fraction in the overlapped region decreased along with an increase in the fraction of martensite in the friction stir processed zone. The kernel average misorientation is observed to decrease after FSP from 1.001 of AB to 0.608. The microhardness test reveals the decrease in the hardness after FSP.

Mechanical properties and microstructure evolution during dynamic compression and cutting of nickel-based superalloys

Superalloy GH4169 has been widely used on account of its excellent property. However, the microstructure characteristics of the machined surface will be changed significantly because of mechanical-thermal coupling effect after the cutting process, which affects the fatigue performance of key components. To clearly understand the microstructure development of surface layer during machining process, the mechanical behavior and microstructure evolution of GH4169 under high strain rate and high temperature conditions are studied in detail by dynamic compression tests. The peak stress can be improved by rising strain rate and decreasing temperature. Cutting experiments show that the depth of low angle grain boundaries distribution and plastic deformation in the machined surface layer are improved by rising cutting parameters. The degree of twin boundaries deviation from the ideal misorientation increases by improving cutting parameters. In addition, the geometrically necessary dislocation density increases first and then decreases by increasing distance from the machined surface. Furthermore, the cutting simulation model based on dynamic compression tests are established.

Experimental and temperature field simulation study of Inconel 718 surface cladding based on vacuum electron beam heat source

By vacuum electron beam welding machine based on simulation of temperature field of NiCrBSi, experiment was conducted by vacuum electron beam surface cladding (VEBSC). The microstructure and friction wear properties of the specimens were analyzed by scanning electron microscope (SEM), electron backscatter diffractometer (EBSD), microhardness tester and friction wear tester under different welding beam currents (IW), respectively. The results show that the grain orientation in Inconel 718 in the rolled state is very dispersed and consists of equiaxed austenite grains (about 15 μm) with a high number of annealed twins. The internal orientation difference becomes relatively uniform after VEBSC, and a large number of elongated needle-like grains (about 200–300 μm) appear in the surface layer. At the same time, the low angle grain boundary of the clad layer (CL) of the specimen increases and then decreases with the increase of IW. When IW = 18 mA, the specimen surface hardness reaches a maximum value of 632.5 HV, and the friction coefficient is at least 0.25. The wear mechanism after cladding is dominated by plowing and abrasive wear to improve the wear resistance of the material. However, the high temperature oxidizing properties do not change much.

Effect of Mg content on the microstructure and properties of high strength, high conductivity Cu–Fe–Cr–Si–Mg alloy

A new strategy for the preparation of Cu–Fe alloy with excellent comprehensive performance is proposed. This strategy combines the construction of dual-scale polymetallic second phases with the addition of trace solid-solution atoms. A series of Cu–2Fe-0.3Cr-0.2Si-xMg (x = 0.1, 0.3, 0.5) alloys are designed and prepared through multi-stage thermo-mechanical treatment. Among the reported Cu–Fe alloys with low Fe content (Fe content less than 3 wt%), the designed Cu–2Fe-0.3Cr-0.2Si-0.3 Mg alloy has the highest product of the tensile strength times the electrical conductivity (PSC). The tensile strength and PSC of the Cu–2Fe-0.3Cr-0.2Si-0.3 Mg alloy are 698 MPa and 43.9 GPa∙%IACS, respectively. The dual-scale polymetallic second phase has been successfully constructed. The submicron-scale Fe8CrSi phase triggers the generation of substructures, such as geometrically necessary dislocation boundaries in lamellar structures and random dislocation boundaries in cell shapes. The high-density low angle grain boundaries (LAGBs) of substructure evolution enhances the substructure strengthening of the alloy. The main strengthening phase in precipitation strengthening is the nanoscale Fe11CrSi4 phase. An appropriate amount of Mg reduces the ultimate solid solubility of Fe atoms in Cu alloys, promotes the formation of a dual-scale second phase and refines the grain size of the alloy. Therefore, the comprehensive performance of the design alloy is significantly improved.

Quasi in-situ investigation on kinking behavior of Fe-Cr-Al alloy during thermal compression

As a rare plastic deformation mechanism, kinking has attracted substantial attention because of its additional softening effect on the matrix and its heterogeneous refinement on the original grains. However, the details of kinking deformation and the influence of crystal orientation on itself have not been sufficiently elucidated yet, thereby resulting in inadequate microstructure optimization associated with kinking. In this work, the kinking formation process in Fe-Cr-Al alloy during thermal compression deformation has been investigated by using quasi in-situ electron backscatter diffraction (EBSD). By employing the average Schmid factor (Average-SF) model, the influence of crystal orientation on kinking behavior is predicted accurately. The results suggest that the softening effect of kinking on the matrix can be attributed to the fact that the nucleation of pre kink bands (pre-KBs, misorientation <15°) leads to a pronounced reduction in geometrically necessary dislocations (GNDs) density within them. Besides, the ripening behavior of pre-KBs is closely associated with the variation of GNDs density and the overall slip activation capability of the matrix, and is essentially dependent on crystal orientation. The harder the crystal orientation, the smaller the Average-SF, the higher the increment rate of GNDs density, the larger the ripening rate pre-KBs. Under equivalent strain, orientations close to 〈111〉// compression direction (CD) have the highest probability of forming mature KBs with high angle grain boundaries (HAGBs, >15°), whereas KBs in those close to 〈001〉//CD almost always remain as pre-KBs with low angle grain boundaries (LAGBs).

Texture tailoring and microstructure refinement induced by {11−21} and {10−12} twinning in an extruded Mg-Gd alloy

An extruded Mg-17Gd (wt%) alloy with < 0001 > fiber texture was compressed along different directions at room temperature. Electron backscatter diffraction characterization was used to investigate the texture tailoring, twinning and dynamic recrystallization (DRX) behaviors. The results indicate that {10−12} and {11−21} twinning activated. The [0001] pole tilted to compression direction with the strain increasing and twinning dominated the texture transition. Activation of {10−12} twinning can be governed by Schmid law, while it is more appropriate to explain the activation of {11−21} twinning by the basal slip transfer condition in the twinned grains according to the slip trace analysis. In addition, operation of {11−21} twinning can facilitate the nucleation of {10−12} twin in the same grain by changing Schmid factor (SF) in the vicinities of {11−21} twin. Grain refinement occurred in the specimens with large strain and the average grain size decrease from ∼27 µm to ∼12 µm. The mechanism of grain refinement can be summarized as follows: the mobility of twin boundaries will be restricted because of the twin-twin and dislocation-twin interaction and converted to random high angle grain boundaries (HAGBs). The random HAGBs offer the nucleation sites and the strain accumulation within twins offer the driving force which facilitate the formation of DRXed grains at random HAGBs. Additionally, the SF difference within parent grain may result in the formation of HAGBs, and these HAGBs can connected the random HAGBs that result from the dislocation-twin interaction which result in the formation of new grains.

High cycle fatigue behavior of inertia friction welded joint of FGH96 alloy at high temperature

High cycle fatigue behavior of FGH96 alloy inertia friction welded joint under axially push–pull cyclic loading was studied at 650 °C. Results showed fatigue strength of welded joint was comparable to pure base metal. Larger grain size, more twin boundaries and high angle grain boundaries induced a higher crack initiation potential at the base metal. Interior crack initiation was dominant while it was transferred from large facet, to twin-assisted facet, and finally to inclusion with stress level decreasing. The Z-parameter model was applied to fatigue life prediction in the case of facet-induced crack initiation for the first time, and good agreement was obtained between prediction and experimental data.

Towards active learning: A stopping criterion for the sequential sampling of grain boundary degrees of freedom

Many materials processes and properties depend on the anisotropy of the energy of grain boundaries, i.e. on the fact that this energy is a function of the five geometric degrees of freedom (DOF) of the interface. To access this parameter space in an efficient way and to discover energy cusps in unexplored regions, a method was recently established, which combines atomistic simulations with statistical methods (Kroll et al., 2022). This sequential sampling technique is now extended in the spirit of an active learning algorithm by adding a criterion to decide when the sampling has advanced enough to stop. In this instance, two parameters to analyse the sampling results on the fly are introduced: the number of cusps, which correspond to the most interesting and important regions of the energy landscape, and the maximum change of energy between two sequential iterations. Monitoring these two quantities provides valuable insight into how the subspaces are energetically structured. The combination of both parameters provides the necessary information to evaluate the sampling of the 2D subspaces of grain boundary plane inclinations of even non-periodic, low angle grain boundaries. With a reasonable number of data points in the initial design, only a few appropriately chosen sequential iterations already improve the accuracy of the sampling substantially and unknown cusps can be found within a few additional sequential steps.

AZ91 alloy nanocomposites reinforced with Mg-coated graphene: Hot-pressing sintering, heat-treatment and microstructure

Graphene has been successfully coated with the nano-Mg through organic magnesium reduction method. The nano-Mg coated graphene (GNPs) was further processed into AZ91 magnesium matrix composites through hot-pressing sintering. The effects of different solution and aging heat treatment on the microstructure evolution and interface bonding of GNPs reinforced AZ91 composites with a content of 2.5 wt% were investigated. The results show that with the increase of GNPs content, the average grain size of the composites decreases gradually, and different forms of β-Mg17Al12 precipitates in the microstructure can heterogeneously nucleate on GNPs. Compared with T4, T5 heat treatment, after T6 heat treatment, a large number of strengthening eutectic phase (β-Mg17Al12) was precipitated in the composite, which promoted the formation of a three-dimensional interlocking tight bonding structure at the GNPs-Mg interface and significantly enhanced the interface bonding strength. During the T6 heat treatment, dynamic recovery and recrystallization occurred in the composites. The area fraction of angle grain boundary increases, and the recrystallization area fraction increases from 24.49% to 75.72%. The texture intensity is obviously weakened, and the maximum texture intensity is reduced from 5.01 to 3.37.

Excellent strength-ductility synergy in a novel medium manganese steel containing 2.4Al-2.9Mn-4.1Ni realized through limited thermo-mechanical processing steps

In this study, the effect of microstructural and micro-textural evolution on tensile properties in hot forged (HF) and hot rolled (HR) medium-Mn steel containing ∼4 wt% Ni is systematically investigated. The microstructures of HF and HR specimens comprise of martensite (α′), nanosized NiAl (B2) precipitates and retained austenite (RA). The excellent tensile properties in HF and HR specimens have been achieved through minimal thermomechanical processing routes. The HR specimen exhibits better strength (∼1868 ± 69 MPa), and ductility (∼16.5 ± 1%) as compared to the HF specimen (∼1765 ± 32 MPa, and ∼12 ± 2%). The prior austenite grain (PAG) size of HR specimen is lower than HF, leading to lower width of lath martensite which contributes to achieving higher strength in the former specimen. Apart from the lath width, the presence of twin martensite along with RA, higher fraction of high angle grain boundaries (HAGBs), and ∑3 boundaries improve the strength-ductility synergy in HR specimen as compared to HF condition. The development of higher intensity γ-fiber <111>//ND during hot rolling has resulted in better ductility in HR specimen, whereas the presence of high intensity Cube texture {001}<100> has deteriorated the tensile properties of HF specimen. Further, visco-plastic self-consistent (VPSC) simulation has been carried out to understand the influence of primary {110}<111> and secondary {112}<111> slip systems activation on strain hardening response of the specimens. It has been observed that the higher activation of primary slip system {110}<111> in HR specimen leads to better strain hardening response at stage IV as compared to HF specimen.

Thermal cycling to improve the impact toughness of 1Cr11Ni2W2MoV steel with banded microstructure

Microscopic segregation of chemical elements results in the banded microstructure of finished steel products, and the harmful effect of banded microstructure on toughness is well recognized. However, typical industrial hot work processing, such as hot rolling and forging, cannot erase the banding, and eliminating it with homogenization heat treatment is also challenging and uneconomical. In this study, the impact energy of a 12%Cr tempered martensitic steel with the banded microstructure was significantly improved from 74 J to 154 J by employing thermal cycling and tempering as the final heat treatment to meet the service performance requirement. Compared to conventional quenching and tempering (Q + T), thermal cycling and tempering (TC + T) simultaneously refined the effective grain size of tempered martensite matrix and coarse M23C6 carbides that were continuously distributed along the δ-ferrite/matrix phase boundaries. As a result, more high angle grain boundaries (HAGBs) were introduced and acted as obstacles in retarding the propagation of microcracks. Additionally, finer carbides precipitated at the banding boundary during tempering, owing to the increasing triple junctions between δ-ferrite and tempered martensite. These finer carbides prevent the initiation of microcracks. The results demonstrate the potential of TC + T as a final heat treatment to remove the detrimental effect of banded microstructure on impact toughness.

Ultrafine lamellar microstructures for enhancing strength-ductility synergy in high-entropy alloys via severe cold rolling process

In the present study, the effect of severe cold rolling (SCR) on the microstructure and mechanical characteristics of X0.25FeCoNiMnV (X: Al, Ti) high-entropy alloys (HEAs) specimens was investigated. It was observed that the as-cast and homogenized specimens exhibited a two-phase microstructure consisting of face-centered cubic (FCC) and a minor body-centered cubic (BCC) structure. Electron backscattered diffraction (EBSD) analysis indicated that SCR could effectively induce a strong rolling texture with stretched grains along the rolling direction and lamellar deformation bands as the major microstructural features for all the HEAs specimens. Moreover, a large number of nano-scale grains are formed within the coarse-stretched microstructure of Al0.25FeCoNiMnV and Ti0.25FeCoNiMnV HEAs specimens due to local fragmentation, resulting in a bimodal grain structure. After 85 % cold rolling, the fraction of high angle grain boundaries and mean misorientation angle of the boundaries are 54 % and 22.15° in the FeCoNiMnV specimen, while they are 61 % and 26.42° for Al0.25FeCoNiMnV, and 57 % and 28.53° for Ti0.25FeCoNiMnV. Ti0.25FeCoNiMnV specimen exhibited the best combination of strength-ductility (tensile strength of 1225 MPa and elongation of 26 %) compared to FeCoNiMnV (720 MPa and 15 %) and Al0.25FeCoNiMnV (970 MPa and 18 %). Observations revealed that the failure mode of HEAs specimens was a ductile type fracture with a combination of deep and shallow dimples. The findings revealed that SCR can effectively improve the strength and microhardness of HEAs, while it highly retains their elongation owing to the formation of a bimodal structure consisting of new grain boundaries and dislocation substructures.

Hot deformation characteristics and microstructure evolution of Al20Co36Cr4Fe4Ni36 eutectic high entropy alloy

Eutectic high entropy alloys (EHEAs) have garnered much research attention due to their excellent mechanical properties, superior castability, good oxidation resistance, and stable microstructure in a wide range of temperatures. Inspires from mature Ni and Co-based superalloys, a novel Al20Co36Cr4Fe4Ni36 EHEA was designed and prepared to obtain good mechanical properties at both room temperature and high temperatures in this study. The novel EHEA consists of a L12 phase and a B2 phase with a superior ultra-tensile strength of 1005 ± 40 MPa and a ductility of 8.8 ± 0.4% at room temperature. The engineering compressive stress–strain curves at 1073 K indicated that the current EHEA performs a high strength of 321 ± 9 MPa without fracture even at a high temperature of ~0.6 TE, which is even superior to some refractory high entropy alloys (RHEAs). The hot deformation behavior and microstructure evolution of the new EHEA was investigated at the temperature range of 1073 K to 1273 K and various strain rates from 10−3 s−1 to 10−1 s−1. The constitutive equation that describes the correlations between the flow stress, strain rate, and temperatures was constructed. Continuous dynamic recrystallization (CDRX) was the primary dynamic recrystallization (DRX) mechanism in both the L12 and B2 phases during hot deformation. However, the proportion of dynamic recrystallization grains in the L12 phase is higher than that in the B2 phase according to the high angle grain boundaries (HAGBs) under the same deformation condition, suggesting that the CDRX kinetics was faster and stronger in the L12 phase. The present study provided a new thought for designing EHEAs and an insight into the hot deformation behavior of EHEAs with lamellar morphologies.

Effect of Initial Orientation on the Anisotropy in Microstructure and Mechanical Properties of 2195 Al-Li Alloy Sheet during Hot Tensile Deformation.

The 2195 Al-Li alloy, as one of the representative third-generation Al-Li alloys, has extensive applications in lightweight aerospace structures. In this paper, the anisotropy in mechanical properties and microstructure evolution of 2195 Al-Li alloy sheets were investigated under a strain rate of 0.01, 0.1, 1 s-1 and a temperature of 440 and 500 °C. Experimental results showed that the hot tensile properties of the 2195 Al-Li alloy sheet exhibited a strong dependence on loading directions. The peak stress (PS) and elongation (EL) along the rolling direction (RD) were larger than the transverse direction (TD). For the tests carried out at 440 °C-1 s-1, the PS values of the sheets stretched along the RD and TD are 142.9 MPa and 110.2 MPa, respectively. And, most of the PS anisotropy values are larger than 15%. The anisotropy in EL is less significant than in PS. All the differences are about 10%. Moreover, dimples in the samples stretched along RD were more and deeper than those along TD at 440 °C. The fracture morphology along RD and TD were similar, and both were cleavage fractures at 500 °C. Particularly, the fractions of high angle grain boundaries (HAGBs) along TD were all about 5% larger than those of RD. And, there were more small-sized continuous dynamic recrystallization (CDRX) grains inside the initial grains and discontinuous dynamic recrystallization (DDRX) grains featured with the local bulge of grain boundaries along TD. This was due to the smaller average Schmid factor and the vertical EL trend of the initial grains when the samples were stretched along TD. A model of grain evolution during the dynamic recrystallization (DRX) along RD and TD was proposed based on EBSD results. The Schmid factor and banded structure had a more prominent effect on the hot ductility of the 2195 Al-Li alloy compared with the degree of DRX, thus presenting a higher EL and better hot ductility along RD.

A novel high nitrogen hot-work die steel 3Cr5Mo2SiVN achieving excellent hardness-strength-toughness balance

A novel high nitrogen hot-work die steel 3Cr5Mo2SiVN was manufactured by pressurized induction melting, possessing excellent hardness-strength-toughness balance under conventional heat treatment process. The results indicated that 3Cr5Mo2SiVN steel exhibited better hardness-toughness synergy compared with H13 steel. Meanwhile, its impact energy and elongation were improved by ca. 50% and 13% compared with H13 steel with similar hardness. The achievement of superior toughness of 3Cr5Mo2SiVN steel could be attributed to the following reasons: firstly, the higher fraction of operators 1 and 2, more uniform and refined close-packets and Bain groups as well as smaller effective grain size obtained a higher ratio of high angle grain boundaries, which resulted in the more changes of crack propagation direction and thus an increase of energy absorption. Secondly, the smaller size and number density of undissolved M(C, N) carbonitrides distributed along the prior austenite grain boundaries required greater tensile stress and then reduced grain boundary brittleness. For the strengthening mechanism, the precipitation strengthening increment was mainly provided by M(C, N) carbonitrides and M3C carbides for 3Cr5Mo2SiVN steel, and MC carbides for H13 steel, respectively. The smaller effective grain size and higher interstitial atom (C and N) content in the martensitic laths of 3Cr5Mo2SiVN steel were crucial factors in enhancing its strength.

Effect of deformation and annealing on microstructure and corrosion behavior of 7075 aluminum alloy with micro arc oxidation coating

The microstructure of cold extruded 7075 aluminum alloy after annealed at various temperatures was investigated by EBSD and TEM. The results show that the grain size, dislocation density and Schmid factors vary significantly by annealing temperature. The alloy annealed at 300 °C owns a more compact and uniform coating with better corrosion resistance compared with those of other temperatures due to the combination of fine grain size and suitable electrical conductivity. The corrosion resistance of the bare alloy decreased with an increase of annealing temperature results from increased grain size, rate of high angle grain boundaries (HAGBs) and Schmid factors.