Low Boundaries Research Articles

Transition of low and high-angle grain boundaries during strain rate-induced dislocation storage and annihilation

This work explores the characteristics of dislocation storage and annihilation behavior in heterogeneous metal welded joints under different strain rates. It forms the micro-strain gradient alternating between the strain and strain-free zones due to the annihilation of dislocations and transition of substructure in welded joints. This gradient structure keeps the strain values at a lower level by limiting the slip of geometrically necessary dislocations (GNDs) in the fine-grained region, while most of the plastic strain is accommodated in the coarse-grained region, thus promoting the storage of GNDs and low-angle grain boundaries (LAGBs). The strain rate enhances the degree of transition among GNDs, LAGBs, and high-angle grain boundaries (HAGBs) by promoting the transition from LAGBs to HAGBs in the fine-grained region, which accelerates the annihilation of GNDs. More importantly, the formation of micro-strain gradient improves the strength and strain hardening ability of welded joints. This work will provide a better understanding of the substructure transition behaviors of heterogeneous metals under different strain rates.

Influence of deformation substructure on the strain compatibility of ferrite and bainite in a commercial pipeline steel

The strain instability due to hardening of the soft phase is a key factor in premature failure of pipeline steels after work hardening. In this work, we reveal the hardening mechanism of ferrite is actually the strengthening effect of deformation substructures on the flow stress, where the strengthening contribution by the low angle grain boundaries (LAGBs) is the main stress enhancement way. It is able to subdivide the ferrite into many sub-grain structures through dynamic evolution between geometrically necessary dislocations (GNDs), LAGBs and high angle grain boundaries (HAGBs) during work hardening. These subgrains have independent orientations and can be further subdivided, thus causing the Hall-Petch strengthening effect. Based on the hardening mechanism of ferrite, the representative volume element model with different levels of work hardening is developed. As the level of work hardening increases, the strain compatibility is gradually reduced between ferrite and bainite. The strain localization factor is increased from 0.023 to 0.061, the plastic deformation difference between the two phases is increased from 6.71% to 10.93%, and strain partitioning coefficient is reduced from 7.11 to 3.07. This work will contribute to better understand the substructure strengthening of dual-phase materials and provide a new idea for the protection of work-hardened metals.

Intermediate temperature tensile behavior and processing map of a spray formed 7075 aluminum alloy

The intermediate temperature deformation behavior and microstructural evolution of a spray formed 7075 Al alloy were performed at temperatures 60–120 °C with strain rates 0.00001–0.1 s−1. The results reveal that the softening mechanism of the intermediate temperature deformation is dynamic recovery (DRV), which can be quantitated by Zener-Hollomon (Z) parameter. The constitutive equation can reasonably predict the stress-strain behavior of the alloy at intermediate temperatures. The activation energy is related to the deformation process and the interactions of the substructure, the average value is 140.1 kJ/mol. The higher efficiency of power dissipation in the processing map is related to the DRV at lower deformation temperatures. The optimum processing domain is located at the temperature of 90–120 °C and strain rate of 0.006–0.1 s−1 with a power dissipation efficiency range of 20%–40%. The processing parameters of the intermediate temperature deformation should be strictly controlled according to the strain. The uniform microstructure of 7075 alloys obtained by spray forming is crucial to overcome the disadvantage of poor deformability. The inferior coordinated deformation facilitates crack propagation at low strain rates. The high-density dislocation provides adequate nucleation sites for dynamic precipitation. The level of dynamic restoration facilitates the transformation from low angle grain boundaries (LAGBs) to high angle grain boundaries (HAGBs). The dislocations move to the grain/sub-grain boundary to form dislocation walls at low temperatures. The contribution to the stress from grain boundary (13.2–17.7 MPa), nanoscale precipitates (65.7–191.5 MPa), and dislocation (5.9–23.4 MPa) increases the difficulty of deformation. The deformation process at higher temperatures (105–120 °C) can be facilitated by the dissolution and coarsening of the nanoscale precipitates, and dislocation motion at the grain boundary.

Effect of intermediate-temperature severe plastic deformation on microstructure evolution, mechanical properties and corrosion behavior of an Al-Zn-Mg-Cu alloy

Grain refinement is an important means to strengthen the properties of alloy. Here the Al-Zn-Mg-Cu alloy was processed by intermediate-temperature equal channel angular pressing (ECAP) to evaluate the grain refinement, mechanical properties and corrosion resistance of the alloy. Ultra-fine grained structure was obtained by 4 passes ECAP applied to Al-Zn-Mg-Cu alloy. With the increase of ECAP passes, some low angle grain boundaries (LAGBs) transform to high angle grain boundaries (HAGBs), forming micron-sized fine grains. Meanwhile, the strength, microhardness and corrosion resistance of the alloy were improved with the increase of extrusion passes. The pinning effect caused by dynamic precipitation and the mechanical fragmentation caused by ECAP refine the grains of the alloy. The microhardness and strength of the deformed alloy were improved under the combined action of fine grain strengthening, dislocation strengthening and second phase strengthening. A high density of dislocations, refined grains and secondary phases were produced by intermediate-temperature ECAP, which together improved the corrosion resistance of the alloy.

Effect of laser energy density on surface morphology, microstructure and mechanical behaviour of direct metal laser melted 17-4 PH stainless steel

Abstract The surface and microstructural characteristics of 3D printed parts play a significant role under mechanical loading. The authors have explored the effect of laser energy densities on the surface morphology, microstructure and mechanical behaviour of 17-4 precipitation hardened stainless steel fabricated under the direct metal laser melting technique. The considered processing parameters were laser energy density and its technical parameters: laser power, layer thickness, hatch spacing and scanning speed. The mechanical and metallurgical properties of the as-printed samples appeared better than the wrought counterpart due to the higher densification level (99.74 %) induced by the rotating scanning strategy. X‐ray diffraction revealed the presence of both the martensitic α phase and austenitic γ phase in the as-printed sample. There is no significant anisotropy in the mechanical behaviour as the build direction has a random texture with a fine columnar grain structure. The high laser energy density with low layer thickness results in an excellent surface finish. The tensile strength (1180 MPa) and the elongation for the as-printed sample (45.0 %) were considerably more significant than that for the wrought sample (1160 MPa and 26.0 %), which is attributed to the combination of low and high-angle boundaries, as confirmed by the electron backscatter diffraction results.

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.

“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.

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.

Laser based additive manufacturing of tungsten: Multi-scale thermo-kinetic and thermo-mechanical computational model and experiments

Coupling a multi-scale thermo-kinetic and thermo-mechanical model with critical experiments, the present study investigates the role of process parameters on the resulting densification, cracking behavior, and microstructural evolution in laser-based powder bed fusion (LPBF) of pure tungsten. Thermal management strategies comprising shorter scan length (1 mm) and/or substrate preheating (873 K) were explored to achieve higher densities and mitigate cracks. A finite element (FE) based thermo-kinetic and thermo-mechanical computational model was developed to simulate the process spanning from the melt pool scale to the component level. FE simulations indicated pronounced heat accumulation with reduced scanning lengths. Such increased heat accumulation resulted in melt pools with larger dimensions (width and depth) and more favorable rheological characteristics resulting in printed tungsten parts with higher densities. Using an input laser fluence (ILF) of 28.6 J/mm2 and a shorter scan length of 1 mm, a relative density of maximum 99.2% was achieved, the highest reported value using LPBF in case of pure tungsten. FE simulations indicated nearly 5-fold reduction in thermal gradients and 2-fold reduction in cooling rates associated with the processing conditions corresponding to low degree of cracking (ILFs:28.6 J/mm2 and ILF 15.9 J/mm2 augmented with substrate preheating at 873 K) compared to the processing conditions of high degree of cracking (ILF: 15.9 J/mm2). Further, high ILF samples were characterized by significant extent of in-grain misorientations and low angle boundaries indicating that process induced in-situ restoration mechanisms are operative, thereby potentially aiding in reduction of cracking.

Optimizing strength and electrical conductivity of Cu–Fe–Ti alloy by pre-aging treatment

Pre-aging treatment before cold rolling and final aging was introduced to optimize the mechanical properties and electrical conductivity properties of Cu-0.75Fe-0.35Ti alloy. The alloy was pre-aged at 540 °C for 6 h, then cold rolled with thickness reduction of 80%, followed by aging at 460 °C for 2 h (SARA). Vickers hardness, electronic conductivity and uniaxial tensile tests were conducted to measure the mechanical and electrical conductivity properties. Electron backscatter diffraction and transmission electron microscope were used to characterize the microstructural evolution. The results showed that the hardness, tensile strength and electrical conductivity of SARA were up to 198.2 ± 4.8 HV, 622.6 ± 6.3 MPa and 63.8% IACS, respectively, which were superior to those achieved without pre-aging. The enhanced mechanical properties of SARA were attributed to an increase in low angle boundaries (LABs), geometrically necessary dislocation (GND) density and volume fraction of precipitates, while the favorable electrical conductivity was ascribed to the enhancement of precipitation and resultant matrix purification.

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.

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).

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.

Improved Perovskite CH3NH3PbI3 Thin Films by ZIF‐67 Additive Assisted Co Ion Doping toward High‐Performance and Stable Photodetectors

AbstractLead halide perovskites are considered important materials for the fabrication of high‐performance optoelectronic devices. However, the poor quality of thin films with abundant grain boundaries and defect states greatly lower the performance and stability of as‐fabricated devices. In the present work, the growth of improved perovskite CH3NH3PbI3 thin films through Co‐based zeolite imidazole framework (ZIF‐67) additive assisted Co ion doping is reported. Investigation of the morphological, structural, optical, and electrical properties shows that the Co ion doped CH3NH3PbI3 (CH3NH3Pb0.98Co0.02I3) thin films exhibit low grain boundaries, low defect states, and improved structural stabilities. The positive roles of suppression of defect states are further demonstrated by improved performance and stability of photodetectors based on CH3NH3Pb0.98Co0.02I3 thin films. Finally, density functional theory calculations are performed to reveal the mechanisms of passivation of undercoordinate Pb2+ vacancy defect states by Co ion doping. The results provided in this work will pave the way toward the fabrication of high performance and stable perovskite photodetectors in the future.

Multimodal microscopy of extended defects in β-Ga2O3 (010) EFG crystals

Beta-phase gallium oxide (β-Ga2O3) has attracted attention in recent years as a potentially low cost, large area substrate and active layer material for high power, high temperature power electronics and sensing devices. However, growth of β-Ga2O3 crystals is complicated by easily activated (100) and (001) cleavage planes, the presence of low angle grain boundaries (LAGBs) and twins, and the potential formation of polycrystalline grains. In this study, β-Ga2O3 crystals were grown by the edge-defined film-fed growth technique with an (010) principal face. Two crystals with apparently randomly formed high angle grain boundaries (HAGBs) were selected and analyzed by electron backscatter diffraction, electron channeling contrast imaging, and cathodoluminescence to investigate the nature of the LAGBs and the source of the HAGB formation. It was discovered that planar LAGBs lying parallel to the (010) plane exist in the region immediately preceding the start of an HAGB. Increased misorientation across the LAGB was observed, approaching the initiation of a new grain. We present multimodal microscopy characterization, correlating misorientation and variation in optoelectronic properties with LAGBs and the associated dislocations.

Effect of strain rate on dynamic recrystallization mechanism of Mg-Gd-Y-Sm-Zr alloy during hot compression

Strain rate plays an important role in the hot deformation process of materials. In this work, hot compression experiments were conducted on Mg-Gd-Y-Sm-Zr alloy at the strain rates of 0.001–1 s−1 by Gleeble thermal simulation machine. The deformation behavior and microstructure evolution were studied in detail, and the dynamic recrystallization (DRX) mechanism was discussed. The results show that the flow stress of the alloy increases with the increasing strain rate. Deformation bands (DBs) are formed inside some grains at the strain rates of 0.1–1 s−1, which increase interfaces in the microstructure and hinder the dislocation movement. DRX grains are formed at the regions of DBs and original high angle grain boundaries (HAGBs). The DRX mechanism at high strain rates includes DBs induced DRX and discontinuous dynamic recrystallization (DDRX). At low strain rates and small strain, the dislocations first accumulate at the original HAGBs, and DDRX occurs in the alloy. The subgrains with low angle grain boundaries (LAGBs) are formed inside the initial grains through sufficient dynamic recovery (DRV) as strain increases, and LAGBs will gradually transform into HAGBs with the increasing misorientation angle. The DRX mechanism shows the synergistic effect of DDRX and continuous dynamic recrystallization (CDRX) at low strain rates.

Optimizing LPSO phase to achieve superior heat resistance of Mg–Gd–Y–Zn–Zr alloys by regulating the Gd/Y ratios

The microstructural features of long period stacking ordered (LPSO) phase in Mg–(11-x)Gd–xY–1Zn–0.3Zr (wt.%) alloys were optimized by adjusting Gd/Y ratios to10:1, 8:3, 6:5, 4:7 and 2:9, respectively. Superior high-temperature strength was successfully obtained in the extruded 6Gd–5Y alloy with ultimate tensile strength (UTS) values of 338 MPa and 286 MPa at 250 °C and 300 °C, respectively. In the as-cast alloys, with the decrease of Gd/Y ratio, the type of intermetallic compound gradually changed from (Mg, Zn)3(Gd, Y) (W) to LPSO phase. After homogenization, the W phase was completely converted to LPSO phase except for the 10Gd–1Y alloy. Importantly, the block-shaped LPSO phases with the smallest size and the abundant lamellar LPSO phases were formed in the 6Gd–5Y alloy, which significantly influenced the microstructures of the extruded alloy. Firstly, the small sized block-shaped LPSO phase before extrusion provided favorable conditions for the formation of thin plate-shaped LPSO phase at DRXed regions, which effectively hindered the grain boundary migration at high temperature due to superior thermal stability. Secondly, the profuse lamellar LPSO phases prior to extrusion contributed to the highest density of low angle grain boundaries (LAGBs) and residual dislocations in the extruded 6Gd–5Y alloy, which obstructed dislocation slip at elevated temperatures. In addition, the abundant lamellar LPSO phase also caused the fine DRXed grain size and strong basal texture to enhance the high-temperature strength of the extruded 6Gd–5Y alloy.

Damage tolerance of low angle grain boundary in a fourth generation Nickel-based single crystal superalloy at 1100 °C service conditions

The effect of different low angle grain boundaries (LAGBs) on the microstructure and mechanical properties in a fourth generation Nickel-based single crystal superalloy was studied. Double seed crystals techniques were used to obtain the specimens with LAGBs. The results showed that there were coarse γ′ phases at LAGBs with the grain boundary angle increasing, the shape of which changed from block to long strip at as-cast and the size of which increased gradually after heat treatment. After thermal exposure at 1100 °C for 100 h, the microstructure stability deteriorated when the angle exceeded 12.6°, where the discontinuous precipitation (DP) colonies region occurred along the grain boundary because of high interfacial energy and mobility at LAGBs. With the grain boundary angle increasing, the ultimate tensile strength of the sample rose slightly and then fell, while the elongation and creep rupture life decreased significantly exceeded 4.9°. Under 1100 °C service condition, the carbides and topological close-packed (TCP) phases were precipitated at LAGBs, which seriously impaired the grain boundary bonding strength. On the other hand, the misorientation at LAGBs led to the stress concentration, which increased the difficulty of grain boundary coordinated deformation. Finally, the damage tolerance of the LAGBs in this fourth generation Nickel-based single crystal superalloy was approximately estimated as about 7° at 1100 °C/150 MPa creep rupture condition based on a 70% creep rupture life standard.