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Twinning-detwinning (TDT) behavior in a strongly basal-textured Mg-Li alloy during two-step compression (RD)-compression (ND) process was investigated using quasi-in-situ EBSD. TDT behavior and TDT variants selection were statistically discussed with the loading path for the first time. Non-Schmid twinning behavior was observed in the first step compression, owing to the local stress fluctuations by neighboring twins; in contrast, Schmid's law well predicted the detwinning variants selection. This asymmetrical TDT behavior was first investigated to date related with the strong basal texture and loading path. Besides, with the progress of compression, Schmid factors for twinning demonstrated a decreasing tendency; however, those for detwinning during the second step displayed an abnormally increasing trend, fundamentally stemming from prior twinning behavior.

The dual-phase Re0.5MoNbW(TaC)0.5 composite, consisting of refractory body-centered cubic (BCC) high-entropy alloy and carbide with many fine eutectic structures, was successfully synthesized by arc melting. The phase stability, high-temperature mechanical properties and strengthening mechanism of the as-cast composite were studied. The microstructure of the composite remained stable after annealing at 1300 ℃ for 168h. It exhibited remarkably high-temperature strength, yield strength ∼ 901 MPa, and true ultimate compressive strength ∼ 1186 MPa at 1200 ℃. The BCC phase and carbide exhibited a semi-coherent interface with good bonding after severe deformation at 1200 ℃. The dipolar dislocation walls in BCC phase, restricted dynamic interaction between defects in carbide, and the pinning effect of semi-coherent interface offered effective strengthening effects.

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth (Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5Y-2Nd-3Sm-0.5Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal II dislocations in the later stage of deformation; these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors: (i) single twin variant ‘parallel’ structure, (ii) single twin variant ‘cross’ structure, and (iii) multi twin variant ‘cross’ structure. In addition, three types of twin-grain boundary interaction behaviors were summarized: (i) twin ‘refracting through’ grain boundary, (ii) twin ‘parallel through’ grain boundary, and (iii) twin ‘fusing through’ grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

We investigated the effects of Al concentration on the reciprocated motion of twin boundaries in pre-strained Mg-Al-Zn alloys through a combination of applied compression and tension, in-situ electron-backscattering diffraction observations, and high-angle annular dark-field scanning transmission electron microscopy observations. The twin growth was restricted by increased Al concentration, which resulted in the occurrence of smaller-sized twins. The reverse motion of twin boundaries was also restricted, resulting in the formation of higher fractions of secondary twins and 2–5° boundaries during reverse tension. The secondary twins and 2–5° boundaries mainly contributed to the increased ultimate tensile strength of the pre-strained Mg alloys. This effect is more significant in Mg alloys with larger pre-compression. Moreover, the increased amount of the Al solute atoms, rather than the precipitates, mainly contributed to the increased strengthening effect on the twin boundary motion. Our research contributes to development of high-strength Mg alloys by stabilizing twin boundaries.

We utilized electron backscatter diffraction to investigate the microstructure evolutions of a newly developed magnesium-rare earth alloy (Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr) during instantaneous hot indirect extrusion. An equiaxed fine-grained (average grain size of 3.4 ± 0.2 µm) microstructure with a weak texture was obtained. The grain refinement was mainly attributed to the discontinuous dynamic recrystallization (DDRX) and continuous DRX (CDRX) processes during the hot indirect extrusion process. The twin boundaries formed during the initial deformation stage effectively increased the number of high angle grain boundaries (HAGBs), which provided sites for new grain nuclei, and hence, resulted in an improved DDRX process. Along with DDRX, CDRX processes characterized by low angle grain boundary (LAGB) networks were also observed in the grain interior due to effective dynamic recovery (DRV) at a relatively high temperature of 773 K and high strain rates. Thereafter, LAGB networks were transformed into HAGB networks by the progressive rotation of subgrains during the CDRX process.

In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rotating electrode process (PREP) have been highly recommended for powder-type additive manufacturing. Most research on PREP is aimed at establishing relationship between PREP parameters and powder size. However, almost no dedicated research on granulation behavior has been conducted so far. In the present study, PREP experiments of Ti64 and SUS316 alloys were carried out. Numerical modeling based on computational thermo-fluid dynamics was developed to analyze the granulation behavior. In particular, the roles of the additionally introduced gas blast and the morphology of the electrode end surface in fluid granulation were preliminarily investigated. The study showed that in addition to the electrode's rotating speed and diameter, manipulating the plasma arc current (i.e., the melting rate) could also be an effective way to control the PREP-powder size. According to the simulation, there were competing actions of the gas blast affecting the powder size. The gas blast created disturbance on the fluid and deepened the depression of the electrode end surface, which facilitated powder refinement. However, the cooling effect enhanced the fluid stability and hindered fluid granulation. The conclusions indicated the possibility of using various methods to manipulate PREP-powder size.

The effectiveness of applying an electron beam melting (EBM) additive manufacturing process to Ti–6Al–2Sn–4Zr–2Mo–0.1Si–1B alloy by comparing the EBM alloy with a corresponding wrought alloy was demonstrated. The EBM alloys exhibited microstructures that had much finer whisker-like TiB precipitates with nanometer width dispersed more uniformly than those of the wrought alloy formed due to the rapid solidification process. The EBM alloys had higher tensile strength than the wrought alloy at elevated temperatures up to 873 K. Furthermore, the oxidation resistance at 1073 K of the EBM alloys was significantly improved because of the dense oxide film formation.

Mo, as one of the most important elements in superalloy, dramatically affected the microstructure and mechanical properties via solution strengthening, forming Mo-enriched phase, and atomic segregation (Suzuki effect). To verify the influence of Mo on the microstructure and mechanical properties of Ni–Co-based superalloy, two types of alloys containing 4 wt% (A4) and 8 wt% (A8) Mo were prepared. The microstructure was examined using X-ray diffraction, scanning electron microscope, and transmission electron microscope, while the mechanical properties were evaluated using tensile tests at both room temperature and elevated temperature (700 °C). The segregation of Mo at stacking faults was observed in both alloys after aging with pre-cold-rolling. At the same time, in alloy A8, direct precipitation of μ phase both along grain boundary and in the interior of matrix was observed after aging. In alloy A4, segregation of Mo along stacking fault ribbons resulted in formation of quasi-μ phase. The quasi-μ phase developed into μ phase during prolonged aging. There was no precipitate along grain boundary in A4 after aging resulting in poor ductility at both room and elevated temperature. The strength was enhanced by net effect of Suzuki segregation, γ′ phase strengthening, and fine μ phase.

Inconel 718 alloy rods were fabricated by electron–beam melting (EBM), where the cylindrical axes (CAs) deviated from the build directions (BD) by 0°, 45°, 55°, and 90°. The microstructures and high-temperature tensile properties of the rods were investigated by taking into account the effect of the BD. Columnar grain structures or mixtures of columnar and equiaxed grains were obtained in the rods. The direction of the grains tended to be oriented not only in the BD but also in the two horizontal EB scanning directions (SDs), which were perpendicular to each other. As a result, the crystal orientation of the rods could be controlled by appropriate choice of the CA. For instance, the rods oriented along the , , directions could be fabricated by choosing their CAs parallel to the BD, face diagonal, and space diagonal of the cubic space defined by the BD and the two SDs, respectively. The highest strength was obtained for the oriented rod. The dependence of strength on the rod orientation could be explained in terms of the anisotropies in the crystal orientation, columnar grain structure, and arrangement of the precipitate particles.

We combine pre-compressive test, reverse tensile test, re-compressive test, in-situ electron back-scattered diffraction, and high-resolution transmission electron microscopy to systematically investigate the effect of annealing on the reciprocal motion of twin boundary (TB) in pure Mg and Mg alloys AZ31 and AZ91. We find that the twin boundary mobility (TBM) can be enhanced by decreasing the dislocation density and increasing the number of coherent TBs after annealing for a short time. On the other hand, after prolonged annealing in Mg alloys, TBM decreases since TBs are stabilized by segregated solute atoms and precipitates. As a result, the TBM significantly depends on both the alloying element content and the annealing time. We demonstrate, for the first time, that friction stress and back stress can be applied to clarify the variation of TBM during annealing in Mg alloys. Our findings show that the TBM can be regulated by annealing, opening up a novel avenue for developing Mg alloys with high damping capacity or enhanced mechanical properties.

We systematically studied the effect of annealing on damping capacity of pre-deformed magnesium alloy and demonstrated the close relationship between damping capacity and twin boundary mobility. The damping capacity of pre-compressed AZ31 alloy can be improved by annealing for short period as a result of increased twin boundary mobility. In contrast, the damping capacity will be deteriorated after prolonged annealing in pre-compressed AZ91 alloy, which is ascribed to the stabilization of twin boundary by significant segregation and precipitation. In addition, we firstly succeeded to distinguish the damping capacity contributed by TB motion from that by dislocation motion in Mg alloys. This study provides the strategy to enhance damping capacity by improving twin boundary mobility after appropriate annealing, the time of which depends on the alloying element concentration.

Electrochemical measurements in 1 M HCl solution were performed to investigate the corrosion behaviour of Ti–6Al–4V alloy fabricated by electron beam melting (EBM) with cylindrical axes deviating from the building direction by 0°, 45°, 55°, and 90°. Microstructure characterization before and after the tests was carried out by using a variety of methods The results suggested that the corrosion resistance of EBM alloy in 1 M HCl solution increased slightly in the order of 45°, 90°, 55°, and 0°. Such difference can be ascribed to the grain boundary density and β phase constitution, which varied with the building direction.

By combining finite element analysis, compressive tests, reverse tensile tests, in-situ electron-backscattering diffraction, and high-resolution transmission electron microscopy, we investigate systematically twinning and detwinning behaviors of {10 1 ¯ 2} twins in pre-compressed pure Mg and AZ31 and AZ91 alloys. We find that the solute elements Al and Zn and the resultant precipitates disrupt the synchronous motion of atoms in twinning boundaries (TBs) during twin formation, yielding a large amount of incoherent TBs and low-angle boundaries from the pre-existing TBs after detwinning. The detwinning in pure Mg and its alloys is found to be linked to both twin boundary (TB) mobility and the interaction of their boundaries with dislocations. At a low strain level (0–1%), comparable detwinning activities in both pure Mg and its alloys are observed due to the selective detwinning in grains with the highest SF, while at high strain level (1–4%), the contribution of detwinning in Mg alloy is higher. This is because a small twin size in Mg alloys leads to strong interactions between basal slip and TBs, and also because in pure Mg, twin size is much larger and its tip is pinned by grain boundaries. In addition, the secondary twins are also found to be formed remarkably in the interior of the primary twins in Mg alloys with a high solute-element concentration owing to the low mobility of incoherent TB and the variation of local stress state by precipitates. The ultimate strength of reverse tensile test is found to increase more substantially in the pre-compressed Mg alloys in comparison to pure Mg, especially in the AZ91 alloy, which is attributed to the hardening effect caused by the formation of many low-angle and secondary TBs and also for the reduced Schmid factor for the basal slip in Mg alloys during detwinning.

Previous research has inadequately elucidated the influence of Nb addition on the tensile properties and oxidation resistance of Ti-based alloys. Herein, the tensile properties and oxidation resistance of Ti–6Al–2Sn–4Zr–2Mo–0.1Si–1.0B–xNb (x = 0, 0.2, and 1.0) alloys were investigated. The oxidation resistance was significantly improved by Nb addition without significantly influencing the tensile properties. The substitution of Ti4+ by Nb5+ in TiO2 and promoted formation of a Sn-rich layer between the oxidation layer and substrate caused an increase in the oxidation resistance, thus highlighting the effectiveness of Nb addition in enhancing the oxidation resistance.

Herein, we studied the microstructural evolution and deformation behavior of aged Mg–5Y–2Nd–xSm–0.5Zr (x = 1, 3, and 5 wt%) alloys during uniaxial compression at room temperature by transmission electron microscopy and quasi-in-situ electron backscatter diffraction techniques. The microstructures of Mg–5Y–2Nd–xSm–0.5Zr alloys are refined by the addition of Sm, with more occurrences of Mg(Nd, Sm) precipitates. The average grain size decreased significantly with increasing Sm content, from 80.4 µm (1 wt% Sm) to 64.8 µm (3 wt% Sm) and 50.0 µm (5 wt% Sm). The Mg–5Y–2Nd–3Sm–0.5Zr alloy exhibited a maximum compressive strength of 390.1 MPa and a maximum strain (e) of 0.103 at room temperature; {10−12} tensile twinning and dislocation slips are the main plastic deformation modes of Mg–5Y–2Nd–xSm–0.5Zr alloys during compression at room temperature. A number of and dislocations were observed in the aged Mg–5Y–2Nd–3Sm–0.5Zr alloy. The optimized ductility value is attributable to grain refinement; elemental Sm promotes the activation of the non-basal slip dislocations and interactions between twins and dislocations. The strengthening mechanism mainly includes solid solution, fine-grain, and precipitation strengthening.

The precipitation behavior of a novel cobalt-based superalloy subjected to prior plastic deformations was investigated in details by using field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), etc. The precipitation of a μ-phase, which appeared at temperatures of 1073–1373 K, was confirmed, while MC-type carbide was observed at the temperatures >1173 K. The prior plastic deformations promoted the precipitation of the μ-phase during subsequent aging. Further, the precipitation of the μ-phase at temperatures lower than 1173 K was confirmed to occur predominantly along the {111} slip bands; this was the dominant deformation mechanism of this alloy during plastic deformation. In contrast, there was no evidence that the precipitation of the MC-type carbide was affected by the prior plastic deformations in the case of the specimens exposed at 1273 and 1373 K. Keywords: μ-Phase, Prior plastic deformations, TCP phase, Cobalt-based superalloy, Suzuki segregation

We manufactured a Ti-based alloy via electron beam powder bed fusion (PBF-EB) and systematically investigated its microstructure and mechanical properties. We compared its properties with those of a conventionally forged alloy via electron backscattered diffraction, high-angle annular dark-field scanning transmission electron microscopy observations, and tensile tests. The formation mechanism, orientation relationship with the matrix, and crystal structures of the nanosized TiB particles were verified. The formation of nanosized TiB particles was mainly attributed to high cooling rates during PBF-EB manufacturing. In addition, the segregation of the solute Zr and Si atoms into the α-phase/TiB interface restricted TiB precipitate growth, which contributed to the formation of fine TiB particles in the alloy manufactured via PBF-EB. The orientation relationships between the TiB particles and α-phase in the PBF-EB-manufactured alloy are different from those in the hot-forged alloy. Moreover, for the first time, the simultaneous existence of the B27 and twinned B27 structures was verified in the nanosized TiB particles. Our study provides general guidelines for the manufacturing of materials strengthened by refined precipitates using PBF-EB or other additive manufacturing methods.

The hot deformation behavior of a Co–Ni-based superalloy was systematically investigated using thermal compression tests. Stress–strain curves showed a typical dynamic softening after peak stress, especially at high temperatures and low strain rates. An Arrhenius-type constitutive equation was developed to reveal the relationship between the flow stress, strain rate, and temperature, while a processing map was constructed based on the calculations from the stress-strain curves combined with microstructural observations to determine the optimum thermal deformation conditions. The extent of recrystallization was found to increase with increasing temperature, a decreasing strain rate, or an increasing strain. A complete dynamic recrystallization (DRX) condition was reached at 1050 °C/0.01 s−1/0.7. In addition, pre-existing annealing twins were replaced by discontinuous dynamic recrystallization (DDRX) grains along the twin boundaries and the twin-DRX (TDRX) grains in the twin interior. In the case of an un-twinned matrix, a combined DDRX and continuous DRX (CDRX) process occurred at high strain rates, in contrasted with a single DDRX process taking place at low strain rates.

An efficient optimization method based on a support vector machine (SVM) is used to optimize multiple process parameters of selective electron beam melting (SEBM) for a non-weldable nickel-base superalloy Alloy713ELC. The global optimum condition and the near optimum conditions are extracted to fabricate SEBM samples. All the SVM optimized conditions lead to near net shaped samples with even top surfaces. The sample fabricated under the global optimum condition for sample dimension of 10 mm exhibits pore-less cross-sections, columnar grains with fine γ′ precipitates and fine substructure, a small amount of grain boundary crack and excellent room temperature tensile properties. The samples fabricated under the global optimum condition and a near optimum condition with increased beam current for sample dimension of 15 mm exhibit excellent creep properties under 980 °C. In both the two situations for sample dimensions of 10 mm and 15 mm, SEBM samples with mechanical properties superior to conventional cast alloys can be achieved by testing only 1–3 SVM optimized conditions. We demonstrate the current method is effective for optimizing SEBM process, especially when multiple parameters need to be considered simultaneously. Besides, this method can rapidly provide not only a batch of conditions leading to samples with good top surfaces but also the optimum conditions leading to good building quality and superior mechanical properties.

Twin boundary mobility (TBM) has significant influences on the mechanical properties of magnesium (Mg). In the current study, the effects of pre-strain on TBM in pure Mg are systematically investigated by evaluating the change of yield stress during compression and tension, and microstructure observations by in-situ electron back-scattered diffraction. Owing to lower dislocation density and less restriction from other boundaries, smaller twins in pure Mg subjected to 2% compressive strain exhibit higher TBM than larger twins in pre-4% and pre-8% compressed pure Mg. Decreased TBM with increasing pre-strain is primarily because of the larger number of dislocations, which impede twin boundary motion. These findings indicate that altering twin size and dislocation density by pre-strain is an effective way to tailor TBM, providing a basis for developing new industrial Mg alloys.

Refined TiB precipitates significantly enhance the oxidation resistance of Ti-6Al-2Sn-4Zr-2Mo-0.1Si-1.0B alloy fabricated by electron beam powder bed fusion (EB-PBF). Refined TiB precipitates in the EB-PBF-built alloy enable finer oxide formation than the larger precipitates in the forged alloy, and the resulting oxidation layers are more compact. Evaporation of scattered B2O3 generated by the refined TiB precipitates in the EB-PBF-built alloy do not significantly accelerate detachment of the oxidation layer from the substrate. However, collective evaporation of B2O3 generated by larger TiB precipitates in the forged alloy accelerate detachment. The oxidation layer on the EB-PBF-fabricated alloy was more stable, preventing further oxidation and improving oxidation resistance.

To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.