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The inconsistencies regarding the fundamental correlation between Gd content and slip (twinning) activities of Mg alloys appeal further investigations. However, the traditional slip dislocations analysis by TEM is time-consuming, and that by SEM/EBSD cannot recognize the partial slip modes. These urge a more efficient and comprehensive approach to easily distinguish all potential slip modes occurred concurrently in alloy matrix. Here we report a modified lattice rotation analysis that can distinguish all slip systems and provide statistical results for slip activities in Mg alloy matrix. Using this method, the high ductility of Mg-Gd alloy ascribed to the enhanced non-basal slips, cross-slip, and postponed twinning activities by Gd addition is quantitatively clarified.

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.

Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic (FCC) structure, whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature (Trx). Microscopically, this hardening effect has been ascribed to several mechanisms, i.e. solute segregation to defects (dislocation and stacking fault) and short-range chemical ordering, etc. However, none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys (HEAs). Here we report the observations, using high-resolution electron channeling contrast imaging and transmission electron microscopy, of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below Trx. The dislocation substructures are observed to be thermally stable up to Trx, which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs. The microstructure feature is markedly different from that of conventional dilute solid solution alloys, in which dislocation substructures gradually vanish by recovery during annealing, leading to a strength drop. Furthermore, dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed (≤500 °C) HEA. This Al induced softening effect, could be associated with the anisotropic formation of dislocation substructure, resulting from enhanced dislocation planar slip due to glide plane softening effect. These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.

Flowability, particle size distribution and particle inner pore features for powders of Ti-6Al-4V, 316-steel, and Co-29Cr-6Mo alloys produced by plasma rotating electrode process (PREP) at various rotation speeds are analyzed by using scanning electron microscopy (SEM) and synchrotron X-ray computed tomography (CT). The results show that powder flowability is related to particle size distribution, surface morphology and dynamic friction coefficient of the alloy itself. This results in the higher powder flowability at low rotation speed than that at high rotation speed. Average particle size is roughly proportional to the square root of the reciprocal of rotation speed. In addition, particle size has an important effect on the particle porosity and the number faction of hollow powder in coarse powder is obviously larger than that in fine powder. Meanwhile, the alloy composition also plays a key role on porosity due to the various surface tension of alloys.

In this study, characteristics of powder for additive manufacturing from three typical alloys Ti-6Al-4V, 316-steel and Co-29Cr-6Mo produced by plasma rotating electrode process (PREP) at 4 rotation speeds are compared in details. The results indicate that for a given rotation speed, alloy with higher ratio of surface tension and density ( γ ρ ) is capable of producing powder with larger particle size and narrower particle size distribution δ (D90-D10). The formation mechanism of the satellite and irregular particle produced by PREP are discussed in detail. The satellite powder is confirmed to be formed by collisions between two particles, and the volume fraction of the satellite powder is lower in the alloy with higher ( γ ρ ) and high thermal conductivity. The formation of irregular powder is possibly related to the peeling of liquid film, which is strongly dependent on the thermal conductivity of alloy.

Iridium (Ir)-based materials are known to be state-of-the-art electrocatalysts for catalyzing the oxygen evolution reaction (OER) in proton-exchange membrane (PEM) water electrolysis. However, it r...

A Cu-10 Ni-2Ti wt.% alloy has been fabricated by induction melting and casting. The microstructure evolutions of the studied alloy treated with thermal-mechanical treatment were characterized. After homogenized at 950 °C for four hours, hot rolling by 80% at 900 °C solid solution treated at 970 °C for four hours, cold-rolled by 50%, aged at 450 °C for one hour, the hardness, electrical conductivity, tensile strength, yield strength, and the elongation of the studied alloy approached 181 HV, 22.1% IACS, 576 MPa, 486 MPa, and 14.6%, respectively. The studied alloy shows large products of strength and elongation (15.4 GPa·%, 9.3 GPa·%, 8.41 GPa·%, and 10.1 GPa·% for the samples treated with a solid solution, under aged, peak aged, and over-aged states), which are much higher than those of other high strength conductive copper alloys. These findings are helpful in designing new high damage tolerance conductive materials. Keywords: Cu alloy, Damage tolerance, Precipitation, Strength, Microstructure

In this study, the selective laser melting (SLM) technology has been employed to manufacture a nickel-based superalloy which was conventionally prepared through powder metallurgy (PM) route. The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart. Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point (γ + γ’) eutectic played a vital role in their formation mechanism. Columnar grains oriented along building direction were ubiquitous, corresponding to strong fiber texture. Solidification cell structures and melt pools are pervasive and no γ’ precipitates were detected at about 10 nm scale before heat treatment. After super-solvus solution and two-step aging treatments, high volume fraction γ’ precipitates emerged and their sizes and morphologies were comparable to those in PM alloy. texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events. Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization. Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.

Developing a rechargeable metal-nitrogen battery is desirable for energy conversion and nitrogen fixation as well as an alternative route for a potential and mild ammonia synthesis. Herein, we realized a proof-of-concept for sodium-nitrogen (Na-N2) rechargeable batteries by introducing the alpha-MnO2 (α-MnO2) nanowire as a catalyst. In addition, we have also demonstrated a reversible reaction of 6Na + N2. ↔ 2Na3N. This system reveals an outstanding reversible capacity of 600 mAh g−1 at 50 mAg−1, runs ∼80 cycles with a controlled capacity of 400 mAh g−1 at discharge terminal voltage below 1.6 V and achieves a high actual specific energy density based on total active materials (about 4900 Wh/KgMnO2) at the current density of 100 mAg−1. In addition, a good efficiency of nitrogen fixation (∼26%) has been achieved. These attractive performances are mainly associated with the distinctive channel structure, high conductivity, and outstanding catalytic activity in terms of in-situ environmental transmission electron microscopy investigations. The success of Na-N2 batteries not only provides an alternative strategy for the next-generation of electrochemical energy-storage devices, but also builds a new trajectory toward green recycling and utilization of N2.

Three Ni-base powder metallurgy superalloys have been developed recently, and tensile tests at temperatures ranging from room temperature (RT) to 815 °C were conducted on them. The results conformed their excellent tensile properties, in comparison with several other existed polycrystalline superalloys. In this work, by means of microstructure characterization, thermal dynamic calculations, and theoretical modeling, different strengthening mechanisms including precipitation strengthening, grain boundary strengthening, solid solution strengthening, and Orowan strengthening, were found to contribute to the yield strength in different degrees, which would help to further enhance the tensile properties of these alloys through composition design and processing optimization thereafter.

A TiNbTa0.5ZrAl0.5 refractory high entropy alloy was prepared by powder metallurgy and subsequently subjected to hot forging and annealing. Precipitation behavior and mechanical properties of the hot-worked TiNbTa0.5ZrAl0.5 alloy were investigated. Results show that the hot forging leads to a formation of (Zr, Al)-rich HCP precipitates in the initial BCC matrix. The precipitates have significant strengthening effect on the TiNbTa0.5ZrAl0.5 alloy after annealing at 1000 °C for 2 h. Owing to the precipitation strengthening, the hot-worked TiNbTa0.5ZrAl0.5 alloy exhibits a high compressive yield strength of 1740 MPa with a compressive stain of 20%. It is interesting that annealing at lower or higher temperatures than 1000 °C causes a degradation in compression properties, mainly due to the decomposition of the BCC matrix and the coarsening of the HCP precipitates.

Corrosion behaviours in hydrofluoric acid solution and mechanical performances of Ni-16Cr-11Mo-6Fe-2Cu and Ni-30Co-16Cr-11Mo-6Fe-2Cu alloys were investigated by using a variety of methods. No precipitate is observed in the present low Mo alloys, suggesting the excellent structural stability. Co leads to grain refinement and localized microstructure after hot rolling, resulting in significant enhancement in both hardness and tensile strength. Owing to the addition of Cu, all alloys with and without hot rolling show near weight loss rates after immersion. This is ascribed to the dominant passive film of Cu, which demonstrates the superior stability among the constitute elements of alloys.

To investigate the microstructure evolution of powder metallurgy (P/M) superalloys during high-strain-rate deformation, a P/M nickel-base superalloy is subjected to thermal compression at temperatures ranging from 1000 °C to 1100 °C and strain rates ranging from 1 s−1 to 10 s−1. The analysis regarding the flow behaviors and microstructure observation indicates that discontinuous softening is significant at high strain rates. Moreover, based on experimental measurement and theoretic estimation, the adiabatic heating under high-strain-rate deformation is hardly neglectable, which enhances the dynamic softening via dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. In order to obtain refined and homogeneous grains, deformation at temperature range of 1025–1050 °C with strain rate rang of 1–10 s−1 or at 1000 °C/1 s−1 is suggested for high-strain-rate compression on the P/M superalloy, where DRX plays more dominating roles.

In this study, single and multi-step aging treatments (SAT and MAT) following a super-solvus solution treatment were employed to tailor the mechanical property of a prototypical powder metallurgy (PM) nickel-based superalloy. Vickers hardness and tensile test at room temperature were used to evaluate the mechanical properties after different heat treatments. Microstructural evolution after heat treatments and tensile tests were revealed through scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) techniques. Our results show that the MAT with a stabilization at 650 °C for 24 h between the solution and aging assists to achieve the highest strength corresponding to microstructure of bimodal distribution of γ’ precipitates. The associated mechanism for variation in microstructures and mechanical properties are discussed based on thermodynamic calculations and precipitation strengthening theory. In addition, an empirical relationship between Vickers hardness and tensile strength of nickel-based superalloys is examined which is believed to be assistance for rapid evaluating mechanical response of materials with different microstructures.

In the present work, the effects of Mo/Cr addition on the microstructure and the resultant property of cemented carbides were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) and Vickers hardness test. The results show that addition of Mo/Cr elements contributed to a significant improvement of WC-Co cemented carbides hardness. The enhanced hardness is ascribed to the decreased grain size of WC with increasing Cr content. In addition, Cr element addition would promote the segregation of M7C3 phase surrounding the η-phase, which inhibits the growth of η-phase, and results in finer and more disperse η-phase. The evolution mechanism of η-phase and the segregation of M7C3 phase during sintering process were discussed systematically.

Optimizing hot workability and processing parameters of the difficult-to-work powder metallurgy (P/M) superalloy is critical to obtain a desired component through thermo-mechanic processing (TMP). In this study, a P/M superalloy with tailored grain structure and γ′ particle was prepared through atomization and hot isostatic pressing, then thermal compression tests were performed to evaluate its hot workability based on processing maps and microstructure evolution. Considering the friction and temperature variations during deformation, the flow data were corrected via an integrated method, to construct dependable processing maps. The results illustrated that the P/M superalloy prepared by the modified processing method presents superior hot workability, comparing with the P/M superalloys produced by different routes.

Uncovering kinetic pathways of non-classical multistep nucleation at the atomic-scale is critical for understanding the complex microscopic mechanism of heterogeneous nucleation and crystallization. However, due to the intricacies in tackling such challenging topics experimentally, the structure of intermediate states and the temperature dependent effect on multistep nucleation at the atomic-scale are still not fully understood. Here, we conduct direct in situ atomic-scale observations of kinetic processes of interfacial multistep nucleation pathways using an aberration-corrected TEM. We provide direct evidence for temperature-dependent multistep nucleation pathways in a supported bismuth system at the atomic-scale: a droplet-crystal two-step nucleation pathway at high temperature close to the melting point of bulk bismuth, a droplet-local ordered structure-crystal three-step nucleation pathway at medium temperature between a critical temperature Tc1 and the lowest size-dependent melting temperature of bismuth nanoparticles, and a cluster-crystal two-step nucleation pathway at a temperature lower than the lowest melting point (Tl). The related critical temperature Tc1 and the lowest melting point in the three pathways are confirmed based on the size-dependent melting phase diagram of Bi nanoparticles. Statistical analyses imply that the nucleation pathway plays an important role in deciding the nucleation and crystallinity kinetics, and the critical size of crystal nucleation is controlled by temperature. The understanding of multiple intermediate steps in nucleation and the temperature effect on nucleation pathways has important implications for the synthesis and growth of clusters, amorphous and crystalline materials, and these findings may further enrich the nucleation theory.

Additive manufacturing is an advanced forming method to fabricate complex components, while cracks appear in the deposited nickel-based single crystal superalloy samples undesirably. Though fabrication of crack-free superalloy were reported, effective solutions to fabricate crack-free nickel-based single crystal superalloy samples are still uncovered, thus the additive manufacturing in single crystal superalloy is still an extremely difficult work. In this work, we fabricated nickel-based single crystal superalloy samples through laser powder deposition, and analyzed the causes of crack during deposition processing. Experimental results demonstrate that the as-deposited thin-wall samples presented single crystal structure and its crystal orientation is well aligned with the [001] orientation of substrate, while some cracks appeared in the as-deposited block samples. Large misorientation angle in the local regions played significant roles in the cracks initiation and the further cracking propagation. Solidification cracks are attributed to the shrinkage cavities and the high-melting point interdendritic carbides, while liquation cracks are attributed to the low melting point compounds. These findings give a guide for causes analysis on cracks in the nickel-based single crystal superalloy fabricated by laser powder deposition additive manufacturing, which has bright prospects in single crystal superalloy manufacturing and repair. Keywords: Superalloy, Single crystal, Crack, Laser powder deposition, Additive manufacturing

To understand how to tailor microstructure of high-Co polycrystalline superalloys during hot processing, the flow curves of a nickel-based superalloy with high Co content was quantitatively analyzed, and the microstructure evolution was studied by a high-throughput method. The results suggested that hot working conditions, especially the temperature, strongly influenced the grain structure at annealing. In specific, deforming under low strain rate and high temperature conditions facilitated the recovery and grain growth to consume the stored strain energy, furthermore, the weakened pinning effects of γ′ precipitates accelerated these procedures, which made the high-Co superalloy more vulnerable to the formation of abnormally large grains during subsequent supersolvus annealing.

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.

The effect of Cu addition varied from 0 to 4 mass% on the corrosion resistance and electrochemical response in Ni–Co–Cr–Mo alloys was investigated using potentiodynamic polarization, electrochemical impedance spectroscopy, and Mott–Schottky analysis. Results indicate that the Ni–Co–Cr–Mo alloy with 2 mass% Cu exhibited the most superior corrosion resistance, and the presence of Cu greatly influenced the outer porous layer. The Ni–Co–Cr–Mo alloys’ corrosion resistance was not simply increasing with copper addition increasing from 0 to 4 mass%. The X-ray photoelectron spectroscopy etching analysis was also conducted to illustrate the fraction of Cu at various depths in the passive film, and the results reveal that a maximum limit on Cu content (appropriately 3.10 mass%) existed in the outermost surface in the present condition. Among the studied alloys, the Ni–Co–Cr–Mo–2%Cu alloy formed the thickest passive film with the lowest donor density.

Oxidation behavior of a novel cobalt-nickel-base superalloy in air from 600 to 900°Cis investigated in details. The results indicate a two-layer oxide film formed on the surface after oxidation: the external layer of oxide film is mainly composed of Cr2O3 and a few of other oxides, and the inner layer dominantly contains Al2O3. During oxidation at higher temperature and long duration, evaporation of Mo-containing oxide leads to void formation and local failure in external oxide layer, characterized by roughening of oxide film. A formation mechanism of oxide film on this alloy during oxidation is proposed for the first time.

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.

In this study, the distribution, proportion and characteristics of internal defects in three kinds of powders of Ti-6Al-4V, 316-steel and Co-29Cr-6Mo alloys, produced by the plasma rotating electrode process (PREP) at various rotation speeds, are characterized by using both scanning electron microscopy (SEM) and synchrotron X-ray computed tomography (CT). The results show that in the powder of a given alloy, internal pores are formed more easily in coarse particles than in fine powder during PREP. The proportion of powder with pores can be reduced by appropriately increasing the rotation speed. In addition, the composition of an alloy has a great influence on the defect formation.

The tradeoff between the strength and the fracture elongation in the high-strength Cu–Ni–Si alloy became a hot research topic recently. Cu–Ni–Si–(Ti) alloys were fabricated in a vacuum induction melting furnace to study the effects of titanium on the microstructure and mechanical properties of Cu–Ni–Si alloys with different thermo-mechanical treatments. After homogenization at 900 °C for 4 h, hot-rolled by 80%, solution treatment at 970 °C for 2 h, cold-rolled by 50%, and finally aged at 450 °C for 180 min, the studied Cu–10Ni–Si–2Ti alloy achieved the hardness of HV 252.4, electrical conductivity of 23.6% IACS, tensile strength of 764.4 MPa, yield strength of 622.26 MPa, fracture elongation of 10.4%, and strength-elongation product of 7.95 GPa%, which are less than those of the studied Cu–10Ni–2Si alloy. The addition of Ti contributed to refining the microstructure, suppressing the decreasing trend in mechanical properties after peak hardening, and arousing a primary substructure strengthening mechanism rather than the precipitation strengthening in Cu–Ni–Si alloys. These findings provide essential understandings of the effects of the Ti on Cu–Ni–Si system alloys, and the designed Cu–Ni–Si alloys with high-strength and fracture elongation could fulfill some requirements of the electronic and electrical industry.

As an advanced material forming method, additive manufacturing (AM) has been rapidly developed in many industry fields. Due to the non-weldability of nickel-based single crystal (SX) superalloys and complex solidification and stress conditions of AM process, hot cracks can easily form in depositions. Previous research focused on the additive manufacturing of single-track thin-walled SX samples, while effective solutions to fabricate crack-free multi-track SX block samples are still limited. Directed energy deposition (DED) technique was used to fabricate multi-track block samples in this work. The effects of the overlapping ratio, scanning velocity, and laser power on the microstructure and cracking susceptibility were analyzed. The experimental results demonstrated that improper process parameters play significant roles in the formation of high-angle grain boundaries and large internal stress, both of which will increase the cracking susceptibility. Crack-free SX multi-track block was successfully fabricated after adjusting the process parameters. These findings provide a guide to reduce the cracking susceptibility by controlling the process parameters, which aids in fabrication of large-scale crack-free SX blocks through DED technique.

This work proposes a novel strengthening strategy for HCP alloy by fully utilizing the strengthening roles from texture and dual-precipitates in both basal and prismatic planes. The binary Mg-7Al (at%) alloy fabricated by such a strategy exhibits a superior combination of strength and strain hardening rate in major plastic strain ranges. Unlike conventionally fixed distribution of precipitate on given lattice plane, both basal and prismatic Mg17Al12 precipitates are firstly engineered to co-exist and reinforce Mg alloy. Additionally, this work reports the presence of a new precipitate morphology with a hammer-like geometric configuration for the first time to date. Further application and extension of this novel strengthening strategy to other HCP alloys, like Ti, Zr, are also discussed.

Owing to its high specific strength and low density, Al&ndash, Cu alloys have been extensively used in aerospace for lightweight components. Additive manufacturing techniques such as selective laser melting, which offers geometric freedom, is suitable for topology-optimized designs. In this study, the effect of processing parameters on the densification, microstructure, and mechanical properties of additively manufactured Al&ndash, Cu alloy 2124 by selective laser melting was investigated. Parameters such as laser power, scanning speed, hatch spacing, and use of a support were studied. The results revealed that a grille support with a hollow structure played a resistant role in the transfer of heat to the base plate, thus reducing the temperature gradient and lessening cracks in the building part. Smaller hatch spacing was beneficial for the achievement of a higher relative density and strength due to track re-melting and liquid phase backflow, which could fill cracks and pores during the building process. An ultimate tensile strength as high as 300 MPa of the vertically built sample was obtained at optimized processing parameters, while the elongation was relatively limited. Moreover, columnar grains were found to be responsible for the anisotropy of the mechanical properties of the as-printed 2124 alloy.

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.

TiC-strengthened Cu alloys with various nominal compositions were prepared by a high-energy ball-milling process, followed by consolidation using spark plasma sintering (SPS), and a subsequent heat treatment. The results indicated that the lower atomic ratio of C/Ti, the more residual Ti in copper matrix, resulting in a lower electrical conductivity but higher microhardness of alloy. The optimal atomic ratio was found to be close to1.0. The reaction behavior of Ti-C in the Cu-Ti-C system during this process was investigated. Microstructural results suggested that TiC formation is through a diffusion-controlled mechanism. A thin TiC layer was formed at the surface of C particles, and the growth of TiC layer was controlled by interdiffusion of C and Ti atoms across the TiC layer. An incomplete chemical reaction between Ti and C, characterized by a core-shell structure, was observed when the initial C particles were large, and the TiC particle size was found to strongly depending on the C particle size.

Electrochemical and static immersion tests in 1 mass% lactic acid solution are performed to investigate the corrosion behaviour of CoCrMo alloy fabricated by electron beam melting with cylindrical axes of samples deviating from build direction by 0°, 45° and 90°. All samples are dominantly composed of columnar γ-fcc phase with preferred crystal orientations. 90° sample, characterized by the largest grain size, lowest grain boundary density and fewest Cr-rich σ phase precipitates, demonstrates the highest corrosion resistance. The results suggest that amount of Cr-rich precipitates and grain boundary density both have much closer relation with corrosion resistance than crystal orientation.

Explosive dynamic recrystallization (DRX) of Co-29Cr-6Mo-0.16 N (CCMN) alloy with extremely low stacking fault energy (SFE) was observed in rapid hot-forging processes; the DRX was considered to be closely related to the prevailing planar-defects formation in parent grains such as mechanical twin and stacking fault bands as well as their interactions, that finally break the parent grains into fine pieces accompanied by instantaneous reorientation and recovery in the refined grains.

Powder metallurgy superalloys are widely used to produce critical rotating parts in turbine engines. However, the issues like what is the driving force of precipitation on prior particle boundary (PPB) during consolidation and how the PPB structure evolves during plastic deformation still remain to be resolved. In this study, with the characterization of the microstructure evolution from powder, consolidation, to plastic deformation, the segregation of alloying elements on the particle boundaries at elevated temperature was analyzed by equilibrium thermodynamic calculations, and the distortion of PPB structure under large plastic deformation was quantitively simulated by finite element (FE) methods. The results indicated that surface energy serves as the main driving force of PPB precipitation and hot extrusion could distort the PPB structure and yield more uniformly refined grains.

In order to investigate the effect of aging on twin boundary (TB) strengthening in magnesium alloys, the cyclic compressions along two orthogonal directions of samples with and without intermediate aging are carried out. The results show that the grains in all Mg alloys AZ31, AZ61 and AZ91 are significantly refined by twin boundaries (TBs). The TB strengthening is strongly dependent on both alloying element content and aging. Although grain refinement by TB formation occurs in AZ31, it exhibits slight strengthening effect even after aging. In contrast, significant TB strengthening after aging is observed in Mg alloys AZ61 and AZ91 with high contents of alloying elements.

Silicon carbide (SiC) coating is deposited on C/C composite substrate for the first time by chemical vapor deposition (CVD) with hexamethyldisilylamine (HMDS, C6H19NSi2) as precursor and N2 as carrier gas in an intermediate deposition temperature range. The effects of deposition temperature on phase constitution, surface morphology and deposition rate of the coating are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating exhibits a β-SiC (3C) phase at 1010 °C, 1060 °C and 1100 °C, coexistence of both α-SiC (2H) and β-SiC (3C) phases at 1130 °C and 1180 °C, as well as a single β-SiC (3C) phase again at 1220 °C. The morphology and microstructure of coating change significantly with deposition temperature. The change of phase constitution and surface morphologies are closely linked to nucleation, growth and defects formation in SiC crystals during CVD process.

Effect of Ti addition on oxidation behavior of Ni-Cr-W alloys was systematically investigated by isothermal oxidation in air. The oxidation rate is accelerated by Ti addition, due to that Cr2O3 vacancy density is increased by Ti4+ occupies Cr3+ position, which promotes the formation of Cr2O3 scales. Ti addition induced the formation of TiO2 but inhibited the formation of Ni oxide. Ti segregated on the external Cr2O3 scales after oxidation for a period time. Besides, internal oxidation and nitridation were found in both alloys containing Ti. The depth of internal oxidation and nitridation increases with the increase of Ti content.

A novel Co-Ni-based superalloy was manufactured by powder metallurgy (PM) process. The microstructure and mechanical properties of PM alloy were analyzed in detail and compared with the corresponding casting alloy. After cold swaging to 66% and subsequent aging for 800 °C for 3 h on solution treated (ST) PM alloy, the yield strengths of PM superalloy can be doubled and tripled to be 1500 and 2265 MPa respectively, as compared with 700 MPa of ST PM alloy. In contrast, for the same casting alloy after subjected to similar treatments, their corresponding yield strengths were 1100 and 1650 MPa, respectively, which are much lower than these of PM alloy. Strengthening mechanism of both PM and casting alloys were quantitatively calculated, unmasking that the higher mechanical property of PM alloy can be mainly ascribed to the higher contribution from grain boundary and precipitation strengthening.

Traditional high-strength wrought aluminum alloys are sensitive to cracks in additive manufacturing (AM) while the eutectic aluminum alloys can be reliably printed. To explore a high-strength heat-resistant aluminum alloys by AM, in this paper, we proposed to retard the solidification range by using an Al−5.7Ni alloy, thus alleviate crack formation tendency. The AM parts of this alloy was featured by uniformly distributed spherical Al3Ni nano-particles with an average size of ~33 nm, showing a novel coherent crystallographic relationship with the matrix, Al// Al3Ni and {111}Al//{211}Al3Ni. Excellent mechanical property was achieved in this crack-free alloy with tensile strength of 410 MPa at room temperature and 140 MPa at 300°C, mainly attributed to the dispersion strengthening from the Al3Ni nano-particles with superior thermal stability.

Co-based alloys are currently being used in a wide range of high temperature applications owing to their high resistance to oxidation and corrosion. However, their oxidation-induced degradation could still occur during the long-term exposure to high temperature. Thus, the continuous development of oxidation-resistant Co-based alloys is of crucial importance. In this research, the influence of Si addition on the oxidation behavior of Co–Cr–Mo–xSi alloys under the isothermal oxidation treatment at 700 °C in air was investigated. The Si concentration (x) was varied from 0.1 to 5.0 wt.%. Surface morphologies and chemical compositions of the oxide films formed were analyzed by using SEM-EDS and XPS. The chemical compositions obtained from the surface analysis revealed that Si has played a role in the stabilization of Cr oxides on the surface of Co–Cr–Mo–xSi alloys. With increasing Si concentration, Co-oxide formation on the alloy surface was suppressed by the presence of Cr-oxide due to the selective oxidation of Cr atoms. Furthermore, SiO2 was both found along the grain boundaries and interfaces between the outmost oxide layer and matrix. It was also found that the oxide thickness was reduced with increasing Si concentration. This was a result of the formation of stable Cr-oxide and SiO2 sub-layer that became a barrier inhibiting the inward and outward diffusion of O and Cr.

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.

In the present research, the microstructure and phase composition of high-Nb TiAl powder, produced by plasma rotating electrode process (PREP) have been investigated in details. PREPed powder with a mean particle size larger than 150 μm demonstrates a dendrite structure, and that smaller than 75 μm indicates a martensitic structure. PREPed powder is mainly composed of a hexagonal a2 phase, while in fine powder and coarse powders minor β phase and γ phase are detected, respectively, which is related to solidification path and cooling rate of molten droplets in PREP process. Phase composition and microstructure of the high-Nb TiAl powder have intrinsic effects on the resultant microstructure of hot isostatic pressed (HIPed) billets. After HIP processing, most martensitic and dendritic particles transformed into near γ structure, while few coarse dendritic powder was prone to introduce defects, such as residual primary particle boundaries (PPBs) or coarse lamellae structure, which were harmful to the mechanical properties of HIPed billets. The residual primary particle boundaries (PPBs) or coarse lamellae structure obviously decreased the ultimate tensile strength at room and elevated temperature. Keywords: Microstructure, High–Nb TiAl, Powder, Densification, Mechanical properties

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.

Improving the oxidation resistance without sacrificing the electrical conductivity of Cu is of great importance for researchers due to its widespread application. In this study, oxidation resistance of Cu-5 wt%Ag alloy under solution treatment (ST) and aging treatment (AT) were studied systematically. It has been confirmed that AT sample demonstrates considerably higher oxidation resistance than ST sample at elevated temperatures. The aging treatment enhanced the formation of a continuous Ag network along grain boundaries, which could act as a barrier for both inward diffusion of oxygen and outward diffusion of Cu along grain boundaries during oxidation.

Influence of Nb on oxidation behavior of novel Ni-base superalloy was investigated in detail at 1000 °C. Although alloy with 2 wt%Nb (2Nb) exhibits significantly higher weight gain than alloy without Nb (0Nb), oxide scale of 2Nb was more continuous and compact. In contrast, 0Nb underwent severe vaporization and spallation in oxide scale. Nb was found to accelerate the formation of compact TiO2 on Cr2O3 layer, leading to inhibited vaporization and spallation in oxide scale. Nb also accelerated the formation of continuous CrTaO4 layer beneath the Cr2O3, enhancing the oxidation resistance via inhibiting the interdiffusion of oxygen, nitrogen and metallic elements.

In this work, the high temperature creep properties of three newly developed P/M superalloys with fine grains were investigated under conditions of 650 °C/1100 MPa, 750 °C/690 MPa, and 815 °C/550 MPa. The creep behavior associated with microstructure were analyzed, considering the interactions of dislocation with grain boundaries and γ′ phase. The results indicated that the cracks were more likely to initiate at grain boundaries, resulting in intergranular cracking. Moreover, the dislocation movement in grains was strongly related with testing temperature and applied stress, which is explained by theoretical calculation on the composition-dependent critical stresses (CSs) to trigger various mechanisms, including dislocation shearing the γ′ phase by stacking faults (SFs) or APB coupled dislocation pairs, and dislocation climbing.

In this study, microstructures, mechanical properties, and electrical conductivities of powder metallurgy (PM) Cu-0.28Cr-0.19 Mg alloy after cold-rolling and aging were investigated in detail. PM Cu-Cr-Mg alloy exhibits a combination of both higher mechanical properties and superior electrical conductivity compared to the corresponding casting Cu-Cr-X alloys. Such excellent performances are closely related to the more uniform and finer microstructures generated in PM process. Strengthening from second phase precipitation, grain boundary, and dislocation in the present PM Cu-Cr-Mg alloy were calculated to be approximately 43.65, 24.50, and 23.12 % of yield strength, respectively. The grain boundary strengthening is the main reason for the higher strength of the present PM Cu-Cr-Mg alloy and is approximately 325 % higher than that of the corresponding casting Cu-Cr-Mg alloy. The gradually decreased activation energy of PM Cu-Cr-Mg alloy with increasing reduction rate suggests that deformation significantly accelerates the precipitation behavior during the subsequent aging process.

Iso–thermal (IF) fatigue behavior of a single e -phase Co–Cr–Mo alloy used as Al die casting mold is investigated in details at temperatures of 400, 500, 600, and 700 °C and applied mechanical strains of 0.1%, 0.2%, 0.3% and 0.4%. The results indicate that for a given applied mechanical strain, higher temperature promotes the occurrence of oxidation on alloy surface, resulting in a shorter fatigue life compared to the tests at lower temperature; at a given temperature, larger applied strain amplitude gives rise to shorter fatigue life. Microstructural observation indicates that grain boundary oxidation plays a significant role for crack nucleation at high temperature. The cracks are found to initiate intergranularly and propagate in a mixed intergranular and transgranular mode.

In this work, Cu-based microcomposites were fabricated by powder metallurgy of gas-atomized Cu-15wt% Fe alloy powder, followed by drawing to various strains. The microstructure evolution and the resultant mechanical and electrical properties were investigated by scanning electron microscopy, electron backscatter diffraction, and tensile testing, among other techniques. The results indicated that during drawing, the Fe phase evolved into nanoscale filaments along the longitudinal direction. With increasing drawing strain, both the Cu grains and the Fe filaments were refined gradually, giving rise to a mean thickness of approximately 25nm for the Fe filaments at a total strain of 4.0. The iron oxides particles formed due to contamination of oxygen maintained unchanged during drawing process. In addition, it was found that an intermediate aging during the drawing can significantly enhance the electrical conductivity of the microcomposites without sacrificing their strength. The present process involving an initial powder with a homogenous distribution of Cu and fine Fe phases makes it possible to obtain microcomposites with an ideal combination of electrical and mechanical properties at lower drawing strain, compared to those produced by conventional casting methods. Keywords: Cu-Fe microcomposite, In situ technique, Hot consolidation, Electrical conductivity, Strength, Microstructure evolution