Main Strengthening Mechanisms Research Articles

Effect of wall thickness on the precipitation behavior, microstructure, electrical conductivity and mechanical properties of copper alloy prepared by electron beam powder bed fusion

Electron beam powder bed fusion (EB-PBF) is one of the most promising technologies for preparing thin-walled copper alloy, because copper alloy have a high energy absorption rate for electron beam, and high preheating temperature can reduce the solidification temperature gradient and reduce the deformation of thin-walled parts. At present, there are few reports on the systematic research work on the EB-PBF of thin-walled CuCrZr alloy parts, and it's impossible to effectively supervise the production of complex thin-walled CuCrZr alloy parts. This work aims to investigate the effect of thickness on the microstructure and mechanical properties of CuCrZr alloy produced by EB-PBF. As the wall thickness decreases from 5.0 mm to 0.3 mm, the grain sizes of the XY and YZ planes decreased from 20.4 μm and 43.1 μm to 14.5 μm and 21.5 μm respectively, and the texture intensity decreased from 16.04 and 23.57 to 7.62 and 10.99. The analysis showed that Cr2O3 nanoprecipitates were precipitated in situ in the sample, and their average size decreased from 65.8 nm to 21.6 nm. Due to the reduction in grain and nanoprecipitate size, the performance of thin-walled samples is significantly enhanced, with yield strength (YS) increasing from 112 MPa to 165 MPa and conductivity increasing from 71.7 %IACS to 86.1 %IACS. Finally, the main contributions to the YS of specimens with different wall thicknesses was discussed. Precipitation strengthening and dislocation strengthening are the main strengthening mechanisms in thin-walled samples, and the gradual refinement of nano-precipitates is the main reason for the improvement of mechanical properties as the wall thickness decreases.

Effect of Al2O3-SiO2 mass ratio and sintering temperature on the properties of water-soluble salt-based ceramic cores formed by binder jetting

Binder Jetting (BJ) technology, known for its cost-effectiveness and efficiency, is widely used for the rapid fabrication of complex ceramic cores. However, fabricating high-strength and water-soluble ceramic cores via BJ technology is particularly challenging due to the balance between mechanical properties and water solubility. In this work, BJ technology was employed to fabricate high-strength and water-soluble salt-based ceramic cores (SCC) regulated synergistically by Al2O3 and SiO2. The effects of the mass ratio of Al2O3 to SiO2 (A/S) and sintering temperature on the properties of Na2CO3-based samples were investigated, with detailed analyses of the regulation mechanism through thermodynamic, kinetic, and microstructural examinations. The results indicated that when A/S was 1:4, the bending strength of samples sintered at 760 °C for 60 min reached up to 62.38 MPa with good water collapsibility. Thermodynamic and kinetic analyses revealed that the formation of Na2SiO3 was prioritized over NaAlO2, and the content of Na2SiO3 as well as the proportion of liquid phase increased with the decrease of A/S and the increase of sintering temperature, resulting in the increase of sample density, as confirmed by microstructure analyses, which was the main strengthening mechanism of samples. Ultimately, complex-structure SCC samples were successfully fabricated. This study provides an insightful method for fabricating high-strength and water-soluble ceramic cores, with significant potential applications in the casting industry.

Deformation behavior of Gd alloyed CoCrFeNi high entropy alloy at high temperature under the influence of local nano-ordered structure and misorientation

The thermomechanical processing (TMP), which is an industrial process that includes deformation and heat treatment, represents a novel approach to enhancing the mechanical properties of metallic materials. However, it requires extensive research on the high-temperature deformation behavior of alloys during its application process. The present study conducts hot compression tests at 700 °C and 800 °C, as well as detailed microstructural characterization of the deformed material after hot deformation, to comprehensively discuss the mechanical properties and deformation behavior at high temperature of the (CoCrFeNi)100-xGdx (x = 0, 0.5, 1, 2, 3 at.%) HEAs. The (CoCrFeNi)100-xGdx HEAs exhibit an excellent combination of strength and plasticity in high temperature. The original dendritic microstructure in the (CoCrFeNi)100-xGdx HEAs was completely fractured and transformed into a mass of deformed structures during thermal deformation. The mechanical properties at high temperature of the alloy system originate from the misorientation in the microstructure. Dislocation slip is the main deformation mechanism of the alloy system at high temperature. The coordinated deformation of the soft orientation for FCC phase and the hard orientation for HS phase affects the dislocation morphology in the microstructure. Dynamic recovery (DRV) plays a dominant role in the flow behavior of the alloy at high temperature, and work hardening is the main strengthening mechanism during plastic deformation. The correlation between microstructure evolution and mechanical properties of Gd alloyed CoCrFeNi HEAs during hot deformation has been established through comprehensive studies on deformation behavior.

Microstructure evolution, mechanical properties, and corrosion behavior of in-situ TiC/TC4 composites through Mo addition

To further improve the mechanical properties and corrosion resistance of the composites, in-situ TiC/TC4-xMo composites were fabricated. The effect of Mo content on the microstructure evolution, mechanical properties, and corrosion behavior of the composites was investigated. The results show that with the increase of Mo content, the amount of β-Ti phase and the size of the TiC particle increase. The mechanical properties and corrosion resistance of the composites can be significantly enhanced through Mo addition. The TiC/TC4 composites containing 10 wt% Mo exhibits superior mechanical properties and corrosion resistance. Grain refinement, load transfer, and solid solution strengthening are the main strengthening mechanisms for improving the strength of composites. In addition, the increase of the β-Ti content, the presence of TiC, and the solid solution of Mo elements contribute to enhancing the corrosion resistance of the composites.

Study on mechanical behaviour and microstructure evolution of wire-arc directed energy deposition Co-free maraging steel under dynamic compression

ABSTRACT The dynamic compression property of maraging steel is an important basis for determining the impact resistance of the structure, and the related studies are few. To enrich the relevant data and elucidate the relevant mechanisms, in this work, a new Co-free maraging steel component is fabricated by wire-arc direct energy deposition (DED), and the quasi-static tensile properties and dynamic compression properties are tested. The results show that the deposition direction exhibits the highest quasi-static tensile properties. Strain rate strengthening effect, grain refinement and grain boundary strengthening are the main strengthening mechanisms during dynamic compression. With the increase of strain rate, dynamic recrystallization intensifies, causing the grain size and dislocation density to decrease. Under dynamic condition, strain-induced martensite phase transformation reduces the austenite content; temperature rise and dynamic recrystallization limit the degree of martensite phase transformation. Strain rate sensitivity of the strength is independent of the direction.

Effect of equal channel angular pressing on microstructure and mechanical properties of a Cu-Cr-Ni-Si-Co alloy

The microstructural and properties of a Cu-Cr-Ni-Co-Si alloy produced by equal channel angular pressing (ECAP) were systematically investigated. After the fourth-pass ECAP deformation, the average grain size was refined from 30 μm (solid solution state) to below 3 μm, resulting in a fine and regularly shaped equiaxed crystalline microstructure. The distribution of grain boundaries became more uniform. During ECAP deformation, the volume fraction of Copper texture {001} <100> decreased, while that of the Brass {011} <211> increased, and the Shear {123} <634> remained nearly constant. An increase in the number of ECAP deformation passes facilitated dynamic recrystallization, leading to an increase in the fraction of the recrystallized texture. After thermomechanical treatment (first-pass ECAP +450 °C aged/120 min + 80 % cold-deformation +450 °C aged/30 min), the alloy exhibited good mechanical properties, with a tensile strength of 546 MPa, a yield strength of 475 MPa, an elongation of 18.2 %, and an electrical conductivity of 52.8 %IACS. The main strengthening mechanisms included precipitation strengthening, dislocation strengthening and grain boundary strengthening, with the precipitation strengthening accounting for 45.6 % of the yield strength.

Investigation on deformation and strengthening mechanisms of Al-Zn-Mg-Cu alloy under a hybrid process of cryogenic incremental sheet forming and bake-hardening treatment

Incremental sheet forming process is featured with high flexibility and short manufacturing cycle, which has great potential in customized forming of complex components. However, the formability of high-strength aluminum alloy is still needs to be improved. In this paper, an efficient cryogenic incremental sheet forming (CISF) process combined with bake-hardening (BH) treatment is proposed to achieve excellent formability and strength for Al-Zn-Mg-Cu alloy. First, the effect of different cryogenic temperatures on the formability of Al-Zn-Mg-Cu alloy is investigated and the mechanism of plasticity enhancement during cryogenic incremental sheet forming was revealed. The fracture forming limit lines constructed from different components show a significant increase in the formability of as-quenched Al-Zn-Mg-Cu at −170°C. Specially, when forming cone component with variable wall angle at −170℃, the ultimate forming height is increased by 70.4 % compared with 25℃. The microstructure shows that dislocation cross-slip is suppressed at cryogenic temperatures, and the large number of subgrain structure and uniformly distributed dislocations reflect the improved deformation uniformity at cryogenic temperatures. Second, the effect of cryogenic incremental sheet forming on the strength of Al-Zn-Mg-Cu is explored. The vertical forming force can be increased by 37.4 % at −170°C compared to 25°C, and the components have higher post-forming strength. Moreover, the strengthening response of the components after the combination of cryogenic incremental sheet forming and bake-hardening treatments is investigated. The cryogenic specimens exhibited a rapid bake-hardening response compared to the room temperature specimens. The yield strength increased by 15.4 % to 387.2 MPa after 40 mins of bake-hardening treatment at 180°C. It has been shown that precipitation strengthening and dislocation strengthening are the main strengthening mechanisms during cryogenic incremental sheet forming and bake-hardening treatments. This study reveals the dual enhancement effect during the cryogenic incremental sheet forming process, and proposes an efficient approach which combines cryogenic incremental sheet forming with bake-hardening treatment to manufacture high-performance components.

Improving the mechanical and tribological properties of Cu matrix composites by doping a Mo2BC ceramic

Cu matrix composites (CMCs) are widely employed for tribological applications such as in sliding bearings and electrical contact fields. However, the Cu matrix needs to be modified to improve its mechanical and tribological properties. In this study, CMCs reinforced with Mo2BC ceramic particles (10–30wt.%) are prepared via the spark plasma sintering (SPS) route method. In comparison with the plain Cu sample, when the content of the Mo2BC particles increased to 30wt.%, the Vickers hardness, yield strength, and tensile strength of CMCs increased by 196%, 157%, and 588%, respectively. Fine grain and load transfer strengthening mechanism are considered the main strengthening mechanism. Introducing the Mo2BC ceramic effectively reduces friction and wear as well as the frictional noise vibration of CMCs. Incorporating Mo2BC ceramic into the Cu matrix can elevate the load-bearing ability, inhibiting mechanical and adhesive wear. In addition, the composite exhibits superior friction and wear properties as the content of Mo2BC ceramic increases gradually up to 30wt. %, where the friction coefficient and wear rate are as low as 0.35 and 3.25 × 10−6 mm3/Nm, respectively. A layer of tribo-film containing Fe2O3 derived from the counterparts and induced by the tribo-oxidation reaction can provide a lubricating effect. Considering the superior mechanical and tribological properties, the as-prepared CMCs could be an attractive alternative material for sliding bearing fields.

Cold gas dynamic additive spraying of functionally graded Cu matrix composites reinforced by high entropy oxides

Cold gas dynamic spraying was attempted as a new technique to additively fabricate functionally graded Cu-matrix composites reinforced by BaFe12O19 and its corresponding BaFe6(AlCrTiGaCo)6O19, BaFe2(AlCrTiGaCo)10O19 and BaFe6(AlCrSnGaCo)6O19 high entropy oxides (HEOs) particles. The BaFe12O19 and HEOs reinforcement particles, prepared by solid-phase sintering and ball milling, exhibited a single BaFe12O19-type structure and homogenous microstructure. Cu-matrix composites with a gradient lamellar microstructure were successfully deposited, in which the coarse Cu layers formed at the bottom of the samples gradually transformed to a more refined microstructure along the deposition direction. The reinforcement particles were incorporated between and within the Cu lamellas. The samples exhibited a gradient microhardness pattern that increased from bottom to the surface of the samples along the deposition direction. Cu-20% BaFe6(AlCrSnGaCo)6O19 composite exhibited the highest microhardness value of 195 HV at its top surface which decreased to about 142 HV at its bottom. Second phase strengthening is considered as the main strengthening mechanism. The as-sprayed Cu alloy exhibited an electrical conductivity of 86%IACS, and this value was about 68.7, 78.8, 64.3 and 69.3%IACS for the composites samples reinforced by 20wt.% BaFe12O19, BaFe6(AlCrSnGaCo)6O19, BaFe6(AlCrTiGaCo)6O19, and BaFe2(AlCrTiGaCo)10O19 respectively. Finally, cold gas dynamic spraying is introduced as an effective additive manufacturing method to fabricate free-standing functionally graded Cu-matrix composites reinforced by different oxide particles.

Quantification of nanoscale precipitation in AA7050 using X-ray scattering, electron microscopy and automated particle counting techniques

Material strength is dependent on several factors, including precipitation strengthening, which is the main strengthening mechanism in AA7050 aluminum alloys. These alloys contain numerous nanometer-scale precipitates that occur throughout grains and along grain boundaries when subjected to a range of heat treatment conditions. In this study, three different characterization methods were used to characterize these nanoscale precipitates: conventional scanning transmission electron microscopy (STEM); laboratory-based small-angle X-ray scattering (SAXS); and a new software analysis tool developed by Thermo Fisher Scientific: automated particle workflow (APW). Each method was used to determine average precipitate size and volume fraction of precipitation in AA7050-T7451 specimens with variable post-T7 heat treatment procedures. STEM techniques measured the average particle size as ranging from 7.6 to 17.6 nm, compared to 6.3–10.7 nm for SAXS, and 7.4–10.8 nm for APW. Volume fraction determinations varied from 0.23 to 16 %, depending on method, and were the most difficult to quantify. Significant outcomes of the current study are that SAXS and APW are the most accurate methods for determining average particle size and that future work is needed to effectively analyze precipitate volume fraction.

Enhancing strength performance of laser welded 7075 aluminum alloy joints with TiC nanoparticle-mixed filler powder

Precipitation-strengthened 7075 aluminum alloy (AA 7075) is a crucial structural material widely utilized in the aerospace industry. Nevertheless, the achievement of highly strengthened joints of this alloy remains a challenge because of its susceptibility to composition dilution and defect formation during welding. In this study, the method of powder-filled laser welding was employed to weld AA 7075-T6 plates, utilizing both AA 7075 powder and a mixed powder of AA 7075 and 5 wt % TiC nanoparticles as the filler. To investigate the impact of the filler on the mechanical properties of the welded joints, their microstructure, phase constitution and tensile strength were measured and analyzed. Moreover, the transient temperature and the forming process of the joints during laser welding were captured. The results indicate that the joints are heat-treatable when the filler contains the AA 7075 powder. The ultimate tensile strength (UTS) of the joints can reach 500 MPa as a result of the η′ phase precipitated during heat treatment. The incorporation of TiC nanoparticles into the AA 7075 powder led to a refinement of grains, a reduction in porosity within the joints and an increase in the UTS by up to 540 MPa, which is 91.5 % of the base metal. Further theoretical estimation suggests that Orowan strengthening as well as grain size strengthening due to the Hall-Petch effect are the main strengthening mechanisms for the improved tensile performance. The powder-filled laser welding method also provides a flexible approach for designing compositions of filler material.

Achieving 1.7 GPa Considerable Ductility High-Strength Low-Alloy Steel Using Hot-Rolling and Tempering Processes.

To achieve a balanced combination of high strength and high plasticity in high-strength low-alloy (HSLA) steel through a hot-rolling process, post-heat treatment is essential. The effects of post-roll air cooling and oil quenching and subsequent tempering treatment on the microstructure and mechanical properties of HSLA steels were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure after hot rolling consists of fine martensite and/or bainite with a high density of internal dislocations and lattice defects. Grain boundary strengthening and dislocation strengthening are the main strengthening mechanisms. After tempering, the specimens' microstructures are dominated by tempered martensite, with fine carbides precipitated inside. The oil-quenched and tempered specimens exhibit tempering performance, with a yield strength (YS) of 1410.5 MPa, an ultimate tensile strength (UTS) of 1758.6 MPa, and an elongation of 15.02%, which realizes the optimization of the comprehensive performance of HSLA steel.

Tuning mechanical behavior of an FCC high entropy alloy: Insights from roll deformation and texture

Severe plastic deformation (SPD) is introduced as a significant approach in designing and tailoring the mechanical characteristics of high entropy alloys (HEAs). This research focuses on investigating the microstructure evolution, deformation mechanisms, and their influence on texture components and mechanical behavior in a single solid solution Ni1.5FeCrCu0.5 HEA subjected to cold rolling. For this purpose, microstructure evolution and texture expansion were studied in 25, 45, 65, and 85 % thickness reductions using electron backscatter diffraction (EBSD) as well as transmission electron microscopy (TEM). The finding illustrates that alongside dislocation density, deformation twins and shear bands increase with higher strain; in particular, nanotwins with a thickness of approximately 50 nm were observed in the 85 % cold rolled (85 % CR) alloy. Bulk texture analysis indicates that the presence of shear bands and twins leads to the Goss {110}<001> and Brass {110}<112> texture orientations in the cold-rolled samples, which is ascribed to the low SFE of the alloy. With increased strain, the alloy's hardness and yield strength increased from ∼150 Hv and ∼236 Mpa (As-cast sample) to ∼467 Hv and ∼1097 Mpa (85 % CR), respectively, while the elongation decreased by ∼66 %. By examining the alloy's strengthening mechanisms, it has been determined that increasing the dislocation density and the presence of twins are the two main strengthening mechanisms of the alloy.

Effects of nano-micron TiB2-Al2Y particles on the microstructural evolution and mechanical properties of extruded Mg-9Y-0.6La composites

In this study, the nano-micron TiB2-Al2Y/Mg-9Y-0.6La composites were prepared by introducing nano-TiB2 and micron-Al2Y particles into Mg-9Y-0.6La alloys via the addition of nano-TiB2/Al master alloy. The introduction of nano-micron TiB2-Al2Y particles refined the α-Mg grains, weakened the basal texture, and promoted the dynamic recrystallization of the composites. The yield strength and tensile strength of the 4.31 wt% TiB2-Al2Y/Mg-9Y-0.6La composite were increased by 69 MPa and 65 MPa, respectively, compared with the Mg-9Y-0.6La alloy. The main strengthening mechanisms of the composites were fine grain strengthening, Orowan strengthening, and coefficient of thermal expansion strengthening caused by the TiB2-Al2Y particles.

Microstructure and Properties of Deformed AlSi7Mg Alloy

Herein, the effects of hot deformation on the mechanical properties and microstructure of AlSi7Mg alloys were investigated. The microstructure of the alloys with deformation were observed. It can be found that the Si grains of the deformed alloy undergo segregation; however, a suitable heat treatment can be employed to effectively eliminate the segregation. The alloy deformation was 40% at 300 °C, and the mechanical properties of the alloy increased significantly after the heat treatment; the AlFeSi phase was found at the inside of the Si grain and the boundary between eutectic Si and α-Al grains. The AlFeSi phase was found at the inner grain and grain boundaries, and the main strengthening mechanisms of the alloy were precipitation strengthening and deformation strengthening.

Gradient microstructures and mechanical properties of WAAMed and aged Al–Cu alloy after laser shock peening

The integration of wire-arc additive manufacture (WAAM) and laser shock peening (LSP) facilitated the study of the gradient microstructure in T6-treated WAAM Al–Cu alloy and the synergistic strengthening mechanisms involving the precipitation phase and LSP parameters. The results indicate that pore closures, increases in kernel average misorientation (KAM), and low angle grain boundary (LAGB) are positively associated with LSP energy density. Residual stress positively correlates with LSP energy density and inversely with spot diameter, where larger spots exacerbate non-uniformity. The values of dislocation density, microhardness and residual stress of the post-LSP specimen gradually increase with the increase of depth, and reach the maximum value at ~100 μm. As the depth continues to increase, the value begins to decrease gradually. Dislocation accumulation near grain boundaries, interacting with θ’-Al2Cu, contributes to strength enhancement. The main strengthening mechanisms enhanced by LSP involve creating a gradient microstructure through significant plastic deformation, pore closure, and the presence of high-density dislocations. Post-T6 treatment, θ’-Al2Cu significantly restricts dislocation slip, pins dislocations, and encourages the emergence of new dislocation sources, thereby enhancing the strengthening effect and stability induced by LSP.

Solution temperature influence on CRSS of the secondary γ′ phase in nickel-based superalloy FGH4096

Nickel-based superalloy components are capable of being manufactured by direct hot isostatic pressing (HIP) process with near-net shape and nearly 100 % raw materials utilization. However, they cannot be applied directly, due to the poor mechanical properties of HIPed components. Fortunately, heat treatment can improve the properties via regulating the microstructure. Therefore, the effects of varied solution temperatures on the tensile properties of FGH4096 nickel-based superalloy disk at room and high temperatures were studied. The results revealed that the disk after sub-solution treatment (1130 °C) has higher tensile properties (room temperature yield strength: 1194 MPa, elongation: 15.5 %; high temperature yield strength: 1075 MPa, elongation: 10.5 %). The antiphase boundary (APB) shear stress, Orowan bypass stress, stacking faults (SF) shear stress, and micro-twins (MT) shear stress affected by the size and distribution of the secondary γ′ phase were calculated. The results show that the main strengthening mechanism at room temperature is APB shear, and the APB shear stress of solution treatment at 1190 °C is the highest. The high temperature strengthening mechanism is due to SF shear and MT strengthening effects. The difference in SF shear stress between various solution temperatures is minimal. 1190 °C has a higher MT strengthening effect than 1090 °C. This is consistent with the actual result that the tensile properties at 1190 °C are superior to those at 1090 °C. The conclusion is that the combination of high strength reflected by several critical resolved shear stresses and considerable plasticity reflected by the MT strengthening effect can lead to a comprehensive improvement in alloy tensile properties.

Microstructure, mechanical properties and interfacial features of graphene oxide reinforced 7075 composites fabricated by ultrasonic assisted casting

Graphene oxide reinforced 7075 (GO/7075) composites were successfully prepared via ultrasonic assisted casting. In this study, the microstructure, mechanical properties and interfacial structure of GO/7075 composites with varying GO contents were investigated. The experimental results showed that GO could refine grain size and produce many dislocations at the GO/Al interface, and was well bonded with Al matrix. Meanwhile, a few rod-like Al4C3 phases were observed at the active prism planes of GO in the composite. When the content of GO reached 1.0 wt%, the optimal mechanical properties of GO/7075 composites were obtained. The yield strength and ultimate tensile strength of 1.0 wt% GO/7075 composites were 212.3 MPa and 285.6 MPa, which were 61.8 % and 41.1 % higher than those of 7075 alloy, respectively. The thermal mismatch strengthening and load transfer strengthening were the main strengthening mechanisms. Moreover, in order to control the interfacial reaction between GO and Al matrix, the thermodynamic/kinetic condition and growth mechanism for the formation of Al4C3 were analyzed in details.

Microstructure and Mechanical Properties of Mg-Al-La-Mn Composites Reinforced by AlN Particles.

The Mg-Al-RE series heat-resistant magnesium alloys are applied in automotive engine and transmission system components due to their high-temperature performance. However, after serving at a high temperature for a long time, the Al11RE3 phase coarsened and even decomposed, while the Mg17Al12 phase grew and dissolved, which limits the service temperature of Mg-Al-RE series heat-resistant magnesium alloys to a maximum of 175 °C. In this study, a new preparation method for in situ AlN particles was presented. The AlN/Mg-4Al-4La-0.3Mn composites were prepared by a master alloy and casting method. The effects of various contents of AlN (0.5-3.0 wt.%) on the microstructure and mechanical properties of the Mg-4Al-4La-0.3Mn (AE44) alloy at room (25 °C) and high temperatures (150-250 °C) were investigated. Microstructure analysis revealed that the inclusion of AlN led to a reduction in both the grain size and second phase size in the AE44 alloy, while also improving the distribution of the second phase. The average grain size, Al11La3 phase, Al2La phase, and Al3La phase of the 2.0 wt.% AlN/AE44 composite were 135.7, 9.6, 1.9, and 12.6 μm, respectively, which were significantly lower than those of the AE44 matrix alloy (179.8, 12.6, 3.3, 17.8 μm). The refinement was attributed to the ability of AlN particles to serve as heterogeneous nucleation cores for α-Mg and, at the same time, impede the growth of the solid-liquid interface, eventually leading to grain refinement. With the increase in the AlN content, the mechanical properties of composites initially exhibited an increase at both room and high temperatures, followed by a subsequent decrease. When the AlN content was 2.0 wt.%, the composite exhibited optimal strength and plasticity matching. At room temperature, the TYS, UTS, and EL values of the 2.0 wt.% Mg-4Al-4La-0.3Mn composite were 96 MPa, 175 MPa, and 7.0%, respectively, which were increased by 26 MPa, 27 MPa, and 0.7% when compared with the base alloy. The TYS of the 2.0 wt.% Mg-4Al-4La-0.3Mn composite at 150 °C, 200 °C, and 250 °C were 17 MPa, 14 MPa, and 22 MPa higher than those of the matrix alloy, respectively. The main strengthening mechanisms were second phase strengthening, load transfer strengthening, and thermal mismatch strengthening. At elevated temperatures, AlN particles effectively pinned the grain boundaries, inhibiting their migration, and hindered dislocation climbing, resulting in excellent mechanical properties of the composites at high temperatures. This study contributes to the advancement of in situ AlN particle preparation methods and the exploration of effects of AlN on the properties and microstructure of Mg-Al-RE alloys at high temperatures (150-250 °C).

Microstructure evolution and strengthening mechanism of pulsed current diffusion bonding TiAl/Ti2AlNb joints with in-situ rapid hot deformation

With the development of aero-engine toward high thrust-to-weight ratio, improving the strength of TiAl/Ti2AlNb joint has attracted considerable attention, as it is the key to manufacturing TiAl-Ti2AlNb composite turbine that combines high temperature resistance and weight reduction. However, the TiAl/Ti2AlNb diffusion bonded joint that is currently receiving the most attention is limited in strength by brittle interface structure and interfacial residual stresses, which cannot meet application requirements. In this study, a novel strategy of employing strong pulsed current to promote diffusion bonding of TiAl/Ti2AlNb joints and in-situ rapid hot deformation treatment was applied to improve this situation. This result shows that TiAl and Ti2AlNb alloys can be efficiently joined by pulsed current diffusion bonding (PCDB) with parameters of 950 ℃/10 min/10 MPa, the shear strength of the joint was 258.3 MPa. The subsequent in-situ rapid hot deformation treatment significantly strengthened the TiAl/Ti2AlNb joint, resulting in a shear strength of 443 MPa, which was 171.1 % of the pre-deformation strength. The formation of nanograined microstructure and precipitates acicular O phases after hot deformation-induced phase transformation of the diffusion bonding layers (DBLs) were the main strengthening mechanisms for the deformed joint. The large number of subgrain boundaries and the high dislocation density formed in the DBLs further increase the strength in this region. This study verified an effective of pulsed current diffusion bonding TiAl/Ti2AlNb joint with in-situ rapid hot deformation strengthening, providing a new strategy for obtaining high-strength dissimilar Ti-Al intermetallics joints.