Precipitation Strengthening Effects Research Articles

Tempering temperature effect on the static and impact properties of 51CrV4 and 55Cr3 spring steels

Abstract This study investigated the impact of tempering temperature on the mechanical properties and microstructure of 51CrV4 and 55Cr3 spring steels, which are commonly utilized in the manufacturing of leaf springs. These springs are integral components of suspension systems in both land and railway vehicles. The steels were hardened through austenitizing at 870 °C for 30 min, followed by oil quenching. Subsequently, they were tempered at 300 °C, 375 °C, 450 °C, and 525 °C for 120 min. Mechanical testing (including hardness and tensile tests) and both micro and macrostructural analyses were carried out to assess the effect of tempering temperature. Additionally, impact tests at various temperatures were performed to evaluate the influence of tempering on ductile-to-brittle transition temperature (DBTT). X-ray diffraction was employed for phase analyses while scanning electron microscopy was utilized to examine fracture surfaces. The quenching/tempering processes resulted in strengths that were two to three times higher and lower hardening capacities below 0.05 in comparison to the normalizing treatment. The steels achieved their maximum ultimate tensile strengths above 1800 N/mm2 at 300 °C, with decreasing values as the tempering temperature increased. The hardness of 51CrV4 steel surpassed that of 55Cr3, attributed to the hardenability and precipitation-strengthening effects provided by the Cr and V content. Furthermore, impact energies increased with increasing testing temperatures (40 °C, 0 °C, 25 °C, and +80 °C), with both steels tempered at 525 °C exhibiting higher and satisfactory impact energies across all testing temperatures, exceeding 15 J and 20 J, respectively. In addition, the tempered steels did not show a well-defined DBTT, while only the DBTT of normalized 51CrV4 steel was clearly determined at 0°C.

Microstructure, precipitates and mechanical properties of Cu-3Ni-1Ti alloy by two-stage cold rolling and aging treatment

The effects of two-stage cold rolling and aging treatment on the mechanical and electrical properties of Cu-Ni-Ti alloy were investigated. The microstructure evolution, precipitation behavior, and strengthening mechanism of the alloy after two-stage cold rolling and aging treatment were analyzed using microhardness, tensile strength, and electrical performance tests, combined with transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques. The research results show that the alloy after two-stage cold rolling and aging treatment exhibits excellent comprehensive properties, tensile strength reaches 635 MPa, yield strength is 597 MPa, hardness is 182 HV, conductivity is 62.1 % IACS, and elongation is 14.5 %. This processing method facilitates the precipitation of more nanoscale Ni3Ti phases and the formation of smaller cellular substructures. Consequently, the alloy maintains its exceptional mechanical properties through the combined effects of dislocation and precipitation strengthening. This study provides a novel preparation process and a solid theoretical foundation for enhancing the mechanical and electrical properties of Cu-Ni-Ti alloys.

Unveiling microstructural evolution and its effect on mechanical performance in a Cu-9Ni-6Sn alloy

Cu-9Ni-6Sn alloy exhibits profoundly potential as a new environmental-friendly conductive elastic material. In this work, the formation and growth mechanism of discontinuous precipitation, as well as its effect on mechanical properties of Cu-9Ni-6Sn alloy are systematically studied. The investigation indicated that the discontinuous precipitation does easily initiate from random grain boundaries but showing opposite result for Σ3 boundaries. The lower frequency Σ3 boundaries relatively, the more intense the solute diffusion, resulting in a higher volume fraction of discontinuous precipitation. The increasing aging temperature and time will accelerate the grain boundary discontinuous reaction, and the fine-grained samples exhibit a higher volume fraction of discontinuous precipitates and smaller lamellar spacing due to the more nucleation sites and increased interfacial energy. The strengthening mechanism of Cu-9Ni-6Sn alloy mainly focus on dislocation strengthening and precipitation strengthening, in which the D022 or L12-γ′ phases exhibit more significantly precipitation strengthening effect but is difficult to guarantee ductility. The localized grain boundary discontinuous precipitation detrimentally affect both the tensile strength and ductility.But the nano-lamellar discontinuous precipitation is beneficial to the strength-ductility trade off when it occupies the entirely Cu matrix. This work establishes a robust foundation for the microstructural optimization and multi-component design of Cu-9Ni-6Sn alloy.

Microstructure and mechanical performances of NiCoFeAlTi high-entropy intermetallic reinforced CoCrFeMnNi high-entropy alloy composites manufactured by selective laser melting

Multi-component alloys have been confirmed to be excellent reinforcing phases in high-entropy alloy-based composite. However, achieving a balance between strength and ductility by controlling the reinforcement content remains a major challenge in the design of high-entropy alloy-based composites. In this study, CoCrFeMnNi HEA/NiCoFeAlTi HER high-entropy alloy composites were prepared using SLM technology. Interestingly, gas-atomized NiCoFeAlTi high-entropy intermetallic (HER) powder was selected as the reinforcing phase, and the CoCrFeMnNi HEA matrix was used for comparison. The influence of the reinforcement content on the composite was investigated, and high-performance HEA/HER composites were obtained, with the strengthening mechanism elucidated. The results showed that the composite with 1 wt% HER reinforcement exhibited the optimal performance, with hardness, yield strength, ultimate tensile strength, and ductility at room temperature being approximately 226.9 HV, 562 MPa, 683 MPa, and 16%, respectively. The excellent mechanical properties at both room and elevated temperatures are attributed to the synergistic effects of heterogeneous microstructure strengthening, grain refinement, and precipitation strengthening. This work provides a foundation for the development of high-entropy alloy composites reinforced by high-entropy intermetallics.

Effect of pre-aging on mechanical reinforcement of the Cu–Fe–Nb composites

Pre-aging treatment was demonstrated to significantly enhance the mechanical properties of Cu–10Fe–1Nb (wt.%) composites. Specifically, after rolling and aging at 400 °C for 8 h, the microhardness, tensile strength, and elongation of the pre-aged samples reached 163 HV, 559 MPa, and 11.2%, respectively. The refinement of Fe-rich phases in pre-aged samples was primarily attributed to the increased shear strain between the Fe-rich phases and the matrix, which was driven by the precipitation-strengthening effect induced by pre-aging. Additionally, the matrix of the pre-aged samples suffered significant refinement as dynamic recovery and recrystallization of the matrix were suppressed during the rolling process, forming a high-density dislocation structure that greatly enhances the work-hardening effect. The main contribution to strength improvement in the Cu–10Fe–1Nb composites was identified as precipitation strengthening, refinement strengthening, and work hardening. Moreover, compared with the sample without pre-aging treatment, the refinement strengthening and work hardening effects of pre-aged samples exhibited substantial improvements, with increment of 49 MPa and 21 MPa, respectively.

Effect of nitrogen on the microstructure and strengthening mechanism of high performance tinplate

In order to meet the high strength and high formability requirements of the tinplate market, the nitrogen strengthening method was used for the chemical composition of the tinplate, and the action mechanism of nitrogen on the full-process tinplate substrate was explored. The present study designed tinplate with different nitrogen contents (named as 20 N steel and 150 N steel), and conducted phase transformation research and recrystallization temperature tests. SEM, XRD, EBSD, TEM, chemical dissolution analysis and tensile tests were carried out, and the influence of annealing process on the microstructure, mechanical properties and precipitation of annealed sheet was also explained. With the increase of nitrogen content, the phase transformation temperature decreases, the hardness increases and the recrystallization temperature increases. The microstructures of 20 N steel and 150 N steel annealed at different annealing temperatures are composed of ferrite and cementite distributed in chains. At the same annealing temperature, the recrystallization degree of 20 N steel is higher, the {111} texture accounts for a larger proportion, and the grain size is larger. With the increase of annealing temperature, the yield strength and tensile strength show a downward trend, while the elongation shows an upward trend. Compared with 20 N steel, the yield strength and tensile strength of the 150 N annealed sheet are significantly improved, and the elongation is slightly reduced. The calculation results show that the mechanism of nitrogen improving the strength of tinplate is mainly solid solution strengthening, and the improvement effect of fine grain strengthening and precipitation strengthening is weak.

Tracking research on the performance changes of S31042 steel under long term service

S31042 is widely used in ultra-supercritical units due to its excellent comprehensive performance at high temperatures. Tracking and monitoring research was conducted on as-received S31042 tubes and tubes in service for 13 k hours, 31 k hours, and 52 k hours at 650 °C. The results show that as the service time increased, a large quantity of M23C6 phase precipitated along the grain boundaries, gathered and grew up into the bulk. This weakened of the grain boundaries, which facilitated crack propagation and significantly reduced the plasticity and toughness of the S31042 heat-resistant steel. After 13 k hours of operation, the dispersion strengthening of the precipitated phase inside the grain and grain boundary led to a substantial increase in hardness, while there was no obvious change in strength. However, after 52 k hours of operation, the dispersed precipitated phase within the grain slowly aggregated and grew, decreasing the effectiveness of dispersion strengthening. Consequently, both the high-temperature strength and hardness of the S31042 steel gradually decreased. As the service time increased, the continuously distributed and similarly sized M23C6 phases on the grain boundaries of S31042 developed into an inhomogeneous coarsening chain structure. Additionally, the appearance of nanoscale secondary NbCrN and M23C6 phases within the grain and their resulting precipitation strengthening effects were the main reasons for the significant change in S31042 hardness values. Based on these findings, the residual life of S31042 heat-resistant steel in service was predicted using an extrapolation method based on high-temperature creep rupture tests and the relationship model between hardness and the P function. A comparison of the L-M parametric life prediction results with the strength extrapolation life prediction results based on the standard creep rupture test shows that the creep rupture strength extrapolation method is slightly conservative. Therefore, the prediction results based on the hardness L-M parameter method, which is simple and easy to perform, can be used for preliminary life prediction.

A molecular dynamics investigation of nano-twins and nano-precipitates effects in CoCrFeNi-based high-entropy alloys

Abstract This study systematically investigates the effects of twin boundaries and precipitates on the performance of CoCrFeNi HEAs matrix using molecular dynamics simulation methods. By constructing corresponding HEAs models and conducting simulations of their structural evolution and mechanical behavior at the nanoscale, the influence mechanisms of nanotwins (NTs) and nano-precipitates (NPs) on the mechanical properties of the material were explored through in-depth analysis of simulation results. The findings suggest that twin boundaries effectively impede the movement and slip of dislocations and stacking faults in the material. As a result, this enhances its mechanical properties and inhibits plastic deformation, ultimately improving its ductility. Meanwhile, precipitates also impact the material's performance, and the shape of precipitates may exert different effects on the material, while the phase interface between precipitates and the matrix can hinder the expansion of defects. The presence of twin boundaries can enhance the strengthening effect of precipitates, further improving the material's performance. This study provides a new perspective for understanding the relationship between the microstructure and mechanical properties of HEAs materials, offering important references for the design and optimization of HEAs materials.

Enhanced ductility via high-density nanoprecipitates driven by chemical supersaturation in a flash-heated precipitation-strengthened high-entropy alloy

Precipitation strengthening is an effective method to strengthen high-entropy alloys (HEAs); however, the incompatible plastic deformation caused by precipitates accelerates the initiation and development of cracks, reducing ductility. Therefore, it is necessary to spare no effort to overcome this strength–ductility trade-off. However, improving the ductility of HEAs without losing the precipitation-strengthening effect remains a daunting challenge. The strategy of this study is to increase chemical supersaturation without introducing other defects to drive the formation of high-density nanoprecipitates to share stress, reduce localization, and alleviate incompatible plastic deformation, thereby fully utilizing the strain-hardening ability to improve the ductility of HEAs without reducing their strength. Specifically, common Ni35(CoCrFe)55Al5.5Ti4.5 (at.%) precipitation-strengthened HEAs were selected as the experimental object. Short-term high-temperature flash heating causes the dissolution of the initial nanoprecipitates, and the separated atoms form chemically supersaturated microregions in the matrix between the initial nanoprecipitates after insufficient short-range diffusion, resulting in the formation of high-density new nanoprecipitates. The density of the nanoprecipitates reaches 5.0 × 1024m−3, representing a two-order increase in magnitude compared to the aging state (1.9 × 1022m−3). By increasing the density of nanoprecipitates, multiple nanoprecipitates share stress together. Sufficient slippage of dislocations maximizes the potential excellent strain-hardening ability, considerably improving the ductility of HEAs. Moreover, the strategy of improving the ductility by increasing the density of nanoprecipitates can be applied to many other precipitation-strengthened alloy systems.

The Influence of the Hot-Rolling Temperature on the Microstructure and Mechanical Properties of Ti-Nb Microalloyed 21%Cr Ferritic Stainless Steel

Microalloying and heat treatment are essential processing techniques for ferritic stainless steel (FSS). Three different compositions of 21%Cr FSS with 0.28Ti, 0.21Ti + 0.05Nb, and 1.05Ti + 0.17Nb were prepared. The interaction effects of the Nb and Ti contents and hot-rolling annealing on the microstructure, mechanical properties, and precipitate phases of FSS were studied. The microstructure, crystal structure, and precipitation phase of steel at 930, 980, and 1030 °C with Ti-Nb microalloying were investigated using an optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM). The room-temperature tensile properties, surface roughness, and hardness were tested separately. This study found that the composite addition of Ti and Nb had a dual effect of fine-grain strengthening and precipitation strengthening. The 1.05Ti + 0.17Nb steel specimen had a moderate grain size and the best uniformity after hot rolling at 980 °C. The tensile strength and elongation were 454 MPa and 34.2%, which achieved an optimal balance between strength and plasticity.

Preparation of graphene/AlCoCrNiTiMox coatings by laser parameter optimization based on TRIZ theory and its application in extending the service life of surveying and mapping drones

During the use of surveying and mapping drones, their aluminum (Al) alloy body is prone to corrosion and has certain requirements for mechanical properties. In order to extend the service life of surveying and mapping drones, this study uses laser cladding technology to prepare graphene (Gr) reinforced AlCoCrNiTiMox high entropy alloys (HEAs) coatings on the Al alloy surface of the drone body. In order to optimize the laser preparation parameters, the Invention Problem Solving Theory (TRIZ) is introduced. By utilizing innovative tools such as physical contradictions, technical contradictions, and material field models of this theory, the preparation process parameters such as laser power, laser scanning speed, laser spot diameter were optimized. The microstructure of the coatings were analyzed using scanning electron microscope (SEM), transmission electron microscope (TEM), electron back scatter diffraction (EBSD), etc. The corrosion resistance, hardness and wear resistance of the coatings were tested. The experimental results show that the microstructure of Gr/AlCoCrNiTiMox alloy is mainly composed of equiaxed grains and molybdenum-chromium (Mo-Cr) precipitates. The Gr/AlCoCrNiTiMox HEAs exhibits excellent corrosion resistance in 0.5 mol/L H2SO4 solution. The corrosion resistance of alloys is related to their composition, the effects of Gr and Mo elements. Under the combined effects of solid solution strengthening, precipitation strengthening, Mo element and Gr, the mechanical properties of Gr/AlCoCrNiTiMox alloys are excellent. The hardness of Gr/AlCoCrNiTiMo0.1 and Gr/AlCoCrNiTiMo0.2 alloys are 685 HV and 736 HV, respectively, with average friction coefficients of 0.64 and 0.58, respectively. The effect of Mo element makes the grain size of Gr/AlCoCrNiTiMo0.2 alloy smaller than that of Gr/AlCoCrNiTiMo0.1 alloy, resulting in better mechanical properties and corrosion resistance. The research results indicate that laser cladding of Gr/AlCoCrNiTiMox coating on the Al alloy surface of drone fuselage provides assurance for extending its service life.

Altering tensile and crack initiation behavior in a metastable β titanium alloy via designing grain size and precipitates

Microstructure can play a vital role in defining mechanical properties of metallic materials. To elucidate this correlation in the case of Ti–15Mo–3Nb–3Al-0.2Si (TB8) alloy, herein, we designed various microstructures via heat treatment exploring the effects of grain size, precipitates and segregation on crack initiation behavior during tensile tests in metastable β-Ti alloy. After solution treatment at 830 °C, the TB8 alloy with equiaxed β grain displayed a good fracture elongation of 30.2 ± 0.63%. The adiabatic shearing band and β→α phase transformation were activated to increase the compatible deformation capability during tensile testing; however, the phase transformation caused the stress concentration in the boundary, resulting in crack initiation. For the samples prepared using solution and low aging at 440 °C, large grain, elements segregation at grain boundary and incomplete precipitates induced a slight reduction in ultimate tensile strength and elongation. After solution and aging at 520 °C, the short-rod or/and lamellar α phase precipitated in β grain effectively enhancing ultimate tensile strength (1398.71 ± 15.6 MPa). The increased boundaries provided the interface or precipitation strengthening effect, but high-density dislocations were also accumulated at the β/α interface, causing unstable deformation and crack initiation. These findings advance our understanding of the correlation between microstructure and crack initiation, and provide a basis for designing and customizing the mechanical properties of metastable β-Ti alloy.

Preparing gradient microstructure to achieve enhanced performance in GH4586 superalloy: Experiments and models

In this present study, a combined method of controlling strain deformation (CSD) and gradient-temperature heat treatment (GTHT) was adopted to prepare gradient microstructure in GH4586 superalloy. Consequently, it brought enhanced tensile strength from 1313.1±18.7 MPa at intermediate transition (IT) region to 1506.3±15.7 MPa at high-strain and low-temperature (HS-LT) region, presenting an improvement of 11.6%. And it also enhanced fracture toughness from 77.7±0.3 MPa⋅m1/2 at IT region to 86.7±3.7 MPa⋅m1/2 at low-strain and high-temperature (LS-HT) region, presenting an improvement of 14.7%. The formation of gradient microstructure along the longitudinal direction of CSD+GTHT-treated sample was thoroughly investigated from perspectives of precipitates distribution, activated slip systems and dislocations evolution. During CSD, the size and volume fraction of γ′ precipitates decreased under the effect of strain-induced dissolution behavior. The activation of (1‾ 11)[101] and (11‾ 1)[011] slip systems dominated dislocation movement at each region and there retained increasing dislocation density due to more extensively activated slip systems at HS region. During GTHT, the high-density dislocations promoted recrystallization while the concentratedly distributed γ′ precipitates obstructed movement of recrystallized front, leading to formation of heterogeneous structure with high-density grain boundaries and dislocations at HS-LT region. Finally, a model reflecting the effect of grain boundary strengthening, precipitation strengthening and dislocation strengthening on mechanical property of gradient microstructure was established and successfully applied to property prediction of GH4586 superalloy.

Effects of Co and Mo on the microstructure properties of GH4282 and the mechanical properties of metal bellows

Abstract In the present study, a new type of Ni-Cr-Co-Mo (GH4282) precipitation-hardening superalloy was prepared on the basis of GH4202 alloy. By adding Co and Mo elements to enhance the solution-strengthening effect, good creep resistance and toughness were obtained. The results show that the strength and microstructure stability of GH4282 alloy are significantly higher than those of traditional GH4202 alloy under the condition of 700~900°C. The precipitated phase of GH4282 alloy is dominated by γ′ phase, which contains M23C6, MC, and a small amount of M6C carbide. The precipitation is mainly distributed along the grain boundaries, and has good stability at temperatures below 900°C, which can effectively pin the grain boundaries. The thermal stability of the alloy is increased. The metal bellows made of GH4282 alloy have significantly higher fatigue life and high-temperature stability, the increase of Co and Mo element content leads to the enhancement of precipitation, and the combined effect of solution strengthening and precipitation strengthening leads to the above results.

Microstructure and Properties of Mg-Gd-Y-Zn-Mn High-Strength Alloy Welded by Friction Stir Welding.

Mg-Gd-Y-Zn-Mn (MVWZ842) is a kind of high rare earth magnesium alloy with high strength, high toughness and multi-scale strengthening mechanisms. After heat treatment, the maximum tensile strength of MVWZ842 alloy is more than 550 MPa, and the elongation is more than 5%. Because of its great mechanical properties, MVWZ842 has broad application potential in aerospace and rail transit. However, the addition of high rare earth elements makes the deformation resistance of MVWZ842 alloy increase to some extent. This leads to the difficulty of direct plastic processing forming and large structural part shaping. Friction stir welding (FSW) is a convenient fast solid-state joining technology. When FSW is used to weld MVWZ842 alloy, small workpieces can be joined into a large one to avoid the problem that large workpieces are difficult to form. In this work, a high-quality joint of MVWZ842 alloy was achieved by FSW. The microstructure and properties of this high-strength magnesium alloy after friction stir welding were studied. There was a prominent onion ring characteristic in the nugget zone. After the base was welded, the stacking fault structure precipitated in the grain. There were a lot of broken long period stacking order (LPSO) phases on the retreating side of the nugget zone, which brought the effect of precipitation strengthening. Nano-α-Mn and the broken second phase dispersed in the matrix in the nugget zone, which made the grains refine. A relatively complete dynamic recrystallization occurred in the nugget zone, and the grains were refined. The welding coefficient of the welded joint exceeded 95%, and the hardness of the weld nugget zone was higher than that of the base. There were a series of strengthening mechanisms in the joint, mainly fine grain strengthening, second phase strengthening and solid solution strengthening.

Nanoceramic particle reinforced high alloying Al-Zn-Mg-Cu composites

In the metal matrix composites (MMCs) reinforced with nanoparticles, the enhancement of composite performance is often closely associated with the types of nanoparticles. It is vital to explore the influences and differences of various nanoparticles on the microstructure evolution and mechanical properties of highly alloyed aluminum-based composites. This study is the first to employ hot pressing for preparing Al-Zn-Mg-Cu composite materials reinforced with SiC and Al2O3 nano-ceramic particles, respectively. By employing hot extrusion and T6 heat treatment techniques, a systematic investigation was conducted on the effects of the addition of nano-ceramic particles on the microstructure evolution and mechanical properties of the Al-Zn-Mg-Cu composites, and a comparative analysis was performed on the two types of composite materials. The experimental results demonstrated that the Mg atoms in the α-Al matrix were consumed by the addition of SiC/Al2O3 nano-ceramic particles, and thereby the grain size primary η’ (MgZn2) strengthening phase was refined, leading to an enhanced overall mechanical performance of the Al-Zn-Mg-Cu composite. In particular, the Mg2Si phase was generated by the addition of SiC nanoceramic particles, through the interfacial reactions, and thus with the synergistic effects of precipitation strengthening and Orowan strengthening, the ultimate compressive strength and compressibility of Al-Zn-Mg-Cu composite can be increased to 844 MPa and 27.5%, respectively. On the other hand, the addition of Al2O3 nanoceramic particles can bring about the formation of an oxygen-rich phase and refined precipitates at grain boundaries. Accordingly, the elongation is increased to 43.5% while a high ultimate compressive strength in the composites can be maintained. It is anticipated that nanoceramic particle-reinforced Al-based metal composites have significant potential for achieving both high strength and exceptional ductility for the application in the industry field.

The Influence of Aging Temperatures on the Microstructure and Stress Relaxation Resistance of Cu-Cr-Ag-Si Alloy

Copper alloys used in connectors rely significantly on stress relaxation resistance as a key property. In this study, a heavily deformed Cu-Cr-Ag-Si alloy underwent aging at varying temperatures, with a subsequent analysis of its mechanical properties and microstructure, with a particular emphasis on understanding the mechanism of improving stress relaxation resistance. As the aging temperature rose, the Cr precipitated into a Cr-Si composite element precipitated phase. Both work hardening and precipitation strengthening played vital roles in enhancing the stress relaxation resistance of the Cu-Cr-Ag-Si alloy, with the latter exerting a more pronounced impact. The notable performance enhancement observed after aging at 450 °C can be attributed to the synergistic effects of work hardening and precipitation strengthening. Following aging at 450 °C, the alloy demonstrated optimal performance, boasting a tensile strength of 495.25 MPa, an electrical conductivity of 84.2% IACS, and a level of 91.12%. These exceptional properties position the alloy as a highly suitable material for connector contacts.

Effect of chromium and molybdenum addition on the matching mechanism of strength−ductility−toughness of low carbonmedium manganese steel

Low carbon medium manganese steel with chromium and molybdenum addition (4MnCM), alongside a control group without these additions (4Mn), was developed. The quenching (860 °C) and tempering (620 °C) heat treatment process was applied to both steel groups. The influence of Cr and Mo on the microstructure evolution and mechanical properties was examined. The findings revealed that, the low−temperature impact energy values (KV2) of 4MnCM at −60 and −84 °C were enhanced by 104 and 107 J, in comparison to 4Mn. The yield and tensile strength saw an increase of 100 MPa. The higher low-temperature toughness of 4MnCM is attributed, on one hand, to the addition of Cr and Mo, which inhibiting deformation twinning through enhances the stacking fault energy (SFE) of reversed austenite. On the other hand, it is due to the absence of coarse ε−martensite (ε−m) along the crack propagation path. Additionally, the phase transformation of reversed austenite occurs at the crack nucleation stage during the impact process of both steels, which helps alleviate stress concentration and increases the energy required for crack formation. The primary reason for the higher strength of 4MnCM lies in the significant enhancement of precipitation strengthening effects through the precipitation of M6C, M23C6 and Nb/TiC co-precipitation with M6C.

Microstructure evolution and strengthening mechanism of Al–Cu–Li–Mg–Ag VPPA welded joint during local three-stage and natural aging treatment

Local three-stage (LTA) and natural aging (NA) treatments were proposed to address the challenges associated with the aging treatment of Al–Li alloy fuel tanks and improve the mechanical properties of VPPA welded joints. The microstructure evolution and mechanical properties of aging welded joints were investigated, followed by a discussion of strengthening mechanisms. The results demonstrated that LTA treatment significantly improved the mechanical properties of VPPA welded joints, with an elongation of 6.8 % and a tensile strength of 501 MPa, representing 88.6 % of the base metal. Following the local double-stage solution (LDS) treatment, the eutectic phases in the welded joints dissolved. During the LTA treatment, there was a consistent decrease in elongation. Initially, the θ'' phase precipitated, followed by the T1(Al2CuLi), σ(Al6Cu5Mg2), and S′(Al2CuMg) phases. With increasing aging time, some θ'' phases transformed into θ′(Al2Cu) phases, while others combined with Mg and Ag to form Ω phases. The precipitated phases continued to grow, increasing the precipitation-free zone (PFZ) width. After 30 h of aging, three types of <110> twinned dendrites were observed in the weld zone (WZ) with (111) twinned planes. The improvement in performance was credited to the elimination of eutectic segregation, as well as the enhanced solid solution and precipitation-strengthening effects. The increase in strength of the LDS state welded joints was primarily due to solid solution strengthening. The strength of the precipitated phase was mainly achieved through the bypass mechanism. The eutectic phases did not dissolve after NA treatment, which resulted in limited strength increment and unchanged elongation (EL). Numerous T1 phases precipitated contributed to the precipitation-strengthening effect.

Achieved strength-plastic trade-off in HfMoNbTaTi refractory high-entropy alloy via powder metallurgy process

The practical application of refractory high-entropy alloys (RHEAs) has been limited by their brittleness at room temperature. This study prepared fine-grained HfMoNbTaTi refractory high-entropy alloys using a powder metallurgy process combining mechanical alloying (MA) and spark plasma sintering (SPS). This method effectively addresses the issue of room temperature brittleness and achieves a favorable balance between strength and plasticity. The resulting sintered alloy comprises two body-centered cubic (BCC) solid solution matrices and a range of nanoprecipitates, including the γ-phase, σ-phase, and α-phase. With increasing sintering temperature, the average grain size increased from 0.45 μm (at 1400 °C) to 4.56 μm (at 1700 °C). Simultaneously, the content of the γ-phase gradually decreases due to the greater influence of mixing entropy. Under quasi-static loading, the fine-grained multiphase structure not only benefits from grain boundary strengthening and precipitation strengthening effects, but also enhances the deformation uniformity, ultimately improving both the strength and plasticity. At 1450 °C, the sintered alloy exhibited a compressive yield strength of 2325 ± 8.40 MPa and a plastic strain of 26.44 ± 1.17 %. These values represent a 35.73 % increase in yield strength and a 120.33 % increase in plastic strain compared to those of the as-cast alloy. The deformation process at room temperature is mainly governed by the multiplication of screw dislocations and cross-slips. In the middle and late stages of deformation, the grains exhibit a preferred orientation, further enhancing the plastic strain. In summary, this study presents a practical approach for overcoming the issue of room temperature embrittlement in RHEAs.