Precipitation Strengthening Effects Research Articles

Pre-precipitating promoted by microshear bands effectively circumvents strength-ductility trade-off of RT-rolled Al–6Zn–1Mg alloy

The aging precipitated phases are of great significance for the strengthening and toughening of Al–Zn–Mg alloys. In this work, pre-precipitating was realized through equal channel angular pressing (ECAP) immediately followed by solid solution treatment at 475 °C for 6 h. Its corresponding strengthening and toughening effect on the room temperature (RT)-rolled Al–6Zn–1Mg alloy was systematically studied. The results indicate that pre-precipitating by ECAP can effectively improve the strength of the RT-rolled Al–6Zn–1Mg alloy and circumvent the strength-ductility trade-off. Microstructure characterization reveals that ECAP induces a large number of microshear bands, which can not only encourage recrystallization but also effectively promote the precipitation of the η′ phase. The synergistic effect of precipitation strengthening and dislocation strengthening contributes to the high strength of Al–6Zn–1Mg alloy, with UTS and YS increasing to ∼522 MPa and ∼400 MPa, respectively. Meanwhile, the ductility remains almost unchanged mainly due to the coordinated grain boundary sliding and the uniform and finely dispersed second phase particles. As a result, the strength-ductility trade-off is effectively avoided. The findings shed some light on optimizing the traditional aging treatment.

Achieving 2.2 GPa Ultra-High Strength in Low-Alloy Steel Using a Direct Quenching and Partitioning Process.

Advanced high-strength steels (AHSS) have a wide range of applications in equipment safety and lightweight design, and enhancing the strength of AHSS to the ultra-high level of 2 GPa is currently a key focus. In this study, a new process of thermo-mechanical control process followed by direct quenching and partitioning (TMCP-DQP) was developed based on Fe-0.4C-1Mn-0.6Si (wt.%) low-alloy steel, and the effects of microstructure evolution on mechanical properties under TMCP-DQP process and conventional hot rolled quenched and tempered process (HR-QT) were comparatively studied. The results show that the TMCP-DQP process not only shortened the processing steps but also achieved outstanding comprehensive mechanical properties. The TMCP-DQP steel exhibited a tensile strength of 2.23 GPa, accompanied by 11.9% elongation and a Brinell hardness of 624 HBW, with an impact toughness of 28.5 J at -20 °C. In contrast, the HR-QT steel exhibited tensile strengths ranging from 2.16 GPa to 1.7 GPa and elongations between 5.2% and 12.2%. The microstructure of TMCP-DQP steel primarily consisted of lath martensite, containing thin-film retained austenite (RA), nanoscale rod-shaped carbides, and a minor number of nanoscale twins. The volume fraction of RA reached 7.7%, with an average carbon content of 7.1 at.% measured by three-dimensional atom probe tomography (3DAP). Compared with the HR-QT process, the TMCP-DQP process resulted in a finer microstructure, with a prior austenite grain (PAG) size of 11.91 μm, forming packets and blocks with widths of 5.12 μm and 1.63 μm. The TMCP-DQP process achieved the ultra-high strength of low-alloy steel through the synergistic effects of grain refinement, dislocation strengthening, and precipitation strengthening. The dynamic partitioning stage stabilized the RA through carbon enrichment, while the relaxation stage reduced a small portion of the dislocations generated by thermal deformation, and the self-tempering stage eliminated internal stresses, all guaranteeing considerable ductility and toughness. The TMCP-DQP process may offer a means for industries to streamline their manufacturing processes and provide a technological reference for producing 2.2 GPa grade AHSS.

Microstructure, properties and natural ageing behavior of friction stir welded dual-phase Mg-Li alloy

In this study, the super-light α + β dual-phase Mg-9Li-3Al-3Zn alloy was successfully jointed by friction stir welding (FSW). The microstructure, mechanical properties as well as corrosion resistance of the welds were specially explored. The natural age-softening of the welds was found and the underlying mechanism for the properties durability was revealed. FSW resulted in significant grain refinement as well as dissolution of α-Mg phases into β-Li phases in the stirred zone (SZ). Meanwhile, Mg17Al12 (λ) and AlLi (γ) precipitates were dissolved into the matrix while orderly-arranged MgLi2Al (θ) precipitates were formed, which contributed to a precipitation-strengthening effect. Therefore, the welds exhibited extensive hardening and demonstrated superior tensile strength than base metal (BM). Under the optimal welding parameters, the ultimate tensile strength (UTS) of the weld reached 165.8% of that of the BM. On the other hand, the welds obtained at the various rotation speeds also exhibited lower corrosion rates than that of the BM, and higher rotation speeds led to superior corrosion resistance. Furthermore, owing to increase in γ precipitate content during natural ageing, the precipitation-strengthening effect in the weld was weakened, resulting in reduction of the strength of the welds. Meanwhile, the increasing γ precipitate content aggravated the local micro-galvanic coupling corrosion, causing the decrease in corrosion resistance of the welds during natural ageing. This welding method might be useful for obtaining the dual-phase Mg-Li alloys with high mechanical properties and corrosion resistance.

Strong room temperature work hardening in a dual heterogeneous structured ferrous medium entropy alloy

In this work, a novel (Fe65Ni15Cr10Co10)92Ti5Al3 medium-entropy alloy (MEA) with a dual heterogeneous structure was produced by adding Ti and Al in our previous reported Fe65Ni15Co10Cr10 MEA. The dual heterogeneous structure consists of the partial recrystallization matrix and heterogeneous nanoprecipitates. The tensile tests show that the MEA displays an ultra-high ultimate tensile strength (1519 MPa) and work hardening extent (559 MPa) at room temperature. The addition of Ti and Al produces precipitates while lowering FCC stability. This contributes to a continuous FCC→BCC martensitic transformation and a consequent transformation-induced plasticity (TRIP) effect during whole plastic deformation. Therefore, the excellent mechanical properties are attributed to the synergism of multiple mechanisms, including the hetero-deformation induced (HDI) strengthening, precipitation strengthening, and TRIP effects. The alloy design strategy in this paper has guiding significance for the development of high-performance ferrous MEAs.

Ultrahigh strength and ductility combination in Al40Cr15Fe15Co15Ni15 triple-phase high entropy alloy

In this work, we developed the as-cast triple-phase Al40Cr15Fe15Co15Ni15 HEA composing of B2, BCC and σ phases. The triple-phase Al40Cr15Fe15Co15Ni15 HEA shows an excellent strength-plastic combination, with yield strength of 1454 MPa, compressive strength of 3628 MPa, ductility of 38 % and specific yield strength of 266.4 MPa cm3/g. These mechanical properties of the Al40Cr15Fe15Co15Ni15 HEA were attributed to the combined effects of solid solution strengthening, precipitation strengthening and grain boundary strengthening caused by high Al atom concentration. The outstanding mechanical performance makes the triple-phase Al40Cr15Fe15Co15Ni15 HEA an aussichtsreich engineering material.

Design crystallographic ordering in NbTa0.5TiAlx refractory high entropy alloys with strength-plasticity synergy

To make up for the poor strength of high plasticity NbTa0·5Ti refractory medium entropy alloys (MEAs), light metal Al was introduced as alloying element. In this work, the NbTa0·5Ti-Alx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) series non-isoatomic refractory high entropy alloys (RHEAs) were prepared by arc melting, and phase equilibrium was predicted by CALPHAD. The effects of Al content and annealing temperature on microstructure and phase evolution, and its mechanical properties were studied. The NbTa0·5Ti-Alx alloys changed from single phase BCC to two-phase A2+B2 crystal structure after adding Al. The hardness and strength of the as-cast alloys are increased by the solution strengthening and precipitation strengthening effect, but the brittleness is increased. The precipitation of Laves of plate-like NbAlTi2 and finer-scale A15 of (needle, particle)-like AlTi3 precipitates at and near grain boundaries after annealing. Higher annealing temperature is beneficial to eliminate dendrites formed by element segregation in the arc melting cooling process and promote grain growth (up to ∼200 μm). This work designed a new alloys with excellent compression plasticity and enriched the field of composition design and aging treatment of the Al-containing second generation RHEAs， so that their microstructure can be better controlled to achieve a balance of strength and plasticity.

Effect of Co content on microstructure and mechanical properties of maraging steel

Effect of Co element on the microstructure and mechanical properties of maraging steel is studied and summarized. The microstructure and mechanical properties of five different compositions of maraging steels were characterized and tested in three heat treatment states, with the heat treatment schemes of homogenization treatment at 1300 °C × 10h, solution treatment at 800 °C × 2h, and aging treatment at 480 °C × 5h. The research has discovered that increasing the Co content in three different heat treatment states can lead to a reduction in the size of the original austenite grains and the width of martensite laths. In the aged state, higher Co content causes the precipitation phase to transform from an ellipsoidal Mo-rich phase to a needle-like Ni3(Mo, Ti) phase. These changes in the size, shape, and density of the precipitated phases result in a more pronounced effect of precipitation strengthening, leading to a significant increase in the material's strength.

Combined strengthening from nanoprecipitates, stacking faults and nano-twins enables a strong and ductile medium-entropy alloy

Single-phase face-centered cubic (fcc) alloys typically exhibit remarkable strain-hardening capability and large ductility but possess low yield strength. Conventional strengthening strategies inevitably lead to the loss of ductility. Here, an Nb alloyed CrCoNi-based medium-entropy alloy with a superb yield strength of ∼1.82 GPa, an ultimate tensile strength of ∼1.94 GPa, and good ductility of ∼24 % at 93 K is designed through the combined strengthening from nanoprecipitates, stacking faults, and nano-twins. The cold-rolling is employed to introduce high-density stacking faults and nano-twins, while subsequent low-temperature annealing generates numerous Nb-rich γ″ nanoprecipitates in the fcc matrix. The combined effects of precipitation strengthening, stacking fault- and nanotwin-induced strengthening contribute to an increase in flow stress to surpass the critical stress for the formation of stacking faults and deformation twinning. Moreover, the precipitation of high-density Nb-rich γ″ nanoprecipitates reduces the stacking fault energy of the fcc matrix. Therefore, the precipitation of γ″ nanoprecipitates produces more stacking faults and nano-twins during tensile deformation. The formation of stacking faults and nano-twins in turn, by dynamically introducing high-density interfaces for reducing the mean free path of dislocations, generates a high strain-hardening rate, thus high ultimate tensile strength and large ductility. More importantly, the coherent interfaces between γ′′ nanoprecipitates and the fcc matrix facilitate the transmission of stacking faults and nano-twins across the γ′′ nanoprecipitates. This mechanism could relieve the stress/strain concentrations at interfaces that could otherwise lead to premature crack initiation. This kind of combined strengthening from nanoprecipitates, stacking faults, and nano-twins provides a new avenue to further enhance the strength of nano-twinned materials or precipitation-strengthened materials while maintaining good ductility.

Effects of the microstructure and texture characteristics on mechanical properties of the Al–Ni–Cu–Fe alloy manufactured by laser powder bed fusion

In this study, the Al–Ni–Cu–Fe alloy was fabricated using laser powder bed fusion (LPBF), with controlled Fe content, to investigate its metallurgical effects. Furthermore, the microstructural differences between the coarse-grained (CG), fine-grained (FG) regions, and the melt pool boundary were compared. The results showed that the FG region exhibited grain-boundary and precipitation strengthening effects. The grain orientation in the FG region was randomly distributed, leading to the randomly oriented columnar grains with a small ratio of low-angle misorientations in the CG region. This prevented the CG region from developing a temperature gradient texture and provided a solid foundation for the high strength. Both high (Al–4Ni–2Cu–1.4Fe) and low (Al–4Ni–2Cu–0.7Fe) Fe-addition alloys exhibited strengths of over 600 MPa. The FG region of the 1.4Fe contained more strengthening phases, contributing to its higher strength. However, the presence of more incoherent interfaces between Al9FeNi and the α-Al lattice resulted in reduced ductility. On the other hand, the 0.7Fe alloy exhibited lower brittleness and improved ductility. Overall, the LPBF Al–Ni–Cu–Fe alloy shows promising potential for aerospace applications, and a comprehensive understanding of the strengthening mechanisms and microstructure can expand its application scope.

A Comparative Study on Microstructure, Segregation, and Mechanical Properties of Al-Si-Mg Alloy Parts Processed by GISS-HPDC and SEED-HPDC.

There are multiple routes to prepare semi-solid slurries with a globular microstructure for semi-solid forming. The variations in the microstructure of semi-solid slurries prepared using different routes may lead to significant differences in the flow behavior and mechanical properties of rheo-diecasting parts. Therefore, it is crucial to have a comprehensive understanding of the microstructure evolution associated with different slurry preparation routes and their resulting effects. In this study, the gas-induced semi-solid process (GISS) and the swirl enthalpy equilibrium device (SEED) routes were employed to prepare semi-solid Al-Si-Mg slurries for their simplicity and productivity in potential industrial applications. The prepared slurries were then injected into the shoot sleeves of a high-pressure die casting (HPDC) machine to produce tensile test bars. Subsequently, the bars underwent T6 treatment to enhance their mechanical properties. The microstructure, segregation, and mechanical properties of the samples were investigated and compared with those of conventional HPDC. The results indicated that the GISS and SEED can produce semi-solid slurries containing a spherical α-Al primary phase, as opposed to the dendritic structure commonly found in conventional castings. The liquid fraction had a significant effect on the flow behavior, resulting in variations in liquid segregation and mechanical properties. It was observed that a higher solid fraction (>75%) had a suppressing effect on surface liquid segregation. In addition, the tendency for liquid segregation gradually increased along the filling direction due to the special flow behavior of the semi-solid slurry with a low solid fraction. Furthermore, under the same die-casting process parameters, the conventional HPDC samples exhibit higher yield stress (139 ± 3 MPa) compared to SEED-HPDC and GISS-HPDC samples, which may be attributed to the small grain size and the distribution of eutectic phases. After undergoing the T6 treatment, both SEED-HPDC and GISS-HPDC samples showed a significant improvement in yield and tensile strength. These improvements are a result of solution and precipitation strengthening effects as well as the spheroidization of the eutectic Si phase. Moreover, the heat-treated SEED-HPDC samples demonstrate higher ultimate strength (336 ± 5 MPa) and elongation (13.7 ± 0.3%) in comparison to the GISS-HPDC samples (307 ± 4 MPa, 8.8 ± 0.2%) after heat treatment, mainly due to their low porosity density. These findings suggest that both GISS-HPDC and SEED-HPDC processes can be utilized to produce parts with favorable mechanical properties by implementing appropriate heat treatments. However, further investigation is required to control the porosities of GISS-HPDC samples during heat treatment.

Partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium-Mn steel

This study reports the partitioning temperature-dependent microstructures and mechanical properties of precipitation-hardened medium Mn steel processed by room-temperature quenching and partitioning. The hot-rolled Fe–7Mn-0.3C–3Ni-1.5Si–1Al–1Cu-0.3V (wt.%) steel was processed by austenitization, water quenching to room-temperature, and partitioning treatment at 200, 300, 400, and 500 °C for 600 s. The samples before and after the partitioning process at temperatures below 400 °C show a microstructure consisting of lath martensite and retained austenite, whereas the sample partitioned at 500 °C shows a notably high fraction of retained austenite, owing to the formation of austenite grains during the partitioning process. The C partitioning during the partitioning process led to higher C contents in the samples processed by austenitization, quenching, and partitioning than in the sample processed by austenitization and quenching. The co-precipitation of VC precipitates and dispersed fine particles could exhibit a precipitation-strengthening effect. The sample partitioned at 200 °C shows an excellent strength-ductility combination, i.e., a yield strength of 740.9 MPa, an ultimate tensile strength of 1623.6 MPa, and a total elongation of 23.8%. Film-like retained austenite stabilized by carbon partitioning could exhibit high mechanical stability, leading to a gradual transformation induced plasticity (TRIP) effect and superior mechanical properties. In contrast, the samples partitioned at temperatures above 400 °C show poor ductility with brittle fracture, which could be related to the formation of martensite/retained austenite (M/A) constituents.

Corrosion behavior and mechanical property of Mg-4Li-1Ca alloys under micro-compressive stress

Biomedical materials may suffer from stress-induced corrosion when performing as implant materials at load-bearing sites, bringing about variations in the microstructure, corrosion resistance, and mechanical properties. In this study, the corrosion behavior and mechanical properties of an extruded Mg-4Li-1Ca alloy were investigated under different micro-compressive stresses (0–6 MPa) using a novel homemade loading device. Under 0–3 MPa of micro-compressive stress, the strong basal texture of extruded Mg-4Li-1Ca alloys was weakened and the internal stress gradient stimulated grain boundary migration to induce grain growth. Meanwhile, increased stress resulted in the precipitation of second-phase particles and the accumulation of residual stress, accelerating the corrosion rate due to preferential corrosion. However, with increasing stress, the volume fraction of the second phase increased, becoming the dominant factor controlling the corrosion rate, and residual stress was released for samples under 4.5–6 MPa of micro-compressive stress. Hence, surface corrosion product films rapidly formed and served as effective physical barriers, weakening the microstructural effect on the corrosion behavior. The yield strength of Mg-4Li-1Ca alloy reached 95.48 MPa under 3 MPa of micro-compressive stress owing to the dual effects of precipitation strengthening and shear-band strengthening. The relationships between microstructure, corrosion behavior, and mechanical property provide a theoretical foundation for understanding the degradation characteristics of the Mg-4Li-1Ca alloy under physiological loading and practical application.

Strain-rate-dependent behaviors of CoCrNiSi0.2 medium-entropy alloy: Deformation mechanism and microstructural-based constitutive modeling

Abundant strengthening mechanisms in medium-entropy alloys (MEAs) endow them with excellent comprehensive properties, rendering them promising candidates for use in extreme service conditions, such as low/high temperatures and strong impact environment. Investigation of the mechanical behaviors and micro deformation mechanisms of MEAs under intermediate/high strain rate loadings should therefore help to explore their application potential in impact-resistant structures. In this study, the tensile mechanical properties of a single-phase face-centered cubic (FCC) CoCrNiSi0.2 MEA were tested over low, intermediate, and high strain rates, and the micro deformation mechanisms at different strain rates were analyzed systematically. Dislocation evolution, deformation twinning, and phase transformation (from single FCC to FCC + hexagonal close-packed (HCP)) were observed in the three groups of tensile specimens loaded at low, intermediate, and high strain rates. The dislocation densities and twin volume fractions in the three groups were comparable, while the volume fraction of phase transformation increased significantly with the increasing strain rate. Therefore, more HCP phases generated during high-strain-rate deformation should lead to a stronger precipitation-strengthening effect on the material, which made the strain-rate-strengthening effect more significant to the extent that it could not be ignored in constitutive modeling. Finally, the contribution of the phase transformation was introduced for the first time into a constitutive model in which dislocation slip and deformation twinning were also considered to describe the viscoplastic flow behavior of the CoCrNiSi0.2 MEA over a wide range of strain rates.

Effect of heat treatment on the hydrogen embrittlement susceptibility of selective laser melted 18Ni-300 maraging steel

The effects of heat treatment on the hydrogen embrittlement (HE) susceptibility of selective laser melted 18Ni-300 maraging steel were investigated. The results demonstrate that the as-built steel has a microstructure consisting of an Fe–Ni martensite matrix and a cellular structure. Substantial microstructural changes are observed following various heat treatment processes (solution, direct aging, and solution-aging), resulting in varying degrees of increased HE susceptibility. Specifically, after solution treatment, the complete elimination of cellular structure and the growth of grains concurrently increased hydrogen diffusion coefficient of the steel. However, its lower proportion of ∑3 boundaries and higher proportion of ∑11 boundaries played an advantageous role that lead to a marginal increase in the HE susceptibility. In contrast, direct aging and solution-aging treatments enhance the HE susceptibility significantly, which is primarily related to the decomposition of the cellular structure and reversible hydrogen trapping and precipitation strengthening effects of the newly formed Ni3(Ti, Mo) precipitates. Compared with the solution-aging treatment, the direct aging treatment results in the formation of a localized austenite aggregation zone near the molten pool boundaries, which promotes the initiation of hydrogen-assisted cracking and leads to a higher HE susceptibility.

Effect Mechanism of Sc Addition and T6 Heat Treatment on Precipitated Phase and Mechanical Properties of Al‐Cu‐Mn Alloy

The microstructure and mechanical properties of cast Al‐Cu‐Mn alloy influenced by various Sc additions (viz. 0.1, 0.2, 0.3, 0.4, and 0.5 wt%) are investigated. It is found that the grain size of α‐Al and the area proportion of the second phase are decreased with the addition of Sc. In addition, the morphology of the precipitated phase in Al‐Cu‐Mn alloy also evolves at different stages, and the solidification temperature range and initial enthalpy change of the alloy are changed, resulting in the formation of a variety of second‐phase containing Sc. Owing to the grain refinement and precipitation strengthening effects, the hardness of as‐cast alloys is better than T6 alloys. Moreover, it is observed that when the Sc content is 0.3%, the mechanical properties of the as‐cast Al‐Cu‐Mn alloy are the minimum, and the Al‐Cu‐Mn alloys after T6 heat treatment are the maximum. These results confirm that the mechanical properties improvement effect of the as‐cast Al‐Cu‐Mn alloy is greater than that of the T6 heat treatment Al‐Cu‐Mn alloy with the addition of Sc. Meanwhile, the Orowan strengthening mechanism is found to have lost its dominant position.

Chemical core-shell metastability-induced large ductility in medium-entropy maraging and reversion alloys

Maraging structural materials have been traditionally indicated as essential metallic alloys for hundreds of years. The highest strength of the aged martensite alloys requires the formation of high nanoprecipitate density; however, it often results in insufficient ductility (< ∼12%) which limits their application. Here, we describe how these alloys obtain enhanced ductility at high strength by injecting reversion-induced metastable austenite into the brittle microstructure, in which we develop a novel dual-phase medium-entropy Fe68Ni10Mn10Co10Ti1.5Si0.5 (at%) maraging alloys with a strength of 1.6 GPa and ductility of ∼25%. Generating the large fraction of austenite metastability with a chemical core-shell microstructure during a simple process of reversion drives profuse heterogeneities at chemical and structural states, including additional precipitation strengthening and transformation-induced plasticity effect. The combined metastability and heterogeneity, realized with heat-treatment techniques that are accessible processing routes in a wide range of academic and industrial applications, can provide a breakthrough to develop sustainable maraging materials with large ductility.

Variation of BCC/B2 coherent microstructure and resultant improvement in strength of Al0.7CrxFe3−xCoNi high-entropy alloys

The effects of Cr and Fe contents on the microstructure and mechanical properties of B2 nanoparticle-enhanced BCC-based Al0.7CrxFe3−xCoNi (x = 1.5–3.0) high-entropy alloys (HEAs) were studied. It was indicated that the addition of Cr and reduction of Fe in the alloys can increase the lattice misfits between the BCC and B2 phases, inducing the variations of BCC/B2 coherent microstructure and the significant improvements of the compressive yield strengths from ∼ 1485 MPa to ∼ 2252 MPa. The calculations of the yield strengths based on the microstructure of the alloys suggested that the improvements of the strengths were mainly due to the enhancements of the precipitation-strengthening effects. The interactions between dislocations and B2 particles with various shapes and sizes were also discussed on the basis of the calculations. It is demonstrated that adjusting the contents of Cr and Fe is an effective way to control BCC/B2 coherent microstructure and improve the mechanical properties of Al–Cr–Fe–Co–Ni HEAs.

Dependence of Tensile Properties and Fracture Behaviors on the Fractions of Continuous and Discontinuous Precipitates in Peak-Aged AZ80A Magnesium Alloy.

After T5 (forging + aging) and different T6 (forging + solution + aging) heat treatments, the AZ80A Mg alloys exhibited microstructures with different fractions of continuous precipitate (CP) regions and discontinuous precipitate (DP) regions. The effects of the fractions of DP regions and CP regions on the tensile properties and fracture behaviors were investigated using microstructural characterizations and analysis. The results showed that increasing the fraction of DP regions enhanced the yield strength and tensile strength at room temperature. However, at the same high temperature, increasing the fractions of DP regions improved the elongation but deteriorated the tensile strength significantly. The different resultant tensile properties at different temperatures were caused by the different precipitation-strengthening effects in the CP and DP regions. The strengthening contribution of the DP regions was more effective at room temperature but became inferior to the effect brought about by the CP regions at high temperatures. Micro-cracks were usually initiated and propagated in the CP regions at room temperature. At high temperatures, however, micro-voids formed more easily in the DP regions, and the fracture path preferred to locate there.

Laser welding of SiCp/2024Al composites with novel TiB2/2024Al composite filler

The effect of TiB2/2024Al filler on the microstructure and mechanical properties of SiCp/2024Al joint was investigated in this paper. The results indicated that needle-like Al4C3 phase with a content of 18.9% was the dominant precipitate in the weld zone (WZ) of direct welded (DWed) joint, which weakened the tensile strength (109.3 MPa) of DWed joint. The introduction of TiB2/2024Al filler changed the microstructure of WZ. In the molten pool, TiC precipitate was precipitated preferentially due to the higher chemical affinity of Ti and C atoms and lower Gibbs free energy of TiC formation. The dispersed TiC blocks presented strong fine grain strengthening and precipitation strengthening effects. Compared with DWed joint, the strength of SiCp/2024Al joint with TiB2/2024Al filler increased to 268 MPa, approximately 72.9% of that of base metal (BM).

Effect of twin assisted precipitation on the microstructure and mechanical properties of gradient structure Mg-Gd-Zn-Zr alloy

Nano-precipitation can effectively pin the dislocations and inhibit the dislocations’ motion, making precipitation strengthening one of the most crucial strengthening mechanisms in magnesium (Mg) alloys. Consequently, controlling the segregation of elements at the coherent interfaces to induce the phase precipitation, has become an innovative way of designing Mg alloys. In this study, a novel twin-precipitation composite gradient structure GZ151K (Mg-15Gd-1Zn-0.4Zr) alloy was prepared using ultrasonic surface rolling process (USRP) and heat treatment. The results indicated that the USRP-treated samples subjected to aging treatment demonstrated high thermal stability without significant twin degradation. Additionally, the twin boundaries and grain boundaries could act as the nucleated points for the (Mg, Zn)5Gd phases during the subsequent aging treatment, and further formed the twin-precipitation composite gradient structure. This unique composite gradient structure of alloys treated by USRP and aging treatment was caused by an extra promotion-precipitation strengthening mechanism. Under the effects of promotion-precipitation strengthening, strain hardening, and precipitation strengthening, the mechanical properties of the GZ151K alloy were significantly improved.