Second-phase Strengthening Research Articles

Effect of Nb addition on microstructure and mechanical properties of TiC/Ti-7.8Cr composites

The morphology of TiC is not difficult to control when the titanium matrix composites with high TiC content are prepared by the melting-casting method. TiC tends to grow into dendrites, increasing the brittleness of the Ti matrix composites. In this study, in-situ TiC-reinforced titanium matrix composites with varying Nb content were prepared by adding Nb. The effects of adding Nb on the microstructure evolution and mechanical properties of the composites were studied. The results show that with the increase of Nb content, the grain size of the composites decreased gradually from 276 μm to 122.8 μm, the primary TiC content decreased, and the eutectic TiC content increased. On the other hand, the orientation relationship between TiC (111)//β-Ti (110) and TiC [10 1‾]//β-Ti [1‾ 11] was studied by HTEM. The two-dimensional mismatch of TiC (111)//β-Ti (110) is only 9.06 %, which means that TiC can be used as a nucleation particle of β-Ti. Furthermore, there was a gradual increase in the Rockwell hardness of the composites as the Nb content was incrementally raised from 0 wt% to 1.5 wt%, with the highest recorded value being 47.4 HRC. Notably, when the Nb addition reached 1.0 wt%, the composite achieved its maximum tensile strength of 1352.9 MPa, signifying a 69 % enhancement compared to the absence of Nb. This is due to the combined effect of fine grain strengthening, solution strengthening, and second-phase strengthening in the composites. It introduces a novel approach to regulate the in-situ formation of TiC morphology during the preparation of titanium matrix composites through melting and casting.

Influences of Si-Ti combined alloying on microstructures and properties of laser welded socket joints of molybdenum

This study selected silicon (Si) and titanium (Ti) as alloying elements to explore influences of the Si:Ti mass ratio on performance of molybdenum bead on plate laser welded joints. When the Si:Ti ratio was 1:1, the silicides exhibited a uniform continuous distribution, and significantly increased room temperature tensile strength of molybdenum bead on plate from 48 MPa to 231.6 MPa. In addition, Si-Ti combined alloying was applied to Mo tube-end plug socket joints with the optimal Si:Ti ratio. Results showed that Si and Ti were uniformly distributed in the weld of laser welded socket joints. Addition of Si and Ti in the weld increased the fraction of low-angle grain boundaries (LAGBs), which was conducive to improving the bonding strength of grain boundaries. The second-phase particles Mo3Si and Ti5Si3 were observed in the weld after adding Si and Ti, which played a role in second-phase strengthening. The room-temperature tensile strength of laser welded socket joints of Mo with the Si:Ti mass ratio of 1:1 reached 301 MPa, and the joint showed mixed fractures. The fractures in the FZ exhibited morphologies of cleavage fractures, while those in the heat affected zone (HAZ) showed mixed morphologies of intergranular and cleavage fractures. Finally, the results of high-temperature nanoindentation creep testing showed that after applying a load of 8 mN for 600 s at 400℃, the creep displacement of the LW-socket joint was measured at 20.28 nm, whereas the LW-SiTi socket joint exhibited a creep displacement of only 6.63 nm. The research findings of this paper provide references for alloying methods in laser welding of molybdenum and molybdenum alloys.

Designing eutectic medium-entropy alloys CoFeNiTax with outstanding mechanical properties

The eutectic medium-entropy alloys CoFeNiTax (x = 0.4, 0.5, 0.6) were designed successfully based on binary eutectic systems and thermodynamics calculations in this study. With the increase of Ta content, the solidified microstructures evolved from hypoeutectic to full eutectic and finally to hypereutectic. The primary phases in the hypoeutectic and hypereutectic were FCC solid solution and C14-Laves phase, respectively. The solidified microstructures were coarsened and spheroidized by the heat treatment, and a high-density of needle-like Laves phase precipitated in the FCC phase of the CoFeNiTa0.4. The crystallographic orientation relationship between the precipitated phase and matrix phase was [2‾ 42‾ 3]Laves//[011]FCC, and the crystal plane (1 1‾ 1)FCC was almost parallel to (0 1‾ 12)Laves, just with a mismatch angle of about 6°. The compressive strength and fracture strain of the heat-treated alloys were within 2232∼2465 MPa and 22.5–32.8 %, respectively, being superior to most high-entropy alloys. The excellent combination of strength and toughness was attributed to the synergistic effects of various strengthening-toughening mechanisms, including the second-phase strengthening, the solid solution strengthening, the interface strengthening and toughening, and the twinning toughening.

Reinforcing effects of nano-WC in AlSi10Mg alloy assisted by in-situ surface modification approach

A series of nano-WC surface modified AlSi10Mg powders with different WC contentswere synthesized by the electrostatic assembly method. After the selective laser melting (SLM) processing, various Al−W intermetallicswere in-situ formed in the SLM nano-WC/AlSi10Mg composites. A well-coherent Al/Al5W interface with a low lattice mismatch of 4.7% was confirmed with an orientation relationship of (002)α(Al)//(104)Al5W. The nano-WC particles and Al−W intermetallics triggeredthe columnar-to-equiaxed transition, leading to a fine equiaxed microstructure. As a result, the SLMed 3 wt.% nano-WC/AlSi10Mg composites showed remarkable mechanical properties, with an ultimate tensile strength of (464.1±8.68) MPa and elongation of (5.6±0.95)%. Additionally, the SLMed 3 wt.% WC/AlSi10Mg samples also achieved the lowest average coefficient of friction (0.429) and wear rate of 3.842×10−5 mm3/(N·m) compared to other WC/AlSi10Mg composites. The remarkable mechanical and wear properties are ascribed to fine-grain and second-phase strengthening.

Effect of WC on the microstructure and mechanical properties of laser-clad AlCoCrFeNi2.1 eutectic high-entropy alloy composite coatings

AlCoCrFeNi2.1/WC eutectic high-entropy alloy composite coatings were prepared on a 316 L stainless steel substrate via the laser cladding method. The effects of the WC content on the phase composition, microstructure, and mechanical properties of the composite coatings were investigated. The results revealed that the laser-clad AlCoCrFeNi2.1/WC eutectic high-entropy alloy composite coating mainly consisted of FCC and BCC phases. As the WC content increased from 0 wt% to 30 % wt%, the Cr7C3 and Cr21W2C6 phases were gradually precipitated from the composite coating. Consequently, the microhardness of the composite coating increased from 318.6 to 572.3 HV1.0. The strengthening mechanism of composite coatings mainly comprised solid solution strengthening, second-phase strengthening, and fine-grain strengthening. Additionally, the high dislocation density, caused by rapid condensation during the laser cladding process, contributed to the improvement in the coating hardness. Wear experiments revealed a significant improvement in the wear resistance of the composite coating with increasing WC content. The composite coating with 30 wt% WC exhibited the highest hardness and wear resistance with an average friction coefficient of 0.535, as well as a wear rate and wear volume loss of 4.4 × 10−7 mm3N−1 mm−1 and 5.44 mm3, respectively.

The microstructures and mechanical properties of CoNiCrx medium-entropy alloys

The CoNiCrx (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr1.8 and CoNiCr2 alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr2 alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr2 alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L12) phase could primarily contribute to the enhance of yield strength of CoNiCr1.4 alloy.

Strength and ductility dual-enhancement of Ni36Co30Fe11Cr11Al6Ti5Nb1 high entropy alloy by directional solidification

Ni36Co30Fe11Cr11Al6Ti5Nb1 high entropy alloys (HEAs) were prepared by directional solidification. The effect of solidification rate on microstructure evolution and mechanical properties was systematically investigated. Results show that the microstructure has a dendritic morphology with uniform orientation. FCC phase rich in Co, Cr and Ni is formed in the dendrite. Laves phase rich in Nb and Ti and FCC phase rich in Ni and Ti are formed between dendrites. With the increase of solidification rate, the dendrites are gradually refined. Tensile results show that the yield strength and elongation increase with the increase of solidification rate. The yield strength and elongation at 100 μm/s are 606.2 MPa and 27.6 %, respectively, achieving an excellent combination of strength and plasticity. Fine-grain strengthening, second-phase strengthening and solid-solution strengthening are the dominant strengthening mechanisms. The dendrites growing along the directional direction reduce the transverse grain boundaries and avoid the rapid propagation of cracks.

Evolution in microstructure and tribological behavior of laser cladded MoxNbTiZr high-entropy refractory alloy coatings with varying Mo content

The effect of the Mo element on the Ni-Ti-Zr refractory high-entropy alloy (RHEA) coatings synthesized by laser cladding were explored in terms of microstructural evolution and tribological behavior assessed by material characterization techniques and drying sliding wear tests. The findings reveal that MoxNbTiZr RHEA coatings, comprising of body center cubic (BCC) matrix phase and several hexagonal close packed (HCP) phases, have a dendrite microstructure with Mo, Nb-rich dendritic region (DR) and Zr-rich interdendritic region (IR), affected by Mo-induced thermodynamic instability. The micro-segregation between DR and IR becomes serious by the synergy effect of melting point difference, diffusion rate, and mixing enthalpy. The microhardness of the RHEA coating has a linear relationship with Mo content, with the highest value of 732.67 HV at x = 1.0. Nanotwins induced by residual stress, high cooling rate, solid-solution strengthening, fine-grain strengthening, and second-phase strengthening, cause microhardness improvement. Due to the generation of the destructive oxide layer and rising coating brittleness, the wear rate rises from 1.66 × 10−4 mm3/(N·m) to 3.75 × 10−4 mm3/(N·m). The wear mechanism of the MoxNbTiZr coatings shifts from oxidation wear to abrasive wear and eventually to three-body abrasion and severe brittle micro-peeling. Among these coatings, Mo0.6NbTiZr exhibits the optimal performance with excellent comprehensive characteristics.

Effects of carbon addition on the microstructure and mechanical property of in-situ reinforced TiAl matrix composite powders produced by plasma rotating electrode process

To investigate effects of carbon addition on microstructural characteristics of rapidly solidified TiAl alloys, Ti–43Al–6Nb-xC (x = 0,0.5,1.0 at.%) powders are produced by plasma rotating electrode process (PREP). Due to the greater viscosity of composite melts with carbides than those without carbides, the average sizes of carbon containing TiAl powders are much larger than theoretical average diameters. In addition, phase compositions of carbon-free TiAl powders usually consist of α phase and β phase, while α phase is predominant in carbon containing TiAl powders due to α-phase stabilizer. β→α phase transformation during the rapid solidification and Burgers relationships {110}β||(0001)α and <111>β||(11–20)α are illustrated in TiAl powders. Three types of microscopic structures are unfolded in TiAl composite powders: smooth structures, dendrite structures and equiaxed structures, which stem from various cooling rates. With carbon contents rising to 1.0 at.%, the average grain size in TiAl powders with the particle size range of 100∼150 μm is refined from 24.31 ± 0.38 μm to 11.32 ± 0.36 μm, moreover, the distribution ranges of particle sizes vary dramatically. Additionally, the micro-hardness of TiAl powders gradually upgrades as carbon contents ascend. The maximum micro-hardness of TiAl composite powders with 1.0 at.% carbon addition touches at 758.7 ± 15.4HV. Solid solution strengthening, refinement strengthening and second-phase strengthening are main sources for the ascent of micro-hardness.

The microstructure and mechanical properties of novel Fe-rich Fe–Cr–Ni–Ta eutectic multi-principal element alloys

Co–Cr–Fe–Ni–Ta eutectic multi-principal element alloys (MPEAs) demonstrate great potential to replace structural alloys in engineering applications. However, the cost of these alloys is high due to the usage of a number of precious metals. To decrease the cost, the present work developed a set of novel Fe55Cr15Ni30−xTax (x = 0, 5, 8, 10 and 15 at.%) eutectic MPEAs and the effect of Ta on microstructure and compressive mechanical properties were investigated for the first time. As the Ta content increases, the microstructure changes from a single-phase FCC solid solution (x = 0) to hypoeutectic microstructure (x = 6), then to eutectic microstructure (x = 8), and eventually to hypereutectic microstructure (x = 10 and 15). The yield strength and the hardness increase at the expense of ductility reduction with increasing Ta content. The strengthening mechanism of this alloy system is mainly second-phase strengthening caused by Laves phase and grain boundary strengthening produced by eutectic interface, supplemented by solid solution strengthening. The Fe55Cr15Ni22Ta8 and Fe55Cr15Ni20Ta10 alloys present the excellent comprehensive mechanical properties. The yield strength, fracture strength and fracture strain of the former are 838 MPa, 1994 MPa and 45.1%, respectively. The yield strength, fracture strength and fracture strain of the latter are 1029 MPa, 1956 MPa and 43.0%, respectively. The phase formation of Fe55Cr15Ni30−xTax alloys can be predicted by Md criterion. When Md ≥ 0.89, the alloys consist of FCC phase and Laves phase. Otherwise, the alloys consist of FCC single phase.

Effect of extrusion and rotary swaging on the microstructural evolution and properties of Mg-5Li-5.3Al-0.7Si alloy

High-strength Mg–5Li-5.3Al-0.7Si alloys were prepared by applying a combination of conventional extrusion with an extrusion ratio of 25:1 at 320 °C and subsequent rotary swaging (RS) with 0.2 mm reduction per pass in diameter at room temperature. The microstructural evolution was characterized and mechanical properties were investigated by tensile tests. The EBSD results showed that the grain size of the alloy decreased remarkably after extrusion and RS. Meanwhile, the yield strength (YS) of the as-extruded alloy was significantly improved from 161 MPa to 301 MPa by 2 passes RS, with only a slight reduction in ductility. The grain refinement during RS should be attributed to the activation of twins and continuous dynamic recrystallization (CDRX). The texture evolution during RS was investigated by EBSD. The number of grains with <0001> parallel to extrusion direction (ED) decrease significantly after RS. These grains are subjected to multi-directional stress and deformation, which rotate and develop a stronger texture component with <101‾0> - <112‾0> parallel to ED during RS, leading to the optimization of grain orientation. The phase evolution was investigated by SEM, EDS and TEM. Extrusion and RS effectively crush and disperse the second-phase particles of Mg2Si and Al0.89Li0.11, leading to their homogenization and refinement. The main mechanisms for strengthening the as-swaged alloy include grain refinement strengthening, dislocation strengthening, and second-phase strengthening. Grain refinement strengthening plays a dominant role in the as-swaged alloy.

Effect of 211 MAX phase Ti2AlC in situ formed TiCx on properties of high chromium white iron

High chromium white iron (HCWI) matrix composites reinforced with in situ formed TiCx decomposed from 211 MAX phase Ti2AlC (Ti2AlC/HCWI) have been successfully prepared. The influence of Ti2AlC contents on properties of high chromium white iron has been studied. The Ti2AlC/HCWI composites were testified to have a conspicuous promotion in mechanical properties compared with HCWI. The maximal Young’s modulus and electron work function (EWF) were achieved in 20 wt% Ti2AlC/HCWI, while the maximal hardness was acquired in 15 wt% Ti2AlC/HCWI. Strengthening mechanisms about refining the grain, second-phase strengthening and solid-solution strengthening were discussed.

Microstructural evolution and wear properties of in-situ (TiB + TiC)-reinforced Ti6Al4V matrix composite coating by laser melting deposition

In this study, a laser melting deposition (LDM) process was employed to deposit titanium matrix composite (TMC) coatings on a titanium alloy substrate by adding 1–5 wt% nanosized B4C particles, which were annealed at 900 °C. The microstructure and dry sliding friction behavior of TMC coatings were studied before and after the annealing treatment. The experimental results show that the TiB + TiC hybrid reinforced phase formed by the in-situ reaction of B4C and Ti matrix mainly precipitated at the grain boundary, providing the grain boundary and second-phase strengthening. Specific orientation relationships of TiB and TiC phases with the matrix phase were revealed, which had an essential effect on the microhardness distribution along the coatings. The annealing treatment promoted the diffusion of hybrid reinforcement into the matrix, strengthening their interface bonding, and has an significant impact on the wear properties of TMC coating. The room temperature friction coefficient (COF) of all the coatings remained relatively constant at 0.3–0.6. With increasing B4C content, the wear loss of the as-deposited coatings continually increased from 0.024 mm3 to 0.054 mm3, while the wear loss of the annealed TMC coatings are much lower than before, from 0.032 mm3 (TMC1) to 0.020 mm3 (TMC2) and then rise to 0.023 mm3 (TMC3). Scanning electron microscopy (SEM) morphologies determined that annealing reduced the abrasive and adhesive wear of TMC composite coating and improved its wear resistance.

Cost-effective and facile route to ultrafine-microstructure high-entropy alloy for cryogenic applications

Industry applications of the current cryogenic high entropy alloys (HEAs) are limited by their prohibitive costs, relatively low yield strength and unknown corrosion resistance. Here, we present a cost-effective and facile approach to produce cryogenic HEAs with lower cost, exceptional mechanical properties and corrosion resistance. The key is to design a cost-effective Fe35Ni35Cr20Mo5Al5 HEA and introduce ultrafine microstructure (UFM), including ultrafine grains (∼693 nm), high density of annealing twins and nanoprecipitates, into the alloy by manipulating the concurrent precipitation and recrystallization at 940 °C within 2 min. The cost-effective UFM-HEA exhibits a temperature-dependent strain-hardening capacity and a superior strength-ductility synergy at 77 K with a yield strength of ∼1165 MPa, a tensile strength of ∼1412 MPa and a uniform elongation over 20%. The superior tensile properties at cryogenic temperature are attributed to the twining-dominated multiple deformation mechanisms and the integrated strengthening effects, including grain refinement strengthening, twin strengthening and second-phase strengthening. At the fracture strain of ∼20%, extensive microcracks formed within the nanoprecipitates without propagating into the face-cantered-cubic matrix, suggesting a high crack tolerance of the UFM-HEA at both room and cryogenic temperatures. Moreover, the UFM-HEA has a superior corrosion resistance compared to the 316L stainless steel due to the larger passivity region and higher charge transfer resistance. Such outstanding cryogenic mechanical properties and corrosion resistance together with the lower cost make the UFM-HEA superior to most cryogenic HEAs. This strategy not only sheds light on the development of new-generation cryogenic HEAs but also significantly enhances their industrial application potential.

TiBw reinforced titanium matrix composites fabricated by selective laser melting: Influence of energy density on microstructure and tribological performance

For the application of high-performance titanium alloy parts in the additive manufacturing of aerospace, automotive and other fields, titanium matrix composites reinforced by TiBw were fabricated using selective laser melting technology. The influence of different energy densities on the microstructure, microhardness and tribological performance of TiBw/Ti6Al4V composite were studied, and the formation and evolution mechanism of the unique microstructure was analyzed. It is found that the TiBw/Ti6Al4V composite fabricated by SLM is mainly composed of a fine α′-Ti matrix, nano-scale TiBw produced by the in-situ reaction and residual TiB2 particles. The size, morphology and distribution of TiBw and its aggregated network structure are related to laser energy density. As the energy density increases, the morphology of TiBw evolves from tiny acicular to thick rod-like or plate-like. Additionally, the network chain composed of TiBw gradually expands or even breaks. Compared with Ti6Al4V alloy processed by SLM and traditional casting, the microhardness and wear resistance of TiBw/Ti6Al4V composites fabricated under various energy densities are significantly improved, and the friction process is affected by the abrasive wear, adhesive wear, and oxidation wear mechanisms. The good tribological performance is attributed to the fine-grain strengthening of Ti matrix and the second-phase strengthening effect caused by the network structure of TiBw.

Microstructure and second-phase strengthening mechanisms of CuTeCrY alloy

A CuTeCrY alloy prepared by homogenising annealing, hot extrusion, solution treatment, cold drawing, and a heat ageing treatment has tensile strength and electrical conductivity of 430 MPa and 78.9% IACS, respectively. Electron microscopy analyses show that a metastable CrCu2Te phase in the solid solution alloy is eliminated by the ageing treatment, and a 3–5 nm granular bcc-Cr phase becomes distributed in the matrix. This nanometre Cr second phase is the main contributor to the strengthening of the alloy whereas the second-phase strengthening effects of micrometre Cr and Te phases are weak.

Microstructure, hardness and wear performance of quaternary CrNbTiZr refractory medium-entropy alloy coating fabricated on a commercial Zr alloy by pulsed laser cladding

To attain enhanced surface hardness and wear resistance, a quaternary CrNbTiZr refractory medium-entropy alloy (RMEA) coating was fabricated on a commercial Zr alloy (Zr-702) by a pulsed laser cladding technique. Phase constitution, microstructural feature, hardness and wear performance of the RMEA coating were systematically investigated. The specific influence of Zr diluted from the substrate on the coating was explored. The results reveal that the cladding zone is mainly featured by a BCC solid-solution phase and net-like Cr2Zr intermetallics. Such microstructural features agree with those predicted based on atomic radius difference, Allen electronegativity and valence electron concentration. Allotropic solid phase transformation occurs in the heat-affected zone, resulting in a mixed microstructure of blocky grains (α-Zr) and martensitic laths. The hardness of the RMEA coating (637 ± 46 HV) is ∼3.3 times that of the Zr-702 substrate, which results from joint contribution from solid-solution, low-angle grain boundary and second-phase strengthening. The RMEA coating exhibits a wear rate ∼56% lower than the substrate, demonstrating significantly improved wear resistance. Both oxidation and adhesive wear are determined to be dominant wear mechanisms of the RMEA coating, accounting for its enhanced wear performance.

Exploring nonlinear strengthening in polycrystalline metallic materials by machine learning methods and heterostructure design

To improve the strength and plasticity of structural materials, researchers often introduce various strengthening mechanisms such as second-phase strengthening, dislocation strengthening, and back stress strengthening (HDI). Due to the interaction of multiple mechanisms, the linear superposition relationship has a poor fitting effect and is only used for rough calculations of the strengthening mechanisms. In this study, the transfer learning data was used to optimize the deep learning network structure (Re-CNN) based on the residual algorithm, and the yield strength prediction physical neural informed model (PNIM) of polycrystalline metallic materials was established. To promote the industrial application of the heterostructure design method, a medium carbon steel heterostructure design strategy based on the existing equipment of the factory was proposed. Medium-carbon heterostructure materials (MHSM) with mixed strengthening mechanisms were successfully prepared. MHSM exhibits excellent comprehensive mechanical properties. When a linear relationship is used to describe the MHSM yield strength, there is a large error, while Re-CNN shows satisfactory prediction accuracy. The linear relationship is incompatible with homogeneous structure materials and heterogeneous structure materials, and its universality is lower than that of nonlinear Re-CNN. Re-CNN shows high cross-scale prediction ability and can be compatible with homogeneous microstructures and heterogeneous microstructures. Using the heterogeneity evolution characteristics of MHSM, the key factors deviating from the linear relationship were revealed. The overestimation and underestimation of the linear relation are demonstrated by Taylor factor and TEM analysis to be caused by the multiscale properties of ferrite, the behavior of the second phase particles, and the interaction of various mechanisms. This study provides a new idea for the cross-scale calculation of the mechanical properties of polycrystalline metallic materials.

Effect of TiC Particles Addition on Tribological Behavior of Ductile Iron

In order to improve the wear resistance of ductile iron, different contents of TiC particles are added into ductile iron used lost-foam casting and the tribological behavior of ductile iron is studied through a ball-on-disk sliding test. It is found that with the increase of TiC content, the pearlite content gradually increases, which is attributed to TiC promoting the heterogeneous nucleation of cementite and increasing cooling rate of the melt. The increase of pearlite content and the second-phase strengthening caused by TiC improve the hardness and tensile strength of ductile iron. Ductile iron has the best tensile strength (498 MPa) and hardness (168 HV1) with a TiC content of 1 wt.%. The wear resistance also increases with increasing TiC content. The wear volume is reduced from 0.14 to 0.03 mm3, and the main wear mechanism changes from adhesive wear to abrasive wear. The oxide layer produced by frictional heat on the worn surface and the hardened layer produced by strain hardening on the subsurface further improve the wear resistance. Therefore, the wear resistance of ductile iron is affected by the changes of microstructure before and after wear.Graphical Abstract

Microstructure and brazing properties of a novel Ag–Cu-Ga solder

In this study, a novel low-silver Ag-xCu-5Ga (x = 28, 34, 39, 49 and 55) solder with high strength was designed and prepared. The effect of element composition on microstructure evolution, thermal behavior, and the mechanical performance of Ag–Cu-Ga solders were investigated in detail. The results indicated that the melting temperature of Ag–Cu alloy could be reduced effectively with Ga addition, and the typical joint consisted of the block-shaped Cu-rich phase in the welded seam and the ribbon-shaped Cu-rich phase adjacent to the interface. The shear strength of joints brazed with Ag-xCu-5Ga (x = 28, 34, 39 and 49) filler metals can be improved significantly compared to that of commercial Ag–28Cu solder, with a maximum value of approximately 445 MPa in Ag–39Cu–5Ga solder. The best mechanical properties are obtained as the second-phase strengthening and interfacial strengthening reach a balanced coexistence state, determined by the block-shaped Cu-rich phase in the welded seam and the ribbon-shaped Cu-rich phase adjacent to the interface, respectively. This study can provide insights to investigate strengthening mechanisms and develop high-strength solders for vacuum brazing.