Isothermal Solidification Zone Research Articles

Study on microstructure evolution and failure behavior of Ni3Al-based single crystal superalloy joints brazed with Ni-based filler

The microstructural characteristics of brazing seams significantly influence the mechanical properties of brazed joints. In this study, a novel Ni3Al-based single crystal superalloy(Alloy 1) with high Mo content was brazed with a Ni-B-Si-Ti filler. The evolution of microstructure and mechanical properties of brazed joints were investigated after thermal exposure at 1000°C up to 300h. The brazed joint was mainly composed of the athermally solidified zone (ASZ), isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and base metal (BM). As the increase of thermal exposure time, the tensile strength of the joint at 980°C decreases from 625MPa at 0h of thermal exposure to 443MPa after 300h of thermal exposure. It is suggested that as the duration of thermal exposure increases, the skeleton phases gradually aggregate at the ASZ/ISZ interface, reducing the relative barrier effect of the skeleton in ASZ. Simultaneously, the ASZ/ISZ interface becomes smoother over time, which in turn leads to rapid crack propagation.

High-performance brazing of single crystal superalloys with Ni-Cr-Ta filler material

High-performance joining especially brazing has been highly expected in the manufacture or maintenance of single crystal blades. In this paper, a Ni-40Cr-10Ta (at.%) alloy was developed as filler material for single crystal superalloys. High-performance brazing of DD5 superalloy was achieved. The joints brazed at 1260 °C contained an isothermal solidification zone (ISZ) with “γ + γ′” structure and an athermal solidification zone (ASZ) composed of γ, γ', γ/γ′-eutectics, TaCr2 and TaC. Unlike the conventional filler materials with melting point depressants of B and Si, the Ni-Cr-Ta filler material was free of B and Si, and thus the resulting joints contained no B-rich and Si-rich compounds and eutectics with low melting points, which enhanced the brazed joint performance. The tensile strength at 980 ℃ of the heat-treated joints reached 654 MPa, which is 88.5 % that of the base metal.

Effect of MBF-20 Interlayer on the Microstructure and Corrosion Behaviour of Inconel 625 Super Alloy after Diffusion Brazing.

The joining zone includes three main parts, which comprise an isothermal solidification zone (ISZ), the athermal solidification zone (ASZ), and a diffusion affected zone (DAZ). Field emission scanning electron microscopy (FESEM) was used here to observe the microstructure equipped with ultra-thin window energy dispersive X-ray spectrometer (EDS) system. Additionally, electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization tests were conducted to evaluate the effect of the DB process on the corrosion resistance of the Inconel 625 superalloy. In the bonding time period, some Mo- and Cr-rich boride precipitations and Ni-rich γ-solid solution phases with hardened alloy elements, such as Mo and Cr, formed in DAZ and ASZ, respectively, because of the inter-diffusion of melting point depressants (MPD). Moreover, during cooling cycles, Ni-Cr-B, Ni-Mo-B, Ni-Si-B, and Ni-Si phase compounds were formed in the ASZ area at 1110-850 °C. The DAZ area developed by borides compound with cubic, needle, and grain boundary morphologies. The corrosion tests indicated that the DB process led to a reduction in the passive region and increased the sensitivity to pitting corrosion.

Precipitates evolution and fracture mechanism of the isothermally solidified TLP bonding joints between 316LN stainless steel and IN718 Ni-based alloy

In this study, the dissimilar TLP bonding joints of Inconel 718 nickel-base alloy and AISI 316LN stainless steel free of brittle athermal solidification zone (ASZ) were obtained. It was recognized that a good metallurgical bonding is achieved at the interface between the isothermal solidification zone (ISZ) and diffusion affected zone on the side of IN718 alloy (hereafter referred to as DAZ-IN718), while the interface of ISZ and DAZ-316LN is more sensitive to crack initiation. A lot of precipitates were found in DAZ on both sides near the base metal, which were identified as Cr2B, Cr5B3, Nb3B2, and BN. Moreover, two different sizes of voids form at the ISZ/DAZ-316LN interface. The bigger voids are caused by BN phases, while the smaller voids were identified as Kirkendall voids. The tensile test results indicate that the tensile strength and elongation decrease with increasing bonding time. In-situ tensile tests and digital image correlation (DIC) analysis indicate that there are two factors that cause the fracture of dissimilar TLP bonding joints at the ISZ/DAZ-316LN interface. The first factor is the brittle BN phases and Kirkendall voids at the ISZ/DAZ-316LN interface. During tensile tests, dislocations slide along the slip surface until they are blocked by precipitates in the DAZ, resulting in extremely high-stress concentrations at these locations. Moreover, the second factor is the poor ability of DAZ-316LN to resist plastic deformation. The high degree of strain concentration in DAZ-316LN also promotes the formation of microcracks at the ISZ/DAZ-316LN interface.

Diffusion brazing of GH536 polycrystalline superalloy with IC10 single crystal superalloy using BNi-2 interlayer

Joining single crystal superalloy with polycrystalline superalloy has important applications in the aerospace industry. Here, the IC10 single crystal superalloy to GH536 polycrystalline superalloy was diffusion brazed using a BNi-2 filler alloy. The effects of bonding temperature and time on the microstructure evolution and mechanical properties of the GH536/IC10 joints were investigated in detail. The results elucidated that the joint brazed at a lower temperature and time (i.e., 1050 °C/30min) was composed of diffusion affected zone (DAZ), athermal solidification zone (ASZ), and isothermal solidification zone (ISZ). The secondary precipitates formed in DAZ were Cr-rich borides, Cr–Mo borides, and Cr–Mo–W borides. The ASZ was made up of Cr-rich borides, Ni-rich borides, Ni–Cr borides, and Ni-based solid solutions. These borides with high hardness were the potential source of cracks and caused the joints to fracture under a lower load. Raising bonding temperature and time gradually eliminated Ni-rich borides in ASZ and Cr-rich borides in DAZ, and improved the shear strength.

Effects of Post-Weld Heat Treatment on Microstructure and Mechanical Properties of the Brazed Joint of a Novel Fourth-Generation Nickel-Based Single Crystal Superalloy.

A novel fourth-generation nickel-based single crystal superalloy was brazed with Co-based filler alloy. The effects of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of brazed joints were investigated. The experimental and CALPHAD simulation results show that the non-isothermal solidification zone was composed of M3B2, MB-type boride and MC carbide, and the isothermal solidification zone was composed of γ and γ' phases. After the PWHT, the distribution of borides and the morphology of the γ' phase were changed. The change of the γ' phase was mainly attributed to the effect of borides on the diffusion behavior of Al and Ta atoms. In the process of PWHT, stress concentration leads to the nucleation and growth of grains during recrystallization, thus forming high angle grain boundaries in the joint. The microhardness was slightly increased compared to the joint before PWHT. The relationship between microstructure and microhardness during the PWHT of the joint was discussed. In addition, the tensile strength and stress fracture life of the joints were significantly increased after the PWHT. The reasons for the improved mechanical properties of the joints were analyzed and the fracture mechanism of the joints was elucidated. These research results can provide important guidance for the brazing work of fourth-generation nickel-based single crystal superalloy.

Transient Liquid Phase Diffusion Bonding of Ni3Al Superalloy with Low-Boron Nickel-Base Powder Interlayer

As a technology for micro-deformed solid-phase connection, transient liquid phase (TLP) diffusion bonding plays a key role in the manufacture of heating components of aero engines. However, the harmful brittle phase and high hardness limit the application of TLP diffusion bonding in nickel-based superalloys. In this paper, a new strategy in which a low-boron and high-titanium interlayer can restrain the brittle phase and reduce the hardness of the TLP-diffusion-bonded joint is proposed. With this strategy, the Ni3Al joint can achieve a high strength of 860.84 ± 26.9 MPa under conditions of 1250 °C, 6 h and 5 MPa. The microhardness results show that the average microhardness of the joint area is 420.33 ± 3.15 HV and is only 4.3% higher than that of the Ni3Al base material, which proves that this strategy can effectively inhibit the formation of the harmful brittle phase in the joint area. The results of EBSD show that 7.7% of the twin boundaries exist in the isothermal solidification zone, and only small amounts of secondary precipitates are observed at the grain boundaries in the joint, which indicates that twin boundaries may play a dominant role in crack initiation. This study provides a feasible avenue to suppress the brittle phase in TLP-diffusion-bonded joints.

Microstructure evolution and tensile behaviors of dissimilar TLP joint of austenitic steel and high-Cr ferritic steel

In this study, austenitic steel and high-Cr ferritic steel were joined by transient liquid phase (TLP) bonding technique with a Fe–Si–B amorphous interlayer. The effects of bonding time (10–60 min) on joints microstructure evolution and tensile behaviors were investigated. On the high-Cr ferritic steel side of the prepared dissimilar TLP joint, Cr2B particles are precipitated at diffusion affected zone (DAZ-α, the α denotes ferrite) and dissolve subsequently when the bonding time is extended to 30 min. In contrast, Cr2B particles are precipitated at the grain boundaries of DAZ-γ (the γ denotes austenite) on the TP347H steel side, which has an apparent orientation relationship with the base metal (BM). Furthermore, when Ni and Cr atoms diffuse from DAZ-γ to isothermal solidification zone (ISZ), a martensitic layer develops at the ISZ/DAZ-γ interface and widens as bonding time increases. Because DAZ-γ is a Ni/Cr-depleted zone with lower austenite stability, deformation-induced martensite forms in DAZ-γ during tensile testing. The in-situ tensile test results and digital image correlation analysis show that strain localization gradually transfers from the interface to the BM-γ during the deformation process, causing the specimen to fracture within TP347H matrix after tensile tests, indicating that TLP bonding achieves sufficient metallurgical bonding. Furthermore, with the extension of bonding time, the grain size of BM-γ decreases first and then increases, which leads to the similar change of the total elongation and the opposite change of the ultimate strength of the joint.

Transient liquid phase (TLP) bonding of Ti–6Al–4V/AISI 304 stainless steel using Cu/CNT composite interlayer

This paper investigates the idea of using Cu/CNT composite interlayer in the transient liquid phase (TLP) bonding of Ti–6Al–4V/AISI 304 stainless steel. In this investigation, initially TLP was carried out using a pure copper interlayer at 900, 950 and 1000 °C with the holding time 60 min. The TLP joining was then carried out at 900 °C with Cu/CNT interlayer, with the CNT weight percent, ranging from 0 to 3%. CNT was deposited on Cu foils, using electrophoretic deposition technique. Microstructure and mechanical properties of the joints were investigated with optical/electron microscopes, microhardness, and shear tests. Results showed that the interface microstructure can be divided into athermal solidification zone, isothermal solidification zone, and diffusion affected zone. The phase structure and compositions in CNT-containing samples are relatively similar to TLP samples with pure Cu interlayer. Results showed that CNT addition has expanded the diffusion areas and facilitated the diffusion across the interface. CNT addition up to 3 wt. % resulted in a significant increase in hardness up 500 VHN. The increase in hardness in the joint area can be related to the presence of various intermetallic compounds at the TLP region. The correlations between mechanical properties and microstructure and the implications of CNTs for mechanical properties of the joints are discussed.

Microstructural Evolution of the TLP Joints of RAFM Steel during Aging and Creep

In this study, transient liquid-phase (TLP) bonding was adopted to obtain a reliable reduced-activation ferritic/martensitic (RAFM) steel joint with Fe-Si-B amorphous foil. The aging tests and creep tests of the TLP joints were carried out at 550 °C to study the microstructural evolution in the service process. The effect of stress loading on the microstructural evolution of the TLP joint was investigated. The results show that creep fractures in the TLP joints occur in the base material. The main factors affecting the creep performance of TLP joints are the recovery of substructures and the coarsening and deformation of martensitic laths. In addition, the M23C6 carbides in the base material were coarser than in the weld zone. Compared with aging samples and creep samples undergoing the same test temperature, the dislocation density in the isothermal solidification zone (ISZ) increased significantly with increases in the stress level. Furthermore, it is worth noting that the microstructure of the weld zone changed from large-sized ferrite to a mixed, fine microstructure of ferrite and martensite, which increases the heat resistance of the TLP joints, and thus results in creep fractures in the base metal.

Microstructural characteristics and mechanical properties of IC10 superalloy and (CoCrNi)94Al3Ti3 MEA joint brazed using NiCrSiB filler

This study focuses on the brazing of IC10 Ni 3 Al-based superalloy to (CoCrNi) 94 Al 3 Ti 3 medium-entropy alloy (MEA) using a NiCrSiB filler metal. The microstructure of the brazed joint was comprehensively investigated, and the forming mechanism of the joint was elucidated. The mechanical properties and fracture behavior of the joint were evaluated. The diffusion of boron from the filler to substrates led to the diffusion-affected zone (DAZ), and the M 3 B 2 - and M 5 B 3 -type boride were precipitated with the coherence of (3 1 0) BCT //(2 0 0) FCC (M 3 B 2 ) and (0 0 2) BCT //(2 0 0) FCC (M 5 B 3 ) adjacent to the IC10 and MEA substrate, namely the DAZ I and II respectively. In addition, the edge dislocation reduced the mismatch between the M 3 B 2 boride and the γ matrix. The isothermal solidification zone (ISZ) was formed near the DAZ when the liquidus temperature reached the brazing temperature due to the diffusion of boron and Si and the dissolution of substrates. The formation of ISZ was effectively activated at high brazing temperatures (1090, 1120, and 1150 °C). The athermal solidification zone (ASZ) was detected in the joint brazed at adopted temperatures, which mainly dominated by eutectic reactions and driven by cooling. The ASZ decreased shear strength of the defect-free joint, and readily accelerated initiation and propagation of cracks. The maximum shear strength of 554 MPa was obtained when the joint was brazed at 1120 °C for 10 min. The fracture mainly occurred in the ASZ (semi-cleavage), and passing through the DAZ (semi-cleavage) and ISZ (dimples). • The brazing of IC10 and (CoCrNi) 94 Al 3 Ti 3 MEA was achieved using a NiCrSiB filler. • The M 3 B 2 - and M 5 B 3 -type borides in the DAZ exhibited coherent with the matrix. • The microstructural evolution and mechanical properties were evaluated. • .

Boride evolutionary behavior and mechanism in the TLP repaired IN738 superalloy with crack-like defects

Boride evolution in the transient liquid phase (TLP) repaired cracks for IN738LC alloy had important influence on mechanical properties. The microstructure of the repaired crack contained two typical zones: isothermal solidification zone (ISZ) and diffusion affected zone (DAZ). There were blocky and acicular borides with hexagonal crystal structure in the DAZ, and boride morphology depended on the interfacial distance, Cr/B content, and aging time. i) As the distance from the ISZ/DAZ interface increased, blocky borides gradually decreased and acicular borides began to form. ii) When Cr and B were sufficient, blocky borides firstly formed. When the content decreased to a certain extent, borides would not form. iii) As aging time increased, the amount of blocky borides gradually increased due to the diffusion of B. Meanwhile, acicular borides began to form along (111) plane, which could effectively reduce the interfacial energy.

Microstructure evolution and mechanical properties of a Fe, Cr-rich multiphase Ni3Al-based superalloy during transient liquid phase bonding process

In this paper, a Fe, Cr-rich multiphase Ni3Al-based alloy is joined through transient liquid phase bonding (TLP) technique. Different from typical γ+γ′ dual phase Ni3Al-based alloys, the joint can be divided into three distinct zones: isothermal solidification zone (ISZ), gamma prime zone (GPZ) and diffusion affected zone (DAZ). Interestingly, the massive fine grains with γ+γ′ dual phase are observed inside the ISZ, and the GPZ consists of blocky γ′ phase. The precipitation of Cr-rich borides in DAZ provides an advantageous compositional condition for the local coarsening of γ′ phase. In addition, hardness value of the bonded samples fluctuates from the centerline to the BM and reaches the peak at the DAZ. The bonded specimens share equivalent room-temperature tensile strength and elongation to failure with the parent metal after the weld thermal cycle. Moreover, the fracture path propagates along the base metal (BM), which indicates excellent tensile properties of the joint.

Influence of Post-Bond Heat Treatment on Microstructure and Creep Behavior of the Brazed Single-Crystal Nickel Superalloy.

Post-bond heat treatment (PBHT) is an effective way to improve the bonding quality of a brazed joint. Herein, brazing of a nickel-based single crystal superalloy is carried out using a Ni-Cr-Co-B-Si-Al-Ti-W-Mo filler alloy, and the microstructure and creep property of the brazed joint are systematically investigated using scanning electron microscopy (SEM), Thermo-Calc software, an electron probe micro-analyzer (EPMA), X-ray diffractometer, confocal scanning laser microscope (CSLM), and transmission electron microscopy (TEM). The results reveal that the as-prepared joint only consists of an isothermally solidified zone (ISZ) and an athermally solidified zone (ASZ), where the cubic γ′ phase is observed in the ISZ, and skeleton-like M3B2, γ + γ′ eutectic and reticular G phases are observed in the ASZ. Furthermore, the γ + γ′ eutectic and G phases disappear and the M3B2 alters from a skeleton-like to block-like shape in the ASZ after PBHT. Meanwhile, some lath-like M3B2 phases are precipitated at the edge of the ISZ and several M3B2 phases are precipitated in the base metal, forming a new zone in the brazed joint, namely at the diffusion affected zone (DAZ). Owing to the removal of low melting point eutectics from the as-prepared joint, the creep life also increases from 188 h to 243 h after PBHT. The current work provides a method for the optimization of brazed joints based on the Ni-based single crystal superalloy.

Microstructure and properties of the C/C-superalloy brazed joint by Ni-based filler

Stable and reliable C/C-superalloy joints that can withstand high-temperature service will become extremely important. Ni-based filler was employed for successful brazing of C/C composites and In738LC superalloy in this study. The brazed joints contained isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and penetration zone close to the C/C side. The ISZ contained lots of carbide-borides besides Ni(s,s) and Cr(s,s). The specific composition and atomic arrangement of these phases were characterized by the focused ion beam (FIB) method and transmission electron microscopy (TEM) analysis. Results showed the obvious lattice distortion of the Ti–Mo–B compound indicated the presence of the severe internal residual stress. The microhardness distribution of the brazed joint was analyzed in combination with the microstructure. Brittle Cr–Ti–Mo carbide-borides formed in the reaction layer contributed to the high microhardness, and the shape of borides influenced the DAZ microhardness significantly When the brazing temperature was 1100 °C, the highest joint shear strength was obtained due to a matched proportion of the width and the microhardness of ISZ.

Microstructure and mechanical properties of transient liquid phase bonding DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy

• The transient liquid phase bonding of DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy was achieved using a BNi-2 filler interlayer. • The M5B3-type boride (BCT) precipitated in the diffusion-affected zone exhibiting a coherent relationship of [ 2 4 0]BCT // [0 1 1]FCC and (0 0 2)BCT // (2 0 0)FCC with the FCC matrix. • The effect of the reaction phase properties on the joint performance was investigated. • The maximum shear strength of 752 MPa was obtained with the eutectic-free joint. This study focuses on the transient liquid phase (TLP) bonding of DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy (MEA) using a BNi-2 filler alloy. The microstructure and mechanical properties of the TLP-bonded DD5/MEA joint were evaluated, and the microstructural evolution mechanism was investigated. The formation of the isothermal solidification zone (ISZ) depended on the diffusion of the melting-point depressants (Si and B elements) from the liquid filler into the DD5 and MEA substrates, as well as the dissolution of the substrates. Boron diffused along the γ channel of DD5 and reacted to form M 5 B 3 boride, herein referred to as the diffusion-affected zone (DAZ I). Similarly, the Cr 5 B 3 boride precipitated in the Ni-rich MEA matrix adjacent to the MEA substrate (i.e., DAZ II). Additionally, a coherent orientation of [0] BCT // [0 1 1] FCC and (0 0 2) BCT // (2 0 0) FCC was detected between M 5 B 3 boride with a body-centered tetragonal (BCT) structure and the face-centered cubic (FCC) matrix. The performance of the joint was dominated by the properties of the bonding seam. As the bonding time increased from 20 to 80 min, the athermal solidification zone (including eutectic microstructure) was gradually replaced by the ISZ exhibiting excellent plastic deformation capability, and the shear strength of the joint was improved. The maximum shear strength (752 MPa) was achieved when the eutectic-free joint was bonded at 1050 °C for 80 min. The fracture morphology revealed a mixture mode, indicating the initiation of cracks in the DAZ II, mainly propagating in the ISZ, and passing through the DAZ I.

Microstructural and mechanical properties assessment of transient liquid phase bonding of CoCuFeMnNi high entropy alloy

The transient liquid phase (TLP) bonding of CoCuFeMnNi high entropy alloy (HEA) was studied. The TLP bonding was performed using AWS BNi-2 interlayer at 1050 °C with the TLP bonding time of 20, 60, 180 and 240 min. The effect of bonding time on the joint microstructure was characterized by SEM and EDS. Microstructural results confirmed that complete isothermal solidification occurred approximately at 240 min of bonding time. For samples bonded at 20, 60 and 180 min, athermal solidification zone was formed in the bonding area which included Cr-rich boride and Mn3Si intermetallic compound. For all samples, the γ solid solution was formed in the isothermal solidification zone of the bonding zone. To evaluate the effect of TLP bonding time on mechanical properties of joints, the shear strength and micro-hardness of joints were measured. The results indicated a decrement of micro-hardness in the bonding zone and an increment of micro-hardness in the adjacent zone of joints. The minimum and maximum values of shear strength were 100 and 180 MPa for joints with the bonding time of 20 and 240 min, respectively.

Investigation on the microstructure and mechanical properties of CoCrFeNi high-entropy alloy joint bonded with BNi2 interlayer

• The athermal solidification zone and isothermal solidification zone of CoCrFeNi joint are determined to be γ phase and high entropy Ni-rich FCC phase, respectively. No eutectic products containing borides precipitated in the athermal solidification zone. • With increasing the bonding temperature or prolonging the holding time, the growths of ISZ and DAZ present a synchronous trend. The precipitating location of CrxBy is changed from grain interior to grain boundaries. • The maximum bonding strength of CoCrFeNi joint is ∼ 429 MPa, which is about 75 % of base metal strength. • The impact of Cr depletion on the performance of CoCrFeNi joint is less severe than that on conventional alloys joints. The strategy of bonding high-entropy alloys (HEAs) is of vital significance for producing complex engineering components available to extreme conditions. In this work, transient liquid phase (TLP) bonding was applied to join CoCrFeNi HEAs using an amorphous Ni-Cr-Si-B interlayer (BNi2). The interfacial microstructure evolution, mechanical properties and bonding mechanism of the joints were evaluated. Results indicated three reaction products in the different zones of joints, including the γ phase in the athermal solidification zone (ASZ), high-entropy Ni-rich face-centered cubic (FCC) phase in the isothermal solidification zone (ISZ) and a high density of CrxBy precipitates with different morphologies (blocky, punctate and acicular) in the diffusion affected zone (DAZ 7 ). With increasing the bonding temperature or holding time, the width of ISZ and DAZ almost presented a synchronous growing trend, and the location of CrxBy precipitates was changed from grain interior to grain boundaries. Tension shear test showed excellent bonding strength of all the bonded joints due to the formation of the ASZ free of eutectic products containing borides, and the maximum bonding strength could reach 429 MPa, which is about 75 % of the base metal (BM) strength. The joint ruptured at the CoCrFeNi BM adjacent to the lapping area in ductile fracture mode. This work is expected to facilitate the fabrication and repair of complex CoCrFeNi-related components in future.

The effect of bonding time on the microstructure and mechanical properties of dissimilar transient liquid phase bonding between UNS N08825 alloy and UNS S32750 super duplex stainless steel using the BNi-2 interlayer

• The dissimilar transient liquid phase (TLP) bonding of the UNS N08825 alloy to UNS S32750super duplex stainless steel was successfully accomplished using the BNi-2 interlayer. • The complete isothermal solidification occurred at 1050 °C and the bonding time of 300 s. • Shear strength of the TLP joints improved by increasing the holding time. In this study, the microstructure and mechanical properties of transient liquid phase, dissimilar joints between UNS N08825 alloy and UNS S32750super duplex stainless steel was investigated. The joining process was carried out at 1050 °C for 0, 60, 120, and 300 s using an BNi-2 interlayer. The transient liquid phase bonding area was divided into three zones: (i) the isothermal solidification zone which contained γ-Ni solid solution, (ii) the a-thermal solidification zone which consisted of Ni- and Cr-rich borides and Ni 3 Si precipitates, (iii) and the diffusion affected zone in which Cr-Ni-Fe-Mo-Cu-rich borides constituents existed. The hard and brittle eutectic compounds in the bonding zone posed deleterious effects on the mechanical properties of the joints. By increasing the holding time, the amount of these detrimental phases in the bonding zone decreased before finally a eutectic-free joint was achieved at the bonding time of 300 s, which was notably low as compared to the previous studies. The joint held for 0 s had the widest a-thermal solidification zone and the highest hardness was measured at the center of it (546 Hv). The average hardness of the diffusion affected zone on the UNS S3750 super duplex stainless steel side was approximately 450 Hv, which was markedly higher than that of the diffusion affected zone on the UNS N08825 side (302 Hv). The joint held for 0 s showed the lowest shear strength (239 MPa), while the maximum shear strength was obtained for the joint with a holding time of 300 s, approximately 487 MPa. Moreover, it was demonstrated that eutectic compounds were the principal cause of the brittle fracture in the joints containing the a-thermal solidification zone, whereas the joint with a complete isothermal solidification experienced a brittle fracture that occurred at the diffusion-affected zone of UNS N08825 alloy.

Transient liquid phase bonding of Ni3Al based superalloy using Mn–Ni–Cr filler

A new 70Mn–25Ni–5Cr (wt. %) filler with no traditional melting-point depressants (MPDs) of Si/B was used for transient liquid phase (TLP) bonding a directionally solidified (DS) superalloy based on Ni3Al intermetallic compound (IMC) at 1150 °C, a pressure of 2 MPa for 10–120 min. Experimental results show that the joint can be divided into 3 zones, isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion-affected zone (DAZ), which was similar to microstructure of typical TLP joint bonded by Ni based filler. The joint microstructures were mainly composed of γ-Ni, γ′-Ni3(Al, Ta), MC and M23C6 carbides. Besides, in the ISZ, due to the enrichment of Al, which diffused from the base metal (BM), strip-shaped Ni(Mn, Al) phases formed in such region. The ASZ included γ-Ni and compounds of “equiaxed Ni(Mn, Al) + acicular Ni3Ta” owing to the composition segregation during the later stage of solidification. Moreover, as the bonding time increased, the volume fraction of Ni(Mn, Al) and MC carbides decreased owing to homogenization of alloying elements. After bonding for 120 min, sufficient isothermal solidification (IS) of filler can also decrease segregation of Mn/Al in front of the S/L interface and the Ni(Mn, Al)+Ni3Ta compounds were eliminated almost. Thus, the highest tensile strength was 816.9 MPa of the TLP joint of 120 min, which has been equivalent to that of BM (812 MPa). However, due to the existence of a few residual large MC carbides in the joint, which can induce the stress concentration and decreased the ductility of the joints, the highest joint elongation of 4.4% was only about 40% of the BM.