Microstructure evolution and fracture mechanism of diffusion bonded MHC/GH4099 joint by hot isostatic pressing
To form a satisfactory bonding between Molybdenum-Hafnium-Carbon(MHC)and GH4099 alloy,in-terlayers are used to alleviate the huge property difference,such as thermal expansion coefficient of the two materi-als.This study focused on interlayers and different layer composition to investigate the bonding microstructure and fracture mechanism.Direct diffusion bonding between MHC and GH4099 is difficult due to the low diffusion rate and brittle intermetallic compound formed.Hot isostatic pressing diffusion bonding technology(HIP-DB)was ad-opted as the major process in this study by inserting layers between them.The effect of different interlayers(Ni-Cr-Si-B,Ag-Cu-Ti,Ti-Cr-Ni,Nb-Ni)on the mechanical performance of welded joint between MHC and GH4099 alloys by(HIP-DB)was investigated.Optical Microscope(OM)and Field Emission Scanning Electronic Micro-scope(FE-SEM)with energy dispersive spectroscopy(EDS)analysis were applied to analysis the microstructures of the bond,fracture morphology and element distribution.Ni-Cr-B-Si interlayer tends to form brittle Mo-Ni inter-metallic phase that is regarded as the main cause of brittle fracture morphology observed.Bond strength with Ni-Cr-B-Si interlayer is very poor.Ag-Cu-Ti interlayer can both react with MHC side and GH4099 side,which lend bond shear strength of 27 MPa.Microstructure observation shows that there are Kirkendall pores reside in the frac-ture surface which is responsible for the low bond strength.Ti-Cr-Ni interlayer does not form an effective metallur-gical bonding,which is attributed to the intrinsic weak strength of TiCrNi material and low HIP-DB process tem-perature.Nb-Ti interlayer forms good metallurgical bonding between MHC and GH4099.Effective diffusion has taken place in MHC/NbTi bond although some pores exist in this area,due to the significant difference of diffusion rate of Mo and Nb.The diffusion-induced reaction layers are determined to be the Mo-Nb solid solution,Ni8Nb,Ni3Nb and Ni6Nb7.Their strength and thickness are measured in order to establish a correlation between the interfa-cial microstructure and joint properties.It is found that the precipitated phase Ni3Nb shows high strength and good plasticity,which is beneficial to the high-temperature performance of the joint.Cracks are initiated from the Mo-Nb solid solution,and then intergranular fracture propagates across the central diffusion layer.Reliable MHC/GH4099 bonding joints are obtained with Nb-Ni interlayer at 1 150 ℃/130 MPa/180 min in HIP process.The joint tensile strength/shear strength with Nb+Ni interlayer reach 84 MPa/41 MPa,which is higher than that with Ni-Cr-Si-B,Ag-Cu-Ti or Ti-Cr-Ni interlayer.
hot isostatic pressing(HIP)GH4099 alloyMHC alloyfracture mechanismdiffusion bonding
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