Diffusion Bonding of Ti6Al4V and SS 304 with Nb Interlayer

Abstract

Abstract In this investigation, solid-state indirect diffusion couples (IDCs) were prepared between stainless steel (SS) 304 and Ti6Al4V using a niobium (Nb) interlayer of 200 µm thickness from 850°C to 950°C using uniaxial compressive pressure (∼4 MPa) for 60 min under high vacuum of 10−3 Pa. Microstructural analysis along the Nb|SS-304 interfaces of IDCs processed at 875°C–950°C revealed layer-wise iron–niobium-based intermetallics like Fe2Nb and FeNb + (Nb), apart from these reaction layers IDCs processed at 925°C revealed a (Nb) reaction layer, and IDCs processed at 950°C revealed both (Nb) and γFe + σ reaction layers. IDCs processed at all processing temperatures revealed solid solution behavior along Ti6Al4V|Nb interfaces. Thermodynamic analysis showed that the diffusion mechanism at the Ti6Al4V|Nb interface is quite functionally easier than the diffusion mechanism observed at the Nb|SS-304 interface. Empirical atomic diffusivity (D) characteristics of Nb|SS-304 interface and Ti6Al4V|Nb interfaces are ∼1.957 × 10−14 m2/s and ∼9.197 × 10−14 m2/s, respectively, for the IDCs processed at 950°C. The observed maximum hardness at the Ti6Al4V|Nb interface was ∼300 HV; whereas hardness at the Nb|SS-304 interface is found to be ∼200 HV and ∼650 HV for IDCs processed at 850°C and at 875°C and above, respectively. A maximum strength of ∼372 MPa (tensile) and ∼241 MPa (shear), along with ∼4.5 % ductility, was noticed for the IDCs processed at 875°C. The presence of two reaction layers: FeNb + (Nb) and Fe2Nb at the Nb|SS-304 interfaces improved the strength properties of the IDCs. Strength properties of the IDCs processed at 950°C deteriorated because of the presence of σ phase. Using diffraction studies, hardness property, and fracture behavior, it was established that the IDCs failed because of the propagation of crack, apparently along iron–niobium-based intermetallic reaction layers at Nb|SS-304 interfaces.