Development of solid state bonding processes for spallation neutron targets

Solid state bonding techniques are of vital interest to current and future spallation target design efforts for both cladding and wider fabrication requirements. The distinct needs of both water and liquid metal cooled sources were considered in this study. Development of hot isostatic pressing techniques and process controls necessary for successful cladding of tungsten with tantalum as needed for existing water cooled designs constituted the first component of this work. A second independent study performed with an emphasis on high temperature lead bismuth coolants focused on exploration of uniaxial diffusion bonding methods to join tungsten and tantalum to HT9, a ferritic-martensitic stainless steel. A technique for evaluation of the thermal performance of joined interfaces was also developed and employed to study the diffusion bonded systems.;Hot isostatic pressing performed at 1500°C for 3 hours and 200 MPa was found to produce an acceptable tantalum-tungsten bond provided extensive tantalum getter foil was used to wrap the target during the process. Excellent interface coherency was observed along with no oxidation or carburization on the tantalum surface. Uniaxial diffusion bonding at a temperature of 1060°C for 3 hours at pressures below 7 MPa resulted in excessive intermetallic formation at the HT9-tungsten and HT9-tantalum interfaces and significant residual interface porosity. Nickel and NiP interlayers were also observed to impart little benefit but did stabilize austenite with the HT9. A transition to lower temperatures and higher pressures improved bond quality. Conditions of 900°C for 3 hours and 70 MPa significantly improved both the HT9-tungsten and HT9-tantalum interfaces compared with the high temperature bond. An exploratory investigation of vanadium interlayers enhanced the result even further under these conditions and warrants further investigation.;All interfaces produced in this study possessed a thermal resistance well below that needed to affect the target centerline temperatures. The presence of cracking within the tungsten of bonded samples was observed to severely degrade thermal transport. All diffusion bonded samples contained far superior thermal resistances compared to either unbonded or mechanically constrained structures.