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FeAl and Mo-Si-B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe-24Al (wt.%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 m/s to 700 m/s. Mo-13.4Si-2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo-Si-B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity, again due to an increased peening effect. For Mo-Si-B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and α-Mo. The T1 phase was retained after spraying at 350 m/s.

Three types of high-temperature joints were created from alloy 617 base metal: fusion welds, braze joints, and diffusion bonds. The microstructures of all joint types and tensile properties of fusion welds and braze joints were characterized. Sound fusion welds were created by the Gas-Tungsten Arc Weld (GTAW) process with alloy 617 filler wire. Cross-weld tensile strengths were equal to the parent metal at temperatures of 25, 800, and 1000°C; ductilities of the joints were only slightly lower than that of the parent metal. Failure occurred in the weld fusion zone at room temperature and in the parent metal at elevated temperatures. Incomplete wetting occurred in joints produced by vacuum brazing using AWS BNi-1 braze alloy, believed to be due to tenacious Al and Ti oxide formation. Incompletely bonded butt joints showed relatively poor tensile properties. A second set of braze joints has been created with faying surfaces electroplated with pure Ni prior to brazing; characterization of these joints is in progress. Conditions resulting in good diffusion bonds characterized by grain growth across the bondline and no porosity were determined: vacuum bonding at 1150°C for 3 hours with an initial uniaxial stress of 20 MPa (constant ram displacement). A 15 µm thick pure Ni interlayer was needed to achieve grain growth across the bondline. Tensile testing of diffusion bonds is in progress.iii

Fatigue crack growth tests were performed on 2¼Cr-1Mo steel specimens machined from ex-service Experimental Breeder Reactor -II (EBR-II) superheater duplex tubes. The tubes had been metallurgically bonded with a 100 µm thick Ni interlayer; the specimens incorporated this bond layer. Tests were performed at room temperature in air and at 400°C in air and humid Ar; cracks were grown at varied levels of constant K. Crack growth tests at a range of K were also performed on specimens machined from the shell of the superheater. In all conditions the presence of the Ni interlayer was found to result in a net retardation of growth as the crack passed through the interlayer. The mechanism of retardation was identified as a disruption of crack planarity and uniformity after passing through the porous interlayer. Full crack arrest was only observed in a single test performed at near-threshold K level (12 MPa m) at 400°C.