Ductile-Phase Toughened Tungsten for Plasma Facing Materials (Final Report)
Tungsten (W) and W-alloys are the leading candidates for plasma-facing components in nuclear fusion reactors because of their high melting point, high temperature strength, good thermal conductivity and low sputtering yield, however, they are too brittle to serve a structural function. A variety of processing approaches were employed to fabricate ductile-phase-toughened (DPT) tungsten (W) composites, that includes consolidating elemental W powder with Cu- or WC-coated W-wires via spark plasma sintering (SPS). Laminated W-composites with Cu-foils were also fabricated by hot-pressing or brazing. Mechanical testing and analytical modeling were used to guide composite development. The fracture toughness of thin W plate or laminated W-composites was measured in three-point bending (3PB) at room temperature (RT). Crack arrest and crack bridging were observed for the laminated composites, and fracture resistance curves were successfully calculated. An analytical model of crack bridging was developed using the specimen geometry, matrix properties, and the stress-displacement function of a ductile reinforcement (“bridging law”) to calculate the fracture resistance curve (R-curve) and load-displacement curve (P-D curve) for any test specimen geometry. The code was also implemented to estimate the bridging law of an arbitrary composite using R-curve data. In another approach, commercially available liquid-phase sintered W alloys with four different compositions were characterized in terms of microstructure, tensile and fracture toughness at different specimen, size, geometry and testing temperatures. The room temperature (RT) average maximum load fracture toughness values (KIc or KJm ≈ 38 to 107 MPa√m) of WHA, containing only 3 to 10 wt.% of a NiFe-based ductile phase (DP), are ≈ 5 to 13 times higher than for monolithic W (≈ 8 MPa√m). Most RT tests show extensive stable ductile tearing (DT) at all W contents, including 97W (KJm ≈ 69 MPa√m), for small baseline bend bars (B=1.65mm, W=2B). Exceptions to DT, include RT elastic fracture at 97W observed in 3x larger specimens than the baseline specimens. However, the WHA KIc was still almost 5 times higher than that for monolithic W. The other exception to RT ductile tearing was for some of the 6 to 8x larger 95W alloy specimens that contained large ceramic initiating inclusions at the precrack front, along with lower DP%, with a KIc ≈ 59 MPa√m, compare to their stable DT KJm ≈ 82 MPa√m. Small specimen tests down to -196°C, to partially emulate irradiation hardening, also transition to elastic fracture at a temperature (-150 for 90W to -25ºC for 97W) that depends on the W content. However, even at -196°C, the KIc at 97W is ≈ 3 times that of monolithic W at RT. In contrast to classical ductile phase toughening by macrocrack bridging, WHA toughening mainly involves new mechanisms associated with arrest, blunting and bridging of numerous process zone microcracks. Later, these WHAs were used to fabricate W-WHA hybrid composites by coating elemental W powder on WHA using SPS. Three-point bend bars were fabricated and fracture toughness was tested at room temperature. Crack initiated and propagated from W-part to the WHA part for the well-bonded (interface) W-WHA composites for 90 and 92.5WHAs, however, crack arrested and diverted at/or through the interface for loosely bonded W-95/97WHAs with increasing load.