Fracture Behavior of Advanced Structural Materials
Further development of ultra-high structural applications requires new materials that can withstand the extreme environments found in power-generation turbines. Highly refractory materials capable of high strength at temperatures in excess of 1100°C are either inherently brittle (Al2O3, Si3N4) or cannot survive at such temperatures in oxidizing atmospheres (Mo, Nb). Brittle materials may be toughened with the addition of a more ductile phase. Materials that do not form protective oxide layers can be alloyed with Si to form more protective silicate scales. A problem still remains as to how dissimilar materials like Al2O3 and Si3N4 can be joined. A brief discussion of the fracture behavior of ductile-phase-toughened nanocrystalline Al2O3 and functionally-graded material joints of Al2O3 and Si3N4 will precede an in-depth exploration of fracture in two Mo-Si-B intermetallic alloys.
Al2O3-based nanocomposites were fabricated and consolidated via spark plasma sintering. The influence on the fracture behavior of nanocrystalline Al2O3 of single-walled carbon nanotube (SWCNT) and Nb additions were examined by in-situ bend testing within a scanning electron microscope. The addition of 10 vol.% Nb to nanocrystalline Al2O3 provided substantial improvement of fracture toughness (6.1MPa-m½)--almost three times that of unalloyed nanocrystalline Al2O3. Observation of cracks emanating from Vickers indents, as well as bend specimen fracture surfaces, reveal the operation of ductile phase toughening in the Nb-Al2O3 nanocomposites. Further addition of 5 vol.% SWCNTs to the 10 vol.% Nb-Al2O3 revealed a more porous structure and less impressive fracture toughness. The SWCNTs, initially added to form uncracked ligament bridges spanning the crack which would carry load and shield the crack tip from the full extent of the remotely-applied load, acted as crack initiation sites and overwhelmed any toughness gains afforded by the ductile Nb grains.
Nominally crack-free Al2O3- Si3N4 joints, comprised of 15 layers of gradually differing compositions of Al2O3/Si3N4, have been fabricated using SiAlON polytypoids as functionally graded materials (FGM) bonding layers for high-temperature applications. Using flexural strength tests conducted both at room and at elevated temperatures, the average fracture strength at room temperature was found to be 437 MPa; significantly, this value was unchanged at temperatures up to 1000°C. Scanning electron microscopy (SEM) observations of fracture surfaces indicated the absence of any glassy phase at the triple points. This result was quite contrary to the previously reported 20-layer Al2O3/Si3N4 FGM samples where three-point bend testing revealed severe strength degradation at high temperatures. Consequently, the joining of Al2O3 to Si3N4 using functional gradients of SiAlON polytypoids can markedly improve the suitability of these joints for high-temperature applications.
New alloys based on borosilicides of molybdenum have been considered as potential replacements for current Ni-base superalloys, as they show promise as highly oxidation- and creep-resistant materials while still maintaining a moderate level of damage tolerance. Two alloys, each composed of Mo-3Si-1B (wt.%) with nominally similar fine-grained microstructures, have been developed utilizing markedly differing processing routes. Here, we study the influence of processing route on the fracture toughness of alloys containing ~55 vol.% ductile α-Mo and ~45 vol.% brittle intermetallics (Mo3Si and Mo5SiB2 (T2)). The room-temperature toughness of these two alloys is significantly lower than that previously evaluated coarser-grained Mo-Si-B alloys with similar composition; however at 1300°C, the crack-initiation toughness of the fine- and coarse-grained alloys are nearly identical. At lower temperatures, the current finer-grained materials behave in a brittle manner as the smaller grains do not provide much impediment to crack extension; cracks can advance with minimal deflection thereby limiting any extrinsic toughening. Plastic constraint of ductile α-Mo grains from the hard intermetallic grains also serves to lower the toughness. At 1300°C, the increased ductility of α-Mo allows for significant plasticity; the correspondingly much larger contribution from intrinsic toughening results in significantly enhanced toughness, such that the finer grain morphology becomes less important in limiting crack-growth resistance. Optimization of these alloys is still required, however, to tailor their microstructures for the mutually-exclusive requirements of oxidation resistance, creep resistance and damage tolerance.