Creep Behavior of a Zirconium Diboride-Silicon Carbide Composite and Preliminary ZrB2-WC Quasi-Binary Alloy Development for Long Duty Cycle Aerosurfaces and Structural Propulsion Applications
Bird, Marc W. 1984-
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The mechanical behavior of select ultra-high temperature ceramics were studied for extreme environment aerospace applications. Hot-pressed ZrB2-20 vol% SiC composites and ZrB2-WC quasi-binary alloys were developed for assessing room temperature mechanical properties and creep behavior. A thermochemical model describing alloy phase stability and reaction equilibria, for promoting WC dissolution, is presented. Room temperature structure-property relationships were developed correlating fracture strength and KIC with microstructure constituent size. Flexural creep studies of ZrB2-20 vol% SiC were conducted over the range of 1400°C to 1820°C assessing the macroscopic creep behavior using power-law stress and temperature dependent constants. Inert environment creep experiments were conducted for probing the local grain deformation mechanism in anticipation of bridging the deformation length scales. A two decade increase in creep rate, between 1500 and 1600°C, suggests a clear transition between the low temperature (1400-1500°C) diffusion creep and high temperature (>1600°C) grain boundary sliding creep having stress exponents of unity and 1.7<n<2.2, respectively. A novel indentation deformation mapping experiment clearly defined the local ZrB2 grain boundary sliding event with its components of 80% grain translations and rotations and 20% grain deformation. EBSD and texture theory confirmed the direct observation of ZrB2 grains deforming by dislocation flow, confined to near-grain boundary (mantle) zones, accommodating the grain rotation and translation events. A transition from the grain core to mantle deformation deviated from single crystal behavior as a result of extra geometrically necessary dislocations accommodating the deformation gradient. Microstructure observations shows evidence of <5% and <20% SiC grain deformation, contributing to the macroscopic creep strain, for tension and compression bending fibers, respectively. Cavitation accounts for less than 5% contribution to the accumulated creep strain. Preliminary ZrB2-WC quasi binary alloy creep experiments reveal a decade decrease in the steady state creep rate with a 1.1 mol% increasing WC composition. Improved creep behavior is discussed in the context of solute interactions with accommodation dislocations from grain boundary sliding. Alloy creep rates of 10-7-10-6 s-1 were measured contrasting with 10-5-10-4 s-1 for the ZrB2-SiC composite approaching the design creep rate of 10-8s-1 for long duty cycle aerospace applications.