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2019 Research Highlight: Grain Boundary Structures and Compositions of Si Graded Boron Carbide

Grain Boundary Structures and Compositions of Si Graded Boron Carbide

CMEDE Researchers
Dr. Christopher Marvel
Lehigh University
Professor Martin Harmer
Lehigh University
Dr. Anthony Etzold
Rutgers University
Dr. Vlad Domnich
Rutgers University
Professor Richard Haber
Rutgers University
Dr. Kristopher Behler
U.S. Army Combat Capabilities Development Command Army Research Laboratory
Dr. Jerry LaSalvia
U.S. Army Combat Capabilities Development Command Army Research Laboratory

Si-doping has the potential to improve the fracture resistance of boron carbide. For example, it has been shown that Si substitution into the 3-atom chains limits icosahedra disintegration and therefore reduces stress-induced amorphization. While mechanical benefits of Si-doping into the bulk lattice are proven, it is relatively unclear how Si is realistically distributed throughout boron carbide microstructures (i.e. bulk lattice, grain boundaries, and second phases) with given doping concentrations. Considering grain boundaries, another strategy to improve fracture resistance is to engineer Si-rich nanolayer films and promote intergranular fracture, similar to toughening silicon nitride via Si-rich intergranular films (IGFs). The current work aims to explore bulk and grain boundary thermodynamics of Si-doped boron carbide with the ultimate goal to incorporate nanolayer films and maximize fracture toughness of boron carbide.

A diffusion couple of boron carbide (B4C) and silicon hexaboride (SiB6) was hot-pressed using 50 MPa applied pressure at 1600 °C for 24 hours. Fabricating a diffusion couple (i.e. Si graded boron carbide) ensured that varying Si bulk solubilities in boron carbide were achieved and the resulting effect on grain boundary structure and composition could be studied. The hypothesis is that the maximum amount of Si solubility will maximize the probability of the formation of nanolayer films. Finally, aberration-corrected scanning transmission electron microscopy (ac-STEM) was conducted at Lehigh University.

Figure 1a shows the diffusion zone between the B4C and SiB6 polycrystals. The bright bands across the diffusion zone designate where thin specimens were extracted using focused ion beam methods. Typical grain boundary structures are shown in Figures 1b-d where the increased intensities in each image indicate an enhancement of Si. However, while Si clearly segregates to grain boundaries, there is no evidence of the formation of disordered Si-rich nanolayer films. The bulk Si solubility (at.%) and average excess grain boundary coverage (atoms/nm2) from each region were determined using ζ-factor microanalysis. The average excess coverage was determined from at least five grain boundaries and the results are shown in Figure 2. Here it is demonstrated that increasing Si concentration in boron carbide increases the extent of Si segregation. The current observations of this work show that despite the high Si solubility in the boron carbide lattice, Si-rich nanolayer films, which could potentially improve fracture resistance, are likely not stable in boron carbide. Future work may include doping boron carbide with other additives in addition to Si that could promote the formation of disordered nanolayer films and thus improve fracture toughness.