Ductility in Crystalline Boron Subphosphide (B₁₂P₂) for Large Strain Indentation

Abstract

Our studies of brittle fracture in B 4 C showed that shear induced cracking of the (B 11 C) icosahedra leading to amorphous B 4 C regions induced cavitation and failure. This suggested that to obtain hard boron rich phases that are ductile, we need to replace the CBC chains of B 4 C with two-atom chains that can migrate between icosahedra during shear without cracking the icosahedra. We report here quantum mechanism (QM) simulation showing that under indentation stress conditions, superhard boron subphosphide (B 12 P 2 ) displays just such a unique deformation mechanism. Thus, stress accumulated as shear increases is released by slip of the icosahedra planes through breaking and then reforming the P-P chain bonds without fracturing the (B 12 ) icosahedra. This icosahedral slip may facilitate formation of mobile dislocation and deformation twinning in B 12 P 2 under highly stress conditions, leading to high ductility. However, the presence of twin boundaries (TBs) in B 12 P 2 will weaken the icosahedra along TBs, leading to the fracture of (B 12 ) icosahedra under indentation stress conditions. These results suggest that crystalline B 12 P 2 is an ideal superhard material to achieve high ductility.

Models for the behavior of boron carbide in extreme dynamic environments

“…With the intent of mitigating the adverse effects of amorphization in B 4 C subjected to shear loading, we used QM to examine substitutions of these CBC chains with two atom chains such as Si-Si [29], PP [30], OO [31] (note, it has been demonstrated before that the fundamental cause of B 4 C brittle fracture is the reaction of the middle B in the CBC chains). Indeed the QM shows that (B 11 C)(SiSi) and (B 11 C)(PP) leads to very ductile materials, with the single crystal staying stable as the shear is continued to large strains [29,30,31]. Unfortunately the standard techniques for fabricating B 4 C do not lead to incorporating Si-Si chains.…”

Strengthening boron carbide by doping Si into grain boundaries

“…Several approaches, such as microalloying [12][13][14][15][16] , stoichiometry control [17][18][19] , and addition of a 2 nd phase [20][21][22] , have been proposed to mitigate amorphous shear band formation. For microalloying, both non-metal dopants (P 23 and Si 24 ), as well as metal dopants (Li 14,15 , Mg 12,16 , and Ti 13 ) may decrease amorphization in B 4 C. Both experimental and theoretical studies have suggested…”

First-principles study on the helium migration energies in B12X2(X=O, Si, P, As) crystals for neutron absorber use

“…Boron carbide (B 4 C) is the leading candidate for a neutron absorber for next-generation fast reactors, which are entrusted with the important task of providing sustainable nuclear energy [1,2]. However, due to the helium gas produced from the 10 B[n, α] 7 Li reaction, the volume swelling of the B 4 C pellets during use has been a critical safety issue for the longterm use of the control rods [3][4][5]. Moreover, the brittleness of B 4 C makes the pellets very easy to break into small fragments under the internal stress induced by the heat gradient and helium gas production.…”

“…The aim of this change is to avoid partial amorphization of the B 4 C grains under stress by enhancing the slip between the boron icosahedra. A series of investigations based on DFT calculations of (B 11 C p )Si 2 , (B 10 Si 2 )Si 2 , B 12 P 2 , and B 12 O 2 revealed that these compounds with two-atom chains instead of three-atom chains are not likely to form amorphous bands under stress, which normally lead to brittle failure [6][7][8][9]. Compared to B 4 C, these materials have similar boron content and improved ductility characteristics, and are therefore expected to outperform B 4 C as neutron absorbers for fast reactors by reducing the pellet fragmentation.…”

“…Meanwhile, carbon nanotubes (CNTs) are attracting increasing attention due to their excellent abilities to catalyze the recombination of radiationinduced defects and create exhaust paths for helium and other fission gases in their aluminum composites [10]. Therefore, the CNTs are worth investigating further to see if they have an inhibiting effect on the swelling of the B 4 C pellet by efficiently capturing and exhausting the helium gas produced from the 10 B(n, α) 7 Li reaction. Moreover, CNTs could be employed as supplement materials to the carbon content in the carbon-deficient boron compounds such as (B 11 C p )Si 2 , (B 10 Si 2 )Si 2 , B 12 P 2 , and B 12 O 2 , and provide a neutron-spectrum softening effect.…”