Mechanism for amorphization of boron carbide B4C under uniaxial compression
Mechanism for amorphization of boron carbide B4C under uniaxial compression
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DOI：
10.1103/physrevb.84.184112
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发表时间：
2011-11-28
影响因子：
3.7
通讯作者：
Ching, W. Y.
中科院分区：
文献类型：
--
作者：
Aryal, Sitaram;Rulis, Paul;Ching, W. Y.
碳化硼在高速冲击下会经历非晶化转变，导致其强度遭受灾难性损失。失效机理尚不清楚，这限制了提高其抗冲击能力的途径。为了帮助揭示失效机制，我们使用从头算方法对两种多型化学计量碳化硼（B4C）B11C-CBC和B-12-CCC进行大规模单轴压缩模拟，其中B11C或B-12是12原子二十面体，CBC或CCC是三原子链。模拟是在 180 个原子的大型超级晶胞上进行的。我们的结果表明，B11C-CBC (B-12-CCC) 多型体在单轴应变 s = 0.23 (0.22) 和最大应力为 168 (151) GPa 时变成非晶态。在这两种情况下，非晶态都是与三原子链弯曲相关的结构崩溃的结果。对非晶化后结构的仔细分析表明，B11C 和 B-12 二十面体高度扭曲，但仍可识别。不同单轴应变下的弹性系数（C-ij）的计算表明，两种多型体都可能在比单轴应变（应力）小得多的剪切应变（应力）下崩溃。另一方面，在高达 180 GPa 的静水压下对两个模型进行的单独模拟没有显示出非晶化的迹象，这与实验观察结果一致。通过对径向对分布函数、总态密度和原子上有效电荷分布的演化的详细分析，证实了两种模型的非晶化性质。计算非晶化前后碳化硼结构的电子结构和成键，以进一步阐明非晶化的机理，并帮助形成实验观察的合理化。
Boron carbide undergoes an amorphization transition under high-velocity impacts, causing it to suffer a catastrophic loss in strength. The failure mechanism is not clear and this limits the ways to improve its resistance to impact. To help uncover the failure mechanism, we used ab initio methods to carry out large-scale uniaxial compression simulations on two polytypes of stoichiometric boron carbide (B4C), B11C-CBC, and B-12-CCC, where B11C or B-12 is the 12-atom icosahedron and CBC or CCC is the three-atom chain. The simulations were performed on large supercells of 180 atoms. Our results indicate that the B11C-CBC (B-12-CCC) polytype becomes amorphous at a uniaxial strain s = 0.23 (0.22) and with a maximum stress of 168 (151) GPa. In both cases, the amorphous state is the consequence of structural collapse associated with the bending of the three-atom chain. Careful analysis of the structures after amorphization shows that the B11C and B-12 icosahedra are highly distorted but still identifiable. Calculations of the elastic coefficients (C-ij) at different uniaxial strains indicate that both polytypes may collapse under a much smaller shear strain (stress) than the uniaxial strain (stress). On the other hand, separate simulations of both models under hydrostatic compression up to a pressure of 180 GPa show no signs of amorphization, in agreement with experimental observation. The amorphized nature of both models is confirmed by detailed analysis of the evolution of the radial pair distribution function, total density of states, and distribution of effective charges on atoms. The electronic structure and bonding of the boron carbide structures before and after amorphization are calculated to further elucidate the mechanism of amorphization and to help form the proper rationalization of experimental observations.