The Role of Multiple Polytypes in Determining the Catastrophic Failure of Boron Carbide at High Shock Velocities
- PDF / 244,160 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 67 Downloads / 167 Views
0904-BB04-22.1
The Role of Multiple Polytypes in Determining the Catastrophic Failure of Boron Carbide at High Shock Velocities Giovanni Fanchini1, James W. McCauley2, Dale E. Niesz1, and Manish Chhowalla1 1 Materials Science and Engineering, Rutgers University Piscataway, NJ 08854, U.S.A. 2 ARL, Aberdeen Proving Ground Aberdeen, MD 21005, U.S.A. ABSTRACT The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (HEL) has been a puzzle for a long time. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [(B11C)C2B, (B12)C3, etc ā¦] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all the candidate microstructures are less stable than a phase involving segregated boron (B12) and amorphous carbon (a-C) but the energetic barrier between boron carbide and B12 + 3C, is by far lower for the B12(CCC) microstructure, requiring the lowest atomic displacement for a transformation B4Cā3B+a-C, occurring at pressures of 6 GPa ā P(HEL). For such a configuration, segregation of free carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis INTRODUCTION The performance of ceramic armor must be fail proof since it determines the survival and safety of people. Boron carbide [1] is the ideal material for such an application. It sits at the third position on the hardness scale, just below diamond and cubic boron nitride, and it possesses the highest dynamic elasticity, its Hugoniot elastic limit (HEL = 17-20 GPa) surpassing by a factor two all of its competitors, such as silicon carbide and alumina, which are 50% denser [2]. Nevertheless, the use of boron carbide in lightweight armor is not possible now because of a lack of residual plastic strength above the HEL, a compulsory issue for armor survival upon damage. No one has been able to elucidate the physics of such a catastrophic failure [1], a unique behavior in dynamically elastic materials finding counterparts at much lower stress values (
Data Loading...
