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The large compositional space of high entropy alloys (HEA) often presents significant challenges in comprehensively deducing the critical influence of atomic composition on their mechanical responses. We propose an efficient nonparametric kernel-based probabilistic computational mapping to obtain the optimal composition of HEAs under ballistic conditions by exploiting the emerging capabilities of machine learning (ML) coupled with molecular-level simulations. Compared to conventional ML models, the present Gaussian approach is a Bayesian paradigm that can have several advantages, including small training datasets concerning computationally intensive simulations and the ability to provide uncertainty measurements of molecular dynamics simulations therein. The data-driven analysis reveals that a lower concentration of Ni with a higher concentration of Al leads to higher dissipation of kinetic energy and lower residual velocity, but with higher penetration depth of the projectile. To deal with such conflicting computationally intensive functional objectives, the ML-based simulation framework is further extended in conjunction with multi-objective genetic algorithm for identifying the critical elemental compositions to enhance kinetic energy dissipation with minimal penetration depth and residual velocity of the projectile simultaneously. The computational framework proposed here is generic in nature, and it can be extended to other HEAs with a range of non-aligned multi-physical property demands. [ABSTRACT FROM AUTHOR]

Functionally graded materials (FGMs) with a gradual transition from ceramic to metal are believed to have superior ballistic performance compared to ceramic/metal composite armors. Therefore, multilayered SiC/AA7075 FGM samples of size 50 × 50 mm are synthesized using a relatively simple and inexpensive technique, i.e., powder metallurgy. The armor requirements depend upon the type of threat and hence, the region and circumstances. Therefore, unlike most of the available investigations, the present study deals with FGM armors with low areal density (< 30 kg/m2) against 7.62 × 39 mm Hardened Steel Core bullets. Two different FGM designs comprising 3 and 5 layers of SiC/AA7075 composite, with 10-30-50 wt.% SiC and 10-20-30-40-50 wt.% SiC, respectively, are developed herein along with a homogeneous composite with 30 wt.% SiC for comparison. The fabrication process involves the stacking of powder blends pertaining to different layers in the die cavity followed by hot pressing at a temperature of 400 °C and 325 MPa pressure. The green compacts are then subjected to pressureless sintering at 540 °C for 3 h. Solution heat treatment and aging are found to enhance the hardness of each layer significantly. The results of the depth of penetration-based ballistic evaluation profoundly establish the superiority of FGM armors over homogeneous composites. Besides, a one-dimensional stress wave propagation-based analysis is also presented herein to explain the reason for the better ballistic performance of FGMs. [ABSTRACT FROM AUTHOR]

Low velocity ballistic deformation behavior of closed-cell aluminum metal foam targets fabricated by powder metallurgy technique has been investigated. The metal matrix of aluminum (60 micron) was mixed with carbamide (CH4N2O) particles of size one mm. Green samples were obtained by compacting the mixture followed by sintering process using a muffle furnace and the metallic foam thus obtained had an average pore size of 1.5 mm. Compared to metallic material, significant reduction in hardness and density was observed when the porosity was increased from 5 to 30%. Using a gas gun setup, low velocity (100 m/s and conical projectile) ballistic experiments were conducted on metal foam targets to understand the deformation behavior. Not any penetration of the projectile was observed when the percentage of porosity was less than 5%. When the percentage of porosity was increased, the projectile was able to perforate the target as the fracture toughness decreases. The size of fragmented areas of the perforated targets and depth of penetration (DoP) of the projectile were measured. Decrease in DoP (improved ballistic resistance) was observed with the increase in relative density of the closed foam target. Even though the projectile could perforate the foam target, no cracks were observed which enhances the suitability of the metal foams to be used as structural applications of defense vehicles. [ABSTRACT FROM AUTHOR]