Penetrating Power of High-Density Tungsten Fiber/Zr-Based Metallic Glass Matrix Composite

Tungsten-ﬁber/Zr-based metallic glass (Zr 41.25 Ti 13.75 Cu 12.5 Ni 10 Be 22.5 ) matrix composite (hereinafter referred to as WF/Zr-MG composite) is considered a potential new generation of projectile material, while the density of WF/Zr-MG composite is usually around 16.5g/cm 3 and it is diﬃcult to exceed 17.0g/cm 3 . To understand the penetration performance of the composite material, a penetration test was performed on high-density WF/Zr-MG composite with a density of 17.1 g/cm 3 . The penetration depth, the diameter of the crater, morphology, and microstructure have been studied in a residual high-density WF/Zr-MG composite penetrator. A total of 8 penetration tests were performed by ﬁring the projectiles into 603 steel targets based on the 30mm ballistic gun, which included ﬁve high-density WF/Zr-MG composite projectiles and three 93W projectiles. Then, the target was cut through the crater using a wire cutting machine into a sample with 80mm × 40mm × 20mm, which was polished by diﬀerent types of sandpaper. The microstructures were characterized and analyzed by SEM and TEM technology. According to this study, high-density WF/Zr-MG composite penetrators perforated the target with a thickness of 69.7mm on average, which is about 19.8% better than 93W alloy (58.2 mm) when the impact velocity is 1400 ± 20m/s. Meanwhile, the high-density WF/Zr-MG composites exhibited “self-sharpening” behavior in the penetration process. Moreover, the higher penetrating power of high-density WF/Zr-MG composite is attributed to its higher dynamic compressive strength and “self ⁃ sharpening” behavior during penetration.


Introduction
Now, the two materials used for kinetic energy penetrators (KEP) are tungsten heavy alloy (WHA) and depleted uranium alloy (DUA). Unlike WHA, DUA fails by adiabatic shear when loaded in dynamic compression. As a result, when a DUA penetrator collides with the target plate at high speed, its nose remains sharp and about the same diameter as the rest of the penetrator and the sheared material is sloughed off. us, the kinetic energy is focused on a smaller area of the target, and the penetration depth is increased. is phenomenon is called "self-sharping". While, the nose of the WHA penetrator is like a "mushroom head" where the volume is bigger than that in the back, leaving a crater with a larger diameter. However, DUA is always not accepted because it emits low radiation to humans and the environment [1]. Consequently, a kind of better penetrator material is being searched for in the world [2][3][4][5].
Due to its excellent properties, including high-density and high-dynamic compressive strength, the tungsten fiber reinforced Zr-based metallic glass (Zr 41.25 Ti 13.75 Cu 12.5 Ni 10 Be 22.5 ) matrix (WF/Zr-MG) composite is considered as a potential new generation of projectile material to replace DUA and WHA [6,7]. Lots of research on WF/Zr-MG composite, which covers material design and preparation, fracture mode, and static and dynamic mechanical properties, have been reported in the past few years [8][9][10][11]. Conner et al. [6,12], Du et al. [13], and Chen et al. [7,14] conducted penetration tests to investigate the penetration power and failure of the WF/Zr-MG composite rods. e above investigations show that the composite rods failed by localized shear banding and displayed a "self-sharpening" behavior, and their armor-piercing power is about 10-20% better than that of WHA penetrators. It is well known that the two desirable properties of KEP are "self-sharpening" behavior and high density, while the density of WF/Zr-MG composite is usually around 16.5 g/ cm 3 [6,13,14], difficult to exceed 17.0 g/cm 3 due to their inability to achieve close packing of tungsten wires.
In the present study, a penetration test was conducted on the WF/Zr-MG composite penetrator with a density of 17.1 g/ cm 3 , which contains tungsten wire at 0.3 mm in diameter and 85% volume fraction, and Zr 41.25 Ti 13.75 Cu 12.5 Ni 10 Be 22.5 bulk metallic glass at a 15% volume fraction. Meanwhile, the armor-piercing power, crater diameter in the stable penetration stage, and the morphology of the crater bottom of highdensity WF/Zr-MG composite projectiles were analyzed and compared with 93W alloy projectiles.

Materials and Methods
e high-density WF/Zr-MG composite consists of Zrbased bulk metallic glass and tungsten fibers, and its density is 17.1 g/cm 3 . e chemical composition of the bulk metallic glass is Zr 41.25 Ti 13.75 Cu 12.5 Ni 10 Be 22.5 , and tungsten fibers with 0.30 mm in diameter are homogeneously embedded in the matrix as the reinforced phase.
e tungsten fibers occupy about 85% of the total volume. e picture of the high-density WF/Zr-MG composite is shown in Figure 1.
e XRD pattern of this composite is shown in Figure 2(a), it can be seen that only the peaks of crystalline tungsten can be found, and no other crystalline phases are detected within the sensitivity limit of the XRD. Meanwhile, the TEM of the interface between the matrix and tungsten fiber is shown in Figure 2(b). As can be seen, no precipitated phase exists in this area, which is consistent with the XRD test results. e 93W alloy is made of W 93 Ni 4 Fe 3 , and its density is 17.3 g/ cm 3 . e material density was measured by the MD-300S Archimedes density tester.
A total of 8 penetration tests were performed by firing the projectiles into 603 steel targets based on the 30 mm ballistic gun, in which five high-density WF/Zr-MG composite projectiles and three 93W projectiles were included. e sizes of the projectile and target are 93 mm in height and 6 mm in diameter and 300 mm × 200 mm × 80 mm (length × width × thickness), respectively. e structure of the 105 mm simulated armor-piercing projectile as shown in Figure 3 has been used for more than 20 years in this paper, and its rationality has been fully verified. e material information of the projectile and target is given in Table 1, and the schematics of the experiment are schematically shown in Figure 4.
In this experiment, the speed of the projectile was measured by the high-speed camera and speed measurement system (XGK-2002, Xi'an Technological University, Xi'an, China). e speed of the projectile was adjusted to 1400 ± 20 m/s by the weight of the gunpowder. And the angle between the trajectory and target was 90°. After the penetration test, the target was cut through the crater using a wire cutting machine into a sample with 80 mm × 40 mm × 20 mm (length × width × thickness), which was later polished by different types of sandpaper. Meanwhile, the penetration depth was measured. e crater diameter at the stable penetration stage, whose range is 24 mm-50 mm of penetration depth, and once per 2 mm is measured. e microstructure was observed by SEM (QUANTA FEG250, FEI Company, Hillsboro, OR, USA) and TEM (JEOL JEM-2800, JEOL, Tokyo, Japan). Meanwhile, the phase component of the composite is analyzed by XRD (Shimadzu XRD-6100, Shimadzu Corporation, Kyoto, Japan).

Analysis of Penetrating Ability.
In the case of 1400 ± 20 m/s impact, the front view of the target plate with eight craters is shown in Figure 5(a), and the back view of the target plate is shown in Figure 5(b), respectively. As shown in Figure 5, it can be seen that all projectiles are embedded in the target plate, not perforated through the target.
Furthermore, the impact velocity, mass, and depth of penetration of 8 projectiles are shown in Table 2, respectively. From Table 2, it can be seen that there is little difference in the mass of the two material projectiles, and the average thickness of the high-density WF/Zr-MG composite projectiles perforating the target is 69.7 mm, 19.8% higher than that of the 93W alloy (58.2 mm). In other words, the penetrator power of the high-density WF/Zr-MG composite is significantly higher than that of the 93W alloy.
Meanwhile, the kinetic energy of the projectile can be calculated by as follows: where E, m, and v are the kinetic energy, the mass, and the impact velocity of the projectile, respectively. And then E′ is defined as the ratio of E to p, which is expressed as where p is the penetrating depth of the projectile. e E′ of two different material projectiles is shown in Figure 6, it can be seen that the E′ of five high-density WF/Zr-MG composite projectiles is 669.5 J/mm, 615.9 J/mm, 619.3 J/ mm, 628.9 J/mm, and 605.8 J/mm, respectively. However, the E′ of the three 93W alloy projectiles is 756.4 J/mm, 746.5 J/mm, and 748.7 J/mm, respectively, which is significantly higher than that of the high-density WF/Zr-MG composite projectiles. e figures above state that the    Figure 4: Schematics of the ballistic experiment: 1 is a 30 mm ballistic gun, 2 is the sabot collector, 3 is a high-speed camera, 4 is a speed measurement system, and 5 is the target.
projectiles of the high-density WF/Zr-MG composite have higher penetrator efficiency than those of the 93W alloy when they have the same kinetic energy at 43 kJ.

Characteristics of Penetration
Crater. e post-test longitudinal sections of 603 steel targets fired by highdensity WF/Zr-MG composite and 93W alloy projectiles are displayed in Figures 7(a) and 7(b), respectively. It can be seen that the residual high-density WF/Zr-MG composite and 93W alloy projectiles filled the craters. Besides, the bottom of the crater penetrated by 93W alloy projectiles is rough and clearly blunt, mainly resulting from the material plastic flowing and little erosion. Besides, further analysis shows that the average diameter of the 93W alloy projectile penetrating a crater is 11.0 mm, which is about 1.8 times its initial diameter.
By comparison, when the projectile material is a high-density WF/Zr-MG composite, the trajectory is straight and smooth, and a narrow tunnel through the target is created. Results show that the average diameter of the high-density WF/Zr-MG composite projectiles penetrating a crater is 8.2% smaller than the former, which is 10.1 mm, approximately 1.6 times the penetrator diameter.
Overall, the diameter of the ballistic hole bored by the 93W alloy projectile is larger than that of the high-density WF/Zr-MG composite projectile, which indicates that the more kinetic energy of the 93W alloy projectile is used for expanding the diameter of the penetrating crater, resulting in a decrease in the depth of penetration.

Microstructure Analyses on the Residual Penetrators.
e head of the 93W alloy residual projectile, as shown in Figure 8, experienced a series of deformations under the condition of high-speed impact, as shown in Figure 9, and the locations are marked as seen in Figure 8. Clearly,    the strong impact changed the 93W alloy grains on the top of the head of the residual projectile (marked as position A) changed into flat particles, and the long-axis direction of the elongated grain substantially is perpendicular to the direction of penetration, as displayed in Figure 9(a). In positions B, C, and D, the tensile and compressive deformation of 93W alloy grains occurred on the lateral side of the residual projectile head. In the positions closer to the top edge of the head of the residual projectile, e.g., positions B and C, a remarkable deformation of 93W alloy grains is observed. ose elongated grains are displayed as the fibrous shape, and its long-axis direction is the same as the streamlined direction of the head of the residual projectile. en, the 93W alloy grains did not break at the contact location between the head of the residual projectile and the target. It is because of the dislocation movement along the direction of the maximum shear stress that results in the plastic deformation of 93W alloy grains. It indicates that the 93W alloy has good plasticity under high-speed penetration, resulting in the formation of "mushroom head".
By comparison, the head of the residual high-density WF/Zr-MG composite projectile is studied when the impact speed is 1400 ± 20 m/s. e result can be seen in Figure 10. It can be seen that the residual high-density WF/Zr-MG composite projectile head is sharper than that of the 93W alloy. Meanwhile, the microstructure of the positions of the E-F and G-F lines that are drawn in Figure 10 is further studied, which is shown in Figure 11. On the bottom left and right bottom, one can see that the tungsten fiber initially aligned with the projectile longitudinal axis, bends and shears along a line oriented about 45°angle to the longitudinal axis. And these wires are then sloughed off, at 90°i ncluding an angle between the two sheared faces is created, and then the tip is sharpened effectively, which is called the "self-sharpening" phenomenon. e "self-sharpening" leads to a sharper nose for high-density WF/Zr-MG composite projectiles, and it effectively improves the penetration capability.

Discussion
In terminal ballistics, relatively simple engineering models are irreplaceable. No matter how great the achievements of numerical simulation technology could be, an engineering model that is based on the most important and characteristic physical processes of projectile-target interaction could give the main trends and principal results of the projectile-target interaction.
e Alekseevskii-Tate model [15] is the most successful semihydrodynamic model applied to long-rod penetration into semiinfinite targets.
is is a one-dimensional model of long-rod penetration derived from hydrodynamic theory [16,17]. Rod and target strength are included in the modified Bernoulli equations. is     Advances in Materials Science and Engineering where Y p is the dynamic yield strength of the rod material, and R t is the target resistance to penetration. R t is related to material strength and can be calculated approximately with cavity expansion theory. v is the rod velocity. u is the penetration velocity, and u < v. Equation (1), together with the following equations, provides a complete solution of the long-rod penetration problem: Here, equation (4) is the rod deceleration equation, equation (5) is the length of the penetrator consumption equation, and equation (6) is the penetration depth equation. p is the penetration depth, l ′ is the residual rod length, v c stands for the critical tail velocity, and the long-rod semihydrodynamic penetration initiates only if v > v c .
From equation (3), the penetration velocity u can be written as follows: e above equation is the expression that correlates the penetration velocity u and these parameters such as the rod velocity v, the densities of projectile and target material (ρ p , ρ t ), the penetration resistance of target R t , and the dynamic flow stress of projectile material Y p , which are listed in Table 2, respectively.
Penetration velocity u is an important parameter in longrod penetration. If the value of u is greater than the condition of the rod velocity, v is known from equation (5), the rod length is eroded more slowly, and the residual rod length l ′ (mass) is larger, which indicates that the penetration ability of the rod will be better.
As can be seen in Table 3, the densities of high-density WF/Zr-MG composite and 93W alloy are almost at the same value, and 603 steel target density ρ t and resistance R t are 7.85 g/cm 3 and 6000 MPa, respectively. However, the dynamic yield strength Y p of the high-density WF/Zr-MG is 3000 MPa, which is higher than that of 93W alloy with a strain rate of 3000 s − 1 . According to equation (7), it can be found that the penetration velocity u of the highdensity WF/Zr-MG composite penetrator is larger than that of the 93W alloy penetrator when these penetrators have the same velocity v. In other words, the penetration ability of the high-density WF/Zr-MG composite penetrator is better than that of the 93W alloy penetrator with the same penetration conditions, such as mass and impact velocity. Besides, an obvious "self-sharpening" phenomenon occurs in the head of the high-density WF/Zr-MG composite penetrator, as shown in Figures 10 and 11. It decreases the penetration resistance and increases the penetration ability, and thereby it decreases the diameter of the bullet hole and increases the depth of penetration, as shown in Table 2 and Figure 7, respectively. While, the material continuity exactly exists between the mushroom head and the 93W alloy penetrator body, which increases the diameter of the 93W alloy penetrator head and bullet hole.

Conclusions
(1) High-density WF/Zr-MG composite penetrators perforated the target with a thickness of 69.7 mm averagely, which is significantly higher than 19.8% better than that of 93W alloy (58.2 mm), when the impact velocity is 1400 ± 20 m/s. Meanwhile, the penetration efficiency of the high-density WF/Zr-MG composite is higher than that of the 93W alloy. (2) e average diameter of the high-density WF/Zr-MG composite projectiles penetrating the crater is 10.1 mm, approximately 1.6 times the penetrator diameter, which is 8.2% smaller than that of 93W alloy projectiles. Meanwhile, the high-density WF/ Zr-MG composite exhibited "self-sharpening" behavior in the process of penetration, which led to a sharper nose for high-density WF/Zr-MG composite projectiles. (3) e higher penetrating power of high-density WF/ Zr-MG composite projectiles is attributed to its higher dynamic compressive strength and "self•sharpening" behavior during penetration.

Data Availability
e data used to support the findings of this study are plotted within the article, and the raw data files are available by contacting the corresponding author.

Conflicts of Interest
e authors declare that there are no conflicts of interest.