Microstructure analysis and comparison of tungsten alloy rod and [001] oriented columnar-grained tungsten rod ballistic penetrators

doi:10.1016/S0921-5093(98)00997-6

Materials Science and Engineering: A

Volume 262, Issues 1–2, 1 April 1999, Pages 115-128

https://doi.org/10.1016/S0921-5093(98)00997-6 Get rights and content

Abstract

Tungsten heavy alloy (WHA) penetrator rods (93% W particles in a matrix of 4.9% Ni and 2.1% Fe) were compared with [001] columnar-grained W penetrator rods in the as-fabricated condition, and after penetration into thick targets at impact velocities of 1 and 1.5 km s⁻¹, respectively. Light metallography and transmission electron microscopy revealed the initial W microstructures to be similar arrangements of predominantly a〈111〉/2 screw dislocations. The in-target (deformed) microstructures were characterized by recovered and elongated sub-grains having misorientations ranging from 1 to 5°.

Introduction

Large caliber penetrators used in modern military ordnance, such as tank gun ammunition rely on the penetration and perforation performance of high-density alloys. Such kinetic energy (rod) penetrators must normally be high-density, have high strength, and have substantial ductility. High-density materials enable the long rod penetrators to bring a large amount of kinetic energy to bear upon a small presented area of a target. High strength and ductility allow these materials to survive the rigors of their launch from the tank cannon. The most effective materials meeting these requirements have been depleted uranium (DU) alloys (U–0.75Ti; U–2Mo, etc.). These alloys, however, have raised serious, long-term environmental and health concerns, and a considerable effort has been focused on the development of so-called tungsten heavy alloys (WHA) (93W–Ni–Fe, etc.) as alternative penetrator materials, as well as model systems consisting of oriented single-crystal and oriented columnar-grained (directionally solidified) tungsten penetrators. Typical penetration experiments utilizing rods impacting semi-infinite RHA (rolled homogeneous armor) steel targets at 1.5 km s⁻¹ have shown that [100] single-crystal tungsten rods penetrate at least as much or slightly more than U–0.75%Ti rod penetrators, and exceed the penetration of 93% W (WHA) rods by more than 15% [1], [2]. While the high rate deformation behavior of the [001] single-crystal tungsten rods differs from the localized shear failure observed for the U–0.75%Ti rods [3], they exhibit an efficient plastic flow which also produces a narrow, and therefore deep, penetration cavity in the armor.

The prospect of developing single-crystal tank gun ammunition is rather remote. However, other penetration tests and microstructural observations have also been conducted using columnar grain tungsten rods, in an effort to elucidate the role of crystallographic orientation in penetrator flow phenomena. These efforts comparing the microstructures associated with the deformed rods of these various penetrators can begin to provide the information required to develop design strategies for new or improved penetrator materials and their performance. In addition, the details of flow on a microstructural level such as the occurrence of adiabatic shear bands or other deformation mechanisms are necessary to validate computer hydrocodes which can aid in the development of these design strategies.

In the present research, the microstructures associated with [001] columnar-grained tungsten rods are compared with WHA (93%W–Ni–Fe) rods in both the as-fabricated condition, and when deformed by penetration into a target. These microstructures are observed by optical metallography and transmission electron microscopy (TEM).

Section snippets

Experimental details

The WHA penetrator rods used in this study ranged in length (l_o) from 78.2 to 80 mm and diameters (d_o) ranging from 7.82 to 6 mm (length/diameter ratios (l_o/d_o) ranging from 10 to 15); and a hemispherical nose. The rods were composed of 93% tungsten, 4.9% nickel, and 2.1% iron (by weight) and were manufactured by Teledyne Firth Sterling using a liquid-phase sintering process (designated X-21) to produce a density of 17.7 g cm⁻³. The alloy was subsequently cold-worked to 25% reduction-in-area by

Results and discussion

Prior ballistic tests have indicated that the difference in penetration performance for single-crystal (especially [001]) tungsten penetrator rods may be attributed to anisotropy of flow and failure during armor penetration [1]. However, very different flow and failure behavior was observed for [001] oriented, columnar-grained penetrator rods. It was suggested that sub-grain misorientation within the columnar-grained material may have been responsible for the different behavior [2].

Previous TEM

Summary and conclusions

We have compared the microstructures of WHA (93% W, Ni–Fe) rods and [001] columnar-grained tungsten rods in the as-fabricated condition and after being captured and deformed in penetrating thick targets; using light microscopy and TEM. The tungsten precursor microstructures are characterized by poorly-formed dislocation cells and dislocation tangles with a propensity for a〈111〉/2 screw dislocations typical for bcc metals. The deformed, in-target microstructures differ in terms of macroscopic

Acknowledgments

This research was supported in part by a US Army TACOM-ARDEC contract no. DAAE30-97-M-0910, a US Army Research Office augmentation award for science and engineering research training (DAAG55-97-1-0238), and by the US Department of Defense, Defense Logistics Agency, Defense National Stockpile Center (DN-009).

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