Effect of aging state on shock induced spall behavior of ultrahigh strength Al–Zn–Mg–Cu alloy

Highlights

• The Hugoniot Elastic Limit and spall strength in three ageing states are independent of peak stress.

• Al₇Cu₂Fe inclusions play a dominant role in determining the overall spall strength.

• The presence of equilibrium precipitates along the grain boundaries is detrimental to spall failure resistance.

• The size and amount of grain boundary precipitates play a negligible effect on spall failure at the peak stress above 2.4 GPa.

Abstract

The Hugoniot Elastic Limit (HEL) and spall strength of an ultrahigh strength Al-Zn-Mg-Cu alloy in natural aging (NA), peak aging (PA) and over aging (OA) states have been measured under different shock stresses. It is found that the HEL and the spall strength of this alloy following these three heat treatments are independent of peak stress (or strain rate). The PA alloy shows the highest HEL ranging from 1.35±0.03 to 1.40±0.11 GPa, followed by the OA alloy, and the NA alloy exhibits the lowest value. On the other hand, the NA alloy has the highest spall strength ranging from 1.73±0.07 GPa to 1.84±0.03 GPa, while the spall strengths for PA and OA alloys are quite comparable, about 11 % lower than NA. The free surface velocity profiles beyond pullback minima are analyzed, showing that although spall strength and growth rate of voids for the PA and OA alloys are almost identical, the OA alloy shows a higher pullback peak, indicating more damage is produced in the OA alloy at the peak stress of 1.9 GPa. The post-impact characterization of the microstructure of these samples reveals a mixed mode of intergranular and transgranular fracture. It is demonstrated that the Al₇Cu₂Fe inclusions act as the main nucleation sites for voids and determine the overall spall strength. The distribution of equilibrium precipitates along grain boundaries is detrimental to the spall resistance. Hence, the absence of grain boundary (GB) precipitates is responsible for the higher spall strength for the NA heat treatment state. However, the size and amount of GB precipitates has negligible effect on spall response, especially when the peak stress exceeds 2.4 GPa.

Introduction

An understanding of the dynamic behavior of materials is important for their application in military and aerospace industries. In these fields it is common for materials to experience high velocity impacts, such as spacecraft shielding from micrometeoroids and man-made debris [1]. Such impacts can result in spall failure of the materials when two strong rarefaction waves collide and generate a tensile region of the impacted materials [2], [3], [4]. For ductile metals, spall is a kinetic process that generally involves the nucleation, coalescence, and growth of microvoids, which eventually leads to failure and separation of materials [5,6].

A number of studies have been conducted using flyer-plate impact experiments to understand shock induced dynamic properties of aluminum alloys. Due to their low density, high strength, and excellent energy absorption capability, they are ideal for advanced light-weight armor materials [7], [8], [9], [10]. It is widely acknowledged that the dynamic tensile (spall) strength and the Hugoniot Elastic Limit (HEL) are two important mechanical properties associated with blast or impact resistance. Spall strength is not an intrinsic material parameter [11]. In fact, the spall strength depends on not only loading conditions, such as pulse duration, peak shock stress and strain rate, but also microstructural effects, such as grain size, inclusions, secondary phase particles, etc. [12]. Williams et al. [13] studied the effects of peak shock stress on the substructure evolution and spall response of fully annealed 1100 aluminum. Their results showed that spall strength increased with shock stress until it was saturated at approximately 8.3 GPa and then decreased for higher shock stresses. Correspondingly, the dominant fracture mode changed from ductile fracture to brittle intergranular fracture at higher stresses [14]. Whelchel et al. [15] found that the spall strength of Al 5083-O increased with peak stress from 0.84 to 0.92 GPa. However, Boteler et al. [16] found that the spall strength was independent of impact stress in the similar alloy Al 5083-H131. All of these results indicate that the effects of loading condition on spall response are unclear and complicated. It is possible that the differences observed in these studies are dominated by microstructural effects. In fact, the microstructural effects on spall failure are of significant interest and have received significant attention as in the fundamental work of Curran et al [17]. It is widely accepted that spall strength increases with increasing grain size because grain boundaries often act as nucleation sites for voids and cracks [18]. However, at higher stresses (22 GPa), the spall strength is independent of grain size. This is because the initial microstructural effects are overridden by shock induced microstructural changes at high stresses [19]. Whelchel et al. [20] systematically investigated the effects of grain structure, orientation, inclusions, and precipitates on the spall failure of Al-Mg alloys. It was found that inclusion size and distribution were the controlling factors for void formation, and grain size was not the dominant microstructural feature affecting spall strength in aluminum alloys. Similarly, Williams et al. [21] examined the roles of second phase intermetallic particles under shock compression. Rosenberg et al. [22] investigated the shock response of various heat treated Al 2024 alloys, showing that HEL and spall strength follow the same trends as static yield strength, with the solid solution state exhibiting lowest HEL and spall strength. In contrast, Millett et al. [23] studied the effect of heat treatment on dynamic properties of Al 7017 alloy, showing that the peak aged state has the highest HEL, while the spall strength is identical and it is independent of heat treatment. Therefore, the relationships between microstructures and shock induced dynamic properties are not fully understood yet, and need to be further studied.

Al-Zn-Mg-Cu alloys containing high Zinc content (higher than 8.5 at %) exhibit ultrahigh strength and excellent toughness. It has been proved that these alloys show improved ballistic performance when compared to the conventional 7xxx Al alloys [24,25]. Hence, these alloys have great potential to be used for the next generation of light-weight armor or spacecraft shielding. However, there is little information on their dynamic behavior under shock loading. In addition, their microstructure is very complex, containing micro-sized inclusions, nano-sized precipitates, as well as various grain boundary structures depending on heat treatment states [26], [27], [28], [29]. Thus, several questions arise, e.g., what is the dominant factor affecting the HEL and the spall strength among so many microstructural variations? How does the damage initiate and develop? It is known that yield strength and strain hardening rate are highly dependent on precipitate states, and void growth is driven by plastic deformation of the surrounding matrix [30]. Upon void initiation, will the precipitate states affect the coalescence and growth processes? In this study, we choose an ultrahigh strength Al-Zn-Mg-Cu alloy and conduct a series of plate impact experiments in various aging states, aiming to understand the effects of resulting microstructure on dynamic behavior and failure mechanisms during spall failure.