Research paperAtomic insights into shock-induced spallation of single-crystal aluminum through molecular dynamics modeling
Graphical abstract
Introduction
A rarefaction wave generally appears when the shock wave is reflected from the material surface upon impact loading. Subsequently, this reflected wave interacts with the unloading rarefaction wave, in turn, some lamellar-fractured slices fly off the material free surface at high speeds, and this phenomenon is called as spalling (Grady, 1988). When the shock pressure is low, the material remains in a solid state during spalling, and this phenomenon is termed as classical spallation. In contrast, phase transition and partial/full melting occur inside the material under a high impact pressure. Consequently, the material exists in a solid−liquid mixed state and a substantial amount of ejecta escape from the free surface, and this phenomenon is called micro-spallation.
Spallation is strongly affected by external conditions. Agarwal and Dongare (2016) found that loading orientation, shock pressure, and temperature had significant effects on the wave propagation and failure behavior of single-crystal Mg. Generally, the failure of a material is greatly dependent on its microstructures and latent defects. Galitskiy et al. (2018) reported that the evolution of microstructure played an important role in determining the spall failure of singe-crystal Al. Turley et al. (2018) also highlighted the effects of crystal anisotropy on the spall strength of single-crystal Cu. Ductile metals exhibit a dynamic damage behavior (characterized by void nucleation, growth, and coalescence) under extreme loading. Void nucleation mainly occurs due to the fracture of second-phase particles or the interfacial debonding between particles and the matrix (Thomson and Hancock, 1984). Subsequently, these voids continuously grow under plastic deformation and mutually coalesce with adjacent voids to cause the ultimate ductile fracture. Chen et al. (2019) expressed the correlation between dislocation evolution and spall strength. Therefore, a detailed understanding of the spalling mechanism is necessary to promote the applications of advanced materials in the fields of weapon detonation and high-speed impacts.
Previous studies mainly investigated the spalling behavior of materials through experiments and molecular dynamics (MD) simulations. High-speed imaging techniques, photoelectric sensing technologies, and plasma diagnostic methods are generally employed to characterize dynamic damages during spalling. Moritoh et al. (2003) performed hypervelocity impact experiments on polycarbonate/steel by a light-gas gun and examined the spalling fracture through micro-observations. Generally, spalling occurs transiently under extreme conditions; thus, it is extremely difficult to experimentally reveal its micro-mechanism. Therefore, the basic mechanics of spalling can be better understood from the perspective of modeling.
MD simulations cannot be used to describe such a real behavior due to its inherent characteristic, and computational capabilities limit MD simulations to minor length and time scales. However, MD simulations are widely used to investigate the damage evolution during spallation. MD modeling can overcome experimental observation flaws and reveal the nature of dynamic failure at the atomic level. Furthermore, it provides the distributions of different physical quantities, including velocity, mass density, stress, and temperature. MD simulations have been successfully applied to study the spalling behavior of Pb (Yang et al., 2019a) and Sn (Liao et al., 2014). It is well accepted that during micro-spallation, melting generally occurs under high shock pressures. In comparison to classical spallation, micro-spallation experiences a complex physical mechanism. Liao et al. (2015) studied the micro-spallation behavior of single- and nano-crystal Al under impact loading and noticed that heat dissipation due to void formation dramatically increased the ambient temperature around voids and led to material melting. Consequently, the melting behavior decreased the material strength and facilitated void nucleation, growth, and coalescence (Yang et al., 2019b). Xiang et al. (2016) performed an investigation to differentiate between the classical spallation and micro-spallation of single-crystal Pd and reported that spallation was strongly dependent on the loading rise time of the ramp wave. Li et al. (2017) investigated the mechanism of shock-induced spallation in SiC and used the piston velocity to differentiate between classical spallation form micro-spallation. Melting under shock loading can be divided into two stages: release melting and compression melting. Previous studies have mainly concentrated on shock melting upon release; hence, the melting mechanism during compression is rarely reported. In addition, atomic insights into classical spallation and micro-spallation under shock waves are very scarce. As spalling possesses complex and transient characteristics in nature (Aniszewski et al., 2019), it is necessary to develop an atomistic model to describe such a phenomenon.
In the present work, MD simulations were carried out to investigate the dynamic process of single-crystal aluminum (Al) under different impact velocities. A comparison between the predicted shock pressure and the previously published experimental data was provided to demonstrate the validity of the non-equilibrium molecular dynamics (NEMD) method for characterizing the shock wave propagation phenomenon in the material. The damage evolution in single-crystal Al during spalling was investigated at the atomic scale. Release and compression melting were characterized by the radial distribution function (RDF). The relationship between the free surface velocity and time was explored, and the spall strength and the strain rate were determined by the free surface velocity. The dependence of spall strength on strain rate and temperature was also examined. All the acronyms and their definitions in this work are listed in Table 1.
Section snippets
Computational details
All simulations in the present work were performed in the open-source software Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) developed at Sandia National Laboratories, USA (http://lammps.sandia.gov). The embedded-atom-method (EAM) potential was employed to describe atomic interactions in Al because it was found to be well suited for simulating Al under shock loading conditions by comparing the Hugoniot curve and the melting curve with the experimental data (Yang et al., 2018
Shock wave propagation
Fig. 3 displays the shock wave propagation in single-crystal Al, and the corresponding free surface velocity profile is presented in Fig. 4. In addition, typical atomic configurations under different instantaneous stages are provided to study the spalling process. When the piston impacted the target with a velocity of 1 km/s, a compression wave was generated and moved toward the target free surface. It is noticeable from Fig. 3(a) that the compression wave passed through the target and reached
Conclusions
The shock-induced spallation of single-crystal Al under uniaxial impact loading was investigated through MD simulations. The NEMD method was validated by comparing the calculated and experimental shock pressures. The shock wave propagation analysis revealed that void nucleation, growth, and coalescence were dependent on the tensile stress generated by the interaction between two rarefaction waves. When Vp = 1.0 km/s, partial dislocations and stacking faults occurred under the tensile stress,
Author statement
This work attempts to investigate the shock-induced spallation in single-crystal Al under uniaxial impact through MD simulations. The bright point is to distinguish between classical spallation and micro-spallation. Importantly, the spall strength is found to be dependent upon both the strain rate and temperature. The results highlight the capability of the proposed approach for simulating the spalling behavior.
This is the first submission related to our latest contributions for publications.
Declaration of Competing Interest
We confirm that there is no conflict of interest.
Acknowledgements
The authors gratefully acknowledge the financial support of the Natural Science Foundation Project of CQ CSTC under grant number cstc2018jcyjAX0581, the Fundamental Research Funds for the Central Universities under grant number XDJK2018B002, and the Venture & Innovation Support Program for Chongqing Overseas Returnees under grant number cx2018078.
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