The effect of grain boundary structures on crack nucleation in nickel nanolaminated structure: A molecular dynamics study

Highlights

• Grain boundaries structures in nickel nanolaminated structure.

• Dependence of tensile properties on textures.

• Different crack nucleation mechanisms for low-angle and high-angle grain boundaries.

Abstract

Molecular dynamics (MD) simulations were performed to explore the structures, energies and tensile properties of grain boundaries (GBs) as well as crack nucleation in nickel nanolaminated (Ni NL) structures. Four typical textures observed in experiments (Liu et al., 2013) [6] were considered in this paper. Results showed that low-angle GBs consist of a series of periodically arranged dislocations while high-angle GBs are composed of disordered phase for all textures. GB energies increase linearly with increasing misorientation in the range of low-angle GBs (0–10°). Then, the growth rate of energies slows down and finally stabilizes when the misorientation angle reaches 30°. Among four textures, {1 1 1} 〈1 1 0〉 texture has the highest GB energy while {1 1 0} 〈1 1 1〉 texture the lowest one. The tensile properties of NL structures with different textures and misorientations were further investigated. Results showed that the tensile properties depend primarily on textures, while weakly on GB energies and dislocation densities. {1 1 1} 〈1 1 2〉 texture possesses the best combination of high-strength and ductility among the four textures. Furthermore, the evolution of dislocation density and structure as well as the nucleation of crack were analysed. Crack nucleates at the junction between Shockley partial dislocations and twin boundaries generated during deformation for low-angle GBs, while at the location where Shockley partial dislocations and the original disordered GBs intersect for high-angle GBs. Our results provide a fundamental understanding of GB structures and deformation mechanisms of Ni NL structures.

Introduction

Due to the crucial role of microstructures on the properties of materials, many studies have been carried out to explore the optimization of material microstructure. Nanocrystalline (NC) metals usually have ultra-high strength compared to the coarse-grained (CG) counterparts [1], [2], [3]. However, the ultra-high strength of NC metals is achieved at the expense of their ductility, which greatly limits its practical application. Recently, materials with heterogeneous nanostructures, such as gradient structure [4], [5], lamella structure [6], [7], [8], [9], bimodal structure [10], [11], [12] and harmonic structure [13] have attracted growing attention with their great balance of strength and ductility [14], [15], [16], [17], [18], [19], [20], [21]. Among them, lamella structure exhibits an ultrahigh hardness, excellent thermal stability and superior tensile mechanical behaviour [22].

Therefore over the past few decades, tremendous efforts have been dedicated to the fabrication and mechanical characterization of metallic nanolaminated (NL) structures [6], [8], [23]. Ni NL structures have been fabricated via surface mechanical grinding treatment (SMGT) [6], [7] with an average lamellar thickness of 20 nm, which exhibits a hardness of 6.4 GPa higher than any reported ultrafine grain (UFG) Ni [6]. Besides Ni, NL structures are also found in other metal materials, such as aluminium [24] and titanium [22], which also exhibit excellent combinations of strength and ductility. In addition to the experiments, crystal plasticity finite element (CPFE) method has been carried out to study the fundamental principles of the simultaneous enhanced strength and high ductility in heterogeneous lamella structures at the meso-scale [25]. To further study the mechanism at nano-scale, molecular dynamics (MD) simulation is also conducted to find out the critical lamellar thickness and the underlying atomistic deformation mechanisms [26], the interaction mechanisms between incident dislocations and low-angle grain boundaries (LAGBs) [27] and the grain boundary (GB) structures, thermal stability and size effects of NL structures [28].

However, the plastic deformation and fracture mechanisms of NL structures are still unclear and need to be further studied. Furthermore, no research on crack nucleation behavior of Ni NL structures, a crucial issue affecting the integrity and safety of a structure, could be found. Crack nucleation usually starts from local stress concentration regions caused by nano-defects like dislocations [29], [30], GBs [29], [31] and second phase inclusions [31], [32], which is very difficult to be observed in the experiments due to the extreme short time and spatial scale. Compared with experiments, MD simulations, capable of capturing the atomistic information (millions of atoms) for short periods (several micro-seconds), have been widely used to investigate the atomistic deformation mechanism [33], microstructures [34] and dynamical fracture process [35] of NC structures.

Therefore, in this work, a series of large-scale MD simulations were performed on Ni NL structures. Detailed micro analyses were carried out to characterize the microstructures and dislocation arrangements of GBs, the aim of which is to reveal the underlying deformation and crack nucleation mechanisms. The paper is organized as follows. In Section 2, the computational setup and MD method are introduced. In Section 3, simulation results of GB structures and tensile properties with different textures and misorientations are presented, and GB energies and dislocation densities are further analysed. In Section 4, the deformation mechanism and crack nucleation in Ni NL structures are discussed. Section 5 concludes the paper.