Elsevier

Mechanics of Materials

Volume 161, October 2021, 104004
Mechanics of Materials

Research paper
Spalling modes and mechanisms of shocked nanocrystalline NiTi at different loadings and temperatures

https://doi.org/10.1016/j.mechmat.2021.104004Get rights and content

Highlights

  • NEMDs are used to explore the spalling modes and physical mechanisms of shocked nc-NiTi.

  • Microstructure evolution of different spallation modes is taking into the thermodynamic path.

  • Effect of different initial ambient temperature on spalling mode and spall strength is considered.

Abstract

Spalling damage is one of the typical forms of material dynamic failure, of which the damage modes and physical mechanisms are directly related to the design of material impact protection. NiTi alloys have already been used in some dynamic extreme environments. However, its dynamic failure behavior and mechanism under high pressure and high strain rate are still unclear. In this study, nonequilibrium molecular dynamics simulations (NEMD) were used to explore the spalling modes and physical mechanisms of shocked nanocrystalline NiTi (nc-NiTi) at different loadings and temperatures. The results show that its spalling modes include classical-spall without melting state, multi-spall with partial-melting or partial-disorder states and micro-spall with complete melting or disorder states according to the thermodynamic states and melting characteristics of the spall region. It was found that the multi-spall or micro-spall depends on the homogeneity of the main Voronoi polyhedrons (VPs) of the short-range order (SRO) under different shock stresses, and that the spall strength at T0=1000 K decreases more regularly than that of T0=300 K with the increase of shock loading velocity Up. Moreover, increasing the initial ambient temperature T0 from 300 K to 1000 K not only reduced the spall strength of nc-NiTi, but also reduced the critical stress required for different spalling modes. This is attributed to the temperature softening effect occupying a dominant position during the spalling, including the grain boundary diffusion and the change of the thermodynamic paths due to the increase of initial ambient temperature T0.

Introduction

Due to its properties of superelasticity and shape memory effect, the near-equiatomic NiTi alloys have become the most widely used shape memory alloys (SMAs) (Yang et al., 2020), involving engineering machinery, biomedical instruments, aerospace, energy development and other fields (Chowdhury and Sehitoglu, 2017, Nnamchi et al., 2019). In particular NiTi has also been applied to the key components in actuators that adapt lifting bodies and morph the wing structures (Brailovski et al., 2010), the self-deploying solar sail (Costanza and Tata, 2018), and satellites and space vehicles, which suffer some impact things such as birds, dust particles and space debris/meteorites to produce dynamic damage at high pressure and strain rate. Spalling is one of the typical dynamic failure phenomena, and the corresponding spall strength is often used to describe the mechanical properties of metal materials under dynamic loads. To better research and design protection engineering, it is necessary to study the spalling characteristics of NiTi alloys at high pressures and strain rates (e.g., spall strength, spalling modes and related mechanisms).

Millett and Bourne (2004) and Meziere et al., 2006a, Meziere et al., 2006b measured the Hugoniot relationship and spall strength of NiTi by one-dimensional strain loading method on the light gas gun. Due to the lack of real-time observation technology and the difficulty of sample recovery at high pressure, the shock stress is limited to less than 20 GPa. The metallographic analysis of the NiTi incipient spall plane shows that the spall damage of NiTi is a mixed failure mode with ductile void growth followed by transgranular failure (Miller et al., 2000). At present, only limited experimental characterization methods can be used to obtain partial time and space microstructure evolution information under dynamic loadings, which shows the final damage state of experimental samples (Moshe et al., 1996, Collins et al., 2001). Nonequilibrium molecular dynamics (NEMD) simulation is considered to be an important tool and good method for studying the microstructure evolution and failure mechanism under dynamic loading, e.g., Cu, Ti, Al (Luo et al., 2009a, Guan et al., 2020, Wang et al., 2019, Long et al., 2020). However, few researches are focused on the damage modes and failure mechanisms of NiTi under dynamic loadings. Yin et al. (2017) used the Finnis–Sinclair type potential proposed by Lai and Liu (2000) and improved by Zhong et al. (2011) to study the shock behavior of single crystalline nano-pillar NiTi at different initial ambient temperatures, and its deformation mode changes from phase transition and twinning to dislocation as the initial temperature increases. Yazdandoost and Mirzaeifar (2018) and Yazdandoost et al. (2019) studied the role of phase transformation and plastic deformation in energy dissipation of local impact loading through experiments and molecular dynamics simulations, and they affirm that shape memory alloys are superior candidates for dissipating the energy under shock loading. Wang et al. used the second-nearest neighbor modified embedded-atom method (2NN MEAM) (Ko et al., 2015) to simulate the phase microstructure evolution of NiTi under dynamic compression and tension, but they did not make further studies on the spall-related spall strength and cavitation process (Wang et al., 2018).

Generally, it will be accompanied by a significant temperature rise when the shock pressure on solid materials is above tens of gigapascals (GPa). The study of material properties under shock compression should consider both mechanical effects and thermal effects (Graham, 1993, Kanel, 2010). The obvious temperature rise generated by shock compression will affect the subsequent spalling process, and the spalling criterion of dynamic failure considering the temperature effect will be more complicated (Xiang et al., 2021). According to whether melting occurs, Andrio et al. divided metal spalling into classical-spalling and micro-spalling (Andriot et al., 1984). The micro-spalling has been systematically understood in Sn (Xin et al., 2014, He et al., 2014), Pb (Xiang et al., 2013a, Xiang et al., 2013b, Shao et al., 2019) and other low-melting point metals by means of experiments and simulations. For the high melting point NiTi alloy, there are few studies on its dynamic failure modes and mechanisms. Exploring the evolution process of NiTi spalling damage at dynamic high pressure and finding out the connection between microscopic features and macroscopic experiments can help to understand the properties of NiTi at micro-scale, and provide reference data for establish accurate models at meso-scale simulations.

In this work, Nonequilibrium molecular dynamics (NE-MD) simulations of nanocrystalline NiTi (nc-NiTi) under shock loading are carried out to study the characteristics of different NiTi spalling modes (such as density, velocity, microstructure, etc.). Furthermore, the relationship between shock loading velocity Up and different spalling modes is given semi-quantitatively by analyzing the thermodynamic path, the results of the radial distribution function (RDF) analysis and the Voronoi tessellation (VT) method at the corresponding spall region, and the effect of initial ambient temperature T0 on spalling of NiTi is studied. In addition, the spall strength at different shock loading velocities are calculated by the acoustic approximation method and binning analysis method, respectively, and we attribute the effect of the initial ambient temperature T0 on the spall strength of nc-NiTi to the effect of T0 on the thermodynamic paths and the grain boundary diffusion rate in the spall region.

Section snippets

Model and MD setting

The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software package (Plimpton, 1995) is used to conduct the non-equilibrium molecular dynamics simulations (NEMD) in this paper, and all the examples are carried out under the Finnis–Sinclair type potential proposed by Lai and Liu (2000) and improved by Zhong et al. (2011). This potential function has been widely used to study the mechanical-microstructure response under general conditions, including static tension,

Shock-induced spall strength of NiTi

Shock wave is generated by the piston shock method in our simulations (Luo et al., 2009b), the piston moves along the x-axis (shock direction) at a constant speed for 15.0 ps and then it is removed to produce a “square wave”, i.e., the incident unloading rarefaction wave is the shock wave front reaches the free surface on the right side of the sample [Fig. 1(a)]. Then the part of incident compression wave is reflected on the free surface and to be unloaded, leading the particle velocity almost

Conclusions

The spalling of nanocrystalline NiTi (nc-NiTi) is studied by NEMD simulations, including the spall strength, the characteristics of different spalling modes and the damage mechanism of different spalling modes. Furthermore, the effect of initial ambient temperature T0 on spalling of NiTi is studied. The main conclusions are as follows:

(1) The spall strength is calculated by two method: binning analysis (σspb) and acoustic approximation methods (σspc). The overall trend of the spall strength

CRediT authorship contribution statement

Chao Lv: Investigation, Formal analysis, Writing - original draft, Discussion of the results. Guiji Wang: Co-analyze the results, Co-summarize the conclusions, Writing - review & editing, Conceptualization, Supervision, Project administration, Funding acquisition, Discussion of the results. Xuping Zhang: Writing - review & editing, Discussion of the results. Bingqiang Luo: Writing - review & editing, Discussion of the results. Ning Luo: Writing - review & editing, Discussion of the results.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was partially supported by the project of the National Natural Science Foundation of China (Grant No. 11972031 and No. 12002327). We are grateful to the Advanced Analysis and Computation Center of CUMT for the award of CPU hours to accomplish this work.

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