Effects of flaw shape and size on fracture toughness and destructive mechanism inside Ni15Al70Co15 metallic glass
Graphical abstract
Introduction
Metallic glasses (MGs) have received much interest due to their excellent properties such as high elasticity, high strength, and high fracture toughness [1], [2], [3]. However, the macroscopic brittle nature of MGs lead to a part of limited their commercial proliferation [4]. Recently, there are many methods that have successfully improved the ductility of MGs [4]. So, MGs are used in automotive, and electronics applications [5], [6]. On the other hand, the random flaws appearances during the fabrication process or deliberately generated result in a variation in mechanical properties. The effect of flaws on the mechanical properties and MGs can present more plastic deformation with the existence of deformation mechanisms of MGs has been being a major topic because failures probably arrive at these irregularly geometries due to stress concentration or propagation [7]. Many researchers have shown that diversity stress states under either tensile or compressive tests [8], [9], [10]. Sha et al. [11] reported the failure sample changing from shear banding to necking with the increase of the sharp and deep of the symmetric notches. Narayan et al. [12] presented that the strength of the shallow depth notched specimens did not seem to change as decreasing specimen dimensions and rising notch sharpness by 14%.
Moreover, Qu et al. [13] showed that the tensile strength of bulk MGs is insensitive to the notch and the notched MGs enhanced the plastic deformation. However, the notch in specimen causes the local stress concentration, leading to strength weakening as reported by Lei et al. [14]. Besides, a suitable thickness of the sample can help to generate a maxima fracture toughness of BMGs [15]. Pan et al. [16] revealed that the notch leads to the increase in the failure strength and tensile plasticity due to the transition from brittle to ductile fracture in the notched MGs under tension. However, systematic experimental research on the flaw sensitivity of MGs is complicated since it is not easy to control the size and shape of flaws [17]. Therefore, molecular dynamics (MD) simulation is employed to study the systematic and qualitative variation of individual flaw features effectively.
According to Refs. [7], [11], the ratios of geometrical dimensions between the specimens and the notch are constant D1/L = 0.2 and D2/W = 0.4. where L and W are the length and width of specimens; D1 and D2 are the length and width of the notch. According to Ref. [18], these ratios were chosen D1/L = 0.22, then change these ratios in order to evaluate the various stress concentrations. Therefore, we designed the flaw sizes D1/L = 0.22 and D2/W = 0.7. The tensile direction coincides with the direction of L. In order to allow for the examination of the influence of shape and size of flaws on the strain energy, fracture toughness, and brittle to ductile transition mechanisms of Ni15Al70Co15 metallic glass, these ratios D1/L and D2/W were changed.
Al-based amorphous alloys are important engineering materials by comparing to other MGs formers including Zr- and Pd- based and crystalline Al-alloys [19]. The Ni-Co-Al amorphous alloys exhibit high strength and good bending ductility [20], [21]. Hiraga et al. [22] investigate the structural characteristics of Al–Co–Ni decagonal quasicrystals via experimental study. On the other hand, Kbirou et al. [19] studied the local structure of ternary MG Ni15Al70Co15 via MD simulations. However, there have been few researches reporting on the mechanical properties and the deformation behaviors of Ni-Co-Al MGs.
In this study, the effect of flaw shape and size on the fracture toughness and the competition between brittle and ductile of the Ni15Al70Co15 MG are evaluated through the tensile process by using MD simulation. The results are discussed in detail in the following sections.
Section snippets
Interatomic potential and heat treatment process
MD simulations in this study are performed by using the Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [23]. The atomic interactions within Ni-Al, Ni-Co, and Al-Co atomic pairs are described by an embedded atom method (EAM) [24].
A sample consisting of 16,805 Ni atoms, 81,599 Al atoms, and 16,965 Co atoms is prepared to construct a Ni15Al70Co15 MG specimen. The sample is set with periodic boundary conditions (PBCs) in three directions via the heat treatment process, which is
Effect of flaw shape
Fig. 4 presents the tensile stress-strain curves (a) and the energy absorption per unit volume (b) for the monolithic specimen and different flaw shapes specimens. As mention above, a constant strain rate of 108 s−1 along the x-direction, a time step of 2 fs, and a temperature of 100 K are used throughout parts 3.1 and 3.2. For the circle flaw, rhombus flaw, rectangular flaw, and monolithic specimens, the ultimate tension strength (UTS) values are 2.48, 2.28, 2.1, and 3.15 GPa, respectively.
Conclusions
The MD simulation is performed to investigate the effect of flaw shape and flaw size on the strain energy, fracture toughness and BTDT mechanism of MGs. Some important results are listed as follow:
The flaw in the specimen dissipates reserves of energy before a catastrophic deformation occurs. So, the strain energy values of the flawed specimens are significantly smaller than those of the monolithic specimen. The energy stored by a system undergoing deformation shows a clear difference as
CRediT authorship contribution statement
Thi-Xuyen Bui: Formal analysis, Investigation, Software, Writing - original draft, Visualization, Conceptualization, Writing - review & editing. Te-Hua Fang: Data curation, Funding acquisition, Methodology, Project administration. Chun-I Lee: Supervision, Validation.
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
The authors acknowledge the support by Ministry of Science and Technology, Taiwan under grant numbers MOST106-2221-E-992-333-MY3 and MOST106-2221-E-992-343-MY3.
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