Effects of flaw shape and size on fracture toughness and destructive mechanism inside Ni₁₅Al₇₀Co₁₅ metallic glass

Highlights

• The UTS changes with the various flaw shapes specimens: circle flaw, rhombus flaw, and rectangular flaw specimens.

• The fracture toughness can be improved by changing the flaw shape.

• A less smooth flaw shape has a lower energy absorption capacity per unit volume.

• The flaw shape decides the SB or STZs formation at the initial period of the plastic deformation.

Abstract

The influence of shape and size of flaws on the strain energy, fracture toughness, and brittle to ductile transition mechanisms of Ni₁₅Al₇₀Co₁₅ metallic glass material are carried out in this paper through a simulation of a tensile test. It is found that a less smooth flaw shape has a lower energy absorption capacity per unit volume because the wrinkle of the flaws spends the energy generated during the testing process. The flaw shapes strongly affect the fracture toughness while the flaw sizes decide the dominance in the competition between brittle and ductile fracture/deformation. The deformation forms from stress concentrations at flaw root/the sharp corner of the flaw and develops to the SBs, then shifts to shear transformation zones (STZs) within the small size. However, in the absence of the SBs within the large size of the flaw, the STZs are formed from the deformation embryos leading to display the essential characteristics of ductile fracture. Especially, for a circle flaw specimen with a large radius, the STZs plays a dominant role, leading to ductile fracture.

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.