Numerical study of pile-up in bulk metallic glass during spherical indentation

Pile-up around indenter is usually observed during instrumented indentation tests on bulk metallic glass. Neglecting the pile-up effect may lead to errors in evaluating hardness, Young's modulus, stress-strain response, etc. Finite element analysis was employed to implement numerical simulation of spherical indentation tests on bulk metallic glass. A new model was proposed to describe the pile-up effect. By using this new model, the contact radius and hardness of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass were obtained under several different indenter loads with pile-up, and the results agree well with the data generated by numerical simulation

Failure behavior and criteria of metallic glasses

Metallic glasses (MGs) constitute an emerging class of advanced structural materials due to their excellent mechanical properties. However, brittle failure at room temperature and the resultant complicated fracture behavior greatly limit their wide engineering applications. Over the past decades, the deformation and fracture in ductile or brittle mode referring to material compositions, load conditions, sample size, etc., have been widely studied, and significant progress has been made in understanding the failure behavior of MGs. Micromechanisms of fracture have been revealed involving shear banding, cavitation and the nature of the crack tip field. The ductile-to-brittle transition and inherent governing parameters have been found. To well describe and predict the failure behavior of MGs, failure criteria for ductile and brittle MGs have been established empirically or based on atomic interactions. In this paper, we provide a detailed review of the above advances and identify outstanding issues in the failure of MGs that need to be further clarified

Probe Embryonic Damage Evolution in Bulk Metallic Glasses under Plate-impact Loading

Microdamage in very short stress durations of spallation process in Zr-based bulk metallic glass (Zr-BMG) samples were captured by a specially designed plate impact technique. With stress durations vary, microdamage “frozen” in Zr-BMG samples exhibited different damage levels. Based on the morphology and stress environment of the microdamage, a compound microdamage evolution mode is applied to characterize the spallation evolution in Zr-BMGs. Especially the spallation in BMGs originates from cavitation instabilities in the weak regions with higher free volume content, which results in formation of ductile damage zones. The activation of shear transformation zones (STZs) or tension transformation zones (TTZs) between these ductile damage zones finally leads to detached spallation

Shear Strength Measurements in LY-12 Aluminium Alloy During Shock Loading

Lateral stress of LY-12 alummium alloy under plate impact shock loading was measured. Based on the measured data, the Hugoniot relation and shear strength were obtained. The result has demonstrated that the shear strenath of the tested material increases remarkably with the increasing longitudinal stress. This means that the assumption of constant shear strength usually adopted in shock stress calculation is not suitable for the present material

Vortex Evolution Behavior in Self-Assembly of Flow Units in Metallic Glasses

Shear banding in amorphous metals originates from the activation and percolation of flow units. To uncover the self-assembly dynamics of flow units in metallic glasses, a rectangular sample with two flow units embedded in the matrix undergoing simple shearing was analyzed using finite element simulations. The vortex evolution behavior, including activation, growth, and collapse during the self-assembly of flow units, was revealed. It was found that the formation of a mature vortex indicates the onset of yielding, and the collapse of the vortex represents the percolation of flow units or shear localization. The effects of initial free volume distribution and the distance between flow units on vortex behavior were also studied. Increasing the initial free volume concentration within flow units or the matrix leads to a gentler vortex evolution process and better homogeneous plasticity. The shape of vortex tends to be "flatter" with the increase in flow units' spacing, and the optimal spacing was found to maximize the strength of the material

Effective Energy Density of Glass Rejuvenation

Glasses with rejuvenated structures usually exhibit improved room-temperature plasticity, which facilitates their applications. However, glass rejuvenation requires external energy injection to "shake up" the frozen-in disordered structure. In this work, we give the answer to how much the required energy is. According to the constitutive model of amorphous plasticity, we find that the applied stress higher than the steady-state flow value can effectively induce the structural disordering in terms of the generation of free volume. Therefore, the effective energy density (EED) of structural rejuvenation is defined as the integral of this effective stress on the corresponding strain. By tailoring the applied strain, strain rate, temperature and initial free volume, different degrees of structural rejuvenation are achieved, which show a generally linear correlation with the defined EED. This work deepens the understanding of glass rejuvenation from an energy perspective

The martensitic transition pathway in steel

The martensitic transformation (MT) lays the foundation for microstructure and performance tailoring of many engineering materials, especially steels, which are with > 1.8 billion tons produced per year the most important material class. The atomic-scale migration path is a long-term challenge for MT during quenching in high-carbon (nitrogen) steels. Here, we provide direct evidence of (1(1) over bar 2) body-centred tetragonal (BCT) twinned martensite in carbon steels by transmission electron microscopy (TEM) investigation, and the increase in tetragonality with the C content matches X-ray diffraction (XRD) results. The specific {1(1) over bar 2}(BCT) twin planes which are related to the elongated c axis provide essential structural details to revisit the migration path of the atoms in MT. Therefore, the face-centred cubic (FCC) to BCT twin to body-centred cubic (BCC) twin transition pathway and its underlying mechanisms are revealed through direct experimental observation and atomistic simulations. Our findings shed new light on the nature of the martensitic transition, thus providing new opportunities for the nanostructural control of metals and alloys. (c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology

Effects of Chemical Short-Range Order and Lattice Distortion on Crack-Tip Behavior of Medium-Entropy Alloy by Atomistic Simulations

It is well demonstrated that the complex chemical fluctuations on high/medium-entropy alloys (H/MEAs) play critical roles in their deformation process, but there are few reports related to the effect of such complex chemical fluctuations on the crack behavior. In this paper, the effects of chemical short-range order (CSRO) and lattice distortion (LD) on the crack-tip behavior of CrCoNi MEAs under mode I loading at room temperature are investigated by carrying out molecular dynamics (MD) simulation, hybrid MD/Monte-Carlo (MC) simulation and the J-integral method. The results reveal that CSRO can improve the J-integral value without significant changes in the localized deformation zone size. On the contrary, LD can lower the J-integral value with an increase in the localized deformation zone size. The energetic analysis shows that CSRO improves the activation energy barrier of Shockley partial dislocation from the crack-tip while LD reduces the activation energy barrier. Our work is a step forward in understanding the effects of CSRO and LD on the crack-tip behavior and deformation mechanisms of CrCoNi MEAs