Search

The room temperature creep behavior of a newly developed Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 high entropy bulk metallic glass was investigated by nanoindentation measurements with various loading rates in comparison with that of a typical conventional Vitreloy 1 bulk metallic glass. A generalized Kelvin model was adopted to describe the creep curves. It was found that for both the two studied alloys, the creep displacement and strain rate sensitivity increases with increasing loading rate, indicating a positive loading rate effect. Vitreloy 1 BMG exhibits a more obvious loading rate effect compared with Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 HE-BMG. The creep compliance and retardation spectra were also derived to study the creep mechanism. Compared with other developed high entropy alloys and bulk metallic glasses (e.g. Vitreloy 1 alloy), the studied HE-BMG possesses relatively small strain rate sensitivity values, indicating a good creep resistance. Based on the experimental results, the mechanisms of the unique creep resistance of Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 high entropy bulk metallic glass have also been explained. [ABSTRACT FROM AUTHOR]

The effects of sample size and grain size on the surface morphology and flow stress of deformed samples were investigated by means of copper micro-cylinder compression experiments at room temperature. The results of SEM showed that when the grain size increased or the sample size decreased, the deformation non-uniformity of samples' free surfaces increased. Meanwhile, the stress–strain curves showed that during the compression process, the flow stress of the sample also tended to decrease as the grain size increased or the sample size decreased. According to the experimental results of nanoindentation, a surface transition layer model was established on the basis of the surface layer model by considering the mutual constraint of grains and the existence of transition layer grains. The experimental results indicated that the stress–strain curve calculated by the surface transition layer model can more accurately reflect the actual deformation situation of the material compared to the surface layer model. [ABSTRACT FROM AUTHOR]

Here, we present nanoindentation on single-crystal copper via large-scale molecular dynamics (MD) simulation to provide clarification on load drop and the correlated hardness drop during plastic deformation of a workpiece placed beneath an indenter. According to the clear criterion of contact between the atoms in the workpiece and in the indenter, the contact area between the indenter and workpiece is calculated, and the general hardness and the spatiotemporal distribution of the dislocations around the indenter are also obtained. In the elastic stage, MD simulation results are in good agreement with the Hertzian solution. During the indentation, dislocations are dominated by the glissile Shockley partial dislocations (SPDs), while the proportion of other sessile dislocations is low and grows sluggishly. The density of the dislocations that aggregated around the indenter remained at approximately 1017m-2 during the indentation process. Dislocations adhered to the indenter concentrate in the volume of the hemisphere whose radius is three times of the radius of plastic zone. Load drop takes place after hardness drop which marks the abrupt and massive growth of the glissile SPDs. The more and more dense state of atoms around the indenter caused by dislocation behaviors result in the increase of hardness, while the avalanche of glissile SPDs leads to the following drop, which occurs repeatedly and causes a sawtooth pattern of the load-depth and hardness-depth curves over the entire indentation process. [ABSTRACT FROM AUTHOR]

A novel mechanism without involvement of vacancy or Frank loop is put forward to cast light on the formation of stacking fault tetrahedron (SFT) during plastic deformation of bulk single-crystal Cu with pre-existing parallel coherent twin boundaries (CTBs) via large scale molecular dynamics (MD) simulation. The fresh mechanism is totally different with Silcox-Hirsch mechanism and its sequelae, which supports that to produce an SFT during plastic deformation, formation of Frank loop is not essential in metals with low stacking fault energy (SFE). [ABSTRACT FROM AUTHOR]

Despite the intense interest in coherent twin boundary (CTB) of nanotwinned materials, many details of interactions between the complex dislocation (prismatic dislocation loop (PDL)) and CTB in a single crystal remain mysteries. In this study, molecular dynamics (MD) simulation of the nanoindentation on single crystal copper with CTBs has been presented. At the initial stage of nanoindentation, indenter moves downward and PDLs form. Thompson's tetrahedron notation has been adopted to scrutinize the process of PDL's formation without cross-slip and release from the indented place. Dislocations at PDLs' side edge are stair-rod partial dislocations, including two Lomer-Cottrell locks and two Hirth Locks. Then the interactions between PDL and CTB drive the migration of CTB. As a result, a parallelogram step which is bounded by Frank partial dislocations has been left, implying the formation of twinning dislocations. When the indenter moves further, the interactions repeat and twinning dislocations multiply. Our simulation results shed light on the interactions between PDL and CTB and would provide a new insight into understanding the role CTB plays during mechanical deformation. [ABSTRACT FROM AUTHOR]

The effect of alloying elements (e.g. Fe, Al, and Ni) on the room temperature creep behavior of a lightweight Ti 41 Zr 25 Be 34 bulk metallic glass (BMG) was investigated via nanoindentation tests. The generalized Kelvin model was adopted to describe the creep curves. The strain rate sensitivity m has been derived as a measure of the creep resistance. The compliance spectrum and retardation spectrum were also derived. The results show that the creep resistance of Ti 41 Zr 25 Be 34 alloy can be obviously improved with the addition of alloying elements, and the most effective element is found to be Al. The mechanism for enhancing the creep resistance was discussed in terms of the scale variation of the shear transformation zone induced by alloying. [ABSTRACT FROM AUTHOR]

The room temperature creep behavior of newly developed Ti 20 Zr 20 Hf 20 Be 20 Cu 20 and Ti 20 Zr 20 Hf 20 Be 20 Cu 20 Ni 10 high entropy bulk metallic glasses (HE-BMGs) was investigated using nanoindentation technique under different loading rates. A generalized Kelvin model was applied to describe the experimental creep curves. The strain rate sensitivity parameter, creep compliance and retardation spectra were derived to study the creep mechanism. The experimental results show that the studied HE-BMGs possess good creep resistance. Moreover, it has been found that the addition of Ni effectively enhances the creep resistance of Ti 20 Zr 20 Hf 20 Be 20 Cu 20 HE-BMG and the related mechanism has also been discussed. [ABSTRACT FROM AUTHOR]

This work investigated the corrosion behavior of the equiatomic CoCrFeMnNi high entropy alloy (HEA) in a 0.5 M H 2 SO 4 solution. The grain size of the HEA was adjusted by annealing. The electrochemical analysis showed that the corrosion resistance of the CoCrFeMnNi HEA increases first and then decreases with the increase of grain size. When the grain size is lower than a certain value, increasing the grain size can improve the corrosion resistance. Fine-grained (≤1.24 μm) HEA forms abundant grain boundaries, which promotes the occurrence of galvanic corrosion at the grain boundaries with inter-grains, resulting in reduced corrosion resistance. However, coarse-grained (≥145.9 μm) HEA provides too few ionic diffusion channels to form a stable and continuous passive film, which also reduces the corrosion resistance. Nanoindentation and XPS tests showed that a high content of Fe (Ⅲ) oxides and Cr (Ⅲ) oxides are formed on the medium-grained HEA, which improves the chemical stability and mechanical strength of the passive films. Thus, the corrosion resistance of medium-grained HEA is much better than those of fine-grained and coarse-grained HEA. Image 1 • Corrosion behaviors of CoCrFeMnNi high entropy alloys with different grain sizes were investigated. • Galvanic corrosion at grain boundaries will reduce the global corrosion resistance. • Continuous passive films cannot be formed on coarse-grained HEA. • Combination of reduced grain boundary density and continuous passive film can improve corrosion resistance. [ABSTRACT FROM AUTHOR]

In this study, FeCoP nanoglass films were prepared via electrodeposition on the copper substrate. The microstructures, chemical compositions and mechanical properties of deposited films were characterized via XRD, SEM, and nanoindentation, respectively. The results revealed that all films present fully amorphous structures, without any crystalline phases detected. The films have a hierarchical structure, with nano-sized amorphous particles densely packing in micro-sized grains separated by distinct grain boundaries. With the increase of deposition voltage, the grain size increases, the surface roughness reduces, and the concentration of iron rises. These films present favorable mechanical properties, reaching a maximum hardness of 6.89 GPa and elastic modulus of 138.85 GPa at the deposition voltage of 1.1 V. Unlabelled Image • FeCoP nanoglass films are prepared via electrodeposition. • The deposited nanoglass has a hierarchical structure. • FeCoP nanoglass has superior hardness and elastic modulus. • Increasing deposition voltage increases grain size of nanoglass. [ABSTRACT FROM AUTHOR]