As-cast Metallic Glass Research Articles

Surface quality and mechanical property evolution of laser nitrided Zr-based metallic glass induced by laser shock flattening

Laser nitriding is an effective modification technique to improve the comprehensive mechanical properties of metallic glasses (MGs). However, numerous micro-defects, such as pores and humps, usually appear on the laser-nitriding-treated (LNed) MG surface, and therefore additional post-treatment is required. Herein, laser shock processing was implemented to flatten the LNed MG and as well modify its surface mechanical property. The dependence of the laser shock flattening process on the laser energy density was investigated. The results revealed that the surface micro-defects on the LNed MG were significantly reduced after laser shock flattening, and accordingly, the surface roughness (Sa) was decreased, with a maximum reduction of 61.87% (from 0.48 to 0.183 μm) achieved at a relatively high laser energy density of 178.3 J/cm2. Furthermore, nanoindentation measurements indicated that although the surface hardness of the LNed MG was decreased after laser shock flattening, it was still higher than that of the as-cast MG.

Effect of laser shock peening on the plasticity of Zr-based metallic glass under compression

The surface of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG was modified in this study using LSP to enhance its compressive mechanical properties.The microstructures and thermal properties of the as-cast and laser-peened specimens were characterized using X-ray diffraction and differential scanning calorimetry, respectively. The results indicated that crystallization did not occur in the laser-peened area. Compared to the as-cast metallic glass, the LSP-treated specimen had a higher free volume. The LSP-treated specimen, as compared to the as-cast specimen with a slight plastic strain of 8.1 % prior to final fracture, exhibited an improved compressive plastic strain of 18.8 %. Based on results of the hardness tests, a significant change in microhardness was observed in the specimen after the LSP treatment. The improved plasticity and decreased hardness of the laser-peened specimen were due to the excess free volume in the laser treated surface zone. Scanning electron microscopy observations of the fracture surfaces revealed that the LSP-treated specimen formed multiple shear bands and highly dense vein-like patterns. This study provides a novel method for increasing the plasticity of metallic glasses to promote their industrial application.

Improved wear resistance of metallic glacier glass

Metallic glasses (MGs) have garnered significant attention due to their excellent properties, like high wear resistance, making them highly promising for a wide range of engineering applications. However, it is of significance to further increase the wear resistance of MGs, which has rarely been studied. Here we present a remarkable enhancement of wear resistance when some MGs undergo a liquid-liquid transition to form a new metallic glacial glass (MGG) state. Within an identical composition of La65Ni20Al15, the wear resistance of the MGG exhibits a notable superiority of 104% compared to the as-cast MG, and an even more remarkable advantage of 349% compared to its crystalline counterpart. The excellent wear resistance of the MGG is attributed to its ultrastable nature in energy confirmed by dynamic relaxation changes. This work not only offers a unique perspective on MGGs, but also provides an effective technique for developing wear-resistant materials.

Effects of He ion irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses

The effects of He2+ irradiation on the structural stability and deformation behavior in Zr63.5Cu23Al9Fe4.5 and Fe80B13Si7 metallic glasses (MGs) were investigated by TEM and nano-indenter. He bubbles in Fe80B13Si7 MG distributed more narrowly along the depth direction, packed more densely and exhibited larger size. At 4 × 1017/cm2 dose, Fe3B nanocrystals generated in Fe80B13Si7 MG caused by enhanced atomic diffusion due to increased excess free volume and larger compressive stress on atoms of bubbles gaps, while Zr63.5Cu23Al9Fe4.5 MG could maintain the amorphous structure. According to serrated flow phenomenon and bubbles distribution, the underlying mechanism of the transformation in deformation behavior of MGs with increasing He dose was analyzed as follows: localized inhomogeneous deformation in as-cast MGs, homogeneous deformation in the rejuvenated stage, localized inhomogeneous deformation in the stage where small bubbles act as weak pinning, homogeneous deformation in the stage where large bubbles act as strong pinning, localized fracture in the stage where bubbles collapse.

Effects of rejuvenation modes on the microstructures and mechanical properties of metallic glasses

This study systematically investigates the effects of three rejuvenation modes, namely biaxial loading, thermal pressure loading, and cryogenic thermal cycling, on the microstructures and mechanical properties of Cu64Zr36 metallic glasses (MGs) using molecular dynamics (MD) simulations. The simulation results demonstrate that biaxial tension application during loading results in a rejuvenated glassy state with high strength. Specifically, we find that the yield stress, Young's modulus, and shear modulus of the rejuvenated glass obtained by biaxial tensile loading decreased by 2.8%, 2.1%, and 2.7%, respectively, compared to an as-cast MG reference. We also find that MGs achieve the greatest degree of rejuvenation, at the expense of strength, with the use of thermal pressure loading. Interestingly, the application of cryogenic thermal cycling results in a rejuvenated state with a lower strength drop compared to thermal pressure loading. Furthermore, analysis of the potential energy and medium-range-order (MRO) size indicates that the microstructures of MGs achieves a high-energy ordered glass state after thermal pressure loading, but a high-energy disordered glass state after biaxial tensile loading and cryogenic thermal cycling. These findings provide meaningful insights into the effect of rejuvenation modes on the microstructures and mechanical properties of MGs, as well as useful guidelines for designing MGs with high strength and superior plasticity for prospective applications in industry.

Influence of the Hydrogen Doping Method on the Atomic Structure, Mechanical Properties and Relaxation Behaviors of Metallic Glasses

The interaction of metallic glasses (MGs) with hydrogen can trigger many interesting physical, chemical and mechanical phenomena. However, atomic-scale understanding is still lacking. In this work, molecular dynamics (MD) simulations are employed to study the atomic structure, mechanical properties and relaxation behaviors of H-doped Ni50Al50 MGs doped by two methods. The properties of H-doped MGs are determined not only by the hydrogen content but also by the doping method. When H atoms are doped into the molten state of samples, H atoms can fully diffuse and interact with metallic atoms, resulting in loose local atomic structures, homogeneous deformation and enhanced β relaxation. In contrast, when H atoms are doped into as-cast MGs, the H content is crucial in affecting the atomic structure and mechanical properties. A small number of H atoms has little influence on the elastic matrix, while the percolation of shear transformation zones (STZs) is hindered by H atoms, resulting in the delay of shear band (SB) formation and an insignificant change in the strength. However, a large number of H atoms can make the elastic matrix loose, leading to the decrease in strength and the transition of the deformation mode from SB to homogeneous deformation. The H effects on the elastic matrix and flow units are also applied to the dynamic relaxation. The deformability of H-doped Ni50Al50 MGs is enhanced by both H-doping methods; however, our results reveal that the mechanisms are different.

The scratch characteristics of laser nitrided Zr-based metallic glass surface

Improving the surface hardness and tribological characteristics of metallic glasses (MGs) is meaningful for their applications as surface contact materials. In this study, nanosecond laser nitriding was performed on Zr-based MG to improve its surface properties. Three types of laser nitrided surfaces with different hardness were prepared, and their tribological characteristics were systematically investigated by means of the scratch test. The experimental results indicated that compared with the as-cast MG, the hardness of these three laser nitrided surfaces was increased by 47.94%, 114.56%, and 171.99%, and their coefficients of friction were reduced by 29.12%, 65.05%, and 71.84%, respectively. Furthermore, the residual scratches and surrounding chips exhibited various features under different conditions, and the scratch damage mechanisms were analyzed accordingly.

Rejuvenation by enthalpy relaxation in metallic glasses

Structural relaxation can be induced by annealing and usually leads to embrittlement in metallic glasses (MGs). Here, we show that a relaxed MG obtained by annealing an as-cast MG at a temperature (annealing temperature, Ta) below its fictive temperature can be suitably rejuvenated by a thermal treatment of the relaxed MG at a temperature higher than Ta. The effect is driven by enthalpy relaxation, an endothermic reaction upon heating relaxed MG. This rejuvenation lifts the energy state of the relaxed MG towards that of the as-cast MG. Importantly, we found that the plasticity and fracture toughness of the optimally rejuvenated samples exceed those in the as-cast sample, reversing the relaxation-induced embrittlement in the relaxed sample. An analysis of the structural changes shows that the packing density, rather than the ordering of the atomic structure, is responsible for the effects of rejuvenation. The observed rejuvenation may originate from the recovery of loosely packed regions in the MG atomic structure that were densified during annealing. Our findings are significant for tailoring the mechanical properties of MGs.

Surface hardening of Zr-based metallic glass via laser surface alloying with silicon powder

Increasing the hardness and wear resistance of metallic glasses (MGs) is beneficial to prolong their service life in highly aggressive environments. For the purpose of achieving surface hardening, in this work, laser surface alloying technology was employed to modify Zr-based MG with silicon powder as the pre-placed powder. The experimental results showed that the laser alloyed layer with thickness of several tens of micrometers was generated on the MG substrate. The surface hardness of the laser alloyed layers ranged from 11.34 to 17.97 GPa, which was significantly higher than that of the as-cast MG surface (6.36 GPa). Furthermore, the scratch resistance of the laser alloyed layer was improved compared to the as-cast sample. The chemical composition analysis indicated that the laser alloyed layer was mainly composed of in-situ formed ZrSi2 phase and residual Si, and these hard phases were responsible for the high hardness of the laser alloyed layer.

Effects of structure relaxation and surface oxidation on nanoscopic wear behaviors of metallic glass

Both structure relaxation and oxidation of surface layer induced by thermal treatment or friction heat play significant roles in reducing wear of metallic glasses (MGs). To distinguish the effects of structure relaxation and surface oxidation, we prepared Zr-based MGs at different states, i.e., as-cast MG; the annealed MGs under argon atmosphere (referred to as structure-relaxed MG) and oxygen atmosphere (referred to as oxidized MG). Nano-wear tests were carried out by atomic force microscope (AFM) to quantitatively clarify these two effects on the wear performance. Compared with the as-cast MG, the structure-relaxed MG shows an increased adhesive but slightly decreased ploughing friction. While, the oxidized MG shows the best anti-wear performance with dramatic reductions in both adhesive and ploughing friction. Classic molecular dynamics (MD) and ab initio MD simulations revealed that the introduction of O hinders the MG atoms from forming interfacial bonds with diamond, and reduces significantly the adhesion between the diamond tip and passivated MG. In addition, the synergistic effect of high hardness and elastic recovery associated with oxidization reduces ploughing friction and enhances wear resistance. This finding clarifies the importance of surface chemistry over structure modification during wear, and offers a generic pathway for tailoring ultra-wear resistant MGs.

Laser nitriding of Zr-based metallic glass: An investigation by orthogonal experiments

Laser nitriding is a useful method to improve the surface hardness of materials, and it has been newly introduced to harden the surface of Zr-based metallic glass (MG). However, the currently achieved improvement in surface hardness is only 15.7%. To explore the potential of laser nitriding of Zr-based MG, orthogonal experiments were designed and performed in this study. By nanosecond pulsed laser irradiation in nitrogen atmosphere, the effects of laser parameters (average laser power, scanning speed, overlap rate and number of irradiation cycle) on the surface hardness of Zr-based MG were systematically investigated. The experimental results showed the highest surface hardness reached 18.21 GPa (Meyer's hardness) by orthogonal design, which is about 2 times higher than that of the as-cast MG. Furthermore, the laser affecting layer consisting of nitride phase reached tens of microns. The obtained results demonstrated that significant improvement in surface hardness could be achieved by selecting appropriate laser irradiation parameters, which would be beneficial to the engineering applications of MGs as surface contact materials. • The potential of laser nitriding on improving the hardness of Zr-based MG was investigated by orthogonal experiments. • The achieved highest hardness was almost three times of the hardness of the as-cast MG surface. • The effects of laser parameters on the surface hardness were comparatively studied. • The characteristics of shear bands and serrated flows of the laser nitrided MG surfaces were changed.

Reduced strain rate sensitivity by structural rejuvenation in metallic glass under nanoindentation

Through tailoring the energy state of a Zr-based metallic glass (MG) by constrained compression, we revealed that the strain rate sensitivity of MGs strongly depends on the energy state. For the as-cast MG with low energy state, hardness increases with the increasing strain rate, showing a positive m value of 0.0132. However, the hardness is independent on the strain rate in the highly rejuvenated MG with a high energy state. These observations can be well understood from free volume model.

Contribution of cryogenic thermal cycling to the atomic dynamics in a La-based bulk metallic glass with different initial states

Structural rejuvenation in metallic glasses (MGs) induced by cryogenic thermal cycling has been intensively studied. However, the effect of thermal cycling on the atomic dynamics in MGs is still missing. In this work, we present a systematic study on the atomic dynamics in a La-based MG affected by sub-Tg annealing and thermal cycling. We find that the thermal cycling has little effect on the atomic dynamics in both as-cast and annealed MGs although it could significantly increase the free volume and change mechanical properties of the as-cast MG sample. In contrast, both the structure and mechanical properties of the well-relaxed sample are almost unchanged by thermal cycling. The increased open volume, mainly in the form of atomic vacancies, does not greatly promote the atomic dynamics in the as-cast MG sample, indicating that the free volume induced by thermal cycling is not the key factor in stimulating the β-relaxation and the boson peak in the studied MG.

Oxidation feature and diffusion mechanism of Zr-based metallic glasses near the glass transition point

The oxidation behaviors of as-cast, pre-deformed, and crystallized Zr47.9Ti0.3Ni3.1Cu39.3Al9.4 metallic glasses (MGs) were studied near the glass transition point. The oxidation kinetics of the crystallized MGs followed a parabolic-rate law, and the as-cast and pre-deformed MGs exerted a typical two-stage behavior above the glass transition temperature (Tg). Most interesting, pre-deformed treatment can significantly improve the oxidation rate of MGs, as the initial oxidation appeared earlier than for the as-cast MGs, and was accompanied by much thicker oxide scale. The EDS and XPS results showed that the metal Al acted as the preferred scavenger that absorbed intrinsic oxygen in the near-surface region of as-cast MGs. However, a homogeneous mixed layer without Al was observed in the pre-deformed MGs. We speculated the accelerated diffusion of other elements in the MGs was due to the local increase in the free volume and significant shear-induced dilation of the local structure. The results from this study demonstrate that MGs exhibit controllable atomic diffusion during the oxidation process, which can facilitate use in super-cooled liquid region applications.

Effect of residual stress on azo dye degradation capability of Fe-based metallic glass

Fe-based metallic glasses (MGs) are among the most interesting in the area of MG materials, due to their superior properties such as high degradation capability of azo dyes. However, the ways that factors influence the degradation capability of MGs towards azo dyes are still unclear. From this reason, the effect of residual stress on degradation capability of Fe-based MG was investigated. (Fe73.5Si13.5B9Nb3Cu1)91.5Ni8.5 as-cast MG ribbons and annealed MG ribbons were prepared and subjected to degrade Orange II solutions. The as-cast ribbons completely degrade the azo dye within 60min, the ribbons annealed at 673K take 120min, although the annealed ribbons still sustain metastable glassy state. Surface morphology and magnetic domain pattern of the ribbons were characterized carefully. The results demonstrate that the inhomogeneous residual stress could contribute to high degradation capability of the as-cast MG ribbons by generating stress gradient cell and increasing active sites. Therefore, increasing residual stress can be a novel clue to enhance degradation capability of materials.

Microstructural changes of Zr-based metallic glass during micro-electrical discharge machining and grinding by a sintered diamond tool

Micro-electrical discharge machining (micro-EDM) and grinding of Zr-based metallic glass (MG) were performed by using sintered polycrystalline diamond (PCD) as a hybrid tool. The microstructural changes of the workpiece surface layer were investigated. X-ray diffraction (XRD) patterns indicated that the ZrC phase and a few other unknown crystalline phases existed in the surface layer after micro-EDM. After shallow grinding, other new crystalline phases were detected on the surfaces. After deep grinding, however, the crystalline phases disappeared and an amorphous surface was obtained, the XRD and micro-Raman characteristics of which were similar to those of the as-cast MG surface. These results suggested that crystallization of MG in micro-EDM was hierarchical along the depth direction with different mechanisms. The results of surface morphology showed that grinding after micro-EDM not only effectively removed the crystallization layers to obtain an amorphous MG surface, but also improved the surface quality. The hybrid process provides a promising method to create three-dimensional micro features on a MG surface.

Nanosecond pulsed laser irradiation induced hierarchical micro/nanostructures on Zr-based metallic glass substrate

A large effective surface area is beneficial to enhance the applications of metallic glasses (MGs) in heterogeneous catalysis and biomedical engineering. For the purpose of increasing effective surface area, in this study, hierarchical micro/nanostructures were fabricated on a Zr-based MG substrate by nanosecond pulsed laser irradiation. Experimental results indicated that a layer of micron-scale laser pulse tracks covered by a cotton-like MG thin film with nanometer-scale microstructure was formed in the laser irradiated region. This hierarchical micro/nanostructures retained amorphous characteristic and exhibited uniform element distribution. Its formation mechanism was investigated by analyzing the laser irradiation process and morphologies. Nanoindentation results indicated that the cotton-like MG thin film was very loose and soft compared to the as-cast MG substrate, showing different plastic deformation behavior. Results from this study indicate that nanosecond pulsed laser irradiation is an effective method to generate hierarchical micro/nanostructures on MG substrates, which can increase their effective surface areas and improve their potential applications as biomaterials and catalysts.

Probing the evolution of slow flow dynamics in metallic glasses

The dynamics of glass is of paramount importance for understanding glass, while experimental studies of it covering broad time and temperature ranges are fraught with difficulty. We employ a method which can probe the extremely slow dynamics in various glassy states in metallic glass (MG). The flow dynamics of as-cast MG is found to follow a universal Arrhenius behavior in a wide temperature range, and aged MG follows a stretched exponential function with a ``magic'' exponent number of 3/7. Our observations have implications for understanding the structural evolution of the slow flow and the issue of finite temperature divergence in MGs.

Corrosion behavior of ZrTiCuNiBe bulk metallic glass subjected to friction stir processing

The corrosion behavior of a Zirconium-based bulk metallic glass subjected to friction stir processing was evaluated. Processing was done at the tool rotational speeds of 300rpm, 500rpm and 900rpm. Potentiodynamic polarization and electrochemical impedance spectroscopy studies were carried out in 0.1M NaCl solution. A strong correlation was found between the corrosion rate and enthalpy of structural relaxation, which is a measure of free volume. Electrochemical impedance spectroscopy measurements showed lower polarization resistance for processed samples as compared to as-cast metallic glass which is consistent with the DC polarization results. Shear bands in processed specimens were found to be preferred nucleation sites for the formation of corrosion pits. In contrast, pitting was more stochastic for the as-cast metallic glass.

Characterization of the surface film on Zr-based bulk metallic glass using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM)

Using XPS, we have for the first time studied the release of metal ions in the film of the Zr-based bulk metallic glass to the corrosive solutions during immersing. The composition of Al ions in the film of the as-cast metallic glass (41%) is substantially higher than the nominal Al composition of the alloy (9.5%). We proposed that the enriched Al ions can be attributed to the preferential oxidation of Al atoms. After immersing in the NaCl- and HCl-solution, the composition of Al ions in the films decreases from 41% to 28.09% and 21.76%, respectively. This indicates that some of the Al ions in the film are dissolved into the solution during immersion. The composition changes of metal ions in the film of the immersed alloys relative to those of the as-cast metallic glass were discussed using the point defect model. SEM was also used to examine the surface morphology of the samples. No pit corrosion was observed in the SEM images of the immersed metal glass.