Optimizing impact toughness of Zr-based bulk metallic glasses via cryogenic thermal cycling-induced free volume engineering

The tribological properties of Zr-based and Cu-based bulk metallic glasses (BMGs) sliding against discs of SUS 304 and Si3N4 at room temperature under dry and lubrication of fresh plasma, distilled water, and physiological distilled water were investigated on a pin-on-disc testing machine. The results demonstrated the influences of counterface materials, lubrication conditions and the characteristics of BMGs on the frictional behaviors of the Cu- and Zr-based BMGs. It is found that the steady state friction coefficients and the wear rates sometimes are not consistent, and the wear resistance was been improved when the counterface change from SUS304 to Si3N4. Otherwise, wear rates of the BMGs under lubrication conditions are much lower than that of under dry sliding conditions, and the Cu-based BMGs exhibits higher wear resistance than Zr-based BMGs under same experimental conditions because it is a nanocrystalline amorphous alloy and has the higher glass transition and crystallization temperatures.

This paper reports a comparative study of shear banding in BMGs resulting from thermal softening and free volume creation. Firstly, the effects of thermal softening and free volume creation on shear instability are discussed. It is known that thermal softening governs thermal shear banding, hence it is essentially energy related. However, compound free volume creation is the key factor to the other instability, though void-induced softening seems to be the counterpart of thermal softening. So, the driving force for shear instability owing to free volume creation is very different from the thermally assisted one. In particular, long wave perturbations are always unstable owing to compound free volume creation. Therefore, the shear instability resulting from coupled compound free volume creation and thermal softening may start more like that due to free volume creation. Also, the compound free volume creation implies a specific and intrinsic characteristic growth time of shear instability. Finally, the mature shear band width is governed by the corresponding diffusions (thermal or void diffusion) within the band. As a rough guide, the dimensionless numbers: Thermal softening related number B, Deborah number (denoting the relation of instability growth rate owing to compound free volume and loading time) and Lewis number (denoting the competition of different diffusions) show us their relative importance of thermal softening and free volume creation in shear banding. All these results are of particular significance in understanding the mechanism of shear banding in bulk metallic glasses (BMGs).

This paper presents the SEM micrographs for the three-point bending fracture surfaces of Zr-based, Ce-based and Mg-based bulk metallic glasses (BMGs), which show the dimple structures in the three kinds of BMGs. The shapes of the giant plastic deformation domain on the fracture surface are similar but the sizes are different. The fracture toughness K C and the dimple structure size of the Zr-based BMG are both the largest, and those of the Mg-based BMG are the smallest. The fracture toughness K C and the dimple structure size of the Ce-based BMG are between those of the Zr-based and the Mg-based BMG. Through analyzing the data of different fracture toughnesses of the BMGs, we find that the plastic zone width follows w = (K C/σ Y)2/(6π).

Rohith Ambadi S is currently pursuing his Ph.D. in Computational Fluid Dynamics at Kalasalingam Academy of Research and Education. He received his B.E. degree in Mechanical and Automation Engineering and M.E. degree in Cryogenic Engineering from PSN College of Engineering and Technology. He is presently working as an Assistant Professor in the Department of Mechanical Engineering at PSN College of Engineering and Technology, Tirunelveli, India. He is a researcher and academician with strong expertise in Computational Fluid Dynamics, supercritical CO₂ centrifugal compressors, cryogenic systems, nanofluids, and thermal engineering. Driven by a passion for research and innovation, he has contributed to high-impact research with publications in reputed journals including Physics of Fluids and Materials Today: Proceedings. He is also a co-inventor of a registered Indian design patent on an AI and IoT-based sustainable smart farming agricultural robot, reflecting his interdisciplinary approach toward engineering solutions. His blend of advanced research, teaching experience, and innovation positions him as a committed professional contributing to both academia and technological advancement.

Correlations between the relaxed excess free volume and the plasticity in Zr-based bulk metallic glasses

The composition mediated serration dynamics in Zr-based bulk metallic glasses (BMGs) is investigated by statistics analyses of the elastic-energy density, and free volumes during shear-banding are beneficial to understand serrated-flow behavior. The amplitude and elastic-energy density display a gradually increasing and then decreasing trend with increasing the content of Zr. It is based on the free-volume theory describing the atomic-level structure of ternary Zr-Cu-Al BMGs. The good agreement between the molecular dynamics simulation and experimental results provides evidence for the variation of free volumes as the elementary mechanism of composition mediated serration dynamics.

In order to evaluate the influence of bulk metallic glass size on its impact toughness, a typical Zr-based bulk metallic glass is selected for numerical simulation research in this paper. The study uses the JH-2 constitutive model to describe its mechanical response under impact loading, and the impact toughness values are obtained for different cross-sectional areas, heights and spans using the FEM-SPH method. The results show that an increase in cross-sectional area and span leads to an increase in impact toughness, while an increase in height leads to an increase and then a decrease in impact toughness, with the highest value for the impact toughness at the square cross-section. These results provide ideas for improving the impact toughness of large metallic glasses.

Compared with conventional channels, narrow and micro channels have significant characteristic of heat transfer enhancement. With smooth internal surface, such channels can efficiently avoid encrustation at the washing effect of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, much work was focused upon flow patterns, heat transfer and pressure drop. Almost everyone thought the heat transfer enhancement mechanism of narrow and micro channels to be bubbles’ deformation and disturbance, which is insufficient to explain the heat transfer enhancement. In present work, an innovative model of quasi-one-dimensional vapor liquid two-phase concurrent separated flow was proposed for boiling heat transfer in vertical narrow rectangular space. Numerical results such as boiling heat transfer coefficient and liquid film thickness were obtained. Comparison of model results with reported experimental correlation indicates that the proposed model can predict heat transfer in narrow channels correctly, with the relative deviation less than 14%. Numerical simulating result confirms that heat conduction through liquid film is the predominant mechanism of boiling heat transfer in vapor liquid separated flow region in a vertical narrow rectangular space.

Compared with conventional channels, narrow and micro channels have significant heat transfer enhancement characteristic. With smooth internal surface, such channels can efficiently avoid encrustation at the washing of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, many efforts have been done which focused upon flow patterns, heat transfer and pressure drop. Almost every researcher thought the heat transfer enhancement mechanism of narrow and micro channels to be bubble’s deformation and disturbance, but the proposed mechanism is insufficient to explain the heat transfer enhancement of narrow channel. In present paper, an experimental flow visualization study has been performed under pressured conditions. Microscopic high-speed video visualization revealed that initial bubbles growth accompanied with slipping on heating wall in flow direction with slip velocity less than 0.1m/s. Several important parameters, such as heat flux and subcooling of cross-section, have important effects on the behavior of bubbles. At higher heat flux, it resulted shifting of the ONB point to the upstream and thus generated larger population of bubbles. It was observed that small bubbles (d = 0.01∼0.07mm) slipped on heating wall at a speed of 0.1 to 0.2m/s, and the velocity of larger bubbles (d = 0.1∼0.3mm) was increased to 0.25 ∼ 0.7 m/s. From flow visualization, it is showed that the process of large bubbles coalescing with small ones is the dominating mechanism of bubble growing. In some situation, the speed-increasing bubble’s size even formed vapor layer near the heating wall.

Compared with conventional channels, narrow and micro channels have significant heat transfer enhancement characteristic. With smooth internal surface, such channels can efficiently avoid encrustation at the washing of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. These types of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, advanced nuclear reactor, cryogenic, aviation and space technology and thermal engineering. Geometrical size of flow passage-away affects heat exchange of flow boiling, with the result that the bubble in narrow channel acts very different from those in non-narrow channel. This paper experimentally compared the bubble behavior with different heating methods of narrow rectangular channels, and the bubble behavior of subcooled flow boiling of R-12 in the narrow channels both with double side and single heating. Experimental settings are: the heating length of test-section is 400 mm, the cross-section is 35 mm in width and 2mm in gap size, mass flux is 700∼1500 kg.m−2.s−1, the heat flux is 25∼70kW.m−2 and the pressure is 1.3∼2.0 MPa. Comparisons were made on Onset of Nucleate Boiling (ONB) point and bubble characters with various flow patterns. Results revealed that the characteristics of double and single side heating shown good agreement with proper modifications.

A group of plastic Zr-Al-Ni-Cu bulk metallic glasses (BMGs) with low Zr content was developed and their thermal and mechanical properties were investigated. The results show that these Zr-based BMGs have a single crystallization event for all heating rates in the studied temperature region. The glass transition temperature Tg decreases with increasing Zr content for all heating rates. There are two melting procedures for the BMGs whose Zr content is less than 52 at %, while three melting procedures for the other Zr-based BMGs. The second melting procedure is split into two melting procedures for Zr52.5Al12.2Ni12.6Cu22.7 and Zr53Al11.6Ni11.7Cu23.7 BMGs, while the first melting procedure is split into two melting procedures for the other BMGs. The activation energy decreases with increasing sensitivity index β for the studied Zr-based BMGs. The plastic strain εp is in the region of 0.2%–19.1% for these Zr-based BMGs. Both yield strength σy and fracture strength σf are smallest for Zr55Al8.9Ni7.3Cu28.8 BMG whose εp is largest among all studied Zr-based BMGs and reaches up to 19.1%. In addition, the mechanism for the large difference of the plasticity among the studied Zr-based BMGs is also discussed.

Unusual plastic deformability in a Zr-based bulk metallic glass after structural relaxation

Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young's modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr65Al7.5Ni10Cu17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr65Al7.5Ni10Cu17.5 BMG, Ti-6Al-4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti-6Al-4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti-6Al-4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti-6Al-4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with Ti-6Al-4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.

Characterization, adhesion strength and in-vitro cytotoxicity investigation of hydroxyapatite coating synthesized on Zr-based BMG by electro discharge process

Ultrasound-induced liquid/solid interfacial reaction between Zn-3Al alloy and Zr-based bulk metallic glasses