Non-Isothermal Crystallization Kinetics of a Rapidly Solidified as-Cast TiZrHfNiCu High Entropy Bulk Metallic Glass

This paper aims to investigate the thermal behavior and crystallization kinetics of TiZrHfNiCu high entropy bulk metallic glass (HE-BMG) alloy using the standard procedure of Differential Scanning Calorimetric (DSC) annealing technique. The alloy was produced using an arc melting machine with a critical diameter of 1.5 mm. The crystallization kinetics and phase transformation mechanism of TiZrHfNiCu HE-BMG was investigated under the isochronal condition at a single heating run based on the Johnson-Mehl- Avrami (JMA) theory. In isochronal heating, the apparent activation energy for glass transition and crystallization events was analyzed by Kissinger and Ozawa methods. The average activation energy value for crystallization of TiZrHfNiCu amorphous alloys in isochronal modes was 226.41 kJ/mol for the first crystallization and 297.72 kJ/mol for second crystallization stages. The crystallization mechanism of the first step was dominated by two- and three-dimensional growth with increasing nucleation rate, while the crystallization mechanism in the second stage was dominated by two-dimensional crystallization growth with a constant nucleation rate. The diffusion mechanism result proved the theory of sluggish atomic diffusion of HEA at elevated temperature.

Dynamic mechanical relaxation behavior of Zr_(35)Hf_(17.5)Ti_(5.5)Al_(12.5)Co_(7.5)Ni_(12)Cu_(10)high entropy bulk metallic glass

Non-equiatomic high entropy bulk metallic glasses were reported recently and show unique mechanical and physical properties.Dynamic mechanical relaxation behavior of Zr_(35)Hf_(17.5)Ti_(5.5)Al_(12.5)Co_(7.5)Ni_(12)Cu_(10)high entropy bulk metallic glass was investigated by dynamic mechanical analysis(DMA)and the mechanical spectra could be well described by the quasi-point defects(QPD)theory.Compared to typical metallic glasses,the intensity of theβrelaxation of Zr_(35)Hf_(17.5)Ti_(5.5)Al_(12.5)Co_(7.5)Ni_(12)Cu_(10)high entropy bulk metallic glass is lower due to the sluggish diffusion.At the same time,the correlation factor is higher than that of conventional metallic glasses and this is ascribed to the high configuration entropy.In parallel,physical aging below the glass transition temperature leads to a decrease of atomic mobility,caused by a decrease of the concentration of defects.

Insight on Viscoelasticiy of Ti16.7Zr16.7Hf16.7Cu16.7Ni16.7Be16.7 High Entropy Bulk Metallic Glass

High entropy bulk metallic glasses show promising mechanical and physical properties.Dynamic mechanical properties of Ti_（16.7）Zr_（16.7）Hf_（16.7）Cu_（16.7）Ni_（16.7）Be_（16.7）high entropy bulk metallic glass were investigated by mechanical spectroscopy（or called dynamic mechanical analysis）.The main（α）relaxation was observed in the framework of the loss modulus G″,which is related to the dynamic glass transition behaviour for the glassy materials.From physical model point of view,dynamic mechanical properties of the Ti_（16.7）Zr_（16.7）Hf_（16.7）Cu_（16.7）Ni_（16.7）Be_（16.7）high entropy bulk metallic glass show good agreement compared with the quasi-point defects theory.

A Brief Review of High Entropy Alloys and Serration Behavior and Flow Units

Multicomponent alloys with high entropy of mixing,e.g.,high entropy alloys（HEAs）and/or multiprincipal-element alloys（MEAs）,are attracting increasing attentions,because the materials with novel properties are being developed,based on the design strategy of the equiatomic ratio,multicomponent,and high entropy of mixing in their liquid or random solution state.Recently,HEAs with the ultrahigh strength and fracture toughness,excellent magnetic properties,high fatigue,wear and corrosion resistance,great phase stability/high resistance to heat-softening behavior,sluggish diffusion effects,and potential superconductivity,etc.,were developed.The HEAs can even have very high irradiation resistance and may have some self-healing effects,and can potentially be used as the first wall and nuclear fuel cladding materials.Serration behaviors and flow units are powerful methods to understand the plastic deformation or fracture of materials.The methods have been successfully used to study the plasticity of amorphous alloys（also bulk metallic glasses,BMGs）.The flow units are proposed as：free volumes,shear transition zones（STZs）,tension-transition zones（TTZs）,liquid-like regions,soft regions or soft spots,etc.The flow units in the crystalline alloys are usually dislocations,which may interact with the solute atoms,interstitial types,or substitution types.Moreover,the flow units often change with the testing temperatures and loading strain rates,e.g.,at the low temperature and high strain rate,plastic deformation will be carried out by the flow unit of twinning,and at high temperatures,the grain boundary will be the weak area,and play as the flow unit.The serration shapes are related to the types of flow units,and the serration behavior can be analyzed using the power law and modified power law.