| Bulk metallic glasses（BMGs）are metastable materials with a long-range periodicity absence in their atomic structure.They exhibit excellent properties such as high strength,corrosion resistance,and high catalytic activity,showing a wide range of potential applications.However,the nature of metastability is like the Achilles’heel,forcing BMGs to transform into crystalline with thermodynamically stable state to fail under the influence of temperature and time,greatly limiting their practical application space.Therefore,researchers have focused their attention on high-temperature BMGs mainly composed of refractory metals.High-temperature BMGs exhibit extremely high thermal stability with a glass transition temperature exceeding 1000 K,ultra-high room temperature mechanical strength,excellent high-temperature softening resistance,and good thermal formability,making them promising for use in extremely harsh environments such as nuclear reactors,high-temperature reaction vessels,aerospace,etc.Unfortunately,due to the marginal glass forming ability of high-temperature BMGs,there is currently little research on their formation mechanisms and properties.The reported high-temperature BMGs only includes Ir-Ni-Ta-（B）and Os-Co-Ta-B,which contain a large amount of precious metals,resulting them costly and difficult to apply in industry.Therefore,considering the structural characteristics of metallic glasses,a method to improve the glass-forming ability of high-temperature BMGs by combining intermetallic compounds with quasicrystal-like structure and eutectic reaction was proposed in this paper.Focusing on Mo and W,two refractory metal systems with the lowest cost,the design of alloy compositions was carried out,and their glass-forming ability and thermal stability were studied,their mechanical properties were evaluated,their local structural characteristics were analyzed,and their glass forming mechanism was revealed.The main research results are as follows.Firstly,the σ-Co2Mo3 phase with quasicrystal-like structure was selected and Mo-B eutectic point was used to define the B content for designing the compositions of Mo-Co-B BMGs.The Mo-based BMGs was successfully prepared for the first time by water-cooled copper mold suction casting,namely Mo-Co-B BMGs with a diameter of 1.2 mm.The results have manifest that the Mo-Co-B BMGs exhibit high thermal stability with a glass transition temperature exceeding 1000 K,which is attributed to its high atomic packing density and strong medium-range order.In addition,mechanical tests have shown that the Mo-Co-B BMGs possess excellent mechanical strength and long-lasting wear resistance.Their room temperature mechanical strength can reach 5083 MPa,and the wear rate under a 10 N load is as low as 2.62×10-6 mm3/N·m.Furthermore,continuing with the design concept of high-temperature bulk metallic glass,theμ-Co7W6 phase with quasicrystal-like structure was selected and combined with the W-B eutectic line to determine the composition region of the glass forming alloys.Therefore,W-base bulk metallic glass,namely W-Co-Ta-B bulk metallic glass with amorphous forming ability of 2 mm,was first successfully prepared by water-cooled copper mold casting method.This high-temperature W-based BMGs exhibits extremely high thermal stability,with a glass transition temperature up to 1197 K.By extrapolating the Kissinger equation,their lifespan at 500 K is estimated to be up to 1051 years and they can even maintain their properties for 1010 years at a high temperature of 900 K.In addition,the high-temperature W-based BMGs exhibit ultra-high mechanical strength and high-temperature anti-softening properties.Their room temperature compressive strength reaches 5600 MPa and room temperature Vickers hardness are about 16 GPa.Even at a high temperature of 1173 K,they can still maintain a high hardness of 12 GPa.Combined with neutron diffraction,nanoelectron beam diffraction,and molecular dynamics simulations,it was found that the W-Co-Ta-B BMGs possess high atomic stacking density and exhibit strong icosahedral-like structure,as well as a strong medium-range order.The Voronoi polyhedron analysis verifies that the content of coordination polyhedral with quasicrystal-like structure exceeds 60%.The formation of ultra-high content icosahedral-like short range order is conducive to the stability of the supercooled to form an amorphous structure.It is also an important structural factor for their ultra-high thermal stability and excellent mechanical properties.In addition,local structure analysis revealed that the W-Co-Ta-B BMGs inherited many short-range structural features from theμ-Co7W6phase,because the type and coordination number distribution of the polyhedron centered on Co in both are similar.Similarly,the type and coordination number distribution of polyhedron centered on W in both are also similar,suggesting that various icosahedral-like short-range orders derived from the coordination polyhedron in theμ-Co7W6 phase under chemical reactions and dynamic fluctuations construct the dominant structure of the W-Co-Ta-B bulk metallic glass.The above research results demonstrate that the path of designing high temperature bulk metallic glass by combining intermetallic compounds with quasicrystal-like structure and eutectic reaction is feasible,and Mo-Co-B and W-Co-Ta-B high temperature BMGs with excellent properties have been successfully designed.Meanwhile,the combination of experiments and simulations revealed the glass formation mechanism of these high temperature BMGs behind the design concept.It is of great reference significance to promote the development and application of high temperature BMGs,and to deeply understand the glass formation and probe glass structure. |
