Glass-forming Ability And Machanical Properties Of Co-B-Ta Bulk Metallic Glasses

Co-B-based bulk metallic glasses(BMGs)have huge potential for the application in the electro-mechanical system and precise machining field because of their higher strength than other bulk metallic materials known so far.However,the brittleness problem for this class of alloys limits their use for structural applications.Previous researches have shown that the shear deformation ability,plasticity and toughness can be improved by tuning the alloy composition.In this paper,C067.5-xB32.5Tax(x = 6.5,7.5,8.5,9.5 at.%)bulk metallic glasses were designed and fabricated.The glass-forming ability,thermal stability and mechanical properties were studied by X-ray diffraction,differential scanning calorimeter,micro-Vickers hardness tester,universal testing machine and scanning electron microscopy.Ab initio molecular dynamics simulations was used to studied the atomic structure of Co60B32.5Ta7.5 and Co61B32.5Ta6.5 with the best glass-forming ability and ductility,respectively.In adittion,the effects of atomic structure on glass-forming ability and mechanical properties were also analyzed.The following are the conclusions:(1)The glass-forming ability of C067.5-xB32.5Tax(x = 6.5,7.5,8.5,9.5 at.%)alloys was investigated.It was found that,with an increase in Ta content,the glass transition temperature Tg increases from 920 K to 956 K,and the supercooled liquid region ?Tx increased from 41 K to 51 K.This indicates that the thermal stability of glassy alloys can be improved by increasing Ta content.The liquidus temperature(T1)decreases first and then increases with increasing Ta content.When Ta content equal to 7.5 at.%,T1reaches the minimum value of 1496K,indicating that this alloy composition is very close to the eutectic point of Co-B-Ta alloys.With increasing Ta content,Tm is remains almost unchanged.The critical diameter of Co67.5-xB32.5Tax(x =6.5,7.5,8.5,9.5 at.%)alloys is above ?2 mm.Among them,the CO60B32.5Ta7.5 alloy exhibits the best glass-forming ability(? 3 mm).(2)The mechanical properties of Co67.5-xB32.5Tax(x = 6.5,7.5,8.5,9.5 at.%)BMGs were studied.The result shows that,with an increase in Ta content,the Vickers'hardness,yield strength and fracture strength of BGM increases from 1320 HV to 1480 HV,from 4580 MPa to 5600 MPa,and from 5100 MPa to 5850 MPa,respectively.The plastic strain of alloy decreases with an increase in Ta content,and the Co61B32.5Ta6.5 has the largest plastic stain,reaching to 3%.From the fracture morphology,it can be seen that the shape of vein pattern is more regular,the width is about 8?m and the distribution is more uniform With increasing Ta,the fracture toughness of Co67.5-xB32.5Tax(x = 6.5,7.5,8.5,9.5 at.%)BMGs decreases gradually.The fracture toughness of Co60B32.5Ta7.5 is 5.02 MPa·m1/2,which is similar to that of Co59B33Ta8(5.0 ± 2.7 MPa·m1/2).It is noticeable that the fracture toughness of Co61B32.5Ta6.5 alloy is about 7.74 MPa·m1/2,which is higher than that of Co59B33Tag and Co60B32.5Ta7.5.(3)The local structure of Co60B32.5Ta7.5 and Co61B32.5Ta6.5 glassy alloys was investigated by ab initio molecular dynamics simulations.With increasing Ta content,the bonding forces of Co-Co,Co-B,B-B and B-Ta decrease and the atomic pair spacing increases,while the bonding forces of Co-Ta and Ta-Ta increase and the atomic pair spacing decreases.The results of H-A bond pair exponential analysis indicate that the 1551,1541,1431,1661 and 1441 bond pairs are the mian bond pairs.Voronoi analysis results show that the numbers of Co-centered polyhedral clusters are mainly<0 1 10 2>?<0 1 10 3>?<0 2 8 4>and<0 3 6 4>,the numbers of B-centered polyhedral clusters are mainly<0 2 8 0>?<0 3 6 0>?<0 2 8 1>and<0 3 6 1>,the numbers of Ta-centered polyhedral clusters are mainly<0 2 8 5>?<0 1 10 4>?<0 2 8 6>and<0 3 7 4>.The polyhedral clusters in the alloy are mainly deformed prism(<0.360>)and deformed icosahedral(<0.1102>).(4)The relationship of atomic structure with glass-forming ability and mechanical properties were analyzed.In Co-B-Ta glassy alloys,the Co-B and B-B bonds have covalent bond characteristics,and the Co-Ta and Ta-B bonds have ionic bond characteristics,so these BMGs have high thermal stability.Obviously,Co60B32.5Ta7.5 glassy alloy contains more stable structures,such as stable icosahedral structure.Tthe stable short-range structure can increase the thermal stability of alloys and thus increases the crystallization resistance,and improves the glass-forming ability of alloy.(5)On the other hand,the ductility of Co-B-Ta BMGs is related to the type of clusters and the density of clusters in space.The more stable clusters are,the closer they are arranged in space,the worse the plasticity and toughness of the alloys.The clusters in Co61B32.5Ta6.5 bulk amorphous alloys repel each other and distribute more uniformly,and stable clusters such as icosahedral clusters distribute more evenly and loosely,so the shear deformation and plastic toughness of Co61B32.5Ta6.5 alloy are easier to occur.