Study On The Structure And Mechanical Properties Of Zr-Co-Al Bulk Metallic Glasses

Only a few decades just pasted since metallic glass was born.However,the significance cannot be simply judged by the young age of this new kind of materials.Thanks to the excellent mechanical properties such as high strength and large elastic limit,metallic glass found its role in precision instruments and space technology.Considering the high expense and rigorous process conditions,most time metallic glass can hardly fulfill the requirements of civil industries,much less that the intrinsic drawback,lack of plasticity is urgently waiting for improvement.For a long time,bulk metallic glass(BMG)with high strength and ductility is the common target of so many material scientists,until Zr-based,especially Zr-Co-Al BMGs was found and reported due to their strong glass forming ability(GFA)and outstanding mechanical properties.That's also our concern in this thesis,which is based on Zr56Co29Al16 BMG and other similar compositions.The purpose of this research is to explore the structure origin of GFA variation and mechanical properties changes caused by composition tuning in Zr-Co-Al alloys.After hundreds of experiments by copper mould suction method for ZrxCo84-xAl16(x = 48-72),only x = 51-58 whose critical diameter is over 2 mm locates in the range of our attentions.The best glass formers are Zr55Co29Al16 and Zr56Co28Al16 in this alloy system,and glassy metal rods with diameter 7 mm were successfully prepared.Then we performed DSC and calculated some thermal parameters like Trg and ?.Unlike most paper stated,there is no clear relation between these thermal criteria and GFA in this work.But Young's modulus and hardness exhibit the same trend with glass transition temperature.Three-point bending tests revealed a quite interesting finding,eight different compositions can be divided into two groups,brittle BMGs for x = 51-54 and ductile BMGs for the rest.The result was confirmed by SEM observation on fracture surface of samples after three-point bending.As a supplement,nanoindentation tests again verified the decreasing strength along with increasing Zr concentration.Atomic structure was characterized by SRXRD and EXAFS,together with ab-initio molecular dynamics simulation.Massive data provided convincing explanations for GFA distribution and plasticity evolution of ZrxCo84-xAl16(x = 51-58).In the process of Zr addition from x = 51 to x = 56,Zr-Co bond length shortens;Co-centered<0,2,8,1>,<0,3,6,2>and<0,3,6,3>polyhedra decrease rapidly,which leads to the dense packing state around Zr,Co atoms,that is,local structure stabilization contributes a lot to the growing GFA.When it comes to x = 58,previous structure collapses because of the abnormal increase of Zr-centered<0,1,10,2>cluster,GFA becomes worse.On the other hand,the simultaneously improved plasticity is associated with the increased Co-centered<0,3,6,0>,and Al-centered<0,2,8,1>and<0,3,6,2>polyhedra,which provide more fertile sites for the nucleation of shear transformations.However,the plasticity is not enhanced further but remains nearly constant for higher Zr content composition,which may be related to the increased fractions of Zr-centered<0,1,10,3>and<0,2,8,4>and Co-centered<0,2,8,0>polyhedra,even if other clusters with low coordination numbers decrease.Thus,we find the probable explanation of property variations from structure aspect.