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Preparation and Properties of Novel Bulk Metallic Glasses and Glassy Thin Films

Author: JiangQingKui
Tutor: JiangJianZhong
School: Zhejiang University
Course: Materials Science
Keywords: Bulk metallic glass Glass forming ability Structure Glass transition Mechanical properties
CLC: TG139.8
Type: PhD thesis
Year: 2012
Downloads: 271
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Abstract


Bulk metallic glasses (BMGs) have excellent properties, such as high hardness, high elastic strain limit, etc., which make them, promising for industrial applications. Hence, searching BMG compositions with high glass forming ability (GFA) is one of the most important issues of BMGs since it is not only closely related to their pratical applications but also the understanding of the mechanism for their high GFA.In this thesis, starting from ternary LaAlCu alloy system, we developed a series of La-Al-(Cu, Ag)-(Ni, Co) BMGs with critical sizes over 20-30 mm, in which the best glass former is La65Al14(Cu5/6Ag1/6)11(Ni1/2Co1/2)10 with critical size over 35mm by optimizing the compositions with elements of Ag, Ni and Co that have similar chemical properties and atomic radius to the main component. So far, it is still the largest Re-based BMG reported in the world. From the structure, thermodynamics and kinetics consideration, it was found that the crucial factor contributing to the high GFA is the lower Gibbs free energy difference between the crystalline and the supercooled liquid phase.The local structure difference in the glassy, the supercooled liquid and the liquid state of La62Al14Cu11.7Ag2.3Ni5Co5 BMG was studied by using in situ high temperature X-ray diffraction. It was found that the coordination number is of about 15.1±0.1, being independent on the temperature, while it slightly decreases for La62Al14Cu24 and La62Al14Cu2oAg4 BMGs with temperature uprising.Room temperature deformation behavior of the La62Al14(Cu5/6Ag1/6)14(Ni0.5Co0.5)10 BMG was studied using nanoindentation at different loading rates. A trend for the deformation mode variation was suggestioned to be:from homogeneous deformation I region at extremely low loading rates to inhomogeneous deformation at middle loading rates and finally to homogeneous deformation II at extremely high loading rates. The mechanism of loading-rate dependent plastic deformation behavior for La-based BMGs was proposed based on the shear transformation zone (STZ) model.Subsequently, we discovered that the ion-implantation technique can be applied to improve the corrosion resistance of BMGs without damage to their GFA. This idea was proven to be true on the La62Al14(Cu5/6Ag1/6)14(Ni0.5Co0.5)10 BMG. BMG. The improvement of corrosion resistance for the implanted alloys is attributed to the high passivating ability of the Nb-containing surface layer (about 100 nm thick).To overcome the poor mechanical properties and thermal stability of La-based BMGs, we attempted to develop an alloy system simultaneously exhibiting excellent mechanical properties and GFA. Here, selecting CuZr-based alloy and adding Ag in ZrCuAl ternary alloys, fully BMGs with at least 20 mm in diameter and some even about 25 gram amorphous metallic ingots can be prepared in a wide Zr-(Cu,Ag)-Al composition range The origin of high GFA in the studied system is attributed to the denser local atomic packing and the smaller difference in Gibbs free energy between supercooled liquid and crystalline phases after Ag addition. (for Zr46(Cu4.5/5.5Ag1/5.5)46Al8,⊿Gtl-s=1.5 kJ/mol).Last but not least, we experimentally revealed 6.6% elastic strain limit and 40% tensile plasticity for monolithic NiNb metallic glass films confined by frames by in situ high resolution transimission electron microscopy (HRTEM) observations. The origin of the high tensile elastic limit and plasticity was analyzed from the factors of thermal expansion, free volume model, Young’s modulus, size effect and frame confinement, etc.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Amorphous alloy
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