Glass Forming Ability And Mechanical Behaviors Of Zr-Based Bulk Metallic Glasses

Zr-based bulk metallic glasses (BMGs) exhibit high glass forming ability (GFA) and outstanding properties, which are very important qualities for engineering application. In this dissertation, a series of quaternary amorphous alloys in the Zr-Cu-Ni-Al system are explored by the method of proportionally mixing the binary eutectics. The GFA and the mechanical properties of the developed new alloys are systematically investigated. Further, the effect of minor addition of Gd element on the GFA and the mechanical behavior of one developed Zr-based amorphous alloy are also examined.In order to locate good glass formers in the Zr-Cu-Ni-Al alloy system, three kinds of binary eutectics Zr₄₄Cu₅₆, Zr₆₄Ni₃₆ and Zr₅₁Al₄₉ are chosen due to their low melting points and line compound products for eutectic reactions. Based on proportionally mixing binary deep eutectics, the designed composition Cam can be obtained as follows: Cam =x (Zr₄₄Cu₅₆) + y (Zr₆₄Ni₃₆) +z (Zr₅₁Al₄₉) where x, y and z are the coefficients for the three basic units summing up to unity. Changing the coefficients will result in different compositions. Moreover, in combination of the concept of phase competition, compositions corresponding to different GFA can be obtained by following this method. Firstly, x, y and z are all straightforwardly assigned to be 1/3 and Zr₅₃Cu_18.7Ni₁₂Al_16.3 is obtained. The critical diameter of this glassy rod sample correspondingly is about 6 mm. Then, to improve the GFA of this quaternary alloy system, coefficients for the eutectic units are adjusted, i.e. coefficients for Zr₆₄Ni₃₆ and Zr₅₁Al₄₉ are each decreased to 7/24 (y=z= 7/24) while the coefficient for Zr₄₄Cu₅₆, x, is increased to 10/24. This leads to a new glass-forming alloy Zr_51.9Cu_23.3Ni_10.5Al_14.3 with a critical size of 10 mm. Finally, another new composition Zr_50.7Cu₂₈Ni₉Al_12.3 with the critical size of 14 mm is explored by adjusting coefficients y and z to 6/24 and x to 12/24. This alloy is one of the best glass former among the Zr-Cu-Ni-Al quaternary alloy systems up to date.To better understand the different GFA of the three new Zr-based BMGs, the fragility parameter m of BMGs is calculated. The value of m ranks as Zr₅₃Cu_18.7Ni₁₂Al_16.3 (m=55.1) >Zr_51.9Cu_23.3Ni_10.5Al_14.3 (m=53.4) >Zr50.7Cu28Ni9 Al12.3 (m=51.2). In addition, for the three alloys, the critical cooling rate Rc estimates from thermal analysis lists as Zr₅₃Cu_18.7Ni₁₂Al_16.3 (Rc=82 K/s) >Zr_51.9Cu_23.3Ni_10.5Al_14.3 (Rc=46 K/s) >Zr_50.7Cu₂₈Ni₉Al_12.3 (Rc=9.8 K/s). The small values of the critical cooling rate Rc and the fragility parameter m for Zr_50.7Cu₂₈Ni₉Al_12.3 BMG indicate its high GFA among new Zr-based BMGs.Moreover, a quaternary Zr-Cu-Ni-Al BMG structural model is achieved, based on synchrotron radiation high energy X-ray diffraction (HEXRD) analysis and Miracle's efficient cluster packing (ECP) structure model. It is envisioned that the substitution of additional Cu atoms for Zr atoms in the investigated alloys stabilizes the ECP structure of the amorphous alloys, correspondingly, improving the GFA of the three amorphous alloys.For Zr₅₃Cu_18.7Ni₁₂Al_16.3, Zr_51.9Cu_23.3Ni_10.5Al_14.3 and Zr_50.7Cu₂₈Ni₉Al_12.3 BMGs, the compressive plastic strains are 14.5%, 6.7% and 5%, respectively, and the fracture toughness are 84.7 MPa m , 74.3 MPa m and 61.4 MPa m , respectively. It can be seen the Zr₅₃Cu_18.7Ni₁₂Al_16.3 alloy owns the highest plastic strain and fracture toughness among the three glassy alloys. Further, high resolution transmission electron microscopy (HRTEM) analysis indicates that the deformation-induced nanocrystallization is responsible for the improved plasticity and fracture toughness of this alloy.The compressive yield strength and plasticity of a new Zr_50.7Cu₂₈Ni₉Al_12.3 BMG improve evidently as testing temperature decreasing from 298 K to 123 K. The increase of yield strength is attributed to the nucleation of shear bands which requires a higher applied load at low temperature. Dense and multiple shear bands which are contributed to the enhanced ductility are observed on the side surface of the low-temperature fractured samples.The GFA of this newly developed quaternary Zr_50.7Cu₂₈Ni₉Al_12.3 alloy can be further improved by proper addition of Gd element. When adding 1 at.% Gd, (Zr_50.7Cu₂₈Ni₉Al_12.3)99Gd1 can be fabricated into a fully amorphous rod with the diameter of 18 mm. The improved GFA is attributed to the decrease of Gibbs free energy difference between the supercooled liquids and the corresponding crystalline solids.Meanwhile, the Gd addition decreases the fracture strength and plastic strain of the alloy system. With increasing Gd, the fracture mode transforms from pure shear to fragmentation failure. Observation on the fragmentation failure region surfaces for Zr-Cu-Ni-Al-Gd BMG samples reveals the alternation of hackle zone and mirror zone with crack propagation. The hackle region is composed of many dimples while mirror region is composed of nanoscale periodic corrugations. Morphology evolution on the fracture surface is generated by the reciprocity between the elastic waves and the plastic process zone on the crack front.