The Glass-Forming Ability And Mechanical Behavior Of Mg-Cu-Gd(-Al) Bulk Metallic Glass

Bulk metallic glasses (BMGs) have attracted considerable research activities due to their high strength, high hardness and excellent resistance against abrasive wear. The Mg-based BMGs exhibit excellent glass-forming ability, high specific strength and low price. So it is considered to be very promising for engineering applications as structural materials. However, the investigation of the foundational theories, properties and applications for the BMGs is not enough compared to the crystalline alloys. At present, the formation and mechanical properties of Mg-based BMGs, especially the deformation and fracture mechanism, have been intensely investigated but still exhibit many problems, thus it is very important to further study.By means of x-ray diffractometry (XRD), differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM) equipped with an Energy Dispersive Spectrometer (EDS) system, uniaxial compression test, indentation test and wear test, the effect of partial substitution of Al for Cu on the glass-forming ability (GFA), the room temperature compressive properties, deformation behaviors and wear properties of Mg₆₅Cu_25-xAl_xGd₁₀ (x= 0, 1, 3 and 5 at. %) alloys were investigated and the corresponding mechanism was discussed in this thesis. The results indicate that:Mg₆₅Cu₂₅Gd₁₀ BMGs has a significantly improved GFA, enabling the fabrication of the monolithic BMG with a maximum diameter of 8 mm by the Cu-mold casting method. An appropriate substitution of Cu by Al in Mg₆₅Cu₂₅Gd₁₀ BMG can keep the good GFA. The Mg₆₅Cu₂₂Al₃Gd₁₀ BMG with the maximum diameter of 6mm can be fabricated. But the GFA quickly decreased with increasing x up to 5, and the maximal size of fully glassy alloy distinctly decreased to 2mm in diameter. With increasing of Al content, the glass transition temperature slightly increased and the crystallization onset temperature decreased resulting the super-cooled liquid regions gradually decreased, therefore, the GFA decreased. The values of R_c,T_rg,K,γandΔT^* can reflect the changes of GFA in a certain extent. Partial substitution of Cu by Al in the Mg₆₅Cu₂₅Gd₁₀ alloy may be destroy the stability of the super-cooled liquid and promote the nucleation and growth of the competing crystalline AlCuGd phase and other crystalline phases, which decrease the GFA. It is suggested that the composition of alloy with x = 5 is more farer away from the eutectic composition as compared with the other alloy, which may be the reason that the addition of 5%A1 remarkably decreases the GFA of the BMGs.The room temperature compressive properties of Mg₆₅Cu_25-xAl_xGd₁₀(x = 0, 1,3 and 5 at. %) alloys were studied by using an Instron testing machine. The ultimate compressive fracture strength is improved with increasing of Al content, proper substitution Cu by 3% Al can dramatic improve the plasticity of Mg₆₅Cu₂₅Gd₁₀ BMG. The compressive fracture strength and plastic strain of the Mg₆₅Cu₂₂Al₃Gd₁₀ BMG increased to 898 MPa and 0.2%, respectively. Only few shear bands can be observed on the fracture surface of Mg₆₅Cu₂₅Gd₁₀ BMG. however, for Mg₆₅Cu₂₂Al₃Gd₁₀ BMG, multiple shear bands is easier to form by the coalescence of the free volume, some adjacent shear bands intersect with each other and branching, resulting the distinct improvement of the plasticity. The improvement may be also attributed to the addition of Al suppress the crack initiation and propagation during compressive stress. Mg₆₅Cu₂₅Gd₁₀ and Mg₆₅Cu₂₄Al₁Gd₁₀ BMGs break in a very brittle manner at room temperature, but it exhibits the nm-scale dimple structure in the fracture surface, which suggests that plastic deformation might occur.The Vickers hardness values of Mg-Cu-Gd(-Al) BMGs exhibit some fluctuation when the loading time is 5 seconds, however, no evidence of variation in hardness value with increasing the loading time up to 10 seconds, which indicates that the elastic and plastic deformation under indentation was accomplished well up to this time duration. The Mg-based BMG exhibit creep deformation behavior under indentation due to its lower T_g, but it is not distinct in this experimental condition. The observation of slightly higher hardness values at low loads can be attributed to the higher resistance to shear band nucleation and propagation at these loads. The Vickers hardness values of Mg-Cu-Gd(-Al) BMGs gradually increase with the Al content, which can be attributed to the interaction of the composition elements enhanced and a higher deformation resistance on the surface.The micro-region mechanical properties and deformation behavior during Vickers indentation in Mg- based BMGs were investigated by using bonded interface technique. At a small load, semi-circular slip-steps of shear bands partially surrounding one edge of the indentation are observed on the top surface, which formed by the propagation of shear bands. At a load of 1000g, it can be observed that some semi-circular slip-steps of shear bands partially surrounding three edges of the indentation the top surface. A lot of semi-circular primary shear bands can be found in the subsurface deformation zone with different loads, and it also observed some secondary radial shear bands in this region. Some tertiary shear bands which develop inside and close to the boundary of the primary shear bands can be observed in Mg₆₅Cu₂₂Al₃Gd₁₀ BMG with a load of 1000g, indicating better plastic deformation ability. The deformation zone size (δand h) increase with the applied loads, but the ratio to the total deformation zone size (δ/D and h/H) is independent of the applied loads. The plastic deformation surrounding the Vickers indentation is achieved by some highly localized shear bands formation and propagation, the primary shear bands are the main nature of the deformation pattern, the secondary shear bands and tertiary shear bands may be formed with a higher load. Therefore, the primary shear bands formation has been known to be the dominant micro-region deformation mechanism during Vickers indentation in Mg- based BMGs.The wear mass loss was found to increase with the sliding distance and applied loads in Mg₆₅Cu_25-xAl_xGd₁₀ (x = 0, 1,3 and 5 at. %) BMGs. The addition of Al element can distinctly improved the wear properties of Mg₆₅Cu₂₅Gd₁₀BMG. The wear mechanism for the Mg₆₅Cu₂₅Gd₁₀ BMG is micro-cutting and localized break, with increasing Al content from 1% to 3%, the wear mechanism gradually turn to the synthesized of micro-cutting and plastic deformation. With increasing Al content up to 5%, the wear mechanism is change to micro-cutting and block fracture. The wear rate is inverse proportional to the Vickers hardness in Mg₆₅Cu_25-xAl_xGd₁₀ (x = 0, 1, 3 and 5 at. %) BMGs, but the simple abrasive wear model used for crystalline alloy can not be appropriate used for the amorphous alloy. A modified parameter correlating to the plasticity is employed to optimize the simple abrasive wear model for the Mg-based BMGs.