The Fabrication And Properties Of Mg-based Amorphous Composites

Magnesium-based amorphous alloys have attracted considerable attention due to light-weight, high-strength and a good bulk glass forming ability (GFA). However, the applications have been limited because the Mg-based bulk metallic glass (BMG) suffers from low ductility problems. In this work, it was demonstrated that a composite design strategy to achieve bulk metallic glass based light-weight Mg alloys. Carbon nanotubes (CNTs), Co, Ti were added into the Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass forming alloys to form the bulk metallic glass composites. The cross-sectional surfaces of the as-cast rods were examined using X-ray diffraction (XRD). The thermal stability of the glass and composites was investigated using differential scanning calorineter (DSC). Mechanical behavior in uiaxial compression of these composites was measured using universal material testing machine. The main results are follwed:1. Mg₆₅Cu₂₅Gd₁₀ metallic glass with diameter of 2mm and 3mm and its CNTs reinforced composites with diameter of 2mm were achieved by means of the copper mould casting. It was shown that carbon nanotube phase existed in Mg-based glass matrix. The apparent active energy of Mg-based bulk glass and its amorphous composites were 139 KJ/mol and 154 KJ/mol, respectively, using thermal stability measurement and analysing. crystalline rate constant is 0.582 s^-1. It was uncovered that addition of CNTs into the glass matrix improves the compression strength to 828 MPa, which increases about 27.7%, compared with the Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass and the CNTs reinforced composites.2. Mg₆₅Cu₂₀Gd₁₀Co₅ alloys rod with diameter of 2mm or 3mm were achieved by means of the copper mould casting, whoes maximum diameter is 2mm when the whole rod is amorphous phase. The critical cooling rate is about 250 K/s. On futher increasing the diameter, there are some ctystalline phase on the Mg-based metallic glass matrix, whice means that the composites include both amorphous structure and nano-crystalline structure. When the Co added to the Mg-based alloys, the width of super-cooled liquid regine (SLR) has no sharp variation., but the multilevel crystallization is apparent, which suggests that mechanics of crystallization is more complex.The addition of 5 at% Co induces the glass transition temperature, Tg, and onset temperature of ctystallization, Tx, shifting to the high temperture area.3. The Mg-based master alloy ingots were obtained by means of arc furnace and induction furnace under an argon atmosphere. And then the cylindrical composites with different Ti content up to 7.5 at% were prepared by the copper casting method. The structure of the amorphous composites had been changed from fully amorphous to glass matrix with some crystalline phase with the Ti content increased. DSC analysis results showed that crystalline peaks from two to three while the Ti content arising up to 7.5 at% in the glass transition term in (Mg₆₅Cu₂₅Gd₁₀)_100-xTi_x (x=0, 2.5, 5, 7.5) and that tendency became more and more obvious. Also, the results revealed that the single melting endothermic peaks splited to two with different degrees in the melting term. It is exhibited that, in the process of super-cooled, Mg-based solid solution as a one-up competing phase precipitated from the liqued phase firstly, and then the liquid phase remain had a high glass formation ability (GFA), so the precipitation of intermetallic compound were restrained, as a result, that liquid phase turned to amorphous structure. The addition of Ti to the glass matrix improves the fracture strength and plasticity of the composets with different levels and the work-hardenable was apparent in the process of plastic deformation. Especially in the metallic glass composite of (Mg₆₅Cu₂₅Gd₁₀)_100-5Ti₅, the compression strength was exhilaratingly gotten up to 977MPa, and the plastic strain reached to 0.5%.