Formation And Properties Of Al-based Bulk Metallic Glasses With Wide Supercooled Liquid Region

Al-based metallic glasses ?MGs? exhibit high strength, high toughness and good corrosion resistance. Especially, Al-based MGs possess high special strength, which is about 1.5 times of high-strength Al alloys. Therefore, Al-based MGs are promising for applications as a new type of structural materials in aerospace fields. However, the preparation for Al-based bulk MGs with low glass-forming ability ?GFA? is difficult, which limits its practical use. Taking advantage of the viscous flow of MGs in the supercooled liquid region ??Tx?, the bulk samples can be produced by superplastic processing. In order to develop Al-based MGs with good superplastic formability which have large ?Tx, high GFA and low viscosity in the supercooled liquid state, we selected Al-Ni-Er MG as a base alloy, and investigated the effects of similar elements Co and other RE ?RE=Gd or La? additions on the thermal properties as well as GFA. In addition, the mechanism of elements addition on the stability of supercooled liquid and GFA was disscussed in terms of the crystallization behavior of MGs. New Al-based bulk MGs with large ?Tx, high GFA and low viscosity in the supercooled liquid state were successfully developed, which can be suitable for superplastic processing. The thesis consists of the following conclusions mainly:1. Proper Co and/or Gd additions enhance the stabilization of supercooled liquids and GFA of the Al-Ni-Er MGs. The Al85Ni5Co2Er6Gd2 MG exhibits a large ?Tx of 35 K, high GFA with critical thickness of 340 um, and low viscosity of 1.3×10 pa-s in the supercooled liquid state. Imprinting patterns in micro-scale were precisely formd by imprinting the A185Ni5Co2Er6Gd2 MG, indicating its good superplastic formability.2. Proper Co and La additions significantly enhance the thermal stability of supercooled liquids and GFA of the Al-Ni-Er MGs, and the effects are superior to that of Co and Gd additions. The ?Tx, critical thickness and the minimum viscosity of Al85Ni8Co2Er6La2 MG reach 40 K,450 ?m and 2.9×107 pa-s, respectively. The micro-/nano-imprinting patterns were successfully achieved in the supercooled region of this alloy, indicating its excellent superplastic formability.3. Study on the crystallization behavior of the MGs reveals that the precipitation phases change from a single a-Al to mixed ?-Al+Al3Er after Co and RE ?RE=Gd or La? additions, and the crystallization activation energy increases, which can explain the enhancement mechanism of the stability of supercooled liquid.4. The Al₈₅Ni_6.8Co_2.2Er₄La₂ bulk MG with a critical diameter ?dc? of 1.2 mm is successfully fabricated through adding appropriate amount of Co and La elements into the base alloys of Al-Ni-Er system. The bulk alloy exhibits good stabilization of supercooled liquid with the ?Tx of 28 K, and excellent mechanical properties with the compressive strength of 1240 MPa, special strength of 348 kN. m/kg, and plastic strain of 0.4%. Bulk samples were consolidated of the Al₈₅Ni_6.8Co_2.2Er₄La₂ MG powders by SPS, suggesting the potential for application as a new lightweight structural material.