Synthesis And Properties Of Al-based Bulk Metallic Glasses And Their Nano-composites

It is a keypoint to produce the Al-based metallic glass with high glass-forming ability (GFA). Compared to crystalline materials, Al-based metallic glasses have significant advantages such as high specific strength and good ductility. The properties will be enhanced by partial crystallization. The main limitation of the Al-based metallic glass in application is its lower GFA contrast to other BMGs. The ribbons and powders of full amorphous have been obtained in Al-based metallic glass but up to now the bulk samples are not obtained. Therefore, it will be necessary to developing the high GFA of Al based metallic glass and prepared the bulk samples as the engineering structure materials.One of this works is focused on elucidating the characteristic of GFA in Al-based metallic glasses. Based on the understanding of the existent structure models and medium-range order of metallic glasses, along with the chemical factor, especially the electronegativity factor, the Al-based metallic glasses with high GFA are potentially designed.Another aim of this research is to prepare amorphous Al86Ni6Y4.5Co2La1.5powders by gas atomization and analyse its micro structure and thermal stability. The influences of cooling rate and oxygen content on microstructure and primary phase as well as consolidation process of Al86Ni6Y4.5Co2Lai.5powders are investigated. The microstructure and mechanical properties of bulk samples are evaluated to establish the relationship between the consolidation variables and the mechanical properties. The main results are summaried as follow:(1) In Al-TM-RE systems, it contains the TM-centered cluster and RE-centered cluster in which the TM-centered cluster could enhance the GFA of systems. The types and numbers are beneficial to improve stacking cluster density and the stability of local intrinsic structure, further affect the GFA of metallic glasses effectively. Meanwhile, the average electronegativity of the best GFA in Al-based metallic glasses is1.61. Through combining the cluster stability lines model with average electronegativity the best glass forming compositions are developed. We first prepared the Al-based bulk metallic glasses on the base of the cluster stability lines model. Their compositions were Al86Ni6Y4.5Co2La1.5, Al86Ni7Y4.5Co1La1.5and Al86Ni7Y5Co1La1, respectively. The critical glass forming size is1mm. No obvious glass transition during crystallization is detected. It is found that the supercooled liquid region and Trg are not in accordance with classical glass forming criterions.(2) The compressive fracture strengths of Al-based metallic glasses are1050-1140MPa, the specific strength is about3.4×105N m kg-1, which are higher than the usual engineering structure materials. The prospect of its application will be extensive.(3) The Al86Ni6Y4.5Co2La1.5amorphous powders produced by gas atomization are near spherical. The XPS results indicate Al, Y and La formed on the powder surface, the powder surface is oxidized and covered by Al2O3with the thickness of about30nm, resulting in the decrease of powders GFA. The critical cooling rate of powders is1000K/s, the thermal stability are high by calculating apparent activation energy. When the powder size less than25μm, they exhibit full amorphous or partially crystalline structure up on devitrification. The powders present three crystalline phases, Al2Y phase, Al phase and Al71.6Ni13Co4.4Y7.4O3.6phase, and the Al2Y phase is formed as the primary phase.(4) When the consolidation temperature is530K, the density and hardness of samples reached the best values, the fracture strength is800MPa. The sample is composed of amorphous phase and fcc Al phase. The strength exhibits a decreased trend due to a large number of existent holes. When the casting temperature and frequency of vibration are1300℃and50Hz, the microstructure of the semi-solid samples are refined. Meanwhile, the strength and ductilities decrease due to the increased needle-like structure.