Elemental Effect on Glass Forming ability of RE-TM-AL Alloys

Abstract

Metallic glasses are metals or alloys without long-range atomic order. In the previous decades metallic glasses have gained considerable interests due to the fact that a great number of new glass-forming alloys have been found with a critical cooling rate of less than 103 K/s and can be made into bulk glassy ingot with a dimension of 1 mm or more. It has been summarized that most of these alloys are multi-component with more than three elements. The pinpoint strategy, which was recently developed based on the phase selection process of the glass over all the competing crystalline phases in the undercooled liquid, has been successfully applied in binary, ternary and quaternary alloy systems to locate the best glass forming compositions. The purpose of the present study is to obtain a better understanding of the above two issues. Firstly, systematic research was performed in RE-TM-Al (RE = La and Ce, TM = Co, Cu and Ni) alloys. Effects of transition metals on the glass forming ability of La-TM-Al ternary alloys were discussed. The best metallic glass formers in each alloy system (La-Co-Al, La-Ni-Al and La-Cu-Al) were identified by applying pinpoint strategy. Compositions, maximum sizes of fully glass rods and several indicators related to glass forming ability were compared. Although transition metals such as Co, Cu and Ni have similar atomic sizes as well as interactions with the solvent atom La, their effects on the glass forming ability differ sharply, with the critical size ranging from 16 mm to 5 mm in the sequence of Co > Ni > Cu. In order to explain the phenomenon that transition metals significantly affect the glass forming ability of La-rich alloys, efficient cluster packing atomic structural model was employed to describe packing details of the metallic glasses. This structural model predicted a number of compositions for good GFA, which matched well with experimental results. For example, in La-Co-Al alloy system, the optimum for glass formation compositions, La69Co17Al14 and La67Co17Al16 are quite close to the one given by ECP model, La69.4Co15.3Al15.3, which indicated that the two alloys had achieved high packing efficiency. Therefore, this model can help to provide both effective hint for locating the best glass former in the La-TM-Al (TM = Co, Cu and Ni) ternary alloy systems and reasonable description of short to medium range atomic arrangement of metallic glasses. Secondly, the effects of rare-earth elements (such as La and Ce) on glass forming ability of RE-Al-Co (RE = La and Ce) alloys were investigated. The best glass former Ce61Co19Al27, with critical size of 6 mm in diameter in Ce-Co-Al ternary alloy system was located. Studies on glass forming ability were moved forward to (La,Ce)-Co-Al system by replacing certain amount of La with Ce. With the help of the developed pinpoint method for quarternary alloys, the optimized glass formation was finally achieved in alloy (La0.7Ce0.3)64Ce21Al15, which can form glassy rod with 25 mm in diameter. This result demonstrated the effectiveness of the pinpoint strategy and also showed contribution of elemental effect study on glass forming ability. Modified efficient cluster packing model was employed to explain how atoms packed in these fully metallic glasses and it is suggested that volume strain caused by atomic size would affect the atomic packing method, and hence resulted in the discrepancy in the optimized glass forming compositions.