Study of Al-RE-Ni Bulk Amorphous/Nanocrystalline Hybrid Composites Synthesized by Spray Forming and Ribbons by Melt-Spinning

• Main Authors: Ming-Lang Guo, 郭敏郎
• Other Authors: Chi-Yuan Tsao
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/23936256281407524941

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 94 === Owing to the requirement of high critical cooling rate, Al-RE-Ni amorphous alloy can only be generated in the form of ribbons or powders. After secondary processing step, like warm extrusion, ribbons or powders can then be consolidated into a bulk material and act as a nano-structured aluminum alloy with strength much higher than the conventional aluminum alloys. However, the high processing cost makes it not feasible for mass production. In this study, a powder metallurgy technique, spray forming, was employed to produce a bulk amorphous/nanocrystalline alloy in one step without any precursor. The unique microstructure and improved mechanical properties are also observed. The results show that spray-formed Al-RE-Ni composites, in which 36–64% amorphous phase combined with retained crystalline phase, are characteristic of "primary crystal" coexisting with "amorphous matrix". The influence of spray forming process on the Al-RE-Ni alloy are "parallel defects generation" and "blunt-shaped primary crystals". During spray forming, the severe temperature gradients and the large stirring and impacting stress during droplets depositing, together with the mismatch of the thermal expansions between the primary crystals and adjacent amorphous matrix, are sufficiently large to trigger the formation of defects in the primary crystals. Additionally, the growth of primary crystals can also be adjusted by multi-direction (about 60o) defects and end up with a blunt-shaped form, for example the Al11La3(Ni) primary crystals in Al89La6Ni5 specimen. However, some alloy (e.g. Al85Nd5Ni10) can only generate defects without intersection or at 90o intersection in the primary crystals. Therefore, the Al7NdNi2 primary crystals in the Al85Nd5Ni10 specimen are still dendritic in shape. The spray forming process can be divided into three stages. In the first stage, droplets are generated by atomization in the form of completely undercooled liquid, primary crystals in liquid, amorphous solid particles and completely crystallized particle. In the second stage, upon droplets impacting on the substrate or free surface of deposit, a semi-solid layer is formed consisting of undercooled liquid+liquid+primary crystal, solidified particles with primary crystals being deformed upon impacting, solidified amorphous particles and completely crystallized particle. In the third stage, amorphous phase devitrifies to form secondary crystals and semi-liquid layers solidifies into a mixture of amorphous phase, primary and secondary crystals. The corresponding melt-spun ribbons are completely amorphous, and can devitrify to nanocrystals upon heating. Because the sluggish crystallization mechanism, it is suitable for the investigation of nucleation and growth reaction in the amorphous solid. Upon isothermally held at sub-Tg temperature, Al89La6Ni5 ribbon prefers concurrent precipitation of fcc-Al nanocrystals and intermetallic intermediate phases; Al85Nd5Ni10 ribbons prefers primary crystallization of fcc-Al nanocrystals. While produced at lower wheel velocity, both ribbons become harder. Concurrent precipitates in the former induces embrittlement, but primary crystallization in the later remains relatively stable. As subjecting the spray-formed amorphous/crystalline nanocomposite to high temperature dynamic tensile test, viscous flow of supercooled liquid is inhibited by the primary crystals. More primary crystals provide increased resistance to flow, as well as decreased loss modulus. The extension of specimen length is also difficult and the maximum value achieved is only 1%. Moreover, it is also observed that the thermal extraction coefficient of melt-spun ribbons are large at sub-Tg temperature, and dramatically increases four times at supercooled liquid region. However, the coefficient of spray-formed nanocomposite is extremely small at sub-Tg temperature, and slightly increases at supercooled region, about one-tenth of the one for ribbons. More primary crystals will retard further extraction. In summary, spray-formed Al-RE-Ni amorphous/nanocrystalline nanocomposites are of great potential for high temperature application. In addition, due to the presence of primary crystals together with the nanocrystal precipitate in the matrix, hardness and modulus of the composites are improved and the thermal extraction behavior is also adjusted. It makes another process route for manufacturing Al-RE-Ni amorphous alloy possible. In the future, applications can be thermal-resistant precision parts, wear and corrosion resistance surface coating or sporting goods, etc.