| Transition metal elements Zr and Ti play important roles in the strengthening of aluminum alloys by forming primary intermetallics and precipitates. The behavior of Zr and Ti in aluminum alloys were investigated by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and atom probe tomography (APT), etc. The work focuses on both the primary intermetallics formed during solidification process and the precipitates formed during heat treatment process. The structural, compositional and morphological evolutions, the growth pattern of intermetallics, as well as the strengthening effect of precipitates were investigated. The effect of intermetallics and precipitates on the microstructure and mechanical properties, especially the properties at elevated temperature were conducted. The main results can be describes as follows:(1) The formation and evolution of Zr- and Ti-containing intermetallics in aluminum alloysThe intermetallics formed in ternary Al-Si-Zr/Ti, Al-Zr-Ti and quaternary Al-Si-Zr-Ti alloys have been characterized. It was found that the phases formed in Al-Si-Zr/Ti alloys are ternary ZrAlSi and TiAlSi. With the increase of Si content in the alloys, the phases evolve from tetragonal TiAl3 to Ti7Al5Si12 and from (Al, Zr, Si) to orthogonal ZrSi2. Also, the composition of the intermetallics evolves synchronously, i.e. leading to a change from chemical formula M(Al1-x,Six)3to M(Si1-x,Alx)2 (M=Ti, Zr), due to the replacement of Al and Si atoms to some extent in the crystal lattice.The intermetallics formed in Al-Zr-Ti and Al-Si-Zr-Ti alloys are AlTiZr and AlSiTiZr, i.e. Ti and Zr participate in the formation of intermetallics at the same time. Similar with the replacement law between Si and Al atoms, Ti and Zr can also replace each other to some extent in the crystal lattices. Besides, the composition of Zr/Ti in the intermetallics has a linear relationship with the content of Zr/Ti in the alloys.Except the structural and compositional evolution, the morphologies and formation process of the intermetallics closely relate to the melting temperature, raw material and composition, etc. of the alloy. Primary ZrAlSi and TiAlSi intermetallics can form through several mechanisms, e.g. reaction and diffusion. When the intermetallics form from the melt, a higher ratio between Si and M (M=Zr, Ti) is more inclined to result in the formation of M(Si1-x,Alx)2, while a lower one to M(Al1-x,Six)3. High melting temperature makes flake-like intermetallics form more easily, while low melting temperature make block-like ones. Besides, using K2TiF6or K2ZrF6 as raw materials tends to form blocky intermetallics. However, in solid-liquid reaction, the intermetallics form through a gradual diffusion process. Si and Al atoms diffuse into solid particles, and the final compounds keep similar morphology with the solid particles. Based on the above, synthesis of the intermetallics can be controlled by changing the melt condition and solidification process.(2) Growth mechanism of the Zr- and Ti-containing intermetallics in aluminum alloys and their control and applicationsThe formation and growth mechanisms of ZrAlSi and TiAlSi crystals can be obtained and the growth pattern can be built by analyzing the three-dimensional morphologies of the intermetallics during different growing stages, with a combination of first-principles calculation. ZrAl3, (Al, Zr, Si) and ZrSi2 crystals were investigated, respectively. As for the intermediate growing stage of ZrAl3, the side faces are formed by eight groups of planes which belongs to{101} and{111}. Due to the different growing rate along<101> and<111> directions,{111} crystal planes disappear when V(111)/V(101)=(?). The final morphology of the crystal is compressed cubic, the upper and lower planes of which are (001) and (001), while the side planes are formed by eight{101} planes. In the growing stage of (Al, Zr, Si) crystal, which also belongs to tetragonal system, the upper and lower planes are also (001) and (001). However, for the side planes, eight{101} planes and four{110} planes which are perpendicular to the upper and lower planes are detected. A similar morphology with ZrAl3, i.e. compressed cubic, forms finally, with the disappearance of{110} planes, due to the diversity of growing rate along different side planes. As for ZrSi2 crystal, the initial growing pattern is same with that of ZrAl3, the upper and lower planes are (001) and (001) and the sides are eight groups of{101} and{111} planes. However, since the crystal has similar growing rate along differnt side planes, in the final crystal, all of the eight groups of side planes reserve, and the final morphology is octadecahedron.TiAlSi crystals perform similar growing pattern with ZrAlSi, i.e. TiAl3 grows into compressed cubic shape like ZrAl3, and Ti7Al5S12 has a final morphology of octadecahedron, with a similar growing pattern with ZrSi2-Based on the investigation about the synthesis and growing pattern of the crystals,TiAlSi and ZrAlSi intermetallics can be controlled and their applications in alloy strengthening has been investigated. It was found that blocky TiAlSi and ZrAlSi particles can improve the mecahnical properties at elevated temperature of an Al-Si-Cu-Ni-Mg alloy, i.e. tensile strength, yield strength and elongation, while flake-like particles exhibit opposite effect. Besides, ZrAlSi particles were introduced into A390 alloy to modify primary Si, and with the combined effect of element Sr, complex modification effect can be obtained along with the obvious increase of properties.(3) Precipitation of Zr-and Ti-containing phases and their effects on microstructures and properties of aluminum alloysRapidly solidified Al-0.5Zr and Al-3Si-0.5Zr alloys were ageing at 525 ℃ for 24h, it was found that β’(L12-ZrAl3) particles precipitate from the supersaturated solid solution, exhibiting coherent relationship with a-Al, while β (DO23-Zr(Al,Si)3) particles were detected in ternary alloy, whose orientation relationship with α-Al is ’cube-on-cube’.The effect of 0.15wt.% Zr on the ageing process of Al-12.2Si-4.1Cu-2.0Ni-1.3Mg alloy was investigated. Zr has changed the condition of the supersaturated solid solution, resulting in the formation of β’ phase during solution treatment (505℃/8h). Besides, it also affects the formation of S (Al2CuMg) phase during ageing process, resulting in the decrease of average size and the increase of volume fraction.An experimental Al-6Zn-2Mg-2.5Cu-0.2Zr alloy has been selected to investigate the effect of Zr and Ti on the precipitations during ageing process, β’particles form during the solution treatment (470℃/8h). When 0.2%Ti was added in the alloy, the combination effect of Ti and Zr modifies the condition of the supersaturated solid solution, i.e. increase the volume fraction of β’(L12-(Zr,Ti)Al3) and decrease the average size. Besides, the change of supersaturated solid solution as well as the precipitation of β’ affect the nucleation of GPⅡ and η’. The η’ particles have a higher volume fraction and smaller average size in the Ti-and Zr-containing alloy, and the hardness, wear property and tensile strengths of the alloy increase synchronously. |
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