Laws And Mechanisms For The Effects Of Pr_xO_y On The Microstructures And Properties Of The Cast Al-Cu Alloys

Recently, there is growing interest in Al alloys because of their interesting properties such as high strength, good ductility, excellent mechanical properties at high temperature and cast ability. Generally, the Al alloys with good mechanical properties applied for the structural materials are used at high temperature, such as Al alloy engine body. It is very important to investigate the creep resistance at high temperature and understand the hot processing parameters. In comparison with the study of the creep behavior for the wrought Al alloys, the creep behavior of the cast Al alloys is not well understood. However, in the cast Al-Cu alloys (such as ZL205A), the precipitations of small coherent Al₂Cu particles increase their tensile strength, but Cu and other alloy elements or impurities, like Fe, which also precipitate as larger intermetallics particles forming a very heterogeneous microstructure and leading to the bad corrosion resistance of these alloys. In order to meet the demand of aircraft construction, space technology, high-speed train, automobile manufacturing and the sailing vessel engine body, it is necessary to investigate the mechanical properties, creep resistance and corrosion resistance of the cast Al-Cu alloys.In the present study, it is expected to increase the strength and ductility, improve the creep resistance at high temperature and enhance the corrosion resistance of the cast Al-Cu alloy by Pr_xO_y addition. Therefore, the research of the effect of Pr_xO_y on the microstructure and properties of the cast Al-Cu alloys and their mechanisms has great significance. The major research efforts of the present study are as follows:(1) Pr_xO_y was decomposed to form AlPrO₃, which acted as the effective heterogeneous nuclei for the crystallization of the primaryα-Al phase by measurement of the cooling curves and analysis of two-dimensional lattice misfit. It is because there are more nuclei in the modified Al-Cu alloy by Pr_xO_y that the crystal grains and dendrites in the modified Al-Cu alloy are much finer than those of the unmodified Al-Cu alloy.(2) The rosette-like microstructure formation of the cast modified Al-Cu alloy was investigated under different cooling rate due to different rapid solidification conditions, i.e. the melt-spun and spurt casting method. At the initial solidification stage, PrAlO₃ phase forms prior toα-Al phase crystal from the liquids, acting as the effective heterogeneous nuclei forα-Al crystals and enhancing the amount of nuclei forα-Al crystals. At the cooling rate of 3.65×106 K/s, only a few uniformly distributed refinedα-Al crystals have the chance to join together and form large grains. When the cooling rate is 105¹03 K/s, some refinedα-Al crystals have more opportunity to coalesce each other forming even larger grains to reduce the surface energy. As the cooling rate is 102 K/s, the coalescence and growth of many refinedα-Al crystals lead to the rosette-like microstructure formation.(3) The average width and length of the regular, needle-networkθ′phase in the modified Al-Cu alloy by Pr_xO_y were about 10 and 120 nm, respectively. At the same time, the distribution of the nanoscaleθ′phase was regular and homogeneous. However, there were only a few needle-shapedθ′phases with a width of 10-15 nm and length of 120-140 nm in the unmodified Al-Cu alloy.(4) A modified cast Al-Cu alloy with ultrahigh tensile strength and ductility of about 520 MPa and 13.5% was obtained by Pr_xO_y addition. The simultaneous increasing of the strength and ductility of the modified alloy may be mainly attributed to the effect of a large number of regular and homogeneous nano-scaleθ′phase precipitates and more crystal grain and dendrite boundaries formed by their refinement on restricting and impeding the dislocation actuation and movement.(5) The modified cast Al-Cu alloy is found to have higher ductility at lower strain rates (10-3s-1 and 10-4s-1). The results show that the nano-scale precipitates growth phenomenon is mainly responsible for the higher ductility of the present alloy at lower strain rates. The nano-scale precipitates which grow during the deformation process decrease the sites of the stress concentration, helping further plastic deformation to enhance the ductility. Besides, the dislocation motion is mainly responsible for the tensile plastic deformation of the present alloy at relatively high strain rates (10-1s-1 and 10-2s-1).(6) The apparent stress exponents and creep activation energies of the modified and unmodified Al-Cu alloys are 12.4-18.5, 173.5 kJ/mol and 12.9-17.2, 150.2kJ/mol, respectively, which has been explained by means of introducing a threshold stress. The induction of a threshold stress in the analysis leads to a stress exponent of 5, which suggests that the creep behavior of both the present alloys is associated with the lattice diffusion-controlled dislocation climb (n=5). Creep behavior of the unmodified and modified cast Al-Cu alloys was investigated at temperatures from 393 to 483K in the tension test. The creep resistance ability of the Al-Cu alloy modified by Pr_xO_y is almost 3-5 times as high as that of the unmodified Al-Cu alloy, which is attributed to a large number of nano-scaleθ′precipitates with high thermal stability in the modified Al-Cu alloy restricting and impeding the dislocation movement during the creep.(7) It was found that the average weight loss of the alloys modified by Pr_xO_y was lower than that of the unmodified alloys under the same conditions. The corrosion potential Ecorr (^–1.07V) of the modified alloy by 1.5wt.%Pr_xO_y shifted positively about 440 mV compared with that of the unmodified alloy. The impedance value of the modified sample by Pr_xO_y was much higher than that of the unmodified sample. These results indicated that the modified Al-Cu alloy by Pr_xO_y had better corrosion resistance than the unmodified Al-Cu alloy.(8) The modified Al-Cu alloy by Pr_xO_y had better corrosion resistance than the unmodified Al-Cu alloy, which was mainly attributed to a proposed mechanism, i.e. the dominated existence of continuous and compact protective Al₂O₃ and Pr₂O₃ films enhanced the corrosion resistance of the modified sample by Pr_xO_y during corrosion, while no continuous and compact protective Al₂O₃ films led to the poor corrosion resistance of the unmodified sample.