| The Al-0.2Sc-0.04Zr (wt.%:weight percentage, the same as the following) alloys were prepared by melting method with appropriate amounts of99.991wt.%high pure Al, Al-4.38Zr and Al-2.12Sc master alloys. All cast alloys were homogenized at650℃for48h, and water quenched. According to the optimizing results of tensile properties at room temperature and electrical conductivity, the alloys were treated with pre-ageing at330℃for189min+88.5%cold-rolling+re-ageing at330℃for60min and88.5%cold-rolling+ageing at330℃for60min, respectively. The uniaxial tensile creep tests of Al-0.2Sc-0.04Zr alloys after two treating methods (pre-ageing+cold-rolling+re-ageing and cold-rolling+ageing) were conducted at225,250,275, and300℃, respectively. The influences of temperatures and stresses on the creep behaviour (the minimum creep rate εmin and creep life) were obtained. Combined with the OM, SEM, and TEM observations, the creep mechanism of Al-0.2Sc-0.04Zr alloys was analyzed.For Al-0.2Sc-0.04Zr alloys after the two kinds of treating methods, the minimum creep rate εmin increases rapidly with the increasing stresses and tempertures. For alloys after pre-ageing+cold-rolling+re-ageing, the εmin is in the range of1.0×10-8to1.2×10-4s-1with the applied stress of30～103.7MPa. The apparent stress exponent of creep n decreases quickly from about20at both225and250℃to about10at both275and300℃. At the same time, for alloys after cold-rolling+re-ageing, the εmin is between2.8×10-8and3×10-5s-1for the applied stress of55～117MPa. The n is in the range of18～24. For both treating methods, the n (10～24) is apparently larger than that of pure Al (4.4), which suggests that the threshold stress σth should exist for the creep of Al-0.2Sc-0.04Zr alloys after both pre-ageing+cold-rolling+re-ageing and cold-rolling+ageing.Following the method provided in literature, the expression of εmin of Al-0.2Sc-0.04Zr alloys after pre-ageing+cold-rolling+re-ageing is obtained by assuming the same n (4.4) of pure Al:εmin=1.763×10-7[σ-σth(T)]4.4exp(-(47905J/mol)/RT); furthermore, the σth decreases linearly with increasing temperature, and σth=351.9-0.5731T was obtained by linear fitting. Using the same method, the εmin is εmin=3.17×10-6(σ-σth(T)]4.4exp(-64268J/mol/RT) for Al-0.2Sc-0.04Zr alloys after cold-rolling+ageing. The σth=351.9-0.5731T was also obtained by linear fitting.For Al-0.2Sc-0.04Zr alloys after the combining treatment of cold-rolling and ageing, there exists many fine cell structure and subgrains. In addition, small amounts of Al3(Sc,Zr) precipitates can be observed in the cell structure and subgrains. Most Al3(Sc,Zr) precipitates are located at the cell boundaries and subgrain boundaries. After creep rupture, the (sub)grains changes from the lath shape at225℃to equiaxed grains at300℃. Furthermore, the dynamic recovery and recrystallization is more apparent. The diffraction pattern of TEM observation on the creep rupture specimens shows that the Al3(Sc,Zr) precipitates are coherent with the Al matrix.Although the room temperature tensile strength σb (184MPa) of cold-rolling+ageing Al-0.2Sc-0.04Zr alloys is smaller than that (213MPa) of pre-ageing+cold-rolling+re-ageing Al-0.2Sc-0.04Zr, the σth (50MPa) of the former is much higher than that (23MPa) of the latter. However, the ath of Al-0.2Sc-0.04Zr after both treating method is apparently higher than that (17MPa) obtained in Al-0.23Sc-0.04Zr (Al-0.14Sc-0.012Zr at.%). It suggests that the combining treatments of cold-rolling and ageing can improve not only the tensile strength at room temperature, but also the creep resistance at high temperature. It should be noted that the activation energy (64.3kJ/mol for cold-rolling+ageing alloys and47.9kJ/mol for pre-ageing+cold-rolling+re-ageing alloys) is much smaller than that (close to the activation energy of pure Al142kJ/mol) obtained in the literature. It is equivalent to the activation energy of vacancies and grain boundaries. The much smaller activation energy may be associated with the fine grain sizes (～μm). The mechanism is not clear yet, and needs further investigations.In all, the obtained results show that the combining treatment of cold-rolling and ageing can not only further improve the tensile strength at room temperature but also enhance the creep resistance at high temperature. However, the better creep-resistance at high temperature can be obtained for the treating method which can improve less the tensile strength at room temperature. |
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