| The Al-0.2Sc-0.04(Zr, Yb) alloys(wt.%, weight percentage, the same as following) were prepared with high pure Al, Al-1.85 Sc,Al-4.38 Zr and Al-5.08 Yb master alloys by conventional ingot metallurgy method. After homogenization at 640 oC for 24 h, samples were water-quenched to the room temperature and aged at different temperatures for 189 min. The effects of the partial substitution of Yb on Zr on the creep property and electrical conductivity of Al-0.2Sc-0.04 Zr alloys were studied. Optical microscopy(OM), scanning electron microscopy(SEM), and transmission electron microscopy(TEM) were used for the microstructure observation. The creep mechanism of Al-0.2Sc-0.04(Zr, Yb) alloys was investigated.The ambient-temperature tensile tests of Al-0.2Sc-0.04(Zr, Yb) alloys aged at different temperatures for 189 min show that there includes three aging period: under-aging, peak-aging and over-aging period without incubation period. The partial substitution of Yb on Zr can further improve the strength of Al-0.2Sc-0.04(Zr, Yb) alloys. The optimum ageing region is 300-350 oC.The effect of the different heat treatment on the electrical conductivity of Al-0.2Sc-0.04(Zr, Yb) alloys was studied. It was found that with the increase of aging temperature, the electrical conductivity was continuously improved significantly even aging at 400 oC. The improvement of the electrical conductivity is associated with the formation and growth of secondary Al3(Sc, Zr, Yb) precipitation. Some effects of the partial substitution of Yb on Zr on electrical conductivity were observed, which varies with the aging condition. With the partial substitution of Yb on Zr, the electrical conductivity is improved only after peak aging, and decreases at any other aging condition. It should be associated with the thermal stability of Al3(Sc, Zr, Yb) precipitations, and the distortion caused by the solid solution elements in Al matrix. At peak aging, the relative electrical conductivity of Al-0.2Sc-0.04 Zr and Al-0.2Sc-0.02Zr-0.02 Yb is 59.8%IACS and 60.8%IACS, respectively. The corresponding relative electrical conductivity is 63.6%IACS and 63.3%IACS after over aging. All the electrical conductivity exceeds the national standard(58%IACS).The uniaxial tensile creep tests of peak aged Al-0.2Sc-0.04(Zr, Yb) alloys at 150-300 oC shows that the minimum creep rate ε& of Al-0.2Sc-0.02Zr-0.02 Yb alloy is much smaller then that of the Al-0.2Sc-0.04 Zr alloy at the same creep conditions. It suggests that the partial substitution of Yb on Zr in Al-0.2Sc-0.04 Zr alloy can improve the high-temperature creep resistance. The surface persistent slip bands(PSBs) observation shows that the spacing among PSBs increases with increasing temperatures. The intersection of different PSBs observed at higher temperatures indicates that there exists the operation of several slip systems. At the same creep temperature, the spacing among PSBs increases and becomes curved with the partial substitution of Yb on Zr.Using the stress increment method, the creep rates at different stresses were obtained for Al-0.2Sc-0.04(Zr, Yb) alloys. It is found that the apparent stress exponent(n) of Al-0.2Sc-0.04(Zr, Yb) alloys increases rapidly with increasing temperatures. At the same creep temperatures, the n increases apparently with the partial substitution of Yb on Zr. The n is 24-56 and 38-86 for Al-0.2Sc-0.04 Zr alloy and Al-0.2Sc-0.02Zr-0.02 Yb alloy, respectively, which is remarkable larger than that of pure Al(n=4.4). The results suggest that there should exist the threshold stress thσ for the creep of Al-0.2Sc-0.04(Zr, Yb) alloys. Using the traditional method, the 1/4.4tε& should increase linearly with increasing tσ by assuming the same n(4.4) of the pure Al. The creep activation energy Q and thσ at different temperatures can be obtained. Since the creep data at 150 oC is apparent deviated from those data of 200-300 oC, only the latter was considered for the discussion of t1/4.4t&~ σε relationship. The obtained activation energy Q for Al-0.2Sc-0.04 Zr alloy and Al-0.2Sc-0.02Zr-0.02 Yb alloy is 134.82 k J/mol and 139.64 k J/mol, respectively, which are all slightly higher than that(124 k J/mol, the self-diffusion energy) of the pure Al. The thσ deceases almost linearly with the increasing temperatures for both alloys. By liner fitting, the thσ for Al-0.2Sc-0.04 Zr alloy and Al-0.2Sc-0.02Zr-0.02 Yb alloy can be expressed as T1314.01.100th-=σ,and T1275.02.100th-=σ, respectively. The results show that the partial substitution of Yb on Zr in Al-0.2Sc-0.04 Zr alloy can indeed increase the threshold stresses.In all, although the partial substitution of Yb on Zr can only improve the electrical conductivity of peak aged Al-0.2Sc-0.04(Zr, Yb) alloys, it can improve both the ambient-temperature tensile properties and high temperature creep resistance. The creep of Al-0.2Sc-0.04(Zr, Yb) alloys can be attributed to the high temperature creep controlled by dislocation climb with the existence of threshold stresses. |
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