| By melting the mixture of high purity Al, Al-1.85 Sc, Al-4.38 Zr, and Al-5.08 Yb master alloys, the Al-0.2Sc-0.04 Zr and Al-0.2Sc-0.02Zr-0.02Yb(wt.%,) alloys were obtained. The as-cast alloys homogenized at 640 oC for 24 h were water quenched. The optimum ageing region was obtained by room temperature tensile tests of alloys annealed for 189 min at different temperatures with under the same strain rates. The tensile tests of peak aged alloys were carried out at different temperatures and strain rates. With the observation of OM, SEM, and TEM, the influence of deformation on microstructure was observed. The effect of heat treatments and substitution of Yb on Zr on the tensile properties of alloys at room and high temperatures was analyzed. The deformation mechanism at high temperatures of Al-0.2Sc-0.04(Zr, Yb) alloys was also discussed.The OM observation shows that the grain size of Al-0.2Sc-0.02Zr-0.02 Yb is about 0.6mm, which is apparently smaller than that of Al-0.2Sc-0.04Zr(~1mm). It indicates that the substitution of 0.02%Yb on Zr can result in the refinement of alloys. The room temperature tensile tests(1.7×10-3s-1) show that the yield strength σ0.2 and tensile strength σb initially increases, and then decreases with increasing ageing temperatures. The optimum ageing region is 300~350oC. At the same ageing temperature, the substitution of 0.02% Yb on Zr can improve apparently the room temperature strength of alloys. The amount of improvement varies with ageing temperatures. The maximum improvement is obtained in under ageing, less in over ageing, and the minimum in peak ageing. The level of the improvement of strength is in the range of 10~30%.The tensile tests at constant strain rates(1.7×10-4s-1) in 100~300oC of peak aged Al-0.2Sc-0.04(Zr, Yb) show that both σ0.2 and σb decrease markedly with increasing deformation temperatures. At the same temperature, the substitution of Yb on Zr can improve both the σ0.2 and σb. As a whole, the amount of improvement of the σb is 10~12.8% while that of the σ0.2 is 5.3~8.7%, which are equivalent to that observed in room temperature tensile tests. The elongation δ(20~40%) of alloys initially increases, then decreases slowly with increasing temperatures, which is apparently higher than that(15~24%) obtained by room temperature tests. In addition, there is some influence of deformation temperature and substitution of Yb on Zr on the ratio of yield strength(σ0.2/σb). The σ0.2/σb of both alloys is below 90% at room temperature, and 90~96% at high temperature. With the substitution of Yb on Zr, the σ0.2/σb will increases at 20 and 100 oC, and decrease at temperature higher than 150 oC. It is to be noted that the total strain before yield is below 0.5% at room temperature, and about 3% at high temperatures. It should be associated with the occurrence of the apparent anelastic deformation at high temperatures.The relationship between the true strain rate tε and true stress tσ at different temperatures was obtained by the method of strain rate increment. It is found that the tt ~ σε relationship cannot be satisfyingly explained by the conventional model of activation volume, which is widely adopted to explain the deformation mechanism of alloys during high temperature tensile test with constant strain rates. Using the creep model with the existence of threshold stress thσ, exp()(A/)k TQn thtt-σ-σ=ε, where A is a constant, n is the true stress exponent, and Q is the activation energy, the tt ~ σε relationship was discussed. Taking n=4.4(the stress exponent of the creep of pure Al), the Q can be calculated, which is apparently smaller than that of the Q of the self-diffusion of pure Al(1.29 e V). For n=8(the stress exponent of alloys with constant substructure), it is found the parameter exp(A-/)k TQ shows apparent abnormality. It indicates that a transformation of deformation mechanism should exist at 150 oC. For temperature higher than 150 oC, the parameter exp(A-/)k TQ changes significantly with the temperature. The obtained activation energy is slightly higher than 1e V, but still some smaller than that of pure Al. If only the data above 200 oC were considered, the Q for Al-0.2Sc-0.04 Zr and Al-0.2Sc-0.02Zr-0.02 Yb are 1.23 and 1.26 e V, respectively, which is in good agreement with that of pure Al. In addition, the substitution of 0.02% Yb on Zr can increase slightly the threshold stress of alloysAbove all, the substitution of 0.02% Yb on Zr can further improve the tensile properties of Al-0.2Sc-0.04(Zr, Yb) alloys. The high temperature deformation mechanism of alloys should be attributed to be the deformation mechanism with constant submicrostructure, not that of pure Al. |
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