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本文进行了淬火状态的、含0.52,0.91,3.46和5.15%Mg的铝合金的扭转疲劳试验,测定了相应的ΔE-N曲线和Tm-N曲线。实验结果指出,对于含镁量为0.52,0.91%的试样来说,当表面扭应变较小时,ΔE在起始时,随着应力循环数的增加而下降。当表面扭应变增大时,ΔE-N曲线始而变平,继而上升,直至达到一较高值才稳定下来。当试样中的含镁量为3.46%时,在扭应变不太大时,ΔE-N曲线的变化情况与Al-4%Cu合金的相象,不过当扭应变足够大时,ΔE起始时上升,并且经过一个峯值又下降。当含镁量增至5.15%时,ΔE-N曲线的表现已完全与Al-4%Cu合金的相象,在所用的最高表面扭应变下也并不表现出明显的峯值。对于所用的各种成分的试样来说,最大抗扭矩Tm起始总是上升的。上述结果都可以根据溶质镁原子在疲劳过程中渐渐进入位错,形成气团来解释。可以认为,在铝镁合金的情形,产生ΔE的因素以及影响ΔE的大小的因素,对于疲劳载荷的起始阶段来说,可能都主要是由于气团的作用。当含镁量较低时,对于足够高的表面扭应变来说,气团较为松动,位错能够拖着气团运动,从而需要作功,使ΔE和Tm都上升。但当合镁量较高时,或表面扭应变不太大时,在疲劳一起始就形成了能够对于位错起钉扎作用的足够浓的气团。继续进行疲劳时,进入位错的溶质原子将使位错的动性进一步降低,导致ΔE起始下降,Tm起始上升。此外,还对于经过不同时效处理的Al-0.52%Mg和Al-3.46%Mg合金进行了疲劳试验,观测到应变时效现象,这与上述的溶质原子气团模型相合。Torsional fatigue experiments were carried out with aluminium-magnesium alloys containing 0.52, 0.91, 3.46 and 5.15% magnesium at the quenched state, and △E-N and Tm-N curves were determined. Experimental results showed that, in the cases of the specimens containing 0.52% and 0.91% magnesium, △E drops with an increase of the stress cycle N when the torsion strain is small. However, when the torsion strain is higher, the △E-N curve keeps flat at the beginning, but △E increases subsequently and becomes stable after a fairly high value is reached. As the magnesium content in the specimen is 3.46%, the change of the △E-N curve is similiar to that of the Al-4% Cu alloy when the torsion strain is not too high. When the torsion strain is sufficiently large, △E increases at the beginning and decreases after it passed through a maximum value. The change of the △E-N curve of the specimen containing 5.15% magnesium is entirely similar to that of the Al-4% Cu alloy and no peak value was observed on the curve with the highest torsion strain used in the experiment. The Tm value shows an initial increase for specimens with all the magnesium contents studied.The results described above can all be explained with the assumption that "atmospheres" of solute atoms are formed because of the gradual migration of magnesium atoms to dislocations during fatigue loading. It is considered that, in the case of aluminium-magnesium alloys, the origin of the energy loss, △E, and the factor that controls the magnitude of △E in the initial stage of fatigue loading are all primarily due to the formation of atmospheres of solute atoms. When the magnesium content is relatively small, the atmospheres formed may be considered as "loose" for a sufficiently high torsion strain, so that a dislocation can drag its atmosphere along and work is done under fatigue loading. This leads to an increase of both △E and Tm. However, when the magnesium content is relatively high or the torsion strain is not sufficiently large, the atmospheres formed at the beginning of fatigue loading become already "dense" enough to pin the dislocations. With the continuation of faligue loading, the mobility of dislocations is further lowered by the migration of solute atoms to these dislocations. This leads to an initial decrease of △E and an initial increase of Tm. Fatigue experiments were also carried out on Al-0.52% Mg and Al-3.46% Mg alloys at various aging stages, and the phenomena of strain aging were observed. Such phenomena are in accord with the assumption of atmospheres of solute atoms described above.
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