不同时效状态的含铜4%的铝合金在疲劳载荷下溶质原子与位错的交互作用

为了进一步研究在疲劳载荷下含铜4％的铝合金中的位错钉扎过程,进行了经过不同时效的试样的扭转疲劳试验,测定了经过各种应力循环数N以后的滞后迴线的形状和面积,从而算出了在每次循环中的能量消耗ΔE和最大抗扭矩T_m。所选择的时效温度和时效时间是使试样中分别有G.P.1]区,G.P.2]区,θ′相和稳定的θ相出现。把所得的ΔE-N曲线和T_m-N曲线的变化情况作比较时可以看出,在疲劳载荷的起始阶段引起位错钉扎的并不是由于相变产物如G.P.1]或2]区的作用。比较并分析了在各种时效状态下的第一周能量消耗值(ΔE)₁的变化,结果指出,在所研究的铝铜合金的情形,产生ΔE的原因是由于在位错附近的点阵中有起伏的内应力场出现,因为位错在这种内应力场中往复运动需要作功。产生这种起伏的应力场的因素有点缺陷(空位和溶质原子)、原子簇、G.P.1]区和G.P.2]区,或者其他种不在位错线上聚集或成核的缺陷。根据上述分析,可以认为,在疲劳载荷中,使位错钉扎的是由于溶质原子气团的形成。溶质原子在疲劳过程中通过空位的帮助进入位错,形成气团,使位错被钉扎。被钉扎的位错的动性减低,因而ΔE下降。在时效过程中,在位错线上成核的θ′和θ相,对于位错线也起着一定的钉扎作用。由上述的图象还可以推知,G.P.1]区和G.P.2]区不是在位错线上成核的,而θ′相和θ相则是在位错线上成核的。

INTERACTION OF SOLUTE ATOMS WITH DISLOCATIONS IN Al-4% Cu ALLOY AT VARIOUS AGING STAGES UNDER FATIGUE LOADING

Torsional fatigue experiments were carried out with Al-4% Cu specimens at various aging stages in order to study further the dislocation pinning process occurring under fatigue loading. The area and the shape of the hysteresis loop after various stress cycles N were determined, from which the energy loss AE in each cycle and the maximum forque T_m of the specimen were calculated.The temperature and the time of aging were so chosen that G. P. 1] zone, G. P. 2] zone, θ′ phase or the stable θ phase appeared respectively in one of the specimens. On the basis of comparing the mode of changes of the experimental curves of △E-N and T_m-N, it is concluded that the dislocation pinning occurring in the initial stage of fatigue loading is not concerned with the transformation products such as the G. P. 1] zone and G. P. 2] zone.The changes of the energy loss in the first cycle of fatigue loading, (△E)₁, of specimens at various aging stages were compared and analysed, and it is shown that, in the case of the Al-Cu alloy studied, the energy loss △E is produced because of the occurrence of a fluctuating internal stress field in the lattice near a dislocation, since work has to be done when the dislocation moves to and fro in such a stress field. This fluctuating internal stress field may appear because of the occurrence of point defects (vacancies and solute atoms), atom clusters, G. P. 1] zone, G. P. 2] zone, or other defects when they do not segregate or nucleate on the dislocation line.On the basis of the above analysis, it may be considered that the dislocation pinning process occurring under fatigue loading is associated with the formation of atmospheres of solute atoms. The solute atoms migrate to dislocations in the process of fatigue loading with the help of the vacancies produced in fatigue loading, so that atmospheres are formed around dislocations. The mobility of these pinned dislocations is reduced, and consequently △E is lowered. The θ′ and θ phases that nucleate on dislocation lines during the process of aging can also pin the dislocations to a certain extent.It can also be contemplated from the above picture that the G. P. 1] and G. P. 2] zones do not nucleate on a dislocation line, while the θ′ and θ phases do.