铝-0.5%铜合金中反常振幅效应内耗峰的进一步实验和位错弯结气团模型

用低频扭摆进一步研究了在Al-0.5％Cu合金中观察到反常位错内耗峯的条件。结果指出,对于完全退火的试样来说,需要有适当的冷加工量,但是对于高温淬火试样则不需要冷加工。用位错气团模型定性地解释了过去所观测到的表现反常振幅效应的时效内耗峯和温度内耗峯;同时指出,简单的气团模型在作定量的解释时,遇到了下述的困难:(i)在测量内耗所用的交变应力的作用下,位错线所能够拖着气团移动的距离太短。(ii)为了气团能够被位错拖着移动,组成气团的溶质原子必须具有比通常大很多个数量级的扩散系数。(iii)根据气团模型,从理论上计算出来的使位错拖着气团以临界速度而移动时,所需的临界应力比观测值大几百倍。提出了溶质原子沿着位错弯结而扩散的气团模型,这个改进模型能够初步解决上述困难,并能定性地解释所观测的结果。这个模型所依据的基本假设是,要观测到反常内耗现象,位错线上必须具有一定数目的弯结。要得到这种弯结,可以对于退火试样进行适量的冷加工,或者把试样从高温淬火。带着弯结的位错线能够通过弯结的沿边运动而实现垂直于位错线方向的移动。可以假定,气团只在弯结两端的直位错段处形成,在弯结本身上并不形成气团。在弯结的沿边振动的过程中,聚集在弯结两端的溶质原子可以沿着位错管道进行来回的短程扩散。已知沿着位错管道的扩散具有比在正常晶体点阵中扩散时大得多的扩散系数。

FURTHER EXPERIMENTS ON THE DISLOCATION INTERNAL FRICTION PEAKS WITH ANOMALOUS AMPLITUDE EFFECTIN Al-0.5% Cu ALLOY AND THE DISLOCATION KINK ATMOSPHERE MODEL

Conditions for the appearance of the anomalous dislocation damping peaks in Al-0.5% Cu alloy were further studied with a low frequency torsion pendulum. Experimental results showed that a suitable amount of cold working is necessary for a fully annealed specimen but not for specimens quenched from an elevated temperature.Previously observed internal friction peaks versus strain aging time and versus temperature which exhibit an anomalous amplitude effect were elucidated qualitatively by the simple dislocation atmosphere model. However, in making quantitative explanations, this simple model meets the following difficulties: (i) Under the action of the alternating stress used in internal friction measurements, the distance that the atmosphere of solute atoms can be dragged along with the dislocation is too small, (ii) For the atmosphere to catch up with the dislocation, the diffusion coefficient of the solute atoms constituting the atmosphere has to be many orders of magnitude larger than the ordinary value, (iii) The critical stress necessary to move the dislocation dragging the atmosphere along with a critical speed as calculated theoretically is several hundred times greater than the observed value.A modified dislocation atmosphere model in which the solute atoms are assumed to diffuse along dislocation kinks was proposed. This model takes into account the above mentioned difficulties and can explain the experimental results obtained in a qualitative manner. The basic assumption of this model is that in order to be able to observe the anomalous internal friction phenomena, a certain number of kinks must exist on the dislocation line. Such kinks can be formed through cold working or quenching the specimens from an elevated temperature. The motion of a dislocation line in a direction perpendicular to itself can be achieved through the sidewise motion of the kinks along the dislocation line. It is assumed that atmospheres of solute atoms are formed only on the straight portions at the ends of the kink, and not on the kinks themselves. In the process of the to and fro sidewise motion of the kinks, a to and fro short range diffusion of the solute atoms segregated at the ends of the kinks can carry out along the dislocation pipe. The diffusion coefficient for diffusion along a dislocation pipe is known to be much greater than that for the diffusion in the regular crystal lattice.