面心立方体金属中间隙原子内耗理论

在面心立方体金属中间隙原子一般不发生内耗。只有在合金元素(杂质)或空穴周围的间隙原子才会发生微扩散的内耗。因为空穴或合金原子的存在破坏了邻近间隙位置的对称性,在这样位置上的间隙原子才有可能在往复应力作用下发生应力感生微扩散内耗。本文按以上所述两种情况,利用作者之一所作的内耗热力学理论作计算,发现由于合金原子存在而引起的内耗弛豫强度应该与x_A(1—x_A)·C成正比,x_A是合金原子浓度,C是间隙原子浓度。由于空穴存在而引起的内耗强度应与N·1/(B/C²+1/C)成正比,此处N是空穴的浓度,C是间隙原子浓度,B是一常数。在葛庭隧、钱知强两氏对面心立方系锰钢的内耗强度的实验中,内耗强度的数值基本上与合金原子浓度无关。因此,在高锰合金钢中像是空穴所引起内耗的那种机构。同时,可以估计出碳原子落入空穴放出的能量约为0.14eV。但目前实验数据有限,实际上面心合金钢中是何种机构在起主要作用,尚待进一步研究。

THEORY OF INTERNAL FRICTION IN A FACE-CENTRED CUBIC LATTICE

It is now well-established that stress-induced micro-diffusion of carbon atoms in the interstitial lattice points will take place in body-centred cubic lattices such as in α-iron. This is the result of the lattice asymmetry introduced by an oscillating strain in the crystal. In a pure face-centred cubic metal, this phenomenon could not happen, namely, not in γ-iron. Nevertheless experimental evidences show to the contrary.The present note advances a criterion concerning the possible types of mechanisms which would give rise to internal friction in a face-centred cubic crystal. By the application of the general thermodynamical theory of internal friction, proposed by one of the present authors, we have calculated the relaxation strength for two important cases. If the asymmetry in the interstitial lattice points is to be ascribed to the presence of impurity or alloying atoms, then it can be shown that the relaxation strength will be proportional to the expression Cx_A (1-x_A) , where x_A. is the density in atom percentage of the impurity or alloying atoms and C is the density of the carbon atoms. However, recent experimental result of Ke and Tsien indicates that there is nothing like this sort of density dependence, i. e. it is rather structure insensitive. Hence, it is not likely that this micro-diffusion can be ascribed to the action of the alloying (substitutional) atoms.A second possible mechanism is proposed in the text. The asymmetry is due to the presence of the Schottky defects, in which a pair of carbon atoms settle down and orientates itself in accordance with the direction of external strain; the possible location of the pair may be such that one carbon atom is located in the centre of the hole while the other is in an interstitial point immediate to the hole. The calculated relaxation strength is proportional to the expression AC²/(B +C), where A and B are such constants that B is structure insensitive. A comparison of the calculated and the experimental curve for the internal friction strength is made in the text, which shows a remarkable agreement. Furthermore, one can thereby estimate the free energy evolved in trapping a carbon atom in a Schottky hole to about the order of 0.14 eV. It seems that the above proposed vacancy-induced asymmetry might be a possible correct explanation.The authors like to tend their appreciation to Dr. Ke Ting-sui and his collaborators for the kindness to communicate to us their results before publication and to their valuable discussions.