材料期刊网

四种不同Ti浓度的Fe-Ti合金,加氮到N/Ti>1之后,发现有两个内耗峰,并且随氮浓度之增加而同时升高。20℃处的峰是氮的Snoek峰,160℃附近的是s-i峰。s-i峰的峰高和Ti浓度成线性关系,表明起峰的反应只涉及孤立的Ti原子,与Ti-Ti原子对或杂质原子团都无关系。 提出了产生s-i峰的二种缺陷中心——Ti-N对缺陷和N-Ti-N仨缺陷——的模型(图7)。氮占Ti位就构成对缺陷,其中的Ti,N原子亲和力很强,只要合金中尚存有自由Ti原子,就不可能存有自由氮原子,因此N/Ti≤1以下,不会出现Snoek峰或s-i峰。N/Ti>1之后,多余氮原子要在对缺陷的OⅡ位和T_3位之间以约1:10的比例进行分配,直到绝大部分的对缺陷转化为仨缺陷。N/Ti(?)2以后,几乎所有的多余氮都进入了仨缺陷的OⅡ位,此时s-i峰的弛豫强度突然增加10倍。 淬火时冻结在α-Fe基体中的过饱和氮、要扩散到OⅡ位(扩散距离～10(?)),以期达到室温下的再分配,因此引起Snoek峰室温下的迅速衰减。s-i峰的形状,只取决于多余氮的浓度,与淬火温度、冷却速度无关。

Wire specimens of Fe-Ti alloys were loaded with nitrogen at 580℃, homogenized at 650℃ and then quenched at 570℃. When internal friction of these specimens were measured at a frequency of 1 Hz, two peaks were observed. The peak located at 20℃ is a normal nitrogen Snoek peak, while the one near 150℃ is the s-i peak (activation energy=1.13 eV). The addition of Ti to α-iron makes the nitrogen Snoek peak unstable, the later decays rapidly even at room temperature, In the range of atomic ratio N/Ti≤1, neither the Snoek peak nor the s-i peak is observed. Only after N/Ti>1, both peaks grow simultaneously with increasing nitrogen concentration. The fact that the height of the s-i peak changes linearly with Ti content indicates the peak to be contributed by reactions involving only isolated Ti atoms and has nothing to deal with either Ti-Ti atomic pairs' or clustering of solute atoms.It is suggested that two types of defects, the Ti-N pair defects and the N-Ti-N triplets, are the reaction centers giving rise to the s-i peak. The pair defects are formed by nitrogen occupation in the T_1-sites. The binding between the two constituent atoms of a pair defect is so strong that the alloys can be deprived of any free nitrogen as long as there are excess Ti atoms remaining in solution. This is the reason for the absence of any peak in the range N/Ti≤1. After N/Ti>1, excess nitrogen atoms will be distributed between the O_Ⅱ-sites and the T_3-sites at a ratio of about 1:10 until majority of the pair defects being transformed into triplets. Over a critical point of N/Ti≈2, almost all of the excess nitrogen atoms are only alloted to the O_Ⅱ-sites of the triplets, and in the mean time, the s-i peak begins to show a rapid increase in relaxation strength of about 10 times.The supersaturated nitrogen atoms frozed in the α-iron matrix during quenching tend to redistribute at room temperature by diffusing into O_Ⅱ-sites (diffusion length～10(?)), so as to make the Snoek peak transient. The profile of the s-i peak is solely determined by the concentration of excess nitrogen and is therefore independent of quenching temperature as well as cooling rate.