THE SNOEK-KÊ-KOSTER PEAK IN Fe-P ALLOYS

Transcription

1 THE SNOEK-KÊ-KOSTER PEAK IN Fe-P ALLOYS J. Ji, Z. Zhao, L. He, D. Geng To cite this version: J. Ji, Z. Zhao, L. He, D. Geng. THE SNOEK-KÊ-KOSTER PEAK IN Fe-P ALLOYS. Journal de Physique Colloques, 1985, 46 (C10), pp.c c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C10, suppl6ment au 11-12, Tome 46, dgcembre 1985 page C10-257,. THE SNOEK-KE-KOSTER PEAK IN Fe-P ALLOYS J.W. JI, Z.Q. ZHAO, L.D. HE' AND D.Q. GENG' Baotou Research Institute of Metallurgy, Baotou, Inner Mongolia, China 'Northeast Institute of Technology, Shenyang, Liaoning, China Abstract- The S-K-K peak in Fe-P alloys has been studied by a vacuum KB-pendulum. It is found that phosphorus has an obvious effect on the IF. The experimentil results agree with Schoeckls theory. The mechanism of the relaxation is also discussed further. I - INTRODUCTION Schoeck's result /I/ shows that relaxation strength A (namely QK~) and relaxation time r of the Snoek-Kg-~tjster(abbreviated to S-K-K) peak all depend on the average dislocation length Lo between immoble points. Schoeck ' s calculation gives : A = B A ~ (1) and z = CXTC~L~~X~( H/KT (2) where, oc are constants, A is the density of dislocation, Co is the interstitial concentration in bulk, the other letters having their usual meaning. Solute atom, evidently, may be a strong locking point of dislocation in souq.cases. Phosphorus is a quite strong hardener in W- iron, indicating thats tkere is a very large interaction between phosphorus atoms and dislocations in iron, and a large effect of phosphorus on the 3-K-K damping.therefore, we will be able to obtain some significant experimental results and further understand the nature of the S-K-K IF through investigating the CW-peak in ~e-(c+n)-p alloys. I1 - EXPERIHENTAL PROCEDURE IF measurements were carried out over a temperature range from room temperature to 4000C in a vacuum Kh-pendulum. The average heating rate was about 2 deg./min., and cooling rate was 1.5 deg/min.. The temperature was kept constant during measurements of decay. Frequency range for measurements was 0.3 to 1.3 cps. The maximum strain amplitude at the surface of specimen was less than 2x10-5. The vacuum was higher than 2x10-5 torr. The specimen has a diameter of 0.79mm. The prestrain quantity of the specimen was 26%. The chemical composition of the tested alloy was listed in Table 1. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Table 1. Chemical composition of tested alloys (wt.pprn) alloy C N number P Balance A Fe A-I I) A II A , A , EXPERIMENTAL RESULT It is found that the IF reduced and shifted to lower temperature as an increase in the phosphorus concentration Cp and a decrease in the concentra ion C(N+C) of both carbon and nitrogen(see Fig.1). The IF maximum Qmax -1 reduces by 9x10-3, and the peak temperature Tp lowers by 65OC with increasing Cp from 70ppm to 2400ppm and decreasing C(N+C) from 137ppm to 35ppm. It can be seen, from a comparison with work /2/, that the influence of phosphorus on the IF is predominent because under the same change of C(N+~ the changes of the IF maximum and peak temperature in the pure Pe-A system are only 2x10~3 and 150C, namely 1/4.5 and 1/5 times of these in the Fe-P alloys, separately. a - - Fe-ISTppidC*-70ppnP o - Fe-lmppmCWal0pp.p 8 l2 -.- F.-soppdW89owmp 0 - Fa- ~. ~ D p P " P f 7 Q m / 2 - /I B &xloz 100 m m 4n 'I? ( 'C Fig.2 The relationship between ) the peak height and the concen- Fig. 1 The S-K-K peak in tested tration of phosphorus alloys -1 A QE~-C~ curve is given in Fig.2, where the Qh is the peak height obtained by deducting background from the Q-1-T curves. Th figure shows that there is a linear relationship betx en QEI and when Cp is in a range from 70 to 640ppm(namely Q-273 is in range of I,4x10-2 to 5.4~10'2). Table 2. The activation energy of both relaxation and interstitial diffusion and the binding energy between the interstitial and dislocation alloy H Hd Hb number (ev> (ev) (ev) A f A f A A * A f

4 The values of H obtained from the slopes of an Arrhenius plot are listed in Table 2. It is indicated that the activation energy of the relaxation is variable since composition changes, and the activation energy decreases from 37.3~cal./mole. in alloy A-0 to 28.1~cal./mole. in alloy A-4. - DISCUSSION Peak height Phosphorus obviously reduces the IF peak height. The relationship between Q ~ and I Cp can be expressed by formula (3) when Cp is in a range from 70 to 640ppm (see Fig.2): Q ~ ~ = ~ ~ c ~ - ~ / ~ + r (3) where 6' and r are constants. ~p-~/3 can be written as (~pl/3)-~ and cp1/3 is the line concentration of phosphorus. The phosphorus in the alloys of A-0 to A-3 is in solid solution,and there is no segregation of phosphorus to dislocation. The phosphorus atoms in solid solution can be considered as strong pinning points to dislocation. The length h0 of movable dislocation segment, which is responsible for the IF, can be expressed by a formula: L~~=(oc~c~~/~) -I....a. (4) where oc* is a pinning coefficient of phosphorus to dislocation. of course, the alloy A-4 is an exception because the content of phosphorus in the alloy obviously exceeds its solubility. In the present study, the dislocation density A can be considered as constant, then there will be + r (5) Q~;~=@~AL:~ is a constant. The expression (5) quite agrees with the formula (1) given by Schoeckts theory, indicating that the S-K-K peak height linearly increases with an increase in length square of movable dislocation segment. It will be noticed that Lno given by formula (4) only represents the length of non-screw(edge) dislocation because phosphorus atom as a substitutional solute causing a spherical symmetric distortion in M-Fe only interacts on non-screw(edge) dislocation rather than pure screw dislocation. That is to say: that the dislocation responsible for the S-K-K relaxation is non-screw(edge) one, and phosphorus shortens the dislocation length resulting in the reduction in qgl. Peak temperature If we substitute L for Lo in the formula (2) and take formula (4) into formula (2), as we!?? as take into consideration of C ci~)=co, then the following expression of the relaxation time can be oitained : rewt'tc( C+N) c~-~/~~x~(h/kt) (6) where OL~'=OL/(~~~)~ is a constant. We shall neglect the change in the variable T in front of exponent for convenience of discussion since its effect on 't is much less than T within the exponent. In this way, it is very evident that to keep c constant in peak temperature to the same fre ency used, the temperature T must lowers when the variables C(C+N), Cp- 273 and H in the alloys A 4 to A-3 all decrease successively. Activation energy Scheock considered that the activation energy of the S-K-K relaxation consists of the diffusion activation energy of interstitial atoms and the binding energy between the interstitial atoms and dislocation, namely, H=Hd+Hb. If we take the value of Hd to be 21~cal./mole.(O.glev) for the test alloys and assume that is the same to all the alloys, the values of Hb for alloys A-0 to A-4 can be obtained by H minus Hd. The binding energy decreasing with an increase of Cp (see Table 2) can be explained as follows: The phosphorus atoms on dislocation reduce the

5 C JOURNAL DE PHYSIQUE energy of the stress field of dislocation, resulting in the reduction in the binding energy beween interstitial atom and dislocation. Such the effect of phosphorus increases with an increase of its concentration. In addition, the particles of the second phase in alloy A-4 and their interfaces with matrix will also affect the distribution of interstitials, namely affect Hb, too(of course, phosphorus may reduce the activation energy of the diffusion of carbon and nitrogen in ol-iron). The above discussion indicates that the S-K-K IF in the test alloys conforms to Schoeck's theory, and non-screw(edge) dislocation is responsible for this relaxation. IF mechanism It can be pointed out that "the enhanced Snoek peak" occurred at slightly higher temperature than the Snoek peak temperature of carbon in iron is caused by the motion of the dragging by both screw dislocation and edge dislocation of Snoek atmosphere of interstitials. The Snoek order will not be responsible for the S-K-K IF appeared at even higher temperature because as soon as the teperature is over that of Snoek order, the process in conversion of Snoek order into Cottrell atmosphere will occur, It is well known that Cottrell atmosphere has already formed and may produce a dynamic strain age in bcc materials containing enough amount of interstitials at the temperature, where the S-K-K peak appears. Moreover, the value of the activation energy for the disappearance of the serrations in a strain-stress curve is quite similar to that for the S-K-K IF /3/, It can be pointed out, from these facts, work/4/ and present work, that the physical model, "the dragging by dislocation of a Cottrell atmosphere of solute atoms by migration of the solute atomsw causing the 3-K-K relaxation, advanced by Kg et a1./5,6/, is correct on the whole, but the dislocation moves, as described by Schoek, by the bowing of pinned dislocation segments /1/ rather than by the formation and motion of kinks /7/. REFERENCES /I/ Schoeck, G., Acta Metall. 11 (1963) 617. /2/ Ino, H. and Segeno, T., Acta Metall. (1 967) /3/ Keh, A. S., Nakada, Y. and Leslie, W. C., Dislocation Dynamics, edited by Rosenfield, A. R. et al, Megrow-Hill ( 1968) 381. /4/ Oytana, C. and Varchon, D., Acta Metall. 3 (1979) 17. /5/ Kg, T. S., Yong, B. C. and Wang, Y. N., Acta Phys. Sin. 11 (1955) 91. /6/ Kg, T. S., Scripta Metall. fi (1982) 225. /7/ Seeger, A., Phys. Stat. Sol. (a) (1979) 457.