VARIATION OF THE ELECTRICAL

WITH THERMAL FATIGUE

1~. L. Vrublevskaya

RESISTIVITY OF C O P P E R ,

UDC 621.3.028: 669.35:539.43

The thermal fatigue of metals is investigated on thin wires and massive samples. A method of t e s t - ing for thin samples has been developed, while studies of mass ive samples have thus far led only to de te r - mining general relationships - the influence of thermal cycling pa ramete r s and the condition of the mater ia l on the effect of cyclic heat treatment. One of the reasons for this is the methodological difficulty of m e a - suring small values of e lectr ical res is t ivi ty , the variat ion of which in thermal cycling makes it possible to determine the accumulation of defects in the sample.

In this investigation of the variat ion of the electr ical resis t ivi ty of mass ive samples of copper during thermal cycling the difficulty was eliminated by use of the special apparatus (Fig. 1). The electr ical r e s i s - tivity was measured with an ordinary compensation circuit based on an R-329 s ing le -double bridge, M17/1 m i r r o r galvanometer , and acid s torage bat tery.

The apparatus is designed for testing round samples 2 mm or more in diameter and also fiat samples 1 mm thick or more and 100-120 mm long. The sample 1 is placed over the e lectr ical contact 2 of annealed copper imbedded in a textolite holder 3. The electr ical contacts have flat and recessed surfaces for both flat and round samples . The holder is held by two pins 4 to the base of a thermosta t 5 placed on the table 6 of a TK-2 hardness gage. When a fiat sample is replaced with a round sample the holder is shifted f rom one position to the other. The free-posi t ioning holder with the potential contacts 7, for which we used the Pobedit cones of the TK-2 apparatus, r es t s on four springs on the holder with the current contacts. The cy l indr ica l shaft project ing f rom the top of the holder serves to establish contact between the whole sys tem and the ha rd - ness gage indicator 9. To ensure good contact between the sample and the e lectr ical and potential contacts , the screw of the hardness gage is tightened to p ress the cylindrical shaft against the indicator with a load of 300 g, which corresponds to 10 divisions on the indicator.

To ensure constant tempera ture the entire apparatus is placed in a thermosta t 5 heated with a 12-V lamp 10. The tempera ture is regulated with a contact the rmomete r 11. A mie romotor 12 driven f rom a 1-V bat tery ensures mixing of the air in the thermostat . The body of the thermostat is made of plastic 5 mm thick, with a removable cover. In the side wall there is a slide cover over the opening for changing the po- sition of the sample or changing samples .

For the res is t ivi ty measurements the sample was placed in the thermosta t for 20 min, af ter which 10 measurements were made in different locations. The electr ical res is t ivi ty was calculated by the formula

,~ A R _ { 2 , ~ AV~, (i)

where R0 is the res is tance of the sample in the original condition; AR = Ri - R0 is the change in res i s tance after i cycles; R i is the res is tance of the sample after i cycles; l 0 and V 0 are the length and volume of the sample in the original condition; A l and AV are the changes in length and volume after i cycles.

Since the length of the sample remains constant during thermal cycling and only the d iameter changes (or the thickness) as the result of etching in a salt bath, formula (1) will have the form

, ~ ___ ,~ R + 2~,__po (2) Po Ro D '

where D is the diameter of the sample.

Pe t rozavodst State University. Transla ted f rom Metallovedenie i Termieheskaya Obrabotka Metal - lov, No. 1, pp. 75-76, January, 1970.

01970 Consultants I3ureau, a division of Plenum Publishing Corporation, 227 g/est 17th Street: New Yark, N. Y. 10011. .41l rights reserved. This article cannot be reproduced for arty purpose whatsoever without permission of the publisher..4 copy of this article is available front the publisher for $15.00.

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~_8.: %

,2 7 , ,

a - ~ '~ z -~ ~ -0'50 100 200 300 400 N

Fig. 1 Fig. 2 Fig. 1. Apparatus for measu r ing the e lec t r ica l r e s i s t iv i ty of m a s s i v e samples . 1) Sample; 2) cu r r en t contacts ; 3) holders ; 4) pins; 5) the rmos ta t ; 6) ha rdnes s gage table; 7) potential contacts ; 8) shaft; 9) ha rdness gage indicator; 10) lamp; 11) t h e r - m o m e t e r ; 12) m i e r o m o t o r .

Fig. 2. Var ia t ion of e lec t r i ca l r e s i s t iv i ty with the number of cyc les .

The sp read of the values amounts to 0.2-0.25% and the resu l t s a re reproducib le within l imi t s of 0.03- 0.05%.

We invest igated s am p l e s of M1 copper (99.92% Cu) 5 m m in d i ame te r and 100 m m long. T h e r m a l cy - cling was conducted with a special device, using the following p a r a m e t e r s : T m a x = 650~ Tmin = 5~ h e a t - ing t ime 2 rain, cooling t ime 30 sec . The samples were cooled by a solution of 10% NaC1 that was cooled by running wate r .

The r e s i s t i v i t y was de te rmined on the s ame sample a f te r var ious numbers of cyc les . F igure 2 shows the var ia t ion of r e s i s t iv i ty with the number of cycles for two different samples . Th ree regions a r e vis ible on the r e s i s t i v i t y cu rves with inc reas ing numbers of cycles - 0-6, 6-100, and 100-500 cyc les .

In the low-cycle range one obse rves a complex relat ionship: A p / p o = f(N). The f i r s t two cycles in - c r e a s e the r e s i s t iv i ty by ~0.7%, which indicates a substant ia l accumulat ion of defects . After th ree and four cyc les the r e s i s t i v i t y d e c r e a s e s below the initial value, which averaged 0.5% for two sample s . In the other two regions of the curves the accumulat ion of defects is l inear but the r a t e d i f fers . The comple te change in the r e s i s t i v i t y af ter 500 cyc les amounted to 5%.

It can be a s sumed that in the initial s tage of the rma l cycling a complex in terac t ion of defects with vacancies and dis locat ions occurs , and also in teract ion of these defects with impur i ty a toms. The f i r s t quenching f ixes the quenching vacancies and there is some inc rease of the dis locat ion densi ty (as comp a red with the original value) due to quenching s t r e s s e s . It is poss ib le that newly genera ted dis locat ions induce the fo rmat ion of s t r a in vacancies [2]. Obviously, the second quenching does not change the initial pa t te rn , although the i n c r e a s e of the r e s i s t i v i t y is approx imate ly half that occur r ing a f te r the f i r s t quenching. This reduct ion of the r a t e of accumulat ion of defects may c h a r a c t e r i z e the occu r rence of a new p r o c e s s inhibiting the accumulat ion of defects . Such a p r o c e s s may be grain boundary migra t ion , leading to re laxat ion of quenching s t r e s s e s during subsequent heating. The effect of grain boundary migra t ion gradual ly d e c r e a s e s in the third and fourth cycles and a f te r s ix cyc les the var ia t ion of Ap/po with the numbei" of cyc les becomes l inear . Data in [3] indicate that migra t ion , occurr ing in the f i rs t cyc les , tends to at tenuate and c e a s e s a f te r 20 cyc les . Evidently, under the given conditions this p r o c e s s is init ially m o r e act ive and stops sooner .

In the f i r s t s tage of the rma l cycling the s t ruc tu re is p r epa red for operat ion under conditions of chang- ing t e m p e r a t u r e . F u r t he r t he rm a l cycling induces accumulat ion of defects , as indicated by the l inear s e c - t ions of the curve Ap/po = f(N). Some inc rea se of the ra te in the third sect ion on the curve may be due to the fo rmat ion of mosa ic s t ruc tu re within the g ra ins [4]. X - r a y s t ruc tu ra l and meta l lographic analys is r e - veal polygonization p r o c e s s e s in this range of the rma l cycles [5].

1.

2. 3. 4. 5.

L I T E R A T U R E C I T E D

I. Ya. Dekhtyar et al . , Czech. J. Phys . , 17, No. 1, Section B (1967). J. Takamura , Lat t ice Defects in Quenche"-d Metals , London (1965). H. Bohm, Z. Metal lk. , 55, No. 5 (1964). V. A. Krakhmalev and G. A. Klein, Fiz. Metal. i Metal loved. , 25, No. 5 (1968). r I Vrublevskaya and O. N. Shivrin, in: In te rac t ion of Disloc '~ions and Impur i ty Atoms in Metals and Alloys [in Russian] , TPI , Tula (1969).

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