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THE EFFECT OF STRAIN AND MEASUREMENT TO 540K ON AN ANOMALOUS INVERSE
MODULUS DEFECT SEEN AT 150K IN HIGH PURITY MOLYBDENUM SINGLE CRYSTALS
J. Garcia, J. Lomer
To cite this version:
J. Garcia, J. Lomer. THE EFFECT OF STRAIN AND MEASUREMENT TO 540K ON AN ANOMALOUS INVERSE MODULUS DEFECT SEEN AT 150K IN HIGH PURITY MOLYB- DENUM SINGLE CRYSTALS. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-107-C8-112.
�10.1051/jphyscol:1987812�. �jpa-00227116�
Colloque C8, supplbment au n012, Tome 48, dbcembre 1987
THE EFFECT OF STRAIN AND MEASUREMENT TO 540K ON AN ANOMALOUS INVERSE MODULUS DEFECT SEEN AT 150K IN HIGH PURITY MOLYBDENUM SINGLE CRYSTALS
J.A. GARCIA"' and J.N. LOMER
J.J. Thomson Physical Laboratory, University of Reading, PO Box 220, Whiteknights, GB-Reading RG6 2 A F , Berkshire, Great-Britain
On a mesurd le module dynamlque et le frottement inte'rleur des monocrlsteaux de molybde'ne de trhs haute pureta' en ulltlsant la vibration d'un cantllever 'a une fr6quence de 550Hz et 'a des tempiratures entre 80K et S40K.
L'effet des petltes 'pre8-d6formatlons' a aussl dtd 6tudld. Des changements des courbes de fr&quence se manlfestent pendant la mesure et le recult 'a 540K. Les effets prlncipaux qui se trouvent sont des variations de la fr6quence resonante 'a 80K et des changements Importants dans la valeur d'un d6faut anormal (c'est
'a
dlre Inverse) du module dans la gamme des temperatures du a-complexe. On se propose que I'effek de 'module lnverse' se prodult par un 'dragging' (entralnement) des ddfauts ponctuels par des dlslocatlons. et que la varlatlon de la friquence rdsonante
d
8OK est le result& des changements dans la valeur du d6faut normal (c'est b dlre directe) du module associ66
un plc lntrlnslque\a
des tempdratures lnfirleures
's
80K.Abstract
The modulus and Internal frictlon of annealed hlgh purlty molybdenum slngle crystals vlbrated as cantllevers at 550Hz have been measured between 80K and 540K. The effect of small pre-stralns has also been studled. Changes In the frequency CUNeS occur durlng measurement and annealing at 540K. The maln features observed are varlatlons In the frequency at 80K and In the slze of a large anomalous modulus defect in the temperature region of the a complex. It is suggested that the anomalous modulus defect arlses from the dragglng of point defects by the dlslocatlons while the varlatlon of the frequency at 80K Is caused by changes In the dlrect modulus defect of an Intrinsic peak below 80K.
Relaxation peaks In the BCC transltlon metals have been extensively studled.
The early work. revlewed by Chambers ( 1 ) . showed that after deformatlon there was a composite peak. a. below room temperature. a somewhat Ill-deflned peak.
B. around room temperature. and a further peak. r . at a higher temperature. It is generally accepted that the a and Y peaks are lntrinslc dislocation peaks whlle the B complex involves point-defect dislocation Interactions.
In molybdenum, B peaks associated wlth direct modulus defects (frequency decreasing wlth temperature) have been seen In lrradlated materlal (2) (3). after hydrogen doplng (4) ( 5 ) and after deforrnatlon ( 6 )
.
A number of B peaks wlth Inverse modulus defects (frequency lncreaslng with temperature) also occur In deformed materlal ( 7 ) ( 8 ) ( 9 ) . Slmllar lnverse modulus defects have been reported In tungsten ( 10) and tantalum ( 1 1).
NOW at Universidad del Pais Vasco, Aptdo. 644. SP-48009 Bilbao. Spain
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987812
C8-108 JOURNAL DE PHYSIQUE
The present authors <12)(13)(14) also found a number of B peaks In annealed and lightly deformed molybdenum. some of whlch had an inverse modulus defect. They found llttle correlation between the slze of the damplng peak and its associated modulus defect. In the straln amplltude and frequency range used. the modulus defects were much clearer than the damplng peaks.
Therefore only the frequency curves wlll be presented in the present paper whlch dlscussea varlatlons In the major B peaks whlch occur below room temperature In one of the samples used In the previous work (13) (14).
Slngle crystals of a 6-pass zone refined molybdenum were grown at A. E. R. E.
.
Hamell by Sunon ( 15). They were decarburized by anneallng at 1 6 0 0 ~ ~ for 48 hours In a partial pressure ( 6 x 1 0 - ~ torr) of oxygen. The oxygen was then removed by anneallng at 1 8 0 0 ~ ~ for 24 hours. The reslstlvlty ratios of the crystals were between 7500 and 8500. The sample. M2. used In this work was 25mm x 4mm x 150Lrm with a <110> axls along Its length. It was annealed at 2 0 0 0 ~ ~ for 3 hours before measurement.The Internal frlctlon was measured as the Iogarlthmic decrement of the free decay of the vibration of the sample when mounted as a cantllever. the vlbratlon being excited and detected electrostatically. The modulus Is proportional to the square of the resonant frequency of the cantllever. In order to keep the amplltude of vlbratlon constant and to accurately follow the changes In the resonant frequency, a feed-back loop was used between the detector and the drive system (2). The maxlmum straln amplitudes In the sample were between
lo-"
and 5 x l 0 - ~ . Measurements were made between 78K and 540K at 5K Intervals uslng a roughly llnear warm-up rate of 1 ~ m l n - ' . The efiect of small pre-stralns was also studled. The sample was stralned at 260K uslng an lnstron Universal tenslle machine. Detalls of the method of grlpplng the small samples and of the measurement cell are glven elsewhere (16).Results
5 30
100 150 200 250 300 TEMPERATURE / K
Flgure 1. Frequency as a functlon of Table 1. Detalls of the treatment temperature for the annealed sample as of the sample prlor to each warm- flrst mounted. measured at a straln up curve shown In flg. 1. U lndlcates
amplitude of 3 x 1 0 ~ . Detalls glven In that the loop became unstable.
table 1.
-
RUN
l ( a ) U 2(A)
3( +) 4(x)
PREVIOUS TREATMENT
As mounted
Vibrated I h at 300K Rested 1 h at 300K Measured to 400K
table 1 glves detalls of the vibration and thermal hlstory of the sample prior to each warm-up curve. The feedback loop became unstable durlng measurement of the flrst curve. The sample was therefore re-cooled. It was then found that changes were occurrlng durlng measurement. The lnstabliltles In the loop were later shown to be associated wlth the rapldly varylng frequency caused by a large modulus defect. For some curves shown In this figure and In flgures 2 and 3.
measurements were made 'above room temperature. These results are not Included as they have been discussed In a prevlous paper (13).
Because of the unexpected changes which occurred durlng measurement the sample was remounted. The results obtained from the remounted sample after varlous treatments llsted In table 2. are shown In flgure 2. whlch is dlvlded Into 2A + 28 for clarlty.
61 0
TEMPERATURE I K
Figure 2. Frequency as a function of Table 2. Details of the treatment of temperature for the annealed sample the sample before each warm-up curve after reclamplng. Curves 1 and 6 shown In flg. 2. U indlcates that the were measured at a straln amplltude loop became unstable.
of 3 x 1 0 ~ and the other curves at
lo'*.
RUN
1 (e)
2(A)
3(+)U
It can be seen that the curves shown In 119.28 are reproducible except for a small shlft In the temperature at whlch the largo frequency increase occurs.
The frequency Increase must therefore arlse from a large Inverse modulus defect rather than from annealing. Since the curves were reproduclble. the straining experlments were started.
PRtVlOUS TREATMENT -
A
As remounted
Measured at 530K Rested 15h at 300K
Vibrated l h at 54OK Rested 12h at 300K
A tenslle stress of 10 kg mm-2 was applied to the sample glvlng a straln In the mlcro-deformatlon region. The varlatlon of frequency wlth temperature was
6
4 ( d U 5(A)
6(+) 7(-1 8 ( x )
Rested 20h at 300K Rested 15h at 300K Rested 10h at 300K Rested 170h at 300K Measured at 540K
J
C8-110 JOURNAL
DE
PHYSIQUEagain measured after various treatments llsted In table 3 and the results are shown In flgure 3. Hlgh temperature measurements on curve 2 (not shown In the figure) showed that the frequency rose wlth temperature above 400K. Since thls was the flrst tlme the sample had been heated above 400K. It is thought to arlse from annealing of a point defect.
TEMPERATURE / K
Flgure 3. Variation of frequency wlth Table 3. Details of the treatment of temperature foiiowlng micro deformation. the sample prior to each warm-up The measurements were made at a curve shown in figure 3.
straln amplitude of
.
RUN
I(@)
2(A) 3(+)
4(-)
5 ( x )
Measurements were also made after small stralns In the plastlc deformation reglon. However no large frequency changes were observed In these curves.
The results, after deformations of 0. 1% and 0.4% straln are shown In figure 4.
It can be seen that detect moblllty is again occurrlng above 400K. The shapes of the repeat curves. after the sample has been heated and vlbrated at 530K are thought to be due to the background. The shapes of the background curves for all the stralns were given in the prevlous paper ( 1 4 ) .
PREVIOUS TREATMENT
After pulllng and restlng for 24h at 300K
Rested 15h at 300K Measured to 540K Rested 12h at 300K Measured to 540K Rested 12h at 300K Heated 45min. at 540K without vibrating, and
rested 12h at 300k
Flgure 4. Frequency as a function of temperature for the stralned samples.
A, prestraln of 0. 1%: 6, prestraln of 0.4%. Curve 1, 8
.
and curve 2.+.
measured at a straln amplitude of lo-'. Curve 3. a. measured at a straln amplitude of 5 x 1 0 ~ .signlflcant effect on the frequency, The fact that some of the earlier measurements were made at different straln amplitudes Is therefore thought to be unimportant. Slnce the resonant frequency of a cantilever depends on length. It may change when the sample Is reclamped. However the method of straining dld not Involve de-clamplng the sample and therefore the absolute frequencies i n flgures 2.3 and 4 may be compared. A small correction must be made for the change In length. Thls will decrease f by 0.2% for A and 0.8% for 0 in flgure 4 above.
The results show that the frequency depends crltlcally on the previous hlstory of the sample. As well as the varlatlons due to controlled pre-straln.
changes occurred when the sample was vlbrated or heated above 450K. The maln features are the variations in the frequency at 80K (f(80)) and in the slze of the Inverse modulus defect near 150K (B( 150) )
.
A small Inverse modulus defect was seen at 220K (B(220)). It is well established that lmpurltles and vacancies can mlgrate at temperatures between 450K and 600K and pin the dlslocatlons. whlle small stralns can produce dlslocatlon 'segments free from plnnlng polnts ( 17) ( 18).
The lnteractlon of polnt defects wlth dlslocatlons can cause changes In the background and in the relaxation peaks. It Is suggested that B peaks with lnverse modulus defects. such as B(150). arise from a temperature dependent drag of the point defects by the background dlslocatlons (12) (19). Sutton (15) showed that a small B(150) and B(220) were formed durlng electron Irradlatlon. Thls suggests that self-lnterstltlals are responsible for the B peaks. However In the present work, B(150) grows after anneallng at 540K and more likely involves an impurity. This implies that the nature of the plnnlng by a self-lnterstitlal Is slmllar to one of the Impurltles moblle at 540K.The larger strains will Increase the dlslocatlon density and reduce the loop length. These effects are usually aaeoclated with changes In the background.
However lrradlatlon experlments (15) have shown that plnnlng of the background dlslocatlons only changes f(80) by a few hertz, The changes In f(80) seen In this work must therefore arlse from a modulus defect below 80K. (It must be remembered that a modulus defect changes the frequency at all temperatures above the peak temperature). The varlatlons In f(80) due to measurement and heating become less as the prestraln Increases. The B peaks no longer play an Important role In the stralned sample once It has been heated at 540K. The varlatlon of f(80) for the stralned and heated sample Is shown In flgure 5 together wlth the results for a second sample (S1) stralned In the same way.
Flaure 5. f(80) as a functlon of p&-straln after the sample has been heated to 540K.
as measured corrected for length change
STRAIN / %
C8-112 J O U R N A L DE PHYSIQUE
The llnear decrease In f(80) wlth straln suggests that it arises from the growth of an lntrlnslc dlslocatlon peak with a direct modulus defect. The low temperature components of the cr peak seen by Grau and Schultz (6) would occur below 80K at the frequency used i n this work and could be responsible. Intrinsic dlslocatlon peaks are attributed to kink processes and may be reduced when klnks are blocked by lmpurltles. Such effects have been seen In Ta (20)(21) and nloblum (21) after hydrogen doping. The large varlatlons in f(80) for the annealed sample are attrlbuted to changes In the kink lmpurlty Interactions.
The maln features of the curves shown in the figures can be explained as follows. Prlor to the Initial clamping. the sample had been heated to 2 0 0 0 ~ ~ and lmpurltles wlll have mlgrated to the dlslocatlons glvlng a B(150) and blocklng the kink motlon. Measurement frees some of the kinks. decreasing f(80).
B(150) moves to B(220) due to a change in conflguratlon of the point defect on the dlslocatlon line. On re-clamplng. damage may occur near the clamp. This wlll free the background disiocatlons from thelr plnning points reducing B(220).
No informatlon can be obtalned from f(80) due to the passlble length change on reclamplng. Subsequent measurement at 540K allows point defects to m'igrate back to the background dislocations re-establlshlng B(150). whereas the vlbratlon frees the klnks and increases the lntrlnslc peak (decreases f ( 8 0 ) ) . The repeat curves (fig. 26) show that this configuration Is stable to thermal cycllng except for some re-arrangement of impurlties on the background dislocations causing a shift In B(150).
Durlng mlcro-strain (flg. 3 ) . the impurlties leave the background dislocatlons and B(150) is destroyed. It Is agaln re-established by heating at 540K. However In this case the thlrd curve is very different. The lntrlnsic peak has been reduced and B(150) destroyed. Thls would arlse If the second heating to 540K allowed sufflclent lmpurltles to migrate to the dlslocatlons to completely pln them.
Complete plnnlng may be easler when the dlslocatlon loop length is reduced. Thls would explain why B peaks are not seen after the higher deformations.
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