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INTERNAL FRICTION IN CONCENTRATED ALLOYS
C. Spears
To cite this version:
C. Spears. INTERNAL FRICTION IN CONCENTRATED ALLOYS. Journal de Physique Colloques, 1971, 32 (C2), pp.C2-183-C2-185. �10.1051/jphyscol:1971241�. �jpa-00214567�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 7, tome 32, Juillet 1971, page C2-183
INTERNAL FRICTION IN CONCENTRATED ALLOYS
C. J. SPEARS
Rbum6. - Le frottement intkrieur, Q-1, de monocristaux et de polycristaux de cuivre et de a-laitons contenant 10-30 at. % de zinc, a kt6 ktudik a des amplitudes de deformation d'environ 10-7 a 12,5 kHz en fonction de la dkformation plastique dans l'intervalle 130-300 OK. Aucuns pics bien dkfinis de frottement intCrieur n'ont kt6 observks dans les laitons. Q-1 augmentait graduelle- ment selon l'klkvation de tempkrature, ktant indkpendant de l'amplitude et plus petit d'a peu prks un ordre de magnitude que les valeurs correspondantes du cuivre. Le frottement intkrieur dkcroit a mesure que l'on augmente le contenu de zinc, et s'accroit en prksence d'une dkformation plas- tique ; au-del8 d'un certain niveau d'effort, cependant, on le voyait s'accroitre de nouveau. On a ktudik des Cchantillons a deux largeurs de grains. Le frottement intCrieur s'est rCvkl6 moindre dans les Cchantillons a grains grossiers. Les observations paraissent accordables avec un modkle dans lequel l'effet serait produit par des mouvements de dislocation limites atttribuables a la diffu- sion (suivant l'effort) de dkformations gkomktriques.
Abstract. - The internal friction, Q-1, of single crystals and polycrystals of copper and alpha- brasses containing 10-30 atomic percent zinc was studied at strain amplitudes of about 10-7 at 12.5 kHz as a function of the plastic deformation in the range 130-300 OK. Well defined internal friction peaks were not observed in the brasses. Q-1 increased gradually with increasing tempera- ture ; it was amplitude independent and about an order of magnitude smaller than the correspond- ing values in copper. The internal friction decreases with increasing zinc content, and is enhanced by plastic deformation ; however beyond a certain strain it was again observed to decrease. Speci- mens with two grain sizes were studied ; the internal friction was less in the coarse-grained ones.
The observations appear to be reconcilable with a model in which the effect arises from limited dislocation movements due to stress-directed diffusion of geometric kinks.
1. Experimental. - Although some observations on the internal friction of concentrated solid solutions were made relatively early (I), due to the complexity of the problem theoretical interpretations have not been advanced. The main object of the present work was t o make a systematic study of the internal friction observed in concentrated alloys a t relatively low tem- peratures, and t o attempt to formulate at least in outline, a theory. Variables which seemed most valua- ble from the point of view of identifying any internal friction mechanism appeared to be temperature, prestrain, grain size and composition.
The <( as received )) brasses contained as principal
impurities about 10 p. p. m. of iron and similar amounts of lead and tin. Two grain sizes were used, i. e. 8 and 45 ym ; they were obtained by annealing at 500 OC and 800 OC respectively for periods of a few minutes as specified by Feltham and Copley 121. Rod- shaped single crystals of a-brasses with nominal zinc contents of 10, 20 and 30 atomic percent, as well as some of 99.997 % copper, were grown by Bridg- man's technique. Crystals, 15 cm long and of 6 mm diameter were excited electrostatically in vacuo at their resonant frequency of about 12.5 kHz to an amplitude of about loF7 E. Amplitude dependence was not observed, i. e. an analysis of decay traces showed purely exponential envelopes. The variation
of frequency with temperature was monotone. Fur- ther experimental detail is given elsewhere [3].
A few preliminary measurements of Q-I of O F H C checked with earlier more extensive work by Feltham and Burdett [4]. The internal friction of annealed and prestrained single crystals of 90/10 a-brass, with orientations close t o < 1107 are shown in figure 1.
In the annealed brasses it was 10-20 times lower than the corresponding value in OFHC. Values of the inter- nal friction obtained after prestraining single crystals
UNDEFCUMED
100 200 T E M R N W I E OK
FIG. 1. - Dependence of internal friction of 90/10 alpha-brass single crystals on temperature and strain (numbers indicate tensile strength, and the broken parts of the curve are extra-
polated).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971241
C2-184 C. J. SPEARS at room temperature to the levels indicated by the
points in figure 2 are shown in figure 3. These Q-'/
strain isotherms show a maximum at a value Q-' 4.7 x corresponding to a tensile strain of about
I I 9 8 I
10 20 30 % TFNSlLE STRAIN
FIG. 2. -Work hardening curve of 90/10 alpha-brass single crystals showing points at which internal friction measurements
were taken.
FIG. 3. -Internal friction versus strain isotherms for 90110 alpha-brass single crystals at 0 130 OK, 180 OK, A 230 OK
and C] 280 OK.
19 %. Corresponding isotherms for annealed and prestrained polycrystals of 90110 a-brass, with grain size of 45 pm, are given in figure 4. The maximum
1 I I
2 4 6 2 4 -1. TENSILE STRAIN
FIG. 4. - Internal frictionlstrain isotherms for alpha-brass of 45 pm grain size at 0 130 OK, 180 OK, A 230 OK and V
280 OK * from reference [41.
Q-' of 2.6 x is again observed near the end of the linear hardening stage, which occurs here at a strain of about 2 %. Similar results for 80120 and 70130
a-brasses give 2.4 x and 1.1 x respec- tively. A progressive reduction of Q-' with increasing zinc content is therefore observed.
The effect of grain size on Q-' is not pronounced, and is illustrated for 90110 a-brass in figure 5. One
I I I
0 2 4 6 2 4 -1. TENSILE STRAIN
FIG. 5. - Internal frictionlstrain isotherms for 90110 alpha- brass of differing grain sizes at 0 130 OK,
.
180 OK, A 230 OKand V 280 OK.
90110 single crystal, partly annealed after an extension of 19 % at room temperature, was cut into two une- qual lengths, and Q-' could thus be measured at 12.5 and 15.5 kHz. A small increase was observed at the higher frequency but it did not exceed a few per- cent and was therefore within the range of experimen- tal error.
2. Summary and conclusions. - The observations show that :
a) Q-I is only weakly, if at all frequency dependent, b) it does not depend on the amplitude in the range studied, i. e. below strains of about
c) internal friction increases gradually with tempe- rature, the relation being almost linear,
d ) the Q-'/T curves appear to pass through the origin on extrapolation,
e) there are no pronounced peaks on the &-'IT
curves, thus the internal friction is essentially of the
(( background D type,
f ) an increase in the alloy content leads to a decrease in Q-l, this also appears to be the case below 10 %
z n 111,
g ) prestrain in the 2nd stage of hardening leads t o a progressive in the internal friction, but with further deformation Q-I diminishes ; this applies to single crystals and polycrystals,
h) the internal friction if of the same order of magnitude in single and polycrystals, but the maxima on the Q-'/strain curves occur at lower levels of strain in the polycrystalline specimens.
It appears from these characteristics that even in concentrated substitutional alloys, minor dislocation displacements are possible at stresses of E x l o p 7
INTERNAL FRICTION IN CONCENTRATED ALLOYS C2-185
and lead to an amplitude independent type of internal of dislocated movement facilitated by the diffusion of friction which, as in some pure metals [5] appears to geometrical kinks. The diffusion would be controlled be rather insensitive to frequency changes. It is pro- by the cr lattice friction )) which would become pro- bable that the effect is associated with a limited amount gressively greater with alloy content.
References
[I] GUILLET (L.), Report of a Conference on Strength of [4] FELTHAM (P.) and BURDETT (C.), Realstruktur und Solids, The Physical Society. London 1948, p. 116. Eigenschaften von Reinstmetallen, Akademie [2] FELTHAM (P.) and COPLEY (G.), Acta Met., 1960,8,542. Verlag, Berlin 1967, p. 615.
[31 SPEARS (C. J.), Ph. D. Thesis, Brunel University, [5] ROUTBORT (J.) and SACK (H.), J. Appl. Phys., 1966,
London, 1970. 37, 4803.
