Effect of vibrations on creep behaviour of CMSX-4 single crystals

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Scienceand EngineeringA214 (1996) i67-169

A

Letter

Effect of vibrations on creep behaviour of CMSX-4 single crystals P. Lukfi~, L. Kunz Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic

Received30 October I995; revised 14 March 1996

Abstract

Creep curves at 800 °C and 720 MPa were measured for superalloy single crystals CMSX-4 at various amplitudes of superimposed vibrations. The effect of the vibrations on creep strain rate and on lifetime was found to be strong over the whole range of amplitudes used, being beneficial at the smaller amplitudes and detrimental at higher amplitudes.

Single crystals of nickel base superaUoys show very good resistance to plastic deformation at high temperatures. Critical components made of single crystals experience complex thennomechanical loading which can be reasonably simulated by creep loading during periods of constant load at constant temperature, by isothermal low cycle fatigue and/or by creep/low-cycle-fatigue interaction during slow changes of external load and by thermomechanical fatigue during start up and shut down events. The response of the nickel base superalloy single crystals to these types of loading has been studied to some extent and the basic relevant data are available at least for some of the monocrystalline superalloys. Beyond the listed types of loading some machine parts also undergo small amplitude vibrations. A typical example is a turbine blade which is loaded by a high centrifugal force with superimposed high frequency vibrations. This type of loading can be characterised as interaction of isothermal creep with high cycle fatigue. In the literature there are no data on this interaction for superalloy single crystals, but there are limited data for several classes of polycrystalline materials. A very pronounced detrimental effect (i.e. creep acceleration and reduction of lifetime) was found in the case of two refractory metal alloys subjected to very high frequency vibrations (10 to 19 kHz) during creep testing in the temperature interval 710 to 930 °C [1]. On the other hand, 100 Hz cycling did not affect the lifetime of IN 100 alloy at 900 °C [2]. For nickel (800 °C, 22 Hz), copper (500 °C, 120 Hz) and two chromium-molybdenum steels (600 °C, 120 Hz) it was found [3] that vibra-

tions of very small amplitudes did not affect the creep rate and the lifetime, while the vibrations of larger amplitudes accelerated the creep and shortened the lifetime. In the case of austenitic steel AISI 304 (700 °C, 120 Hz) the small amplitude vibrations led to only a negligible increase in lifetime [3]; vibrations of larger amplitudes were, again, detrimental. The aim of this letter is to present results of the effect of vibrations on the creep behaviour and the lifetime of the superalloy single crystals CMSX-4. Single crystals used in this work were kindly provided by Howmet, England in the framework of COST 501 (Round III). They were supplied in the fully heat treated condition. The microstructure of the single crystals consists of cuboidal ~' precipitates embedded in a matrix. The ~' particle size lies between 0.4 and 0.5 lam and the volume fraction of the ~,' phase is about 70%. The cylindrical specimens were oriented within 10° of < 001 > . The tests were performed in a modified resonant pulsator at 800 °C in air. The specimens were subjected to a static load of 720 MPa with a superimposed cyclic load of an amplitude varying from specimen to specimen; the corresponding R-ratio (_R = Gmm/Crmax)-varied from 1 (pure creep, no vibrations) to 0.64 (vibrations _+ 160MPa). The start-up procedure of the tests was the following. The static load was increased from 0 to 720 MPa after the specimen had reached the desired temperature. The full static load was reached in 70 s; the corresponding elastic strain rate during the increase of the load was about 9 x 10 .5 s -I. The cyclic load (frequency 90 to 95 Hz)

P. Ltd'd~, L. Kunz /Materials Science and Engineering A214 (i996) I67-I69

168

was then applied by switching on the resonant loading system. The full load amplitude was reached within 500 cycles. The typical creep curve in the differentiated form is presented in Fig. 1. Here the total strain rate ~tot is plotted in dependence of the total strain e~ot. At the beginning of the curve there is ~t plateau extending to etot = 6 x 10 -3. This value is eq~:,al to the elastic strain; the corresponding total strain rale is equal to the elastic strain rate. The contribution of the plastic strain to the total strain at this point (i.e. at e,ot=6 x 10 .3 ) is negligible. It can be seen in Fig. 1 that the plastic strain rate at this point is more than two orders of magnitude lower than the elastic strain rate; this is why the presented curve /tot vs. 6or exhibits a sharp minimum at etot 6 X 10 -3. The curve then follows a "hill" which is believed to be due to an interplay between the dislocation creep in the 7 channels and the recovery of dislocation structure by glide/climb processes. The minimum on the ~ot vs. e,tot curve at e,tot "--3% is believed to be related to the onset of the cutting of the 7' particles by dislocations. As the temperature of testing is too low for the directional coarsening of the 7' precipitates (rafting) to occur [4-6], only the dislocation structure can undergo changes during creep and creep/fatigue loading. From this point of view it is important that the basic shape of the differentiated creep curves is independent of the amplitude of vibrations. Thus the basic features of the dislocation structure can be expected not to depend on the amplitude of vibrations. The amplitude of vibrations was found to influence the creep strain rate, the fractare strain and, consequently, the time taken to reach a given creep strain and the time taken to rupture. "['he creep strain rate is affected most strongly in the region of small creep =

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strains. This is documented in the following diagram. Fig. 2 displays the beginning of the selected differentiated creep curves again in the representation ~totvs. e,tot. For e~ot < 6 x 10 .3 the measured strain values consist predominantly of the elastic strain component. The plastic strain component is negligible and the total strain rate is practically equal to the elastic strain rate. For etot > 6 x 10 .3 the static load is constant (the corresponding stress is equal to 720MPa). Consequently the elastic strain rate is zero and the total strain rate is equal to the plastic strain rate. As the plastic strain is due to the creep process, the term "plastic strain" will be replaced in the following by the term "creep strain". It can be seen in Fig. 2 that the creep strain rate decreases with increasing amplitude of the vibrations. This is a surprising result, because increasing the amplitude of vibrations means increasing the maximum stress in the loading cycle; thus we have to state that the creep strain rate decreases with increasing maximum stress. This holds for the region of small creep strains roughly up to 2%. For larger creep strains no systematic differences in the creep strain rate exceeding the scatter band were found. The consequence of this behaviour is that the time taken to reach a given creep strain increases with increasing amplitude of vibrations. This is shown for the time required to reach plastic strains of 1% and 3% in Fig. 3. Results from all the specimens studied are included (with the exception of the specimen 720 + 160 MPa --the fracture strain of this specimen lies below 1%). It can be seen that the time to reach the given creep strain increases with decreasing R-ratio, i.e. with increasing amplitude of the vibration. In other words, vibrations superimposed onto the static load are beneficial in slowing down the creep deformation.

P. Lukfg, L. Kunz / Materials Science and Enghwering A214 (I996) I67-i69 '

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The time to complete fracture is given not only by the rate of the creep defollnation process, but also by the rate of the fracture process. The dependence of the fracture strain on the amplitude of vibrations offers indirect evidence regarding the effect of the cycling on the rate of the fracture process. This diagram is presented in Fig. 4. It can be seen that the fracture strain decreases with increasing amplitude of the vibrations, i.e. with decreasing R-ratio. This is due to a higher chance for a fatigue crack to propagate in specimens subjected to higher cyclic stress components. Thus the rate of the fracture process increases with increasing amplitude of vibrations. As the rate of the creep defor-

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mation process decreases with increasing amplitude of vibrations, the lifetime given by both these processes has to exhibit a maximum when plotted in dependence of the amplitude of vibrations. This is shown in Fig. 5. In spite of the relatively large scatter it can be clearly seen that the lifetime for pure creep (R = 1) is shorter than the lifetime for creep with superimposed vibrations with amplitude up to 40 MPa. For higher amplitudes the lifetime decreases. In summary it can be said that the vibrations superimposed onto static load in the case of superalloy single crystals at 800 °C affect both the rate of the creep process and the rate of the fracture process. In the range of amplitudes investigated the vibrations slow down the creep process and speed up the fracture process. The detailed explanation will be sought from the observation of dislocation structures and fractographic features.

.9 8References

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[I] K.D. Sheffler, Metall. Trans., 3 (1972) 167. [2] P.M. Lesne and G. Gailletaud, in M.G. Yan et al. (eds.) Mechanical Behaviour of Metals, Pergamon, Oxford, 1987, p. 1053. [3] P. Luk/tg, L. Kunz and V. SkIeni+ka, in H. Oikawa et ai. (eds.) Strength of Materials, Japan Institute of Metals, 1994, p. 17. [4] H. Mughrabi, W. Schneider, V. Sass and C. Lang, in H. Oikawa et aI. (eds.), Strength of Materials, Japan Institute of Metals, I994, p. 705. [5] S. Socrate and D.M. Parks, Acta Metall. Mater., 4I (I993) 2185. [6] T.M. Poliock and A.S. Argon, Aeta Metall. Mater., 42 (1994) 1859.