Mechanical Energy Losses in High Purity Polycrystalline Molybdenum at Elevated Temperature

F. G. BONIFACICH; G. I. ZELADA; O. A. LAMBRI; J. A. GARCÍA

Rosario

Congreso; Congreso SAM-CONAMET; 2011

Sociedad Argentina de Materiales - Comisión Nacional de Matriales, Chile

Molybdenum and its alloys are used in nuclear reactor vessels and other reactor components.The knowledge of environment-material reactions in terms of fundamental stability and kinetics arecritical to predict the long time performance of nuclear materials [1].The appearance of five stages of recovery has been proposed for the molybdenum. They can bedefined as stage I, below 120 K; stage II, from 120 K to 330–350 K; stage III, from 330–350 K to600 K; stage IV, from 600 K to 850–900 K and stage V, for temperatures higher than 850–900 K.Indeed, the temperature range around one third of the melting temperature, usually related to thestages IV and V of recovery, is particularly interesting in molybdenum due to the strong influenceon the mechanical properties both in the pure metal, as in technological molybdenum based alloys.We have previously reported several works into which we have analysed the behaviour ofdamping, at temperatures within the stages IV and V of recovery, in high purity single crystallinemolybdenum deformed and irradiated with neutrons. Using mechanical spectroscopy, electricalresistivity, differential thermal analysis, transmission electron microscopy and small angle neutronscattering, we found two damping peaks related to the interaction of dislocations with vacancies, theso called low temperature peak (LTP) and high temperature peak (HTP). The physical mechanismwhich controls the LTP, which appears at around 700 - 800 K, is related to the dragging of jogs bythe dislocation under movement assisted by vacancy diffusion. In contrast, the HTP, which appearsat around 1000 K, is controlled by a mechanism involving formation and diffusion of vacanciesassisted by the dislocation movement [2 - 5].In the present work we extend our study of the interaction process between dislocation and pointdefects, to high purity polycrystalline molybdenum with different thermomechanical states withinthe temperature range room temperature - 1273K; i.e. at temperatures including stages III, IV andV. Annealed and deformed samples have been investigated by means of mechanical spectroscopy,MS, (damping, Q-1, and modulus measurements as a function of temperature) and electricalresistivity studies. Samples were purified by means of 4 pass of zone refining in A.E.R.E., Harwell,UK. The residual resistivity of the samples was about 6000. Annealing treatment of samplesinvolved a heating at 1973K during 24 hours and at 2273K during three hours under high vacuum.Some samples after annealing were stretched at different degree, followed by torsion at roomtemperature (RT).Annealed polycrystalline molybdenum exhibited a group of damping peaks within thetemperature interval room temperature - 700K, see curve 1 in Figure 1. The intensity of these peaksincreased with the plastic deformation, as it is revealed by observing the curve 2 in the Figure. Afteran annealing at 973K, during the first damping measurement, these peaks are absent in the secondrun-up in temperature both in the annealed and plastically deformed samples, see curves 3 and 4 inFigure 1, respectively. However, in the previously plastically deformed samples a small and widedamping peak, labelled P, appears at around 700K, see curve 4.Previously, in plastically deformed single crystalline samples we found an ascendingbackground with peaks in the temperature region 450K - 600K, see curves 5 and 6, but in thecooling spectrum a wide damping peak at around 700K appears which corresponds to the reportedLTP [2 - 5], see curve 5. The observed peaks at around 450K - 600K in the polycrystalline samplescan be related to the γ and γ’ peaks of the molybdenum controlled by the kink pairs formation inscrew dislocations and/or the interaction between dislocations and point defects and impurities [6].The discussion of the present work will be done by comparing with previous results obtained insingle crystalline samples.References1. Y. Guérin, G. S. Was, S. J. Zinkle, (Eds.), “Materials Challenges for advanced nuclear energysystems”, MRS Bulletin, Vol. 34, (2009), p. 10 – 53.2. G. I. Zelada-Lambri, O. A. Lambri, P. B. Bozzano, J. A. García, and C. A. Celauro, “Interactionprocesses between vacancies and dislocations in molybdenum in the temperature range around 0.3 ofthe melting temperature”, Journal of Nuclear Materials, Vol. 380 (2008), p. 111 – 119.3. O.A. Lambri, G.I. Zelada-Lambri, G.J. Cuello, P.B. Bozzano, J.A. García, “Study of the temperatureevolution of defect agglomerates in neutron irradiated molybdenum single crystals”, Journal ofNuclear Materials, Vol. 385 (2009), p. 552 – 558. (paper)4. G. I. Zelada-Lambri, O. A. Lambri and J. A. García, “Mechanical energy losses due to themovement of dislocations in molybdenum at high temperatures (0.3T m)”, Journal of NuclearMaterials, Vol. 353 (2006), p. 127 – 134.5. O. A. Lambri, G. I Zelada-Lambri, L. M. Salvatierra, J. A. García and J. N. Lomer, “Anelasticrelaxation in high purity molybdenum single crystals at medium temperatures”, Materials Scienceand Engineering A, Vol. 370 (2004), p. 222 – 224.6. J. A. García, J. N. Lomer, C. R. A. Sutton and W. G. Wood, “Evidence for the dragging of pointdefects by dislocations from measurements of internal friction and modulus defect in irradiated anddeformed molybdenum single crystals”, Philosophical Magazine. A, 59 (1989), 105 – 121.