Anelastic Phenomena Associated with the Long-range Diffusion of Self-interstitials and Their Interaction with Dislocations in Electron Irradiated Magnesium

The Internal friction and elastlc modulus of high purlty magneslum were studled during irradiation at 2 K with 0 .4 0. 6 MeV electrons and subsequent anneal sequences. The sallent feature of the results is the evidence of a marked plnnlng stage In the temperature Interval 9 to 13 K. This Indicates that long-range diffusion of elementary interstitial defects occurs at these temperatures. Post-lrradlatlon anneals enabled also the dlscovery of two relaxatlon peaks centered at 6 K and 15 K. respectively. These stem from the arrival of the diffusing self-lnterstltlals at dislocations. and presumably from their Interactions wlth geometrlcal kinks. From a general viewpoint, after a long-standing controversy. the state of affairs about the conflguratlonal and migratory properties of the spllt-interstitlals is now settled in a number of metals. However. in a few of them. there Is still some suspense about the very existence of these defects and their observability. This Is precisely the case for magneslum. In thls metal. It was deduced from Huang X-ray scattering measurements that lnterstitlal clusterlng was below 6 K C 11. In contrast, on the basis of electrical reslstlvlty studies between 1. 5 K and 150 K. none of the existing stages In the recovery spectrum of electron irradiated magnesium were Interpreted as due to the random diffusion of self-interstitlals C2-43. The present research is Intended to elucidate the sltuatlon for the freely mlgratlng self-lnterstitlals. using dlslocatlons as sensors. An attractive feature of magneslum is the existence of large anelastlc effects. in the hlghly pure material [5-61. at very low temperatures. 11. IN-LINE MEASURING DEVICE The sample used Is a rectangular strip 2 cm long. 0. 4 cm wide. wlth thickness 300 fim. obtalned by cold-rolling. It Is clamped In a copper block In whlch a heat exchanger Is incorporated. Thls system works between 2 K and 900 K by combined use of a llquld hellum stream and furnace heatlng. The copper block Includes also a helium Ill container. By pumping on the hellum HI bath. temperatures as low as 0. 7 K (with no flux) and about 2 K (for a moderate electron flux rate) are attainable. A Rh-Fe thermocouple is used to monltor the sample temperature. For damplng and dynamlc modulus measurements. the strip is operated In flexural vlbratlons wlth use of electrostatlc excltatlon C71. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19851049 (210-220 JOURNAL DE PHYSIQUE The materlal studled Is hlgh purlty magneslum prepared by zone meltlng at the Centre d'Etudes de Chlmle MAtallurglque de Vitry sur Seine. Neutron actlvatlon lndlcated that the total lmpurlty concentration 1s below one atomlc part per mllllon. After mountlng. the sample was heat treated under vacuum at 440 K for 20 mln. to produce long dlslocatlon iines. III PREIRRADIATION DAMPING AND MODULUS SPECTRA Most features of the overall damplng spectrum of hlgh purlty hcp magneslum are quite slmllar to the ones observed In a number of fcc metals C6-73. In partlcular. after anneals In the range 400 to 500 K followed by cold-work. two Bordonl peaks labelled 81 and B2 exlst at 60 K and 120 K, at a measurlng frequency of about 1000 Hz C83. Subsequent annealing at temperatures hlgher than 340 K produces the dlsappearance of 62. The presently measured temperature variations of Interna1 frlctlon and elastic modulus are shown In Flg. 1 for the non-lrradlated condition. In t,he consldered case of a 440 K heat treatrnent. only the flrst of the Iwo Bordonl peaks exlsts. In agreement wlth prlor work. It 1s seen In Fig. 1 that a large peak termed D shows up around 15-20 K. In addition a large background damplng exlsts at stlll lower temperatures. Further Inspection of Flg. 1 reveals that the relaxation strength of Peak 61 keeps almost unchanged when the vibratlonal straln amplitude 1s varied by a factor of 5. whlle the background damping is markedly lncreased wlth lncreaslng amplitude. Noteworthily. for the lower strains, a small peak. called R I . appears to be superlmposed on D. In prlor work this peak has been traced to Interactions between geometrlcal kinks and vacancles 16.9. 103.