Dedicated to Professor V. I. STARTSEV in honour of his 70th birthday High purity niolybdenum single crystals were deformed in tension and compression along the symmetric double slip orientation [110] in the temperature range from 300 K down to 0.5 K with strain rates between 10-3 and 10-5 s-l. The activation volume was measured by stress relaxation tests. The dislocation structure of the deformed crystals (T 2 1.85 K) was examined by high voltage electron inicroscopy. It was established that the low teriiperature increase of the critical shear stress exhibits three distinct temperature regimes with different temperature dependences. These non-uniformities are discussed in terms of recently developed theories of kink-pair formation and kink-kink interactions on screw dislocations in bcc metals. The HVEM observations suggest that the nlobility of screw dislocations a t very low temperatures should be determined by the combined effects of the PEIERLS barriers and the jog dragging.Molybdiineinkristalle hoher Reinhrit und tier Orientierung [l lo] wurtlen in1 Zug-und Drwkversiich im Temperaturbereich von 300 K bis 0 , 5 K iiiit Verforrnungsgeschwindigkeiten zwischen 10-3 s-l und 10-5 8 -l verfornit. Das Aktivierungsvolurnen wurde mittcls Spannuiigsrelaxation gernessen. Die Versetzungsstruktur der verformten Kristalle (fur T 2 1,85 K ) wurtle rnit Hilfe der Hochstspannungselektronenniikroskopie untersucht. Es wurde festgestellt, dalJ der Tiefteinperaturanstieg der kritischcn Schubspannung drei cleiitlichr Temperaturbereiche mit unterschiedlichen Teiiiperaturabhiingigkeiten aufwrist. Dies wird auf tier Grundlage neuer Theorieii iiber die Bildung von Kinkpaaren untl die Kink-Kink Wechselwirkung auf Schraubenvessetzungeii in krz Metallen tliskutiert. Die elrktrorienrnikroskopischen Beohachtungen legen nahr, daB die Bewrglirhkeit der Schraubcnversetziingen bei schr tiefen Ternperaturen durch den gerneinsanien EinfliiB des hohen PEIERtsreliefs uiid der starken Breinswirkung von Jogs bestimmt wirtl.'24 *

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We discuss the physical nature of electron sound signals excited in molybdenum by an acoustic wave and propagating at the Fermi velocity. The experimental temperature dependences of the amplitude and the phase velocity of these signals have been studied in the normal and superconducting state. We interpret this effect observed earlier in Ga by Burma er al as the exciration of a weakly damped zero-sound wave caused by the Fermiliquid interaction between charge carriers. A dominating role of the electron-electron collisionsin thezero-sounddampingin Mowasestablished, and thecorresponding relaxation time was estimated. Theoretical calculations of the expected zero-sound behaviour in a superconductor are ingood agreement with theexperimentaldata andenable ustodetermine the intensity of the Fermi-liquid interaction.

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The data for the microhardness and fracture toughness of Y–Ba–Cu–O and Bi-based single crystals and ceramics in the temperature range 77–293 K are presented and analyzed. Our study reveals that the microhardness of high temperature superconductors is very sensitive to the oxygen stoichiometry, the phase composition, the temperature, and to the microstructural defects such as impurities, intergranular boundaries, and voids. Attention is drawn to the anisotropy of the micromechanical properties and to the features of the fracture in the vicinity of the indentation. The data available on the plasticity of Y–Ba–Cu–O and Bi–Sr–Ca–Cu–O from micro- and macrotests are compared.