Deformation mechanism and dislocation structure of high‐purity molybdenum single crystals at low temperatures

Abstract

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 *

A new regime for mechanical annealing and strong sample-size strengthening in body centred cubic molybdenum

“…It is known that the athermal shear stress for screw dislocation motion in Mo is 750 MPa (the corresponding uniaxial stress would be of the order of 1.5 GPa) near 0 K (ref. 22). At stresses far above this level, such that the free-flight dislocation velocity is exceedingly high, for example, approaching the speed of transverse acoustic wave, the mobility difference between edge and screw dislocations is expected to diminish.…”

The structural dependence of work hardening in low carbon steels

“…A decrease in the average cell size . 1 9 has been verified in nearly every effort at quantifying cell formation, in bee as well as fcc metals, and for monotonic as well as cyclic loading [4,7,32,33,[36][37][38][39][51][52][53][54][55][56][57]66,71].…”

“…It has been observed in bee metals that the extent of cell formation at a given strain is strongly temperature dependent in certain temperature ranges, with higher temperatures promoting cell development [33,[36][37][38][39][73][74][75]103]. Although designation of a structure as .…”

“…Luft and co-workers for example, have studied instability processes in bee transition metals and alloys, following a temperature change [36,[73][74][75]. Their most extensive work was performed on molybdenum single crystals.…”

Twinning pathway in BCC molybdenum

“…which is rather small, considering the low-temperature macroscopic critical resolved shear stress (CRSS) of BCC Mo is 750 MPa [34]. In real materials, there are many local stress concentrators such as voids and interstitial clusters.…”