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Diffusion processes in the dislocation cores during the high-temperature deformation of intermetallic alloys.

Messerschmidt U., Guder S., Häussler D., Bartsch M.

Inżynieria Materiałowa

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2001

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R. XXII, nr 5

627-632

EN

In situ straining experiments in a high-voltage electron microscope to study dislocation dynamics and macroscopic deformation experiments including stress relaxation tests have been performed on a number of intermetallic alloys. There are many similarities in the dynamic behaviour of dislocations in the studied materials. In the low-temperature range, different mechanisms lead to a normal decrease of the flow stress with increasing temperature. At high temperatures, in several materials the dislocations move either in an unstable or a viscous way. They are then smoothly curved, or they are straight and oriented along crystallographic directions. As the low-temperature mechanisms cease at high temperatures, the viscous motion should be due to an additional friction mechanism which also causes the flow stress anomaly in the respective materials. It is suggested here that this mechanism involves diffusion processes in the dislocation cores, which can be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. All these processes lead to an inverse dependence of the strain rate sensivity on the stress.

2

In situ tensile tests in an HVTEM - a powerful method to investigate dislocation motion in ODS alloys

Wasilkowska A., Bartsch M., Haussler D., Reppich B., Messerschmidt U., Czyrska-Filemonowicz A.

Metallurgy and Foundry Engineering

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2000

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Vol. 26, no. 3

247-258

EN

Reported in silu tensile tests in a high voltage transmission electron microscope (HVTEM) are devoted to oxide dispersion strengthened (ODS) alloys, which have technological importance at high temperatures. First in silu experiments carried out on commercial alloys MA956 and MA754 at temperatures up to 1010°C have been very promising. Continuos recordings revealed the temperature and strain rate dependence of dislocation dynamics and evidenced various individual events, e.g. the interaction of dislocations whh localised obstacles. The results were discussed in the different temperatures in terms of mutual dislocation interactions, the Orowan-, interfacial pinning and solutes drag mechanisms. Quantitative aspects of in situ experiment analyses have been underlined. A comparison with post-mortem observations of deformed and unloaded specimens showed that reliable microstructural data about deformation processes can be obtained from in situ straining at high temperatures.

PL

Opisane w pracy próby rozciągania in situ za pomocą wysokonapięciowego mikroskopu elektronowego (ang. HVTEM) poświęcono stopom umocnionym dyspersyjnie tlenkami (ang. ODS). Stopy te znajdują techniczne zastosowanie w wysokich temperaturach. Pierwsze badania in situ przeprowadzone na handlowych stopach MA956 i MA754 w temperaturach do 1010°C są bardzo obiecujące. Ciągłe nagrania wideo dynamiki dyslokacji pozwalają obserwować jej zależność od temperatury i prędkości odkształcenia oraz zarejestrować różnorodne zjawiska oddziaływania dyslokacji z lokalnymi przeszkodami. Obserwowane w rożnych zakresach temperatur procesy dyskutowano pod kątem wzajemnego oddziaływania dyslokacji, mechanizmu Orowana, kotwiczenia dyslokacji na powierzchni cząstek oraz dyfuzyjnego ciągnienia atomów obcych przez ruchome dyslokacje. Szczególną uwagę zwrócono na ilościową interpretację wyników. Obserwacje odkształconych próbek post-mortem wskazują, że pewniejsze dane mikrostrukturalne o procesach odkształcenia w wysokich temperaturach można otrzymać na podstawie badań in situ.

3

In situ TEM straining experiments on incoloy Ma956 at room temperature and at high temperatures.

Wasilkowska A., Bartsch M., Messerschmidt U., Czyrska-Filemonowicz A.

Inżynieria Materiałowa

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1998

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R. XIX, nr 3

463-466

EN

A direct experimental proof of dislocation motion during plastic deformation of ferritic oxide dispersion strengthened (ODS) alloys is first given for ferritic alloy INCOLOY MA956. In situ straining experiments were performed at room temperature and at high temperatures (up to 1010 degrees centigrade) by means of high voltage electron microscopy (HVEM). Moving dislocations revealed a Burgers vector of a/2<111>type. Deformation at room temperature proceeds by jerky motion of dislocations on {110} slip planes. At high temperatures the dislocations show a completely different behaviour. A strong pinning of dislocations at dispersoids was observed, but the density of interaction points was low. Slip takes place on {112} and {123} planes. No subgrain formation was detected.