No abstract

Pure Ta has been cold rolled and annealed at various temperatures. The crystallographic textures were studied by measuring X -ray pole figures and subsequently calculating the orientation distribution function. The microstructure was investigated via optical microscopy. The rolling textures were explained by dislocation glide on {110} (111), {112} (111), and {123} (111) glide systems. Corresponding simulations were carried out using relaxed constraints Taylor theory. Interpretation of the annealing textures was carried out via continuous recrystallisation in the case of weak deformations and temperatures and via discontinuous recrystallisationfor higher rolling degrees and temperatures, respectively. MSTI1886

The influence of strain rate on the stress-strain curve of materials showing yield point phenomenon are determined in the range of small strains and high strain rates by means of the indirect impact tension test. With increasing deformation velocity, the LOders strain is found to increase considerably. The stress reduction after reaching the upper yield point, is not abrupt and takes place gradually with increasing strain. These phenomena are investigated on Armco iron, on the tempered steel 42 CrV 6, on pure tantalum and tungsten alloys. A simple mechanical model is introduced which shows that the LOders front velocity increases with strain rate approaching asymptotically the value of the plastic wave velocity as an upper limit. With this restriction, the local strain within the LOders band must increase by increasing the rate of elongation of the tensile specimen. The LOders strain could be estimated quantitatively as a function of strain rate. The model describes also the stress-strain function after reaching the upper yield point considering the limited stiffness of the test arrangement. EinfluB der Dehngeschwindlgkeit auf die Spannungs-Dehnungs-Kurve im Bereich der Ludersdehnung. Der EinfluB der Dehngeschwindigkeit auf die Spannungs-Dehnungs-Kurven von Werkstoffen mit ausqepraqter Streckgrenze wird im Bereich kleiner Dehnungen und hoher Dehngeschwindigkeiten im indirekten Schlagzugversuch ermittelt. Mit steigender Verformungsgeschwindigkeit nlrnrnt die LOdersdehnung deutlich zu, Der Spannungsabfall nach Erreichen der oberen Streckgerenze erfolgt bei hohen Dehngeschwindigkeiten nicht abrupt, sondern graduell und erstreckt sich Ober einen groBeren Dehnungsbereich. Diese Erscheinungen werden an Armco-Elsen, am vergOteten Stahl 42 CrV 6. an Reintantal und an Wolframlegierungen untersucht. Anhand eines einfachen, mechanischen Modells wird gezeigt, daB mit steigender Dehngeschwindigkeit die Geschwindigkeit der LOdersfront zunimmt und dem Wert der plastischen Wellengeschwindigkeit als oberem Grenzwert asymptotisch zustrebt. Durch diese Elnschrankunq muB die lokale Dehnung innerhalb des LOdersbandes mit steigender, globaler Verlangerung der Zugprobe zunehmen. DarOber hinaus beschreibt das Modell die Spannungs-Dehnungs-Kurven nach Erreichen der oberen Streckgrenze unter BerOcksichtigung der Nachgiebigkeit der PrOfanordnung.0 ' -------------------1 STRAIN Figure 1. Schematic representation of the stress-strain curve showing a yield point phenomenon the slip plane, the dislocation breaks away from the solute atom atmosphere. An alternative model [7] explains the stress drop at the upper yield point by dislocation multiplication in connection with dependence of the dislocation velocity on the stress.Under quasi-static loading, the localised yielding was studied early by determining the distribution of the reduction of area along tensile specimens [8]. In later investigations, the local strain was determined continuously at different points along the specimen as a function of time using strain gages [9; 10] or inductive ...

Dynamic compression tests are carried out on Armco iron, Cr-Vsteel, Ni-Cr-Mo-V-steel, an austenitic Ni-Cr-Mo-steel, tantalum, nickel and Ni,AI and magnesium. The flow curves are analysed to determine the influence of the deformation energy which is transformed into heat on the flow behaviour and mechanical stability. Not only the material properties but also the conditions of friction between the specimen and the compression tool are found to have a greate influence on the flow stress reduction and stability. High frictional forces promote mechanical instability of materials with low strain hardening and low strain rate sensitivity. Adiabatisches

For dynamic compressive tests, the conditions of Friction between the contact surfaces of the specimens and the bars are found to cause mechanical instability of materials with low strain hardening and low strain rate sensitivity. The adiabatic flow curves are described by a model using a weakening parameter taken into account both damage and thermally induced instability. In this paper, a material model is used, which is able to describe the material behaviour in the whole range of deformation, i.e. in the region of thermally activated processes with gradual transition to the region of damping-controlled processes. RCumB: Dans les essais de compression dynamique, les conditions de frottement des surfaces de contact entre I'tchantillon et les barres sont A I'origine des instabilitts mkcaniques des mattriaux posskdant une faible sensibilitd h la vitesse de dtformation. Les courbes de contrainte d'tcoulement adiabatiques sont dtfinies par un modkle utilisant un facteur d'adoucissement qui tienne compte de I'endommagement et des instabilitts thermiques. Dans cette publication on utilisera un mod6le capable de dtcrire le comportement du mattriau dans tout le domaine de dkformation., c. h d. dans le domaine des mtcanismes d'activation thermique avec passage progressif au domaine des mtcanismes d'amortissement contrdlt.