SUPERPLASTICITY AND GRAIN BOUNDARY CHARACTER DISTRIBUTION IN OVERAGED Al-Li-Cu-Mg-Zr ALLOY

摘要

Nous avons soumis des echantillons d'alliages 8091 a un processus de traitement thermomecanique (TMP) comprenant les etapes suivantes : survieillissement avant deformation, deformation en plusieurs etapes a 300 deg C et tests de superplasticite sous differentes vitesses de deformation. Nous avons utilise la deformation par torsion aussi bien pour developper la microstructure fine que pour tester la superplasticite. La sensitivite a la vitesse de deformation, m, du materiau varie entre 0.30 et 0.45 a 450 degC pour des vitesses de deformation entre 8 X10~(-2) et 10~(-3) s~(-1). Nous avons determine au moyen d'une technique de diffusion retrograde des electrons (EBSD) la distribution du caractere de joints de grains (GBCD) de l'alliage Al-Li-Cu-Mg-Zr (8091) traite thermomecaniquement, ce qui produit une bonne superplasticite. Nous avons classe tous les joints de grains dans l'une des trois categories en fonction de la valeur de sum : petit angle, reseau de coincidence et joints a grand angle aleatoire. Pour obtenir des evidences sur l'evolution de la structure et une indication sur l'augmentation en frequence du I des joints apres le TMP. nous avons effectue des etudes quantiatives sur le caractere des joints de grains apres plusieurs etapes de traitement. L'examen de la diffraction d'electrons sur des aires selectionnees (SAD) a fourni des evidences sur la structure fine pour laquelle la mesorientation des joints de grains augmente de quelques degres apres le TMP jusqu'a 20-30 degres apres les tests de changement de vitesse de deformation. Nous avons montre quantitativement par l'EBSD comment le caractere des joints de grains a change vers une grande valeur de sum. Les analyses au TEM indiquent que la phase T_2 est responsable de la stabilisation de la structure. Il n'y a pas d'evidence de formation de cavites pendant la deformation superplastique par torsion, ce qui suggere que la nature de la contrainte influence grandement la nucleation des cavites.%Samples of 8091 alloy were subjected to a thermomechanical processing (TMP) treatment that included the following stages : overaging before deformation, multistage deformation at 300 deg C and strain rate change tests for superplasticity. Torsional deformation was utilized both to develop the refined microstructure and to test for superplasticity. The strain rate sensitivity, m, of the material ranged between 0.30 and 0.45 at 450 deg C for strain rates between 8X 10~(-2) and 10~(-3) s~(-1). The grain boundary character distribution (GBCD) of thermomechanically processed Al-Li-Cu-Mg-Zr (8091) alloy, which develops good superplastic response, has been determined by an electron backscattering diffraction technique (EBSD). All grain boundaries have been classified into one of three categories in terms of I values: low angle, coincidence site lattice and random high angle boundaries. Quantitative studies of grain boundary character were done after various processing stages to obtain evidence about structure evolution and indicate an increase in X boundary frequency following TMP. Selected area electron diffraction examination (SAD) gave evidence about the refined structure, in which the grain boundary misorientation increased from a few degrees after TMP to 20-30 deg after strain rate change tests. It has been shown quantitatively by EBSD how the grain boundary character was changed to high sum values. EM analyses indicate that the T_2 phase is responsible for substructure stabilization. There is no evidence of cavity formation during superplastic deformation by torsion, which suggests that cavity nucleation is strongly influenced by the nature of stress.