Spreading of extrinsic grain boundary dislocations in austenitic steel

Abstract

The dependence of the spreading process of extrinsic grain boundary dislocations (EGBD's) on grain boundary state and misorientation, and spreading kinetics in austenitic steel is investigated. Spreading of EGBD's at non‐equilibrium grain boundaries obtained by successive cold rolling and annealing treatments at a temperature much lower than 0.8Tm is found to occur at approximately 450 °C. EGBD's in specimens annealed at 0.8Tm however spread at about 550 to 600 °C. This observation seems therefore to indicate that at approximately 0.8Tm an irreversible change in grain boundary structure takes place. It is also found that EGBD's lying at special and incoherent twin boundaries spread at 700 °C compared with 550 to 600 °C for equilibrium, random boundaries in the same material. Observations of spreading kinetics indicate that the latter is not solely controlled by grain boundary diffusion.

Mobility of Dislocations in Grain Boundaries

“…proportional t o (D,+,,,./t)" and independent of S [ 8,16,19,201 , This seems u n r e a l i s t i c , because t h e mutual i n t e r a c t i o n between d i s s o c i a t i o n products is neglected, and was not confirmed by l a t t e r experiments. The more advanced models concerning mechanism by which EGBDs moves a p a r t along t h e i n t e r f a c e during spreading of TLD, i n f a c t , can be t r e a t e d i n the frameworks of more general r e l a t i o n s concerning climb and viscous g l i d e of d i s l o c a t i o n s :…”

“…It should be noted that the disappearance of TLDs image as observed in TEM is solely to indicate the fact that the products of dissociation spread out to such an extent t h a t t h e width of a r r a y has reached a value S,, of some hundreds a n g s t r o m [8,19,321, Therefore t h e occurrence o r non-occurrence of c o n t r a s t should not be used a s t h e determinant of t h e s t r u c t u r a l s t a t e of t h e boundary and the physical s i g n i f i c a n c e of EGBDs does not depend on whether they a r e detectable i n TEM by conventional methods 1 251 .…”

Non-equilibrium state and recovery of grain boundary structure. II. Energetic Analysis

“…(i) impurities increase the TLD stability, hence in alloys it is higher than in pure nietals, which is likely to result from inhibition of diffusion by impurity atoms [15, 18 to 201; (ii) in special GB's the TLD stability is higher t'lian in randoin ones [B, 16,201. Grain boundary diffusivity decreases with the increase of the level of GB structure perfection [all which seems to explain the influence of the concrete GI3 structure on TLD stability;…”

Dislocations extrinsèques, ségrégation intergranulaire et mobilité des joints de grains

“…But du travail. -Certains auteurs ont montré que les dislocations extrinsèques introduites par déformation plastique à température ambiante disparaissent extrêmement rapidement sous l'effet d'un recuit modéré [10,11,12]. Ce comportement est interprété par un étalement du coeur de la dislocation [ 13] ou, ce qui est formellement équivalent, par une dissociation en un grand nombre de dislocations dont les vecteurs de Burgers sont infiniment petits [14].…”