The Dislocation-grain Boundary Interaction and Its Effect on the Boundary Dynamic Properties

The problem o f t h e i n t e r a c t i o n o f d i s loca t ions w i t h g r a i n boundaries i s d i s cussed on t h e bas is o f t h e concept o f g r a i n boundary non-equi l ibr ium s t a t e generat ion and recovery. According t o t h i s concept, trapped l a t t i c e d is loca t ions , being non-equil i b r i u m defects , are absorbed by t h e boundaries as they d issoc ia te i n t o g r a i n boundary d is loca t ions which are then rearranged i n t o equ i l i b r ium con f igu ra t ions . During t h e abso rp t ion o f l a t t i c e d is loca t ions , g r a i n boundaries are shown t o be i n an non-stat ionary, exc i ted s ta te , which causes an abrupt acce le ra t ion o f some o f t h e g r a i n boundary dynamic process such as d i f f u s i o n , migrat ion, and s l i d i n g . The i n t e r a c t i o n o f g r a i n boundaries (GB) w i t h l a t t i c e d is loca t ions (LD) usual ly imply such processes as nuc lea t ion o f LD, penetrat ion o f LD through GB and t h e i r absorpt ion. The l a t t e r process i s o f s p e c i a l importance when s t r u c t u r a l changes and behaviour o f s o l i d s a t h igher temperatures are s tud ied, i n p a r t i c u l a r dur ing t h e s o l i d phase reac t ion , r e c r y s t a l l i z a t i o n , and high-temperature deformation /l, 21. The process o f LD absorpt ion i n t h i s case i s respons i b l e no t only f o r t h e reduc t ion o f t h e densi ty o f d i s loca t ions i n t h e s t ruc tu re , bu t a lso f o r s i g n i f i c a n t changes i n such dynamic p roper t ies o f GB as d i f f u s i o n , migrat ion, and s l i d i ng . These problems have been t h e authors' primary concern l a t e l y and t h e r e s u l t s o f t h e i r s tud ies are summarized i n t h i s paper. The study o f t h e dislocat ion-boundary i n t e r a c t i o n and i t s r e l a t i o n t o t h e GB dynamic propert i e s inc luded both t h e ana lys is o f t h e LD absorpt ion mechanism / 3 , ' 4 / and experiments i n or der t o conf i rm t h e existence o f t h i s r e l a t i o n and t o de f ine i t s nature /l, 21. Let us cons i d e r these two p o i n t s i n t u r n . 1. Our ana lys is o f t h e LD absorpt ion mechanigms i s based on t h e concept o f t h e GB non-equil i b r i u m s t a t e generat ion and recovery 14, 5/ . According t o t h i s concept, trapped l a t t i c e d is loca t ions (TLD), being non-equi l ibr ium defects , are absorbed by t h e boundaries, t h e boundary energy being reduced t o t h e e q u i l i b r i u m s t a t e values, through r a t h e r complicated rear rangement~ i n GB s t ruc tu re . The type o f these rearrangements depends on t h e crystal logeometry o f GBand condi t ions o f t h e LD absorpt ion. It should be noted t h a t t h e term "recovery" used w i t h reference t o the process o f LD absorpt ion i s r a t h e r convent ional s ince i n transformat i o n s o f t h e f o l l o w i n g type: an equ i l i b r ium GB LD enter ing t h e GB absorpt ion n f TLD by t h e GB s t ruc tu re , t h e r f s u l t a n t equ i l i b r ium GB s t r u c t u r e must d i f f e r from t h e o r i g i n a l m /4/. Numerous experiments /2 / prove t h a t dur ing heat ing TLD are unstable i n a l l t h e GB, bu t t h e observed changes however depend on t h e GB crystal logeometry. I n coincidence and near-coincidence GB t h e d i s s o c i a t i o n o f TLD i n t o p e r f e c t e x t r i n s i c g r a i n boundary d is loca t ions (EGBD) w i t h t h e DSc-Burgers vectors o r p a r t i a l ones are o f t e n observed; i n GB w i t h v i s i b l e i n t r i n s i c g r a i n boundary d i s l o c a t i o n (IGBD) networks t h e i n t e r a c t i o n o f EGBD w i t h IGBD and incorporat i o n o f t h e former i n t o equ i l i b r ium IGBD networks were observed. I n random GB it i s hard ly poss ib le t o see t h e d issoc ia t ion o f TLD i n t o EGBD. The experiments on t h e i n s i t u heat ing o f f o i l s revealed t h e spreading o f t h e e lec t ron microscope TLD con t ras t : .dith heat ing, t h e TLD (l-fhe non-equi l ibr ium s t a t e o f GB can be described i n more d e t a i l i n t h e framework o f d i s c l i n a t i o n s t r u c t u r a l u n i t model proposed by t h e authors i n /6/ and developed i n /7 / . Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901108 Cl-680 COLLOQUE DE PHYSIQUE images i n random GB w i l l gradual ly widen and fade u n t i l t h e d i f f r a c t i o n contrast disappears completely. It should be emphasized t h a t spreading and disappearance o f t h e e lec t ron microscope images o f TLD do no t a t t e s t t o t h e complete TLD absorpt ion, i . e . t o t h e recovery o f t h e GB s t ruc tu re . This conclusion was drawn d i r e c t l y from the experiments which demonstrate t h a t some o f t h e GB proper t ies remain non-equi l ibr ium a f t e r the spreading o f TLD images (see below). The e l u c i d a t i o n o f t h e mechanism o f LD absorpt ion i n GB imp l ies t h e considerat ion o f both t h e c rys ta l logeomet r i ca l rearrangements o f GB s t r u c t u r e and energy aspects o f t h i s i n t e r a c t i o n 1 4 , 5, 61. I n 15, B/ v a r i a t i o n s o f t h e GB energy dur ing the TLD absorpt ion was analyzed on t h e bas is o f . t h e model o f t h e d i s s o c i a t i o n o f TLD i n t o EGBD. The energy o f a non-equi l ibr ium GB was represented as a t o t a l o f t h e f o l l o w i n g components: t h e energy o f t h e o r i g i n a l e q u i l i b r i u m GB; t h e energy o f e l a s t i c d i s t o r t i o n s in t roduced e i t h e r by the TLD o r by t h e complex o f EGBD, i . e . products o f TLD d issoc ia t ion ; t h e v a r i a t i o n o f t h e energy o f o r i g i n a l GB due t o t h e change i n m isor ien ta t ions provoked by the defects ; the energy o f t h e i n t e r a c t i o n o f EGBD w i t h each other ; t h e energy o f t h e i n t e r a c t i o n o f GB regions occupied by t h e products o f TLD d issoc iat i o n w i t h t h e GB regions o f t h e o r i g i n a l GB s t r u c t u r e , i . e . t h e energy o f t h e t r a n s i t i o n r e g ion. The energy o f GB decreases dur ing the r e d i s t r i b u t i o n o f EGBD i n GB, i . e . dur ing t h e spreading o f TLD d i s s o c i a t i o n products. Assuming t h a t t h e e l a s t i c i t y theory-appl ies t o highangle GB, t h e energy v a r i a t i o n value f o r TLD w i t h t h e Burgers vector normal t o t h e GB w i l l be as shown i n F ig . 1. During t h e absorpt ion o f TLD, t h e GB energy does no t change monotonously: the re i s a c e r t a i n p o t e n t i a l b a r r i e r a t the beginning o f t h e separat ion o f TLD d issoc ia t ion products. It means t h a t TLD i n GB are r e l a t i v e l y s tab le and t o be absorbed by t h e GB s t ruc t u r e , they have t o overcome'the.potent ia1 b a r r i e r due t o thermal a c t i v a t i o n .