The nature of grain boundary sliding and the superplastic flow

Abstract

Two types of grain boundary sliding are established and investigated during Zn bicrystal deformation: "pure" grain boundary sliding (GBS) which is not associated with intragranular sliding (IS) and GBS which develops simultaneously with IS. Both types of GBS exhibit greatly simi1a.r structural peculiarities but under identical deformation conditions the second type GBS has a much higher rate with a WLGBS paramet'er decreasing from 1 to 0.4. The comparative analysis of GBS regularities in Zn bicrystals and the superplastic Zn-0.4% A1 alloy attests t'o the fact t1ia.t "pure" GBS determines the strain hardening at superplastic deformation while GBS of the second type determines the development of the recovery processes. It is shown that these concepts may serve as a basis for the elucidation of many experiment'ally observed peculiarities of the superplastic flowthe existence of the initial unsteady and steady state &ages, decrease of flow stress after short-term deformation a t elevated rates, and others.Zwei Arten von Korngrenzengleitung werden aufgestellt und wahrend der Zn-Bikristalldeformation untersucht : ,,reine" Korngrenzengleitung (GBS) die nicht mit intragranularer Gleitung (IS) verbunden ist und GBS, die sich gleichzeitig mit IS entwickelt. Beide Arten von GBS zeigen stark ahnliche strukturelle Besonderheiten, jedoch hat unter identischen Deformationsbedingungen der zweite GBS-Typ cine vie1 hohere Rate mit einem mGBs-Parameter, der von 1 auf 0,4 abnimmt. Die vergleichende Analyse der GBS-GesetzmaDigkeiten in Zn-Bikristallen und der superplastischen Zn-0,4y0 Al-Legierung ergibt, daa ,,reine" GBS die Spannungshartung bei superplastischer Deformation bestimmt, wahrend GBS vom zweiten Typ die Entwicklung des Erholungsprozesses bestimmt. Es wird gezeigt, dad diese Vorstellungen als Grundlage fur die Aufklarung vieler experimentell beobachteter Besonderheiten des superplastischen Flusses dienen kanndie Existenz des unstetigen Anfangszustandes und stetiger Zustande, Absinken der F l u b spannung nach kurzzeitiger Deformation bei ausgewahlten Raten und andere.

Effect of extrinsic grain-boundary defects on grain-boundary sliding resistance

“…Dislocations may leave the GB and enter the matrix to relieve stresses at obstacles or lattice dislocations may impinge on the GB, increasing the density of GB dislocations. Several workers (Valiev et al 1983, Sheikh-A li 1995 have related enhanced GB sliding to the impingement or emission of lattice dislocations from a GB.…”

Stress–strain relation during superplastic uniaxial tension at constant tension velocity

“…At present, a consensus has been reached on the dominant mechanism of GBS in Region II at intermediate strain rates. GBS accounts for 50% to 80% of the total deformation during the superplastic deformation process [6,7,[35][36][37], although this ratio varies with material, composition, grain size, and temperature. It is certain that the ratio of GBS is high in superplastic strain rate range [38].…”