THE MEASUREMENT OF INTERNAL STRESSES DURING CREEP OF Al AND Al-M9 ALLOYS* C. N. AHLQUISTT and W. D. NIX? Current experimental techniques are described for measuring average int~erdialocation internal stresses. Measurements of internal stresses during steady stat,e creep of Al and Al-Mg are reported. The predicted reoovery ram based on the measured internal stress dependence of the steady state creep rate agrees with Fried&s network growth model when one accounts for the position dependence of the int,ernal st,ress. The variation of internal stress during normal and inverted primary creep is reported. From transient, creep measurements it, is possible to evaluate the stress dependence of the average dislocation velocity and mobile dislocation density. Results of measurement’s on Al 5% Mg indicate that t’he average dis- location velocity depends linearly on t,he effective stress while the mobile dislocation den&y varies reciprocally with the internal st,ress. ~TI~SURES DES (~O~TRAI~TES INTER?;ES AU CGURS DU FLUAGE DES ALLIAGES Al ET A-Mg Lea autours decrivent les techniques experimentales utilistes couramment pour mesurer les contraintes internes moyennes entre dislocations, et donnent les resultats obtenus pour les mesures des contraintes imernes de fluage pour Al et Al-Mg. La vitesse de revenu prevue it partir de la variation de la vitesse de Auage stationnaire avec la contrainto in&me mesuree est, en accord avec la modele de Friedel pour la croixsance du reseau, si on co&d&e que la contrainte interne depend de la position. Lex auteurs domlent la variation de la contraint‘e intern0 au tours du fluage primaire normal et inverse. A partir des mesures du fluage transitoire il est possible d’evaluer la variation aver la contrain& de la vitasse moyenne des dislocations et de la densite des dislocations mobiles. Les r&&tats des mesures sur AI--5 % Mg montrent que la vitesse moyenne des dislocations depend lineaircment de la contrainte effective alorx que la densite des dislocations mobiles varie inversement aver la cont,raint,e intnrne. MESSUNG INNERER SPA?iNlJ?\‘GEN WdHREND DES KRIECHENS VGN Al UpiD Al-Mg- LEGIER~T~G~~ Experimentelle Methoden zur Messung van mittleren inneren Spannungen zwischen Versetzungen werden beschrieben. Es wird iiber Messungen von inneren Spannungen wahrond des stationiircn Kriechens von Al und van Al-Mg berichtet. Die aus der gemessenen Abhangigkeit der station&m Kriechge- schwindigkeit van inneren Spannungen vorhergesagte Erholungsrate stimmt mit Friedels Model1 iiber das Wachstum von Versetzungsnetzwarken iiberein, wenn man die Ortsabhiingigkeit~ der inneren Span- nungen berii?ksichtigt. Uber die Anderung der inneren Spannung bei normalem und invertiertem prim&rem Kriechen wird berichtet.. Aus Messungen des ~~bergan~skriechens ka.nn man die Spannungsabh~ngigkeit dar mittleren Versetzungsgeseh~~indigkeit und der Dichte beweglicher ~~ersetzungen bestimmen. Ergebnisse an Al-5% Mg deuten darauf hin, da& die mittlere Versetz~lngsgeschwindigkeit linear van der effektiven Spannung abhiingt, wiihrrnd die Dichte beweglicher Versetzungen reziprok mit der inneren Spannung variiert. 1. INTRODUCTION A unified description of steady creep has recently been proposed(l) which incorporates the effects of recovery(2-6) and dislocation glide.(7-11) In this approach the glide process is assumed to be driven by the effective stresso2-22) given by the difference between the applied stress and the interdislocation internal stress. To review briefly the physical basis for the relation between glide and recovery-strain hardening let us consider the plastic deformation of a well annealed polycrystalline metal at high temperature. Applica- tion of an external stress results in a strain rate which is init~i~lly high since barriers to dislocation motion are few and largely ineffective at impeding initial dis- location motion. As deformation proceeds the inter- nal stress builds up in response to strain hardening as shown in Fig. 1. If strain hardening were to occur * Received June 29, 1970; revised August 11, 1970. S&%2 ialifornia. e ar mont of Materials Science, Stanford University, > indefinitely, the internal stress would eventually equal the applied stress and deformation would cease, However, recovery events occur simultaneously and limit the final level of the internal stress. At steady state a dynamic balance exists betlveen strain harden- ing and recovery such that the level of internal stress is independent of time. On a microscopic scale, strain hardening is controlled by the rate at which dislocations come into close proximity through glide while recovery occurs through dislocation rearrangement and annihilation which are controlled by dislocation climb. The applied stress is thus composed of two ‘terms, the internal stress associated with the recovery-strain hardening balance and the effective stress responsible for glide. The present work has as its basis the glide recovery model of creep but is phenomenological in character and should be viewed as supplementary to the more microscopic desc~ptions. (23*24)This paper considers application of the glide recovery model to creep of Al ACTA METALLURGICA, VOL. 19, APRIL 1971 373