,fcro mefall. Vol. 33, No. 9, PP. 1587-1591. 1985 ooo14160/85 s3.00+0.00 Printed in Great Britain. All rights reserved CoPyri&t 0 1985 Pcrgamon Press Ltd THE EFFECT OF GRAIN SIZE ON THE YIELD STRENGTH OF Ni,Al E. M. SCHULSON, T. P. WEIHS, D. V.VIENSf and 1. BAKER Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, U.S.A. (Received IO July 1984; in revised form 4 April 1985) Abstract-Experiments have established that the effect of grain size (d = 2.9-l IOO~m) on the yield strength of Ni,AI at room temperature is given by the relationship or = a, + zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ kd-” where n = 0.80 f 0.05, a,,=93+ l4MPa and k =2080+ lOSMPa.pm am*o.or. The relationship is explained quantitatively in terms of work-hardening which occurs within the Liiders bands which accompany yielding. The Liiders strain, Ed, increases with decreasing grain size according to the relationship Ed= M-O.” where 1 = 8.4 x 10-2~m05r. R&urn&Des experiences ont permis de montrer que I’effet de la taille da grains (d compris entre 2,9 et 1100 pm) sur la limite elastique de N&Al i la tem$rature ambiante est don& par la relation eu = co + kd-” oti n = 0,80 f 0,05, a0 = 93 f 14 MPa et k = 2080 f I05 MPa~pmWfW. Nous exphquons quantitativement cette relation par l’brouissagc qui se produit dans les bandes de Ltiders qui accom- pagnent la d&formation plastique. La dkformation de LCiders augmente lorsqu’on diminue la taille des grains suivant la relation e, = Amass, od L = 8.4 x 10-zpm~. Zwammenfassung-Nach Rxperimenten kann der EinfluB der Komgr6Be (d = 2,9 bis 1 IOO~m) auf die PlieDspannung von Ni,Al bei Raumtemperatur mit folgender B&hung bescluieben werden: rrr = co + kd -“, mit n = 0,80 f 0,05, a0 = 93 f 14 MPa und k = 2080 f 105 MPa*cmass*ar’5. Dieser Zusammenhang wird quantitativ mit der Verfestigung erkhirt. die in den Ltidersbgndern, welche das PlieDen begleiten, auftritt. Die Liidersdehnung sLsteight mit abnehmender KomgriiBe entsprechend r, = 1 d-O.“, mit I, = 8,4 x IO-* pm”.u, 1. INTRODUCTION This paper considers the effect of grain size on the room-temperature yield strength of the strongly or- dered Ll, aluminide Ni,AI. The work is part of a larger investigation of grain size effects on the mech- anical properties of some aluminide intermetallics, but is presented separately because the relationship to be described is a novel one. 2. EXPERIMENTAL Most specimens were processed from rapidly solidified and consolidated powder. The powder, obtained from Pratt and Whitney Aircraft, West Palm Reach, Florida, was of stoichiometric com- position and was consolidated into rod by extruding at elevated temperatures. Table 1 lists the impurities, the extrusion/fabrication schedule and the geometry of the specimens. Multi-stage extrusion was necessaq to produce finely grained aggregates (d < 10 pm). tPresent address: United Technologies Research Center, East Hartford, Connecticut, U.S.A. fBoundaries of annealing twins were excluded because they appeared not to obstruct slip; i.e. slip lines on deformed surface grains changed direction but did not terminate at twin boundaries. Specimens were also produced from ingots of stoichiometric composition. The ingots were supplied by United Technologies, East Hartford, Connecticut, and were processed as noted in Table 1. These specimens contained grains which were more than an order of magnitude larger (i.e. d = 9004100~m) than the grains which could be produced through prolonged annealing (100 h/1573 K) of the consoli- dated powder. Other specimens were prepared from off-stoichiometric (23.9 at.y&) recrystallized single crystals, also obtained from United Technologies. These specimens were processed as noted in Table 1. They allowed a comparison of the strength of me- dium grained N&Al (9 d d < 80 pm) processed in two different ways. The grain size of the consolidated powder and of the recrystalhxed single crystal was increased by annealing at temperatures between 1073 and 1273 K in a flowing atmosphere of dried and deoxygenated argon. In this way, sixes varying from 2.9 to 80 pm were obtained. When combined with the grain size of the homogenixed ingots, the complete range was 2.9 to 11OOpm. The grain size of every specimen was determined using optical metallography and the method of linear intercepta.$ Prior to and aRer the grain coarsening anneal the material was examined by transmission electron mi- croscopy (TEM) and by X-ray diffraction (XRD). 1587 AM W9A