Idea Transcript
Cyclic Hardening and Microstructure 573 and 873K.
M. G. Moscato, I. Alvarez-Armas,
of Zircaloy-4 between
and A. F. Armas
Institute de Fisica Rosario, CONICET-UN,Bv. 27 de Febrero 210 Bis, 2000 Rosario, Argentina The study of the microstructure developed during cyclic deformation is one of the principal subjects of investigations in fatigue. However, the majority of this investigations have been conducted on cubic metals and their alloys. Scarce information is available about the fatigue microstructure of hcp metals such as Zirconium and its alloys, which are important in nuclear technology. The aim of the present work is to analize the dislocation configurations produced in Zircaloy4 during fatigue in order to give microscopic support to the fatigue deformation mechanisms and to establish a correlation between the mechanical behavior and the dislocation arrangements of the material. Strain controlled cyclic tests were carried out using a fully reversed triangular wave with total strain range AE, = 0,Ol and a total strain rate E, = 2 x 10e3s-‘. The tests were performed in air and in the temperature range from 573 to 873 K. Thin-foil discs were prepared from sections cut parallel and perpendicular to the tensile axis. The foils were examined in a Philips EM 300 operating at 100 Kv. This work is primarily concerned with the intermediate temperature range, where abnormal mechanical behavior of Zircaloy-4 have been reported [ 1,2]. Figure 1 shows the cyclic behavior of Zircaloy-4 at various temperatures within the intermediate range. From the figure it can be inferred that the cyclic behavior of this material can be divided in three regions, The first stage, which represents a high but decreasing hardening rate, could be attributed to the typical hardening expected to take place in an annealed material[3]. However, in the second stage, the material presents a pronouced cyclic hardening that has a noticeable linear dependence with the number of cycles instead of reaching a state of saturation, Finally, in the third stage the hardening rate decreases and the stress falls as a consequence of the specimen failure. Figure 2 shows the microstructure developed in a sample fatigued at 713K up to 50 cycles, that correspond to the end of the first hardening stage and the beginning of the linear cyclic hardening. The structure consists mainly of bundles of edge dislocations almost perpendicular to one primary slip direction , and channels containing a lower density of long straight screw dislocations aligned with the primary slip direction. Figure 3, corresponding to a sample fatigued at 713K up to 500 cycles, shows, besides the bundles of edge dislocations, a new set of walls oriented parallel (longitudinal walls) to the primary slip direction. A detailed observation of the figure reveals jogged screw dislocations next to the longitudinal walls and a great number of elongated loops with their long axis perpendicular to the slip direction. Trace analysis of the jogged dislocation reported previously [4] have shown that jogs are aligned along a (101 1)-plane trace. A higher magnification of these walls, Figure 4, shows that they consist effectively of dislocation debris. Thereby, it can be speculated that two mechanisms could account for the formation of the dislocation structures and, consequently, the linear strain hardening of the fatigued material. One is the increase of dilocation acumulation due to stronger dislocation-solute atoms interactions (dynamic strain aging) and the other is a debris
mechanism [6,7] involving multiple cross-glide of screw dislocations. operate, but it needs to be studied further which, if either, is dominant.
Presumably
FIG. 1 - Cyclic behavior of Zircaloy-4 at various temp.
Fig. 2 -Dislocation
struchue
up to 50 cycles
FIG. 4 -Detailed
dislocation
debris structure
FIG. 3 -Dislocation stmcture up to 500 cycles
both mechanisms
References 78,49, (1978) 111W. R. Thorpe and I. 0. Smith, J. ofNuclearMaterials, (21D. Lee and P. T. Hill, J. o/Nuclear Materials, 60, 227, (1976) I31 M. Klesnil and P. L&s, Fatigue ojMetalic Materials. Materials Science Monographs,
Elswier,
Amsterdam,
7,
Conference
on
p.17, (1980)
[4] M. G. Moscato, M. Avalos, I. Alvarez-Armas and A. F. Armas, Proceedings Microscopy ofMaterials, SB, (1996) [6] J. J. Gilman, J. ofApplied Physics, ~01.33, n”9, 2703, (1962) [7] C. E. Feltner, Phil. Mugmine, 12, 1229, (1965)
5” Brazilian