Short-range reordering of heavy interstitials in Ta, Nb, and Fe during relaxation and static strain aging
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I.
INTRODUCTION
THE interaction
between heavy interstitials and dislocations has an important effect on the mechanical properties of the body-centered cubic metals (bcc) of group V (Ta, Nb, Fe, V . . . ) . In particular, the yield stress of such a material when it contains interstitials and has been plastically deformed is generally greater alter it has been aged. If cry is the flow stress at the end of cold work, after aging under stress for a time t,, at a temperature T,., we obtain a new elastic limit 0-~ _>- cry. This phenomenon can be attributed either to a Snoek type reordering of the interstitials in the dislocation stress field j'2 or to their diffusion in volume toward the dislocations, a mechanism of the Bilby-Cottrell type. 3'5 This second type of long-range diffusion aging has often been studied in bcc as well as fcc and cph metals, either by the yield-point increase measurements 5~2 or by the recovery of internal friction, 4'13'~4and the conclusions in reference to these physical phenomena and the kinetics of aging converge in a significant way. The same is not true for the first stage relative to short-range migration. In fact, although Rosinger et al., ~3 Humphreys et al., ~4 and then Quist and Carpenter I~ have introduced the Snoek type reorientation of interstitials during the recovery of internal friction, some essential points of disagreement in the observed kinetics wilt have to be discussed further on. Likewise, there are few complete and convincing results concerning the study of this phenomenon by the yield-point effect technique. 6'7'16-19 This paper deals with a work aimed at showing the occurrence and studying these interstitials ordering mechanisms in the dislocation stress field during the static strain-aging of tantalum, niobium, and iron. The results obtained (kinetics and amplitude of the yield stress variations) are compared with those reached during anelastic isotherrnic relaxation tests for the same temperature range T,. as well as with high temperature internal friction tests. P. DELOBELLE, Engineer C.N.R.S., D. VARCHON, Engineer Fac., and C. OYTANA, Professor, are with Applied Mechanics Laboratory associated with the C.N.R.S., Faculty of Sciences and Techniques of Besanqon, Route de Gray-La Bouloie-25030 Besan~on Cedex, France. Manuscript submitted June 18, 1982.
METALLURGICAL TRANSACTIONS A
11.
EXPERIMENTAL
METHODS
A. Installation and Characterization of Samples
The relaxation tests were carried out on a high precision electrodynamic torsion machine. The relative stability of the torque was ___3 • 10 5 during 10 5 s for a superficial strain of samples of 10-5. 20The samples were wires of 0.5 to 1 mm in diameter and of 600 mm in length. The temperature could vary from - 2 0 to 200 ~ During the test the samples were twisted to a given superficial shear strain which was then kept constant, and the decrease of the shear modulus with time was then measured (insert at the upper right of Figure 3). Parallel to these tests, some internal friction experiments in the neighborhood
