Deformation characterstics of BCC metals - Dislocation mechanisms of plastic flow in polycrystalline a-Fe, Mo, Ta and W in the range 77-1150 K

Abstract

The plastic deformation behavior of polycrystalline body-centered cubic metals ( -Fe, Mo, Ta, and W) was examined in the range 77-1150 K with the following objectives:

(i) To characterize the rate-controlling obstacles to dislocation motion at low temperatures (<0.25 T_m), (ii) To investigate the existence or otherwise of an athermal plastic flow regime at intermediate temperatures (0.25-0.5 T_m), and (iii) To identify the dislocation mechanism for high-temperature (>0.5 T_m) plastic flow.

Tensile tests, along with strain-rate-change tests and differential-stress creep experiments, were employed. The thermal activation strain rate analysis was carried out on the basis of the strain rate equation:

= 0exp ( GkT),

=

0

exp( kT G

),

where activation enthalpy, activation area, pre-exponential factor, and entropy of activation were obtained as a function of stress.

In the low-temperature thermally activated region, the activation areas were in the range 10-100 b² and the zero-stress activation energy was of the order of 0.1 eV/b². The rate-controlling obstacles were found to be “linear elastic” in nature. However, the entropy of activation was quite small, and the pre-exponential factor was almost independent of stress.

Differential creep experiments indicated that the activation areas following stress increments are larger than those following stress decrements below a certain temperature, while the reverse was found to be the case at higher temperatures approaching T_m; the transition temperature, where the two sets of activation areas are the same, was close to the interstitial solid solution softening-strengthening transition temperature in BCC metals.

It is concluded that two dislocation processes control the low-temperature deformation of BCC metals: at lower temperatures, the dissociated-core model is rate-controlling, whereas at higher temperatures, the plastic flow occurs by thermally activated surmounting of the Peierls energy hills.

Regarding the plastic flow of -Fe above 0.5 T_m, the activation free energy at zero effective stress was much higher than the activation energy for diffusion. Besides, the activation areas (90-500 b²) were fairly large and were stress dependent. These data suggest that forest intersection involving attractive junctions is the operative mechanism in this temperature range. This is supported by the observation that the reversible change in flow stress resulting from a change in strain rate during the tensile test was proportional to the total flow stress.

At intermediate temperatures in Mo, athermal flow behavior was observed only with regard to the initial flow stress. At larger strains, the deformation became thermally activated, and the temperature sensitivity of the flow stress increased with strain. These observations are consistent with the view that thermally activated unzipping of attractive junctions occurs at high temperatures. The transition from athermal to high-temperature thermally activated flow, and the absence of the athermal region in some metals, could also be satisfactorily explained on the basis of this model.

In Chapter I, the concepts of the theory of thermally activated glide are presented together with a review of relevant literature. Experimental details are included in Chapter II, and the results and discussion in Chapter III are organized in four sections:

(i) Rate-controlling obstacles for dislocation glide in BCC metals at low temperatures, (ii) High-temperature dislocation mechanism in -iron, (iii) Existence of an athermal region of plastic flow of metals at intermediate temperatures, and (iv) General discussion.

Summary and conclusions are presented in the final chapter.