Some aspects of thermomechanical fatigue of AISI 304L Stainless Steel: Part II. dislocation arrangements
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II.
EXPERIMENTAL
A comprehensive report about the servo-hydraulic fatigue test system used in this study and the experimental realization of the "true" plastic-strain control used in TMF tests can be found in Reference 20. Therefore, only a brief description is given subsequently. The mechanical tests have been carried out in vacuum using solid specimens with a gage length of 12 mm and a diameter of 8 mm. The results reported here are restricted to in-phase TMF cycling with a triangular command signal for temperature and plastic strain. Because of the use of plastic strain control, the plastic strain rate is connected with the rate of temperature change, and the maximum plastic strain rate is determined by the obtainable cooling rate. The value of the plastic strain rate ept = 6.7 9 10 -5 s -~ which was applied in most tests could be reached using the sample geometry described earlier. In order to carry out experiments with an increased plastic strain rate, the efficiency of cooling had to be improved. By the use of hollow specimens which were additionally cooled by means of a flux of inert He-gas through a concentric hole, 11'2~ a value of ~p~ = 5.0. 10 -4 s -I could be obtained. This technique enabled study of the influence of plastic strain rate on the TMF behavior and allowed significantly shortened long-term experiments, e.g., tests at small plastic strain amplitudes AepJ2. Tests with different plastic strain amplitudes were carried out using a constant cycle time of 40 seconds. The temperature interval 400 ~ to 650 ~ (heating/cooling rate: 750 ~ which was applied for these tests and the plastic strain amplitudes of 0,25, 0.5, and 1.0 pct result in plastic strain rates of 2 . 5 - 1 0 -4, 5.0" 10 4, and 1.0- 10 -3 s-~, respectively. Unfortunately, the temperature gradient in the specimen becomes relatively pronounced at high heating/ cooling rates, leading especially at a high-plastic strain amplitude to inhomogeneous deformation of the specimen. As a consequence, the sample's gage length tended to bulge, so that in some cases the tests had to be stopped in the saturation state prior to fracture. Similar effects have been reported earlier in the literature on TMF of AISI 304.12j'22"231 The values of temperature interval, plastic strain rate, and plastic strain amplitude of the tests reported in this article can be found in Table I, which is presented in the Discussion. The microstructures formed in these TMF experiments are related to the dislocation arrangements observed after isothermal LCF tests at room temperature, 150 ~ 250 ~ and 400 ~ which have been discussed in detail in our previous report, tzl The microstructural investigations have been carried 408--VOLUME 25A, FEBRUARY 1994
out by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM), using also the SEM electron channeling contrast. The advantages of the application of the latter technique to TMFdeformed specimens of AISI 304L are demonstrated in Reference 24 on the basis of some typical results. III.
RESULTS
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