7 Method for Characterizing Electric Current Effects on the Deformation of Metals 43 Experimental Zero-current baseline tensile tests were run using each system (SEMTester and Instron) at a cross-head displacement rate of 0.15 mm/min until break for matched comparison of stress-strain responses for each machine. As expected, they were identical. The following tests were conducted on each material (Cu, Fe, and Ti): zero-current baseline runs, non-zero current runs at two current densities for each material type, and temperature-history matched runs at zero-current in the Instron system. Preliminary tests were performed for each material type with SEMTester specimen-fixture cooling to determine the maximum current density levels possible while maintaining a gage-section temperature less than 200 ◦C (to ensure that the gage-section temperatures remained below the 250 ◦C at the ultimate load point of the test where the specimen temperature increases as a result of cross-section area decreases) and grip/fixture temperatures less than 50 ◦C to avoid possible damage to the load cell. The lower test level current densities were selected to be ~2/3 of the maximums. The Cu specimens were tested with constant dc current densities of: 0, 125, and 200 A/mm2; the Fe specimens at: 0, 45, and 70 A/mm2; and the Ti specimens at: 0, 25, and 40 A/mm2. Multiple tests were performed (at least 3) for each material at each current density and at each temperature to assess repeatability. The results showed variations in ultimate strengths below 1.4% for the copper and iron specimens and 2.7% and 5% for titanium specimens at the low and high current densities, respectively. Average values for the multiple tests are reported. Specimens were first tested at the desired current densities. The resulting thermal histories (averaged gage temperature versus time) were then programmed in the Instron environmental chamber to match the current-induced temperature history. This procedure differs from some reported in literature [2, 4] that use a single average or maximum temperature from the electric current test for the temperature control point in matching zero-current tests. Figure 7.8 shows averaged temperature histories for the three metals at the highest applied current densities. Prior to necking, the specimen temperatures increased by: 25, 50, and 35 ◦C respectively for the Cu, Fe, and Ti metals. After necking, a rapid increase in temperature occurs that cannot be simulated in the Instron environmental chamber. The results below are reported up to the ultimate load points (marked on the figure) where the temperature-history is accurately followed. The use of one constant temperature during the zero-current thermal baseline testing leads to unquantified experimental inaccuracies. Results and Discussion The test results for the pure Cu, Fe, and Ti materials are summarized in Table 7.2. No discernable effects of current on the plastic deformation behavior of the pure Cu and Fe materials were detected beyond those due to temperature. Post-test examination of the Cu and Fe microstructures across the cross-section for all test Fig. 7.8 Gage-section thermal histories from the electric current tests (averaged-dashed lines) and matching thermal histories from the zero-current tests (averaged-solid lines)
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