Boron Segregation to Grain Boundaries and Improved Ductility in Pt + 30 Wt Pct Rh + 8WtPctW
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PT
3008 (Pt + 30 wt pct Rh + 8 wt pct W) is a single phase, fcc alloy developed at ORNL ~,2 for use in radioisotope thermoelectric generators. This alloy typically exhibits good ductility in the temperature range 100 to 850 ~ with ductility decreasing below 100 ~ and a ductility minimum in the neighborhood of 1100 ~ Elongation to failure, ~f, in a room temperature low strain rate tensile test (solid curve, Fig. 1) is typically about 15 pct, increasing to almost 40 pct at 200 ~ Failure at room temperature is usually intergranular (see Fig. 2), and while Auger electron spectroscopy (AES) studies have frequently indicated small amounts of phosphorus and silicon segregation to the grain boundaries, no clear correlation between such segregation and the intergranular failure of Pt 3008 has been established. Improvements in the room temperature ductility of Pt 3008 could significantly reduce the cost of alloy production and component fabrication. During the course of an alloy development program at ORNL, in which many heats of Pt 3008 were examined, several heats having unusually good room temperature ductility (cI = 35 pct) and a mixed intergranular plus transgranular fracture mode were produced. Chemical analysis indicated that these more ductile heats had higher bulk concentrations of boron than normal. Experimental results reported in this paper will show that boron is segregated to grain boundaries in these more ductile heats. EXPERIMENTAL PROCEDURE AND RESULTS Pt 3.008 alloys for this study were arc melted and drop cast into water cooled copper molds. Ingots were hot rolled to 1.0 mm (0.040 in.) thick sheets, annealed, cold rolled to 0.5 mm (0.020 in.), then stress relieved at 800 ~ Tensile specimens were punched from the stress relieved sheet, then annealed for 1 h at 1200 ~ in vacuum.
C. L. WHITE, J. R. KEISER, and D. N. BRASKI are Research Staff Members, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, TN 37830. Manuscript submitted September 25, 1980. METALLURGICAL TRANSACTIONS A
Room temperature tensile tests were performed using a strain rate of 3.33 x l0 -3 s -1 (0.2 min-~). Figure 3 shows typical engineering stress-strain curves for specimens from heats 4A, 5D, and EA-11. Curves for 4B and 5E were similar to those for 4A and 5D (respectively) and are not shown. Table I summarizes the tensile test results from five heats of Pt 3008. Heats 4A and 4B are typical o f a large number o f Pt 3008 heats we have examined, with ej = 15 pct, at room temperature. In contrast, heats 5D and 5E exhibited unusually good ductility, having EI = 33 and 39 pct, respectively.* This improved tensile ductility was *Tensile test results on heats 4A, 4B, 5D, and 5E from the work of H. Inouye. 3
accompanied by significantly higher yield and ultimate tensile strengths. Scanning electron microscopy of the failed tensile specimens indicated that those from heats 4A and 4B failed 100 pct intergranularly. Failure in speciments from heats 5D and 5E was mostly intergranular with small regions transgran
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