Gas phase embrittlement by metal vapors
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Gas Phase Embrittlement by Metal Vapors J.P. BECKMAN and D. A. WOODFORD Gas phase embrittlement in Ni and Ni-base alloys at intermediate temperatures due to inter~ranular penetration of oxygen has been well documented 9 ] Oxygen similarly embrittles Cu. 12'3'41 It is also known that gaseous sulfur will 9 [ 5 61 severely embrittle Ni by intergranular penetration, ' yet has little embrittling effect on Cu. ITj The reason for this difference in behavior may be deduced from data on intergranular S segregation from the matrix in these two metals. Although S is a grain boundary segregant in both Ni and Cu, data on mid-range ductility minima of Cu with S additions ~8] suggest that Cu is much less sensitive to intergranular S embrittlement than is Ni. Because both Bi and Pb segregate to Cu grain boundaries and are known to cause more severe embrittlement than S,[8'9] it was thought that Pb and Bi vapors may cause gas phase embrittlement of Cu. Although Pb and Bi are also known to be embrittling when present in the liquid phase in intimate contact with Cu, f~~ in the experiments to be described, the metals were held in the same evacuated chamber with, but separate from, the Cu during exposure. In this case, the damage could only have occurred by vapor phase transport. It was found that exposure of OFHC Cu to Pb or Bi vapor at 800 ~ allows sufficient grain boundary penetration of these elements to result in intergranular embrittlement on subsequent tensile testing in air at 400 ~ Subsize tensile specimens of 2.5 mm gage diameter of O F H C c o p p e r were sealed in e v a c u a t e d quartz tubes together with quartz vials filled with Pb or Bi powders (Johnson and Mathey 99 9 pct). These sealed tubes were then heated in a furnace at 800 ~ for 24 hours9 The partial pressure of Bi at 800 ~ is 2.7 x 10 -5 MPa and for Pb it is 6.7 • 10 -6 MPa. Ul] After exposure, the encapsulated specimens were air cooled and broken out of the quartz tubes at room temperature. Subsequent tensile testJ.P. BECKMAN, Graduate Student, and D. A. WOODFORD, Professor, are with the Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted March 9, 1988. 184--VOLUME 20A, JANUARY 1989
ing at 400 ~ (in the mid-range ductility minimum regime for copper) resulted in an intergranular fracture mode indicative of severe embrittlement. Tensile testing at room temperature did not result in a significant decrease in ductility as compared to the control specimens. Specimens of two different grain sizes, 100 microns and 500 microns, were used. Control samples of each grain size were tested at 400 ~ for comparison. Figure 1 shows the resulting fracture morphologies from the various exposures for the finer grain size, and Figure 2 for the larger. All of the Bi or Pb exposed specimens tested at 400 ~ show intergranular fracture modes. The partially intergranular fracture mode of the vacuum exposed control specimens is wholly expected in the mid-range ductility minimum regime, and probably reflects a degree of oxyg
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