Brittle fracture in polycrystalline Ir-0.3 pct W
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Specimen Description and Preparation In a previous study of brittle fracture in polycrystalline iridium16 it was shown that pure iridium and high p u r i t y iridium alloyed with 3000 ppm (0.3 wt pct) of tungsten suffered from brittle transgranular cleavage and intergranular fracture. The tungsten is present substitutionally in solution and helps t o control g r a i n growth during high temperature exposure. Previous mechanical tests showed that the low l e v e l addition of tungsten does not influence the fracture behavior. ~9 The It-0.3 pct W is used in s p a c e n u c l e a r power a p plications17 and was the only m a t e r i a l readily a v a i l D. L. ROHR and S. S. HECKER are with the Los Alamos Scientific Laboratory, Los Alamos, NM87545, and L. E. MURRis Professor and Head, Department of Metallurgical and Materials Engineering, New Mexico InstituteofMiningand Technology, Socorro, NM 87801. Manuscript submitted February 3, 1978. METALLURGICAL TRANSACTIONS A
able in a form suitable for examination by transmission electron microscopy. We examined two lots of Ir-0.3 pct W prepared at the Oak R i d g e National Laboratories. Sheet specimens, 0.66 mm thick, were fabricated by electron beam m e l t ing, drop casting, hot rolling and grinding to final thickness. The chemical analyses and heat treatments are shown in Table I. Specimens of lot WC were exa m i n e d in the annealed condition and those of lot WG in the annealed and deformed condition (1440°C at high s t r a i n rates). Thin foils for TEM were prepared by spark planing the sheet m a t e r i a l t o about 0.12 mm thickness, punching 3 mm dlam blanks and, finally, electropolishing the blanks in a Fischione jet polisher. The best results were obtained by alternating between two electropolishing solutions: 1) a 20 pct HC1 saturated NaC1 solution at room temperature, and 2) the acidified CaC12 solution and method developed by A h l e r s and Balluffi.18 G r a i n Boundary Analysis by TEM Electron transparent foils of I r - 0 . 3 pct W were examined in a JEOL 200-B transmission electron microscope, operated at 200 kV, employing a goniometer tilt s t a g e . The most striking g r a i n boundary feature found was the presence of numerous ledges. T h e s e l e d g e s were systematically characterized and documented. In a study of g r a i n boundary ledges Murr and Venkatesh 19 concluded the ledges can be identified by t h e i r characteristic contrast features which include: I) straight (nonoscillating) l i n e s of black, black-white, or white contrast, 2) black-white contrast at individual l i n e s or alternating sets of straight b l a c k or white lines in the boundary plane, and 3) boundary fringe displacements associated with straight contrast l i n e s indicative of a boundary plane s t e p . T h e s e features are collectively sufficient t o confidently identify most of the ledges. 19 However, even the simple straight black, black-white, or white contrast lines permit t h e i r identification with considerable confidence. The density of l e d
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