The Cleavage Plane of Pearlite
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D.J. ALEXANDER and I.M. BERNSTEIN are both with the Department of Metallurgical Engineering and Materials Science, CarnegieMellon University, Pittsburgh, PA 15213. Manuscript submitted May l l , 1982. METALLURGICALTRANSACTIONS A
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ture mode in thicker, more constrained specimens. Other investigators ~23 have prepared foils by plating a layer of material onto the fracture surface and then cutting slices to include both the underlying material and the plated layer. Haga and Mimura2~produced foils by plating iron, followed by jet polishing, and used high voltage electron microscopy to study the fracture of a martensitic microstructure, and found {100} and {211} cleavage planes. Naylor and Krahe21 plated nickel onto the steel and used ion milling to produce thin foils in a similar study. Benson and Edmonds22'23plated iron onto steels containing vanadium carbide dispersions produced by interphase precipitation. They also observed {100} ferrite cleavage. In this case, the known habit planes of the ferrite-carbide interfaces were used to corroborate the TEM results. However, there are no reported identifications of the operating cleavage plane in pearlite. Park and Bernsteins did attempt nickel plating and ion milling, for the case of pearlite, but were unable to produce consistently good specimens. The present study extends these approaches with iron plating being used to achieve similar thinning rates for both the plated and base materials, combined with carefully controlled electropolishing. Experimental Procedure. Charpy bar sized specimens of fully pearlitic steel (0.8 wt pct C, 0.17 pct Si, 0.84 pct Mn, 0.013 pct S, 0.018 pct P) were notched with a saw cut and then broken with a hammer blow at liquid nitrogen temperatures. The resulting fracture surfaces were completely transgranular cleavage in nature. To prepare foils for electron microscopy, the fracture surfaces of the specimen halves were plated with iron from a solution consisting of 370 g ferrous chloride (FeCI2 9 4 H20), and 145 g calcium chloride (CaCI2) per liter of water. Approximately 1.5 mm of iron were deposited at 90 ~ in 72 hours, using an Armco iron electrode and a current density of 0.5 A/mm2. 24 The plated specimen was then sectioned perpendicular to the fracture surface and parallel to the direction of crack growth, thus producing slices containing the fracture surface and the plated layer. These slices were mechanically polished to approximately 0.003 in thickness, and 3 mm discs were punched out so as to include the fracture surface, s These discs, half iron and half eutectoid steel, were jet polished in a room temperature solution of 100 g anhydrous sodium chromate (Na2CrO4) and 500 ml glacial acetic acid (CH3COOH) at 42 mA and about 90 V. After perforation, the foil was examined by conventional (100 to 120 KV) TEM and if necessary, repolished to ensure that the thinned region contained a segment of the fracture surface. The unique feature that made this method possible was the slow and controlled polishing action of this particular electrol
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