The Fracture Toughness of a Brittle Nickel Silicide Containing the Ductile Phases Ni(Si) and Ni 3 Si
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THE FRACTURE TOUGHNESS OF A BRITTLE NICKEL SILICIDE CONTAINING THE DUCTILE PHASES Ni(Si) AND Ni 3Si ZHAOHUI LI AND ERLAND M. SCHULSON
Thayer School of Engineering, Dartmouth College, Hanover, NH 03755
ABSTRACT The fracture toughness at room temperature of a brittle nickel silicide containing the ductile phases Ni(Si) and Ni3 Si has been investigated. The microstructure was comprised of a brittle nickel silicide matrix, ductile Ni(Si) particles distributed within the matrix; and Ni 3Si rims airound the particles. It was obtained by annealing hot-extruded Ni-23 at.%Si (with and without 0.19 at.% boron) at 11000C and then air cooling. The fracture toughness values were in the range of 13 MPa m 1/2 to 22 MPa m1/ 2 . Systematic effects of annealing time and of boron were not apparent. It would appear that the incorporation of the ductile particles has a beneficial effect on the fracture toughness of an otherwise brittle silicide.
I. INTRODUCTION The incorporation of ductile inclusions into brittle matrix is an effective method for toughening materials [1-6]. In keeping with this procedure this paper shows that the incorporation of ductile particles within a nickel-silicide matrix imparts significant toughness to an otherwise brittle, but highly corrosion resistant alloy. The particles are a Ni(Si) solid solution which contain a sub-structure of Ni 3 Si precipitates, and the matrix is a complex silicide denoted P2 on the Ni-Si phase diagram. That the matrix is brittle is apparent from the fact that it cracks under thermally induced stresses upon quenching.
II. EXPERIMENTAL The alloys used in this study were the same materials as used in an earlier study [7]. They contained Ni-23 at.% Si ( with and without 0.19 at.% boron ) and had been extruded from powders into rod ( 26 mm dia.). From the as-received rods specimens were machined for three-point, notched bend tests. The dimensions were S = 45.7 umm, W = 11.4 mm and B = 5.8 umm; the ratio of these dimensions is in compliance with ASTM specification E399. The specimens were annealed in dried and de-oxygenated argon at 11000C for 45 minutes to 48 hours to obtain different Ni(Si) particle sizes within the matrix, and then air cooled. (While quenching following the high-temperature anneal would have been desirable to suppress the peritectoid phase transformation (see below), such rapid cooling cracked the material.) Subsequently, the specimens were annealed at 3000C in dried and deoxygenated argon for 1 h to reduce any internal stresses. The specimens were notched at the center of the bottom surface (S x B) using either a diamond saw or the single-edgeprecracked-beam method ( given by Nose and Fujii) [8]. The notches were 5.8mm deep (for both methods) and of root radii 0.1 mm (diamond saw) or 0.0005mm (single-edgeprecracked-beam), measured from optical photographs of the notch roots at 200x magnification. Three-point bend tests were performed at room temperature using a floor model MTS machine. The loading rate was around 1.0 MPa(m)l/ 2/s. A clip displacement gage was mou
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