The fracture toughness and toughening mechanisms of nickel-base wear materials
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I. INTRODUCTION
THE costs and risks that result from nuclear power plant contamination with activated cobalt-wear debris can be significantly reduced by replacement with a low-cobalt alloy.[1–4] Nickel-base wear materials can be used as welddeposited hardfacings that provide a stiff and wear-resistant surface to a base metal that possesses the desired mechanical properties.[4,5] Cast or hot isostatically pressed (HIP) inserts can also be used to provide wear resistance. Although nickelbase wear materials generally do not have the excellent wear resistance and damage tolerance that is associated with cobalt-base alloys, such as Stellite 6,[1–5] lifecycle costs for a nuclear plant would be reduced by replacement with a nickel-base wear material. Nickel-base wear materials contain high levels of chromium, silicon, carbon, and boron, which results in complex microstructures that are comprised of high volume fractions of silicide, carbide, and/or boride phases that provide the needed wear resistance.[4] The volume fraction of nickelphase dendrites typically ranges from 40 to 70 pct, and these dendrite regions are individually separated by a matrix of silicide, carbide, and boride phases. The nickel-phase dendrite regions are prone to adhesive wear behavior and have been observed to be initiation sites for galling wear damage.[2] Reducing the size and volume fraction of the nickelphase dendrites would be expected to improve the wear performance of a nickel-base alloy. However, increasing the volume fraction of the brittle silicide, carbide, and boride
phases can result in a low-damage tolerance and low toughness for nickel-base wear materials. A wear material that has a high fracture toughness is needed for applications that involve high stresses or thermal shock loading. Since less wear resistance and higher fracture-toughness values are expected for a nickel-base alloy that has a higher volume fraction of nickel-phase dendrites,[3] the microstructural features must be optimized to provide the best combination of toughness and wear resistance for a nickel-base wear material. Assuming nickel-base wear materials are comprised of a continuous matrix of brittle carbide, boride, and silicide phases that surround the nickel-phase dendrites, the fracture toughness (KIC) of nickel-base hardfacing alloys has been accurately described by a crack-bridging model.[3] Crack bridging is the toughening of a brittle matrix that contains ductile-phase particles by the plastic deformation and stretching of the ductile particles in the wake of the crack tip.[6,7] The purpose of this work is to complete further KIC testing of nickel-base wear materials and determine if the crack-bridging model is a valid description of the toughness. Coarsening heat treatments are applied to HIP forms of nickel-base wear materials to clarify the influence of nickelphase dendrite size on KIC . This work will show that the crack-bridging model can be used to predict the fracture toughness of nickel-base wear materials and direct processing methods to improv
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