The fracture toughness and toughening mechanisms of nickel-base wear materials

Abstract

Nickel-base wear materials are typically used as weld hardfacing deposits, or as cast or hot isostatically pressed (HIP) inserts that provide the needed wear resistance to a base material with the desired mechanical properties. Most 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. The volume fraction of nickel-phase dendrite regions typically ranges from 40 to 70 pct, and these dendrite-phase particles are individually isolated by a matrix of silicide, carbide, and boride phases. The continuous matrix of brittle silicide, carbide, and boride phases results in a low damage tolerance for nickel-base wear materials, which is a concern in applications that involve high stresses, thermal transients, or shock loading.

Fatigue crack growth (FCG) and fracture toughness (K _IC) testing in accordance with ASTM E399 methods has been used to quantify the damage tolerance of various nickel-base wear materials. Fractographic and microstructure examinations were used to define a generic toughening mechanism for nickel-base wear materials. The toughness of nickel-base wear materials is primarily controlled by the plastic deformation of the nickel-phase dendrites in the wake of a crack moving through the matrix of brittle silicide, carbide, and/or boride phases, i.e., crack bridging. Measured K _IC values are compared with calculated K _IC values based on the crack-bridging model. Microstructure examinations are used to define and confirm the important aspects of the crack-bridging model. This model can be used to predict the toughness values of nickel-base wear materials and direct processing methods to improve the K _IC values.