Mechanics and mechanisms of fatigue in a WC-Ni hardmetal and a comparative study with respect to WC-Co hardmetals

There is a major interest in replacing cobalt binder in hardmetals (cemented carbides) aiming for materials with similar or even improved properties at a lower price. Nickel is one of the materials most commonly used as a binder alternative to cobalt in these metal-ceramic composites. However, knowledge on mechanical properties and particularly on fatigue behavior of Ni-base cemented carbides is relatively scarce. In this study, the fatigue mechanics and mechanisms of a fine grained WC-Ni grade is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to analyze the fatigue susceptibility of the studied material. It is found that the fatigue sensitivity of the WC-Ni hardmetal investigated is close to that previously reported for Co-base cemented carbides with alike binder mean free path. Additionally, fracture modes under stable and unstable crack growth conditions are inspected. It is evidenced that stable crack growth under cyclic loading within the nickel binder exhibit faceted, crystallographic features. This microscopic failure mode is rationalized on the basis of the comparable sizes of the cyclic plastic zone ahead of the crack tip and the characteristic microstructure length scale where fatigue degradation phenomena take place in hardmetals, i.e. the binder mean free path. (C) 2014 Elsevier Ltd. All rights reserved.Peer ReviewedPostprint (author’s final draft

Implementation of an effective time-saving two-stage methodology for microstructural characterization of cemented carbides

Linear intercept on scanning electron microscopy micrographs is the most commonly used measurement method to determine carbide grain size and contiguity in WC–Co cemented carbides (hardmetals). However, it involves manual time-consuming measurements and is critically dependent on the quality of the micrographs as well as on the identification and definition of grain boundaries. In this study a two-stage methodology for microstructural characterization of hardmetals is presented. First, a digital semi-automatic image analysis procedure for grain size determination of the carbide phase is presented. It involves an experimental assessment of grain size on processed images corresponding to a series of WC–Co and WC–Ni cemented carbide grades with different microstructural characteristics. Obtained results are then compared to the values obtained by means of the linear intercept technique. A good correlation between the mean grain sizes determined following both measurement techniques was attained. Based on experimental findings, a series of empirical relations were found to correlate grain size distributions obtained following both methods. Second, an empirical relation for estimating carbide contiguity in WC–Co cemented carbides is proposed. This relation considers simultaneously the influence of the binder content and the experimentally determined mean grain size on contiguity. The proposed equation for contiguity estimation is based on extensive data collection from open literature. An excellent agreement was attained between contiguity values estimated from such equation and those obtained using the linear intercept technique. This validates the two-stage procedure as an effective time-saving methodology for microstructural characterization of WC–Co cemented carbides.Peer ReviewedPostprint (author's final draft

Strength and reliability of WC-Co cemented carbides: understanding microstructural effects on the basis of R-curve behavior and fractography

Strength and reliability of WC-Co cemented carbides (hardmetals) are dependent on effective fracture toughness as well as on nature, size and distribution of processing flaws. Regarding toughness, they exhibit a crack growth resistance (R-curve) behavior, derived from the development of a multiligament bridging zone at the crack wake. Accordingly, successful implementation of fracture mechanics requires consideration of tangency criterion, between applied stress intensity factor and R-curve, in addition to fractographic inspection. It is the aim of this study to evaluate the strength behavior of a series of experimental WC-Co grades on the basis of R-curve failure criteria. Results indicate that microstructural effects on the strength of hardmetals may be satisfactorily rationalized following the referred criterion. The analysis includes consideration of nature and distribution of fracture origins, found to be more diverse and wider, respectively, for the harder fine-grained grades. This experimental evidence, together with the fact that these hardmetals exhibit steeper rising R-curves than tougher coarse-grained ones, leads to lower reliability for the former. This investigation documents and validates the great relevance of R-curve behavior for optimizing the mechanical performance of WC-Co cemented carbides on the basis of microstructural design.Peer ReviewedPostprint (author's final draft

Corrosion damage in WC-Co cemented carbides: Residual strength assessment and 3D FIB-FESEM tomography characterization

The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterization. Experimental results point out a strong strength decrease associated with localized corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenized and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.Postprint (published version

Boración del acero inoxidable AISI 316

En este trabajo se presentan los resultados obtenidos en lacaracterización de capas boradas en el acero inoxidable AISI 316. Paraello se realizó un borado superficial mediante el método de boraciónpor polvos sólidos. La caracterización microestructural consistió en laobservación de la capa borada y medición de espesores para lo cual se emplearon técnicas de microscopia óptica y de barrido, EDX y XRD, así como la determinación de la microdureza superficial con arreglo a los parámetros operacionales del proceso. Los resultados permitieronevaluar la influencia de las variables tecnológicas en la microestructura de la zona tratada, así como en sus propiedades superficiales en términos de microdureza. El análisis de los resultados de los ensayos muestra una mejora de las propiedades mecánicas del material modificado superficialmente

Fatigue behavior of a WC-Ni cemented carbide

Nickel is one of the materials most commonly used as a binder alternative to cobalt in cemented carbides. However, knowledge on mechanical properties and particularly on fatigue response of nickel-base cemented carbides is relatively scarce. In this study, the fatigue behavior of a fine grained WC-Ni cemented carbide is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to evaluate the fatigue sensitivity of the studied material. Additionally, fracture modes under monotonic and cyclic load condition are assessed. Clear differences in fractographic features are discerned and discussed

Fatigue behavior of a WC-Ni cemented carbide

Nickel is one of the materials most commonly used as a binder alternative to cobalt in cemented carbides. However, knowledge on mechanical properties and particularly on fatigue response of nickel-base cemented carbides is relatively scarce. In this study, the fatigue behavior of a fine grained WC-Ni cemented carbide is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to evaluate the fatigue sensitivity of the studied material. Additionally, fracture modes under monotonic and cyclic load condition are assessed. Clear differences in fractographic features are discerned and discussed.Postprint (published version

Fatigue behavior of a WC-Ni cemented carbide

Nickel is one of the materials most commonly used as a binder alternative to cobalt in cemented carbides. However, knowledge on mechanical properties and particularly on fatigue response of nickel-base cemented carbides is relatively scarce. In this study, the fatigue behavior of a fine grained WC-Ni cemented carbide is assessed. In doing so, fatigue crack growth (FCG) behavior and fatigue limit are determined, and the attained results are compared to corresponding fracture toughness and flexural strength. An analysis of the results within a fatigue mechanics framework permits to validate FCG threshold as the effective fracture toughness under cyclic loading. Experimentally determined data are then used to evaluate the fatigue sensitivity of the studied material. Additionally, fracture modes under monotonic and cyclic load condition are assessed. Clear differences in fractographic features are discerned and discussed

Notch effects on the fatigue endurance of cemented carbides

Postprint (published version

Fracture and fatigue behavior of a WC-Ni cemented carbide

Nowadays, one of the main research areas in cemented carbides is focused on the replacement of cobalt as a binder for more economical materials with similar or improved properties. Nickel is one of the materials most commonly used to replace cobalt, especially in applications involving corrosion. However, knowledge on mechanical properties, and particularly on fatigue response, of nickel based cemented carbides is relatively scarce. In this study, fracture and fatigue behavior of a nickel-base (11%wt) hardmetal grade with fine-grained tungsten carbide is assessed. With this purpose, fatigue crack growth (FCG) resistance and fatigue limit are determined. The study has been complemented with a fractographic inspection for each testing condition. An analysis of the results from a fatigue mechanics perspective, considering the correlation existing between fracture toughness and flexural strength for the studied material, permits to validate FCG threshold as the effective fracture toughness under cyclic loading