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The main goal of the present work is to study the influence of the microstructure on the corrosion behavior of cemented carbides WC-Co in two corrosive media. Corrosion kinetics were determined by immersion tests while the electrochemical evolution of the Surface was analyzed using impedance testing. Damage tolerance to corrosion was evaluated by assessing fracture strength on specimens previously subjected to corrosion. Results pointed out that for both grades the corrosion rate was higher in seawater, being more significant for the grade with a medium grain size. The corrosion phenomenon that took place in both media was caused by the oxidation reaction of cobalt. In seawater, the polarization resistance decreased for both grades whereas in mine water increased, due to the formation of a layer of corrosion products, which slowed down the cobalt dissolution process in Surface. In both media, a greater strength loss of the ultrafine grades was evidenced.

WC–Co cemented carbides are geometrically complex composites constituted for two interpenetrating networks of the constitutive ceramic and metal phases. Accordingly, assessment of microstructural effects on the local mechanical properties of each phase is a challenging task, especially for the metallic binder. In this work, it is attempted by combining massive nanoindentation, statistical analysis, and implementation of a thin film model for deconvolution of the intrinsic hardness and flow stress of the metallic phase. Plotting of yield stress values as a function of the binder mean free path results in a Hall-Petch strengthening relationship with a slope (ky) of 0.98 MPa m1/2. This value points out the effectiveness of WC–Co phase boundaries as strong obstacles to slip propagation; and thus, for toughening of the brittle phase (WC) by means of crack-bridging ductile (Co) reinforcement.

The influence of microstructure on the R-curve behavior of hardmetals and its implication on their strength and reliability are studied. In doing so, an already validated model for R-curve description is implemented for rationalizing the fracture behavior of five microstructurally different WC-Co cemented carbides. Results indicate that hardmetals with R-curves developing smoothly over relatively long multiligament zones (i.e. lower slopes) exhibit reduced strength variability than those with steeper and shorter (i.e. higher slopes) ones. The reduced strength scatter associated with a low-slope R-curve behavior is discussed on the basis of the effectively developed toughness and the subcritical crack extension reached before unstable growth is triggered, both being dependent on microstructural length scale and initial flaw size. The results of this investigation highlight the relevance of an in-depth knowledge of R-curve characteristics of hardmetals in order to optimize the performance of engineering parts by means of microstructural design. Keywords: Cemented carbides, Fracture, Strength, R-curve, Reliability

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.

In an attempt to improve the material selection, design and reliability of rock bit WC-Co cemented carbides (hardmetals), an extensive and detailed study is conducted with the main goal of characterizing the fracture and fatigue crack growth (FCG) behavior of four hardmetal grades. Work includes basic microstructural and mechanical characterization of the materials, assessment of fracture toughness and FCG kinetics. It is found that rock bit cemented carbides exhibit relatively high fracture toughness values (between 17 and 20 MPa root m) in direct association with their specific microstructural characteristics, i.e. medium/coarse carbide grain size and medium cobalt content. The influence of microstructure on the measured crack growth mechanics under monotonic loading may be accounted by considering the effective operation of ductile ligament bridging and crack deflection as the prominent toughening mechanisms. Regarding FCG behavior, it is observed to exhibit a significant Km influence. Furthermore, relative increments in toughness are maintained, in terms of crack growth threshold, under cyclic loading. As a consequence, fatigue sensitivity for rock bit cemented carbides is found to be lower than that extrapolated from data reported for fine-grained grades. Crack growth resistance under cyclic loading for the hardmetals studied may be understood on the basis that prevalent toughening mechanisms (ductile ligament bridging and crack deflection) show distinct susceptibility to fatigue degradation and are thus critical in determining fatigue sensitivity. (C) 2014 Elsevier Ltd. All rights reserved.

The influence of the microstructure on the tolerance of WC-Co cemented carbides to corrosion damage was studied by using residual strength as the critical design parameter. In doing so, samples were immersed in synthetic mine water solution for different times, and changes induced by corrosion exposure were assessed. A detailed 3D FIB/FESEM tomography characterization of corrosion damage-microstructure interactions is included. Results reveal that corrosion damage may result in relevant strength degradation on the basis of stress rising effects associated with the formation of surface corrosion pits. Thus, as immersion time increases strength gradually decreases. Fractographic examination reveals the formation of semi-elliptical and sharp corrosion pits for studied medium- and ultrafine-sized cemented carbides, respectively. The latter has a much more pronounced stress rising effect, and consequently higher strength losses were determined for ultrafine grades. Corrosion process consists of a selective attack of the binder that is dissolved in the corrosive media. Initially, it is located at centres of binder pools and as exposure time in the media increases, corrosion evolves consuming the rest of the pools which finally leaves an unsupported WC grain skeleton at the surface. Keywords: Cemented carbides, Cobalt, Corrosion, Fracture, 3D FIB/FESEM tomography

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 characterisation. Experimental results point out a strong strength decrease associated with localised 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 homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.

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.

In this study, systematic nanomechanical andmicromechanical studies have been conducted in threemultilayer TiN/CrN systems with different bilayer periods (8, 19 and 25 nm). Additionally, experimental work has been performed on corresponding TiN and CrN single layers, for comparison purposes. The investigation includes the use of different indenter tip geometries aswell as contact loading conditions (i.e. indentation/scratch) such to induce different stress field and damage scenarioswithin the films. The surface and subsurface damage under the different indentation imprints and scratch tracks have been observed by atomic forcemicroscopy, field emission scanning electron microscopy and focused ion beam. Multilayer TiN/CrN coated systems are found to exhibit higher adhesion strength (under sliding contact load) and cracking resistance (under spherical indentation) than those coated with reference TiN and CrN monolayers. The main reason behind these findings is the effective development of microstructurally-driven deformation and cracking resistant micromechanisms: rotation of columnar grains (and associated distortion of bilayer period) and crack deflection of interlayer thickness length scale, respectively.

The intrinsic hardness of the constitutive phases in WC-Co composites is investigated by combining experimental and statistical analysis nanoindentation techniques. It is done on the basis of considering the cemented carbide material as effectively heterogeneous at the microstructure scale, i.e. consisting of three phases defined by either different chemical nature (carbides and binder) or distinct carbide crystal orientation (i.e. with surface normal perpendicular to either basal or prismatic planes). As main outcome, experimentally measured and statistically significant intrinsic hardness values for the defined phases (WC and constrained metallic binder) are analyzed and determined. Besides the evidence of crystal anisotropy for the WC phase, they permit to identify and account the expected strengthening of the plastic-constrained metallic binder, a critical input parameter for hardness and toughness modelling as well as for microstructural design optimization of ceramic composites reinforced by ductile metallic ligaments. (C) 2015 Elsevier Ltd. All rights reserved.