Fracture and fatigue behavior of WC–Co and WC–CoNi cemented carbides

Abstract

The fracture and fatigue characteristics of several cemented carbide grades are investigated as a function of their microstructure. In doing so, the influence of binder chemical nature and content (Co and 76 wt% Co-24 wt% Ni), as well as carbide grain size on hardness, flexural strength, fracture toughness and fatigue crack growth (FCG) behavior is evaluated. Mechanical testing is combined with a detailed inspection of crack-microstructure interaction, by means of scanning electron microscopy, in order to evaluate crack-path tortuosity and to discern fracture and fatigue micromechanisms within the metallic binder. Results show that CoNi-base hardmetals exhibit slightly lower hardness but higher toughness values than Co-base grades. Meanwhile, flexural strength is found to be rather independent of the binder chemical nature. Regarding FCG behavior, experimental results indicate that: (1) FCG threshold (K th ) values for coarse-grained grades are higher than those measured for the medium-grained ones; and (2) fatigue sensitivity levels exhibited by CoNi-and Co-base cemented carbides, for a given binder mean free path, are similar. These findings are rationalized on the basis of the increasing relevance of crack deflection mechanisms as microstructure gets coarser and the evidence of similar fatigue degradation phenomena within the binder independent of its chemical nature, respectively.

FIB/FESEM experimental and analytical assessment of R-curve behavior of WC–Co cemented carbides

“…The fitting curve yields values for K Ic , K t and t of 13.7 MPa·m 1/2 , 7.5 MPa·m 1/2 and 3.5, respectively. The first two values are quite close to those experimentally determined for plane-strain fracture toughness (K Ic = 13.9 MPa·m 1/2 ) and fatigue crack growth threshold (K t = 7.2 MPa·m 1/2 ) values for the investigated cemented carbide [50] while the t parameter is related to the microstructural size of the hardmetal under consideration. Figure 9.…”

“…This value of strength is significantly higher than that expected for bulk Co, as expected from the solid solution strengthening contribution associated with dissolved tungsten and carbon [48,49]. It may be assumed that the K t value is equal to the corresponding fatigue crack growth threshold (for a load ratio of 0.1), which in its turn is linearly dependent on the grain size [50] due to the action of crack-deflection as an additional toughening mechanism in cemented carbides [27,50]. Under these assumptions, equation (2) yields a value for D* of 2.8, in satisfactory agreement with previous estimations reported in medium-and coarse-grained WC-Co cemented carbides [27].…”

In-Depth Understanding of Fatigue Micromechanisms in Cemented Carbides: Implications for Optimal Microstructural Tailoring

“…Figure 1 summarizes the fatigue mechanics framework just described on the basis of different experimental testing and data anes and those determined through stair-case fatigue testing in selected grads (optional "input"), as well as identification of similar critical flaws, as discerned from fractographic analysis of broken specimens, independent of the loading mode. Tarragó and coworkers [13,15,16] have implemented the above protocol in several microstructurally different cemented carbides in terms of the binder chemical nature (either Co or Ni or CoNi) and content (between 10 and 15 wt%), as well as carbide grain size (between 0.4 and 2.4 μm). In general, an excellent agreement is obtained between the predicted, using σf = σr(Kth/KIc), and experimentally determined fatigue limits, validating the FCG threshold-fatigue limit correlation as general for all the cemented carbides studied.…”

Corrosion resistance of WC–Co and WC–Ni type sintered carbides in acetic acid water solution

“…Investigations concerning sintered carbides with alternative metallic binders, mainly nickel, have been performed for quite a long time and described broadly e.g . These issues also proved to be of recent interest, both in terms of microstructural‐mechanical properties as well as electrochemical corrosion …”