In-Situ Response of WC-Ni Composites under Compressive Load

Abstract

The in-situ strain response of WC-Ni cemented carbides (5, 10, and 20 wt pct Ni) to uniaxial compressive load was measured using neutron diffraction. Strain was measured in both phases parallel and transverse to the loading axis of cylindrical samples. Plasticity is observed in the Ni binder from the lowest levels of applied load. The plasticity occurs locally in the Ni phase, on the scale of the microstructure, and leads to continuous curvature of the WC-Ni stress-strain curves and significant toughness of the material. The plasticity results from the interaction of the thermal residual microstresses created during sample production with the applied macrostress. It also leads to anisotropic relaxation of the initial residual stress and the creation of a residual stress state with cylindrical symmetry in the material. This process was observed over three loadunload cycles. Analysis enables phase-specific stress strain curves to be constructed. Finally, strain distributions were observed through peak breadth responses.

Residual Stresses

“…However, the situation becomes complicated if the preexisting stresses change during mechanical or thermal treatment (Paggett et al, 2007). High-temperature measurements may be particularly problematic as compositional changes can occur, in which case the initial values of the d-spacings are no longer meaningful.…”

“…The role of binder fraction is shown in Figure 5, for both WC-Co (Coats & Krawitz, 2003;O'Quigley, Luyckx, & James, 1997) and WC-Ni (Paggett, 2005;Paggett et al, 2007). The square symbols are from samples of a different source than the triangle symbols.…”

“…Second, for a given composition, as the WC particle size decreases, the stress magnitude in both phases increases. For example, as the WC particle size is (Paggett et al, 2007) and WC-Co (squares (Coats & Krawitz, 2003;O'Quigley et al, 1997) and downward triangles (Paggett, 2005)). The "x" symbols are calculated values using Eqn (1).…”

“…Figure 9 shows the axial response of the Ni phase in WC-20 wt% Ni as well as the macroscopic stress-strain curve for uniaxial compression to À2000 MPa (Paggett et al, 2007). The Ni curve shows only the elastic strain measured by diffraction while the composite curve shows the sum of the elastic and plastic strain response.…”

Fracture and Strength of Hardmetals at Room Temperature

“…On the other hand, the large residual tensile thermal stress on the binder phase causes the binder regions to deform plastically even at very low external compressive stresses that contributes to the elastic strain energy absorption of the alloy and might constitute yet another reason for the high toughness of cemented carbides 8 (Paggett, Krawitz, Drake, et al, 2006;Paggett, Krawitz, Drake, et al, 2007).…”