PSI - Issue 23

Keng Jiang et al. / Procedia Structural Integrity 23 (2019) 451–456 Keng Jiang et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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(a)

(b)

[21̅ 1̅ 0] [1̅ 21̅ 0] [0001] tungsten atom carbon atom

Fig. 1. (a) SEM micrograph of WC-Co hardmetals; (b) morphology of single ideal WC grain and lattice structure of WC unit cell.

2.2. Co phase

The metallic binder phase is treated as an elastoplastic material. A group of binder like alloy specimens were produced via hot isostatic pressing (HIP) technique with 85.69 ± 1 wt. - % Co, 14.18 ± 1 wt. - % W and 0.13 ± 0.02 wt. -% C. The energy dispersive X-ray (EDX) measurement validated the successful production of 100 % -Co without undesirable presence of graphite or -phase (Özden et al. (2015)) . The specimens were then tested under both monotonic and cyclic loading to determine elastoplastic material parameters. In this study, the cyclic strain hardening behavior is represented by Chaboche plasticity model (Chaboche et al. (1979)). This model decomposes the backstress into three parts, and the evolution of each part is produced by Armstrong-Fredrick hardening model (Armstrong and Frederick (1966)), 3 (3) where 0 is the equivalent stress defined the size of the yield surface at zero plastic strain and remains constant, , are material constants, and ̅̇ pl is the equivalent plastic strain rate. The basic idea of this superposition is to represent the initial hardening modulus, transient nonlinear stage and quasi-linear part by three components, respectively. The ductile damage evolution for Co phase is described by Lemaitre damage model: (4) where eq is the equivalent von Mises stress, is the stress triaxiality function and is the accumulated equivalent plastic strain. The model relates the accumulative damage in a material to its internal stress and strain state. When the level of damage exceeds its critical value cr (Lemaitre and Desmorat (2005)), the material fails. The material parameters for Co phase are summarized in Table 2. Table 2. Material parameters for Co phase. [MPa] 0 [MPa] 1 [MPa] 1 2 [MPa] 2 3 [MPa] 3 cr 227 280 0.3 198 210 000 1 900 85 000 400 22 000 30 0.3 1 k   X X = k (2)  k C            X X   pl pl 0 k k k X   2 eq   2 cr 2 1 cr d , if = 1, if R E D p D D  D D D       