FIGURE 5 A crack tip and the intensely deformed plastic region of a size approximating the spacing L of holes formed at second-phase particles.

undergoes an incipient shear instability, the larger local strain rate and stress can induce martensite transformation, selectively hardening the region and inhibiting continued strain localization.71 If the degree of metastability is properly adjusted so that these effects take place in the intensified stress region ahead of a crack, the toughness of the material can be improved.72

Contained ductile rupture can occur at low toughness values, as indicated by Kc or Jc.73 A thin continuous ductile layer between two harder regions can fail by a ductile mechanism with very little volume of plastic flow. This phenomenon occurs in precipitate-free zones near grain boundaries in improperly heat-treated, precipitation-hardened alloys.

Brittle Fracture

Brittle crack propagation occurs with little energy release below a ductile-to-brittle transition temperature (DBTT). Yet, experimental estimates of J

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