Fatigue Crack Propagation in a (Co,Cr)-(Cr,Co)7C3 Composite.
DREXEL UNIV PHILADELPHIA PA DEPT OF MATERIALS ENGINEERING
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Fatigue crack propagation in the Co,Cr-Cr,Co7C3 in-situ composite has been characterized at room temperature in the as-grown condition and following post-solidification isothermal exposure or thermal cycling. The dependence of dadN on delta K follows the Paris-Erdogan relation over the range delta K approx 10 MNmsuperscript 32 to 50 MNm superscript 32. Fracture toughness is low in the as-grown condition and is attributed to restricted matrix slip and a low stacking fault energy in the cobalt-rich matrix coupled with the absence of delamination. The heat treatments enhance fracture toughness and this is shown to be the result of precipitation of Co,Cr23C6 in the matrix andor fiber coarsening with an attendant increase in the interfiber spacing. At the lower end of the delta K range fatigue cracking is primarily crystallographic stage I in nature. Stage II cracking, with the fracture surface normal to the stress axis, is operative at the upper end of the delta K range.
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