Corrosion-fatigue crack propagation behavior of Mn-Ni-Al bronze propeller alloys

The fatigue propagation behavior in air and in 3.5% NaCl aqueous solution of Mn-Ni-Al bronzes containing 12 and 14% Mn was studied for three material conditions: as-cast (AC), normalized (NSC) and quenched (HT). Accelerated cracking occurred in the salt solution at higher growth rates. At lower rates, corrosion-product induced crack closure caused decelerated propagation in this solution. The corrosion-fatigue mechanism was associated with facilitated initiation of fatigue cracking in α-grains and, in the NSC and AC materials, of decohesion at α-β interfaces proceeding ahead of the macroscopic crack front. The fatigue threshold increased with increasing β grain size as a result of greater roughness-induced crack closure effects. The effective value of ΔKTh, was constant within the experimental accuracy. The logda/dN vs log ΔKeff data more clearly showed the accelerated cracking in the salt solution. The propagating crack experienced difficulty in crossing β-grains, which, in the alloys of larger grain size, resulted in oscillations in the lower portion of the crack propagation curves.