Effect of Short-Range Ordering on the Tension-Tension Fatigue Deformation Behavior and Damage Mechanisms of Cu-Mn Alloys with High Stacking Fault Energies

The cyclic-deformation mechanism of face-centered cubic (fcc) pure metals or single-phase alloys, i.e., decreasing the stacking fault energy (SFE) of materials through alloying could lead to the transition of dislocation slip mode from wavy slip to planar slip, thereby, improving fatigue properties has been achieved after extensive research. However, except for diminishing SFE, alloying treatment can increase the degree of short-range ordering (SRO) in the alloy, which could equally promote the activation of planar slip just as the lower SFE does in alloys. However, most studies only emphasized the unilateral effect of SFE but ignored the action of SRO. For some single-phase fcc alloys, such as Cu-Mn, Cu-Ni, and some high-entropy alloys, the effect of SRO cannot be ignored. Therefore, in this study, the high SFE Cu-Mn alloys with different SRO degrees were selected as the target materials and general rules and micromechanisms for the effect of SRO on their tension-tension fatigue deformation and damage behavior were investigated under different stress amplitudes. The results show that with the increase of SRO degree, the dislocation slip mode changes from wavy to planar slip. Fatigue-cracking mode changes from dominating intergranular cracking to slip-band cracking, and the tension-tension fatigue life of Cu-Mn alloys is improved. The abovementioned effects are manifested as a synchronous improvement of fatigue strength coefficient (sigma(l)') and fatigue strength exponent (b) in the Basquin relation. The analysis shows that the enlargement of sigma(l)' is mainly owing to the solid solution strengthening of Mn element, and the planar-slip enhanced work-hardening capacity, whereas the increase in b stems from the higher deformation uniformity and slip reversibility governed by planar slip. In summary, this study provides guide for improving the fatigue properties of fcc metals.