Slip versus twinning in low and very low stacking-fault energy Cu-Al alloy single crystals

The effect of stacking fault energy on slip to twin shear changeover in soft oriented single crystals of single phase Cu-Al alloys loaded in tension, was investigated. The single crystals of stacking fault energy between 4 mJ/m2 and 7.5 mJ/m2, or alternatively of a solute content between 11.5 at.%Al and 7.5 at.%Al, were able to twin during room temperature tensile deformation performed at a strain rate of 10−4s−1. For other stacking fault energy values, lower than 4 mJ/m2 and higher than 7.5 mJ/m2, the crystal lattice shear was produced by slip. It was also found, that as the stacking fault energy decreases from 7 mJ/m2 to 4 mJ/m2, the twinning stress increases from 90 MPa to 110 MPa and for the lower values approaching 3 mJ/m2 is expected to exceed 160 MPa. To rationalize the existence of the stacking fault energy window and the inversely proportional dependence of twinning stress, a conversion of mutual work hardening rates of twin and slip systems was found via latent hardening type experiments. It is postulated, that the conversion results from the stacking fault energy controlled transformation of forest dislocations from un-extended into extended dislocation configurations. Additionally, a dual role of material stacking fault energy on the activation of twinning and twinning stress of face-centered cubic materials is emphasized.

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