Effects of increased alloying element content on NiAl-type precipitate formation, loading rate sensitivity, and ductility of Cu- and NiAl-precipitation-strengthened ferritic steels

Abstract Two experimental bcc -Cu- and B2–NiAl-precipitation-strengthened ferritic steels with 6.3 at. % and 12.4 at. % Cu + Mn + Ni + Al, 950 MPa and 1600 MPa yield strength respectively, were studied. Atom probe tomography showed that the volume fraction and number density of NiAl-type precipitates in the heavier alloyed steel (designated as CF-9 ) is ∼60–70 times greater than those in the lighter alloyed steel (designated as CF -2). This is attributed to the smaller lattice misfit between these NiAl-type precipitates and the ferritic matrix in CF- 9 due to more incorporation of Mn atoms on the Al sub-lattice in the B2 NiAl unit cell. Loading rate sensitivity of hardness was measured for CF -2, CF -9 and SAE -1090, which does not have bcc -Cu precipitates. Results show that even though CF -2 and CF -9 have double and triple the strength of SAE -1090 respectively, their hardness shows weaker dependence on loading rate. This is attributed to the presence of bcc -Cu precipitates in CF -2 and CF -9 providing athermal activation of nearby screw dislocation motion. Auger electron spectroscopy studies of CF -9 samples reveal Cu segregation on grain boundaries. The observed Cu segregation is believed to be partly responsible for the lower elongation-to-failure of CF -9 compared with CF -2.