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

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.

CF-9 has a higher amount of alloying elements (Cu, Mn, Ni, and Al) than CF-2. More Mn is incorporated into NiAl, displacing Al. This substitution reduces the lattice misfit with the Fe matrix and hence nucleation barrier, resulting in more NiAl-type precipitates in CF-9. This increases the loading rate sensitivity in CF-9 because the precipitates impede the motion of edge dislocations. Additionally, Cu segregation is observed by in situ Auger studies which partially explains the lower ductility of CF-9.Figure optionsDownload high-quality image (272 K)Download as PowerPoint slide