Structures and properties of lath martensite in rapidly solidified and conventionally processed iron-nickel-manganese-(titanium) maraging alloys

The microstructural changes, precipitation reactions and the mechanical properties of rapidly solidified and conventionally processed Fe-Ni-Mn-(Ti) martensitic alloys aged at 400-550$\sp\circ$C up to 600 hrs have been studied. Upon aging from 400 to 550$\sp\circ$C, the alloys showed classical age-hardening behavior and the activation energies for precipitation were calculated as 24 Kcal/mole for Fe-Ni-Mn alloys and 38-46 Kcal/mole for Ti-containing alloys, which supports precipitate nucleation and growth occurring on dislocations by a pipe diffusion mechanism.Aging results in a refined distribution of spherical precipitates at the initial stage which gives coherency strain and maximum hardening. The spherical precipitates transformed to rod-shaped and disc-shaped face centered tetragonal $\theta$-NiMn precipitates in Fe-Ni-Mn alloys depending on the aging condition. The strengthening precipitates in the Ti-containing alloys are needle-like hexagonal $\eta$-Ni$\sb3$Ti whose orientation relationship with the matrix is (0001)$\sb\eta$//(011)$\sb{\rm M}$ and (1120) $\sb\eta$// (111) $\sb{\rm M}$. Widmanstatten austenite appeared in the lath martensite at longer aging times along with lath-like reverted austenite and globular austenite.The optimum combination of strength and ductility was obtained in alloy A-1 (Fe-20.8Ni-2.13Mn-0.84Ti) with 198.9 ksi YS and 31.2% RA after aging at 450$\sp\circ$C for 1 hr. The strengthening behavior was explained by a modified Orowan relationship. The amount of Mn and Ti seems to play a critical role in the strengthening and embrittlement. Based on the present study and previous reports, it is proposed that Fe-(18-20)Ni maraging steels containing a combined amount of (Mn + Ti) higher than 3.5% might be severely brittle.The HIPed maraging steel powders show almost the same age-hardening behavior as the cast and wrought form. The prior austenite grain size of HIPed powders is much smaller than that of conventionally processed alloys, and this enhances the homogeneity of the alloy. The higher strength results from the smaller grain and lath packet size, whereas the loss of ductility results from the heterogeneous formation of Ti-rich precipitate zones along powder boundaries. Preaging at above 525$\sp\circ$C for 10 min improved ductility dramatically without a severe loss of strength probably due to the homogeneous formation of precipitates and/or reduction of Ti content along powder boundaries.U of I OnlyETDs are only available to UIUC Users without author permissio