Effect of aging and deformation on the microstructure and properties of Fe−Ni−Ti maraging steel

Abstract

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550°C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni 3 Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.

Observations of decomposition of martensite during heat treatment of steels using atom probe tomography

“…It could be suggested that Ti+Al-rich clusters together with substrucure were preferential nucleation sites for this precipitation. Based on TEM data, there was a long standing debate regarding the exact shape of Ni 3 Ti precipitates in maraging steels being plate-like [16], needlelike [23][24][25] or rod-like [24,26]. As we now could conclude, both plate and rod morphologies of these precipitates co-exist at certain stages of decomposition.…”

“…FeNiTi (Mn,Al) maraging steels exhibit an unusual rapid hardening phenomenon after just 5 s ageing at 550 o C [14][15][16][17]. In this condition it achieves approximately 50% of the maximum increase in strength without significant sacrifices in ductility.…”

Precipitate modification in PH13-8 Mo type maraging steel

“…The g-phase exhibits a hexagonal lattice with a = 0.255 nm and c = 0.42 nm [22]. The orientation relationship between the g-phase and the martensitic matrix was found to be (0 1 1) M ||(0 0 0 1) g ; [1][2][3][4][5][6][7][8][9][10][11] M || [11][12][13][14][15][16][17][18][19][20] g [21,[23][24][25]. In the literature some debate exists on the mechanism of formation of the g-phase.…”

Observation of Precipitation Evolution in Fe-Ni-Mn-Ti-Al Maraging Steel by Atom Probe Tomography

“…In the as-quenched condition, the microstructure contained martensite with~5 pct retained austenite in the form of thin layers between the martensite laths. [11,16] The samples were aged in a salt bath for 5 to 3600 seconds at 550°C. Needle-shaped atom probe specimens were prepared with a standard two-stage electropolishing procedure.…”

“…[4][5][6][7][8] The improved properties achieved in these newer steels are associated with the formation of Ni 3 Ti and Fe 2 (Mo,Ti) precipitates, which strengthen the alloys through the operation of Orowan-type mechanisms. The morphology of the Ni 3 Ti particles was found to be needle-like, [7,[9][10][11][12] plate-like, [11] or rod-like. [4,7] In addition, unidentified spherical precipitates were detected in the matrix.…”