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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.

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.cases of maraging alloys containing Ni, Cr, Mo, Ti or Ni, Mn, Ti, hardening has been associated with precipitation of Ni 3 Ti. [3,4,[8][9][10][11][12][13] Developing an understanding of the microstructural evolution and strengthening processes, particularly during the early stages of aging at a low homologous temperature, is a central theme of this work.Observations of Ni 3 Ti precipitation have been made in Fe-25.67 wt pct Ni-1.68 wt pct Ti alloy by Garwood and Jones [12] and Pitler and Ansell. [13] The Ni 3 Ti precipitates, found in all maraging alloys, have a DO 24 structure (a ϭ 0.5093, c ϭ 0.8306 nm) with an hcp lattice and a needlelike morphology. [1][2][3][4][8][9][10][11][12][13][14] The orientation relationship between Ni 3 Ti precipitate and martensite was found to be that of Burgers: [9] The Garwood-Jones [12] study was made on specimens aged at a homologous temperature of only 0.43, where volume diffusivities should be very low. Such rapid hardening has also been observed in Al-base alloys and has been shown experimentally, by means of the three-dimensional atom probe (3DAP), to result from solid solution clustering (e.g., Reference 15). Clustering in Al-base alloys can take place at homologous temperatures as little as 0.33, [16] despite the generally lower diffusivities at comparable homologous temperatures in fcc than in bcc alloys. [17] However, quenched-in vacancies clearly expedite clustering in Al alloys. [15] While the matrix in maraging steels is highly dislocated and so quenched-in vacancies are unlikely to play any role, the potential for effects from precipitate solute clusters is high. We have thought to ascertain the mechanism through which rapid hardening is achieved in mar...