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.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...

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

“…Some maraging steels were reported to show hardening because of solute clustering even after heat treatments of only a few seconds in duration. 8,9 The chemistry of the precipitate phase(s) and the amount of martensite-to-austenite transformation depend on the alloy composition. For the classical Ni maraging steels introduced by Decker et al, 10,11 typically, the high content of Ni in combination with Ti leads to precipitation of the Ni 3 Ti phase.…”

Development of New Cobalt-Free Maraging Steel with Superior Mechanical Properties via Electro-Pulsing Technology

“…Furthermore, the results of Figure 4d demonstrate that there also exist some stacking faults in the EPS sample. It is worth noting that some layered retained austenite is also detected in EPS sample (Figure 4a,c), and the crystal orientation between retained austenite and martensite is the K-S relationship [33]: (011)α//(111)γ, [-100]α// [1][2][3][4][5][6][7][8][9][10]γ. whereas in the TS sample, almost no retained austenite can be distinguished.…”

“…In other words, less Ni 3 Mo and more Ni 3 Ti will be formed after aging regime is applied. Whereas, due to the higher c/a (lattice constant) value of Ni 3 Ti, the lattice coherence between Ni 3 Ti and the matrix is also lower than that between Ni 3 Mo and the matrix [9,10]. Thus, compared with maraging steels with cobalt content, the ductility of cobalt-free maraging steels is unsatisfactory.…”

Effect of Heat Treatment on Microstructure and Mechanical Properties of 12Cr–10Ni–0.25Ti–0.7Mo Stainless Steel

“…Precipitation hardenable (PH) stainless steels are widely used in the aerospace industry due to their high strength, weldability, good toughness, and adequate ductility. These steels are most commonly strengthened by the formation of fine intermetallics consisting of Ni, Ti, Al, and Mo [1][2][3][4]. However, a few PH steels gain strength by the precipitation of a copper-rich phase, with copper as the major precipitating element.…”

“…Molybdenum is added to improve the resistance to tempering [8]. Titanium plays a key role as a strengthening element by formation of Ni 3 Ti precipitates during aging treatment [1][2][3][4]. In addition to increasing strength, titanium also fixes the carbon and nitrogen by forming fine Ti(C, N) [9][10][11].…”