Precipitation and fracture behaviour of Fe–Mn–Ni–Al alloys

Abstract

The effects of Al addition on the precipitation and fracture behaviour of Fe-Mn-Ni alloys were investigated. With the increasing of Al concentration, the matrix and grain boundary precipitates changed from L1 0 h-MnNi to B2 Ni 2 MnAl phase, which is coherent and in cube-to-cube orientation relationship with the α′-matrix. Due to the suppression of the h-MnNi precipitates at prior austenite grain boundaries (PAGBs), the fracture mode changed from intergranular to transgranular cleavage fracture. Further addition of Al resulted in the discontinuous growth of Ni 2 MnAl precipitates in the alloy containing 4.2 wt.% Al and fracture occurred by void growth and coalescence, i.e. by ductile dimple rupture. The transition of the fracture behaviour of the Fe-MnNi-Al alloys is discussed in relation to the conversion of the precipitates and their discontinuous precipitation behaviour at PAGBs.

Predicting microstructure and strength of maraging steels: Elemental optimisation

“…Similarly, at 460°C Lean7Mn reaches the peak hardness after 100 h, whereas Lean10Mn and Lean12Mn reached the peak hardness after ∼50 and ∼2 hours, respectively. However, the model underpredicts the hardness during underaging; these discrepancies can be due to the partial formation of NiMn, especially with higher Mn content, which can later transition to Ni 2 AlTi [24], or due to the interdiffusion of additional elements to the intermetallic increasing the values of r during early stages of precipitation [25]. Nevertheless, the model describes the experimental trends in the peak hardness values and overaging.…”

“…Additionally, no carbide formation was reported in M350 [21], 5Mn [22], Fe12Ni6Mn [15], LeanLAl and LeanLAl [23], and the Mar6-13 grades [7]. No carbon content in Fe8Ni8Mn was reported [24]. Coarse TiC particles were observed in Lean7Mn, Lean10Mn and Lean12Mn in as-quenched conditions, having volume fraction of 0.22% and a mean size of 500 nm; nevertheless, Qian [25] concluded that they have no influence on hardening due to their size.…”

“…For the ternary Fe Ni Mn system, ordered face-centred tetragonal NiMn (θ) particles of lenticular shape form; Heo et al [16] have reported that they transform into austenite after long ageing periods. He & Lee [24] observed that Al additions in Fe Ni…”

Precipitate transformation from NiAl-type to Ni2AlMn-type and its influence on the mechanical properties of high-strength steels

“…In comparison, in medium-and high-Mn steels with more than 5 w.% Mn, the main strengthening agent has been found to be L2 1 -type Ni 2 AlMn Heusler nanoparticles, which is a further ordering of the B2 structure [22,2326,27]. It is found that the Ni(Al,Mn) nanoparticles with a fine dispersion are effective in strengthening steels without significantly compromising the ductility [21,22], whereas the Ni 2 AlMn nanoparticles usually lead to an increase in strength but a loss in ductility, the degree of which depends strongly on the microstructure [27]. To date, however, the underlying mechanism of how the nanoparticles transform from NiAl-type to Ni 2 AlMn-type is not clearly understood yet, and its influence on the mechanical properties of the Fe-Ni-Al-Mnbased steels remains elusive.…”

“…Given the smaller atomic radius of Mn compared with that of Al, Mn substitution would result in a better lattice mismatch with the bcc Fe matrix and hence a smaller nucleation barrier for the formation of NiAl-type nanoparticles, which dramatically increases the particle number density and enhances the precipitation strengthening response [16,17,20,218,9,22]. In comparison, in medium-and high-Mn steels with more than 5 w.% Mn, the main strengthening agent has been found to be L2 1 -type Ni 2 AlMn Heusler nanoparticles, which is a further ordering of the B2 structure [22,2326,27]. It is found that the Ni(Al,Mn) nanoparticles with a fine dispersion are effective in strengthening steels without significantly compromising the ductility [21,22], whereas the Ni 2 AlMn nanoparticles usually lead to an increase in strength but a loss in ductility, the degree of which depends strongly on the microstructure [27].…”