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

The precipitate transformation from NiAl-type to Ni2AlMn-type and its influence on the mechanical properties of the Fe–5Ni–1Al−xMn (x = 0, 1, 3, and 5 wt.%) alloys were studied thoroughly through a combination of three-dimensional atom probe tomography (APT), first-principles calculations and mechanical tests. APT reveals the precipitation of NiAl-type nanoparticles in the 0-3Mn alloys and the co-precipitation of fine NiAl-type and coarse Ni2AlMn-type nanoparticles in the 5Mn alloy, in which the Ni2AlMn-type nanoparticles are formed as a result of the coarsening of the NiAl-type ones. The first-principles calculations indicate that the Ni2AlMn-type nanoparticles are energetically more favorable than the NiAl-type ones, but their nucleation and growth kinetics are relatively slow, resulting in the initial precipitation of the NiAl-type nanoparticles and the later precipitate transformation from NiAl-type to Ni2AlMn-type. A quantitative analysis of the strengthening mechanism reveals that although the Ni2AlMn-type nanoparticles have a higher strengthening efficiency than the NiAl-type ones, they make a minor contribution to the total strengthening effects due mainly to their small number density. In addition, the Ni2AlMn-type nanoparticles with a composition consistent with that of the Heusler phase are likely to promote the brittle cleavage and intergranular fracture of the 5Mn alloy, resulting in a significant decrease of the alloy ductility.