Deformation behavior of Fe-Al single crystals containing Ni₂Al(Ti,V) precipitates

Ferritic heat-resistant steels are widely used for steam turbine of thermal power plants due to their low thermal expansion coefficient, high thermal conductivity and low cost, compared with Ni-based superalloys [1]. However, the application limit temperature is around 630 C. Thus, extensive efforts have been made to develop new ferritic heat-resistant alloys with high application limit temperature. It is well known that precipitation hardening is one of the effective ways to increase material strength. In ternary Fe-Al-Ni phase diagram, there is a two-phase field composed of the bcc phase and the NiAl phase with the B2 structure. The precipitation of the NiAl phase leads to a huge increase in strength in the wide temperature range [2,3]. Recently, our research group carried out a systematic study on the strengthening mechanism of Fe-Al-Ni alloys using the single crystals and found that the difference in primary slip system between the bcc matrix and the NiAl precipitates is closely related to the strong precipitation hardening [4]. The primary slip systems of the bcc matrix and the NiAl precipitates are {101}<111> and {110}<001>, respectively. For example, at <149> orientation, {101}<111> slip occurs, which is favorable for the bcc matrix. However, the NiAl precipitates are also sheared by <111> slip, resulting in strong hardening. On the other hand, at <557> orientation, {110}<001> slip occurs, though <001> slip is generally impossible to take place in bcc metals. This also leads to strong hardening. Thus, we call this type of precipitation hardening ''slip frustration hardening (SFH)''. SFH is very effective in increasing high temperature strength in Fe-Al-Ni alloys. In the present study, Ti and V were doped to improve the mechanical properties of Fe-Al-Ni alloys at high temperatures. Ti or V doping led to the formation of the Ni₂Al(Ti, V) precipitates with the L2₁ structure. In general, the Ni₂Al(Ti,V) precipitates have the high dissolution temperature, which is favorable for high temperature strength. It is also noted that Ti and V were co-doped to control the characteristics of the L2₁ precipitates and to improve high temperature strength.