The influence of minor alloying additions on material behavior in FeAl and Ni(3)Al-based intermetallics

The binary FeAl and Ni₃Al materials have primary physical and mechanical properties that can be significantly enhanced through minor alloying. Although significant progress has been made in efforts to commercialize the FeAl and Ni₃Al alloys, important issues related to their processing and weldability, for example, have not been completely resolved. In this study, a systematic approach was taken to better understand the influence of minor elemental additions (i.e., Mo, Zr, B) upon the microstructure, thermal stability, and the potential weldabilities of the FeAl and Ni₃Al-based materials.; A series of FeAl and Ni₃Al-based materials were prepared by arc melting and drop cast into copper chill molds. Differential scanning calorimetry (DSC) measurements from ambient to 1500°C were used to identify transition temperatures associated with their thermal evolution characteristics by employing continuous heating and cooling cycles. The as-cast materials were further characterized using techniques such as; optical microscopy, X-ray diffraction, Gleeble simulation, and 57Co Mössbauer effect measurements on selected materials with iron as a constituent.; Differential scanning calorimetry results for both the FeAl and Ni ₃Al-based materials showed distinct metastable transitions that occurred below 850°C during heating for varied scan rates, but were irreversible upon cooling. In addition, melting, solidification and invariant reactions associated with their structural changes were observed at higher temperatures. Based on DSC results it is evident that certain characteristic peaks associated with the metastable transition processes are diffusion controlled or thermally activated. On the basis of non-isothermal kinetics studies, the activation energies for these processes were determined. It was found that metastable transitions observed below 850°C were associated with artifacts introduced during sample preparation prior to DSC measurements.; Optical microscopy of the as-cast materials revealed distinct microstructures, where the FeAl materials, in particular, can be discussed under two different categories. The binary FeAl, ternary Fe-Al-Mo and quaternary Fe-Al-Mo-B materials revealed varying grain sizes averaging between 195 and 272 $m$m. The FeAl-based materials containing Zr and C additions ( i.e., Fe-Al-Mo-Zr-C and Fe-Al-Mo-Zr-C-B) revealed dendritic growths in their microstructures. All of the Ni₃Al-based materials showed the existence of dendritic type growths in the microstructure.; Initial X-ray diffraction patterns of the FeAl and Ni₃Al-based materials were used to identify the phases present in their as-cast conditions. The structural evolutions discerned with heat treatments were corroborated with X-ray diffraction and Mössbauer effect studies in the as-cast and aged states of the FeAl-based materials.; Gleeble simulation studies of the weldability of these materials suggest that for the FeAl-based materials, the addition of B, Zr and C are beneficial, (individually or in combination), to crack resistance following weld thermal cycles. However in the Ni₃Al-based materials evaluated, results suggest that for an optimum level of Zr, (i.e., Zr levels within 1.7–3.0 wt. %), intergrannular cracking is prevented.