Modulated structures in Ni-Al and Fe-Ni-Al-C alloys

Abstract

The primary goal of this thesis is to investigate the modulated structures found in Ni-rich Ni-Al and austenitic Fe-Ni-Al-C alloys. Ordering and clustering in rapidly-solidified Ni-14\% Al have been analyzed by transmission electron microscopy and X-ray diffraction investigation. To examine the formation processes of $\gamma\,``$ phase($L1_2$ structure), the disordered state of starting specimen was sought through the rapid solidification process by melt spinning. In the highly undercooled condition, the supersaturated $\gamma$ follows its characteristic mode of $Ll_2$ ordering/clustering with respect to experimental variables. At relatively low temperature($500\,^\circ\!C$), $\gamma$ firstly goes through continuous ordering and subsequently undergoes phase separation. While at intermediate ageing temperature($710\,^\circ\!C$), $\gamma\,``$ phase precipitates via the nucleation and gorwth mechanism. At this temperature, the morphology of $\gamma\,``$ precipitates evolves as follows:quasi-randomly distributed cuboids $\rightarrow$ rows of cubic particles $\rightarrow$ plates. gowth kinetics of $\gamma\,``$ particles obeys the LSW (Lifshitz, Slyozov and Wagner) theory. Such a coupled ordering/clustering phenomenon fairly well is explained qualitatively by the metastable phase diagram subdivided by the loci of $T_i$+(disordering instability), $T_o$ (critical ordering), $T_i$-(ordering instability) and Ts(spinodal instability) temperatures. The phase decomposition of Fe-42.4wt\% Ni-4.15wt\% Al-0.45wt\% C alloy was investigated on ageing at 823K by means of micrographs, selected area diffraction patterns, micro-beam diffraction patterns and microprobe chemical analyses obtained by conventional transmission electron microscopy and scanning transmission electron microscopy. In the matrix, the K phase with $L``1_2$ structure has formed via the zone formation process. This matrix perovskite carbide is gradually repslaced by the discontinuously precipitated grain boundar...