The purpose of this research was to understand microstructural changes that occur in NiAl alloys as the alloy composition and aging treatments are altered. Much of the study investigated precipitation of the ordered beta ' phase. The composition and misfit of beta' and effect of alloying elements on lattice parameter were measured. The BFS thermodynamic model was compared to experimental observations of beta ' precipitation, site occupancy, and lattice parameter and shown to accurately predict the observations.; Examination of NiAl-Ti alloys has shown that dramatic changes occur in the precipitation behavior of beta' with small changes in Ti content. In NiAl-7% Ti (at. %), beta' plates formed after cooling from the homogenization treatment (1644 K/32 h). Reducing the Ti content to 4.3% required an aging treatment of 1000h at 1000K to produce well-defined beta' plates. At these concentrations and aging treatments, precipitation occurs by nucleation and growth. No evidence of beta' precipitation was observed in NiAl with 3% Ti, however, the addition of 0.5% Hf to this alloy allowed for beta ' precipitation during aging at 1255 K. Examination of a series of NiAl-3Ti-X alloys (X = 1, 3, 6% Cu) with the Ni concentration constant at 50 at. % showed that the alloy remained single-phase beta after 1255K/6hr aging. In an NiAl-3Ti-3Cu alloy with Cu replacing 3% of the Ni, however, beta' plates precipitated homogeneously. Examination of an alloy containing NiAl, a high volume fraction of beta ', and alpha-Cr was found to have Cu partition to the NiAl phase from the beta'.; Statistical ALCHEMI (Atom Location by CHannelling Enhanced MIcroanalysis) experiments showed both Ti and Hf were predominantly on the Al lattice sites. In NiAl-alloys with 50% Ni, both Ti and Cu reside predominantly on Al sites, however, alloys with less than 50% Ni contained Cu on Ni sites while Ti remained on the Al sites. By adding Cu to Ni sites, the lattice parameter of NiAl increases, while a decrease occurs when Cu resides on Al sites. These results suggest that a decrease in the misfit occurs when Cu is on Ni sites, causing the precipitation of beta' at a lower Ti concentration than in ternary NiAl-Ti alloys.
展开▼
