Phase transformations in the central portion of the Nb-Ti-Al ternary system

Intermetallics in the Nb-Ti-Al ternary system have been considered for high temperature aerospace applications, and their development requires a thorough understanding of phase equilibria and phase transformations. In this study, transmission electron microscopy (TEM) was primarily used to investigate the phase equilibria and phase transformations of two alloys with compositions of 27Nb-33Ti-40Al (alloy 2) and 42Nb-28Ti-30Al (alloy 4). Small arc melted samples were thermally aged at temperatures between 400{dollar}spcirc{dollar}C and 1550{dollar}spcirc{dollar}C for four to sixteen hours (long-term), and at 1000{dollar}spcirc{dollar}C and 1200{dollar}spcirc{dollar}C for two to five minutes (short-term), followed by either water quenching, air cooling, or furnace cooling.; The equilibrium phase study showed that both alloys solidified as the {dollar}beta{dollar} phase, which becomes ordered to the B2 phase during solid state cooling. The {dollar}sigma{dollar}-Nb{dollar}sb2{dollar}Al phase precipitated from the {dollar}beta{dollar} phase slightly below 1400{dollar}spcirc{dollar}C in alloy 2 and 1550{dollar}spcirc{dollar}C in alloy 4. The {dollar}sigma{dollar} phase formed as isolated grains above {dollar}approx{dollar}1300{dollar}spcirc{dollar}C in both alloys. Colonies of {dollar}sigma{dollar} grains formed below {dollar}approx{dollar}1300{dollar}spcirc{dollar}C in alloy 4. A eutectoid transformation from {dollar}beta{dollar} to {dollar}sigma{dollar} + {dollar}gamma{dollar}-TiAl occurred at 1200{dollar}spcirc{dollar}C in alloy 2. A discontinuous transformation from B2 to {dollar}sigma{dollar} + {dollar}beta{dollar} occurred at 1000{dollar}spcirc{dollar}C in alloy 4.; A metastable {dollar}omega{dollar}-D phase formed by the collapse of {dollar}{lcub}{dollar}111{dollar}{rcub}sb{lcub}beta{rcub}{dollar} planes and chemical ordering from the B2 phase in alloy 2 during slow cooling. The {dollar}omega{dollar}-D phase consisted of the Al{dollar}sb4{dollar}Ti{dollar}sb3{dollar}Nb{dollar}sb2{dollar} stoichiometry and P6{dollar}sb3{dollar}/mcm space group. A proposed model showed aluminum and niobium on single layers, and titanium and aluminum on double layers. The transformation path was described using subgroup and symmetry relations as: Pm3m(B2) {dollar}to{dollar} P31m({dollar}omega{dollar}) {dollar}to{dollar} P6{dollar}sb3{dollar}/mcm({dollar}omega{dollar}-D).; A martensitic transformation of the {dollar}beta{dollar} phase to plates occurred during fast cooling in alloy 2. The observed habit plane of the plates agreed with that calculated using the invariant line theory. The {dollar}beta{dollar} composition affected the formation of structurally related orthorhombic (Pmma and Cmcm) and HCP (P6{dollar}sb3{dollar}/mmc) plates. The proposed site occupancy showed the Al{dollar}sb2{dollar}TiNb stoichiometry for the Cmcm phase. Analysis of domain structures, stacking faults, and electron diffraction suggested two possible transformation paths: Im3m({dollar}beta{dollar}) {dollar}to{dollar} Cmcm(disordered) {dollar}to{dollar} P6{dollar}sb3{dollar}/mmc(disordered) {dollar}to{dollar} P6{dollar}sb3{dollar}/mmc(DO{dollar}sb{lcub}19{rcub}{dollar}) for HCP plates and Pm3m(B2) {dollar}to{dollar} Pmma {dollar}to{dollar} Cmcm(ordered) for orthorhombic plates.