Phase equilibria and development of beta(o)(+)gamma two phase microstructures in gamma-based Ti-Al-Mo ternary alloys

A new class of TiAl alloys based on the {dollar}gamma{dollar}(L1{dollar}sb0){dollar} phase has been studied with the {dollar}rmbetasb{lcub}o{rcub}{dollar}(B2) as a second phase constituent instead of the usual {dollar}alphasb2(DOsb{lcub}19{rcub}){dollar} phase. The development and stability of {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} two phase microstructures have been evaluated in the Ti-Al-Mo system. The results from a series of microstructural investigations, heat treatments, X-ray diffraction, and compositional analysis were combined for the establishment of the relevant phase diagram in the Ti-Al-Mo system. A partial {dollar}beta{dollar} liquidus projection was determined in the composition range of practical interest (40at% {dollar}<{dollar} Al {dollar}<{dollar} 50at%, Mo {dollar}<{dollar} 15at%). Also, partial isothermal sections have been developed in the conventional thermal processing temperature range for {dollar}gamma{dollar}-TiAl alloys(1140-1450{dollar}spcirc{dollar}C). The mechanism for the formation of the {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} two phase field in this alloy system was identified from experimental evidence as a temperature maximum reaction.; Based upon the phase equilibria and phase transformation information, a model alloy of Ti-50Al-5Mo was selected to define the range of possible {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} two phase microstructures as a function of thermal processing procedures. Two distinct {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} two phase microstructures, a discontinuous lamellar structure and an equiaxed structure, were produced in the model alloy by changing the thermal processing strategy. Since only {dollar}(alphasb2+gamma){dollar} two phase microstructures are available in binary {dollar}gamma{dollar}-TiAl alloys, these two new distinct {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} microstructures expand the microstructural options available for y-TiAl alloys.; In the long term annealed samples with an equiaxed {dollar}rm(betasb{lcub}o{rcub}+gamma){dollar} microstructure (1100{dollar}spcirc{dollar}C, 376 hours), interfacial features were investigated to provide insight into the microstructural evolution. An important microstructural feature in the two phase microstructure: the quadrijunction {dollar}rm(gammabetasb{lcub}o{rcub}gammabetasb{lcub}o{rcub}){dollar} involving alternating {dollar}gamma{dollar} and {dollar}rmbetasb{lcub}o{rcub}{dollar} phase grains with a common intersection was clearly identified for the first time. A theoretical analysis using relative interfacial energy ratios was successfully applied to assess the existence and stability of quadrijunctions. The development of quadrijunctions significantly affects microstructural evolution. Indeed, the {dollar}gamma{dollar} matrix grain size remained less than 30{dollar}mu{dollar}m even after the long term annealing.