The Research On The Effect Of α₂ Phase Critical Size In High-Temperature Titanium Alloy

The effect of Al content and heating treatments on the precipitation and distribution ofα₂ phase and silicide in bimodal microstructure of near-αhigh-temperature titanium alloys was investigated. Thermal strength (creep and endurance) and thermal stability (room-temperature tensile after blank thermal exposure) of high-temperature titanium alloys under different aging time was also systematically investigated. On this basis, the process of precipitation and growth of the second phase particle under different aging time induced properties changes was studied. Summarizing the cooperation action ofα₂-ordered phase and silicide on thermal strength and thermal stability, the good match of thermal strength and thermal stability was gained. Further more, the optimally precipitation-combination ofα₂-ordered phase and silicide, achieved match range of Al and Si elements content were realized.Acorrding to the Al atom 12 percent which the critical electron concentration needed for theα₂-ordered phase, the equations of the volume fraction of orderedα₂ phase were founded. For multi-component alloy system with more than one of the forming elements of orderedα₂ phase, the electron concentration is the only determinative factor for the volume fraction of orderedα₂ phase no longer. The different atom ratio of Al: Sn may cause different volume fraction of orderedα₂ phase but the same electron concentration. The addition of Al element increases the volume fraction of orderedα₂ phase to a larger extent than the addition of Sn element for the same electron concentration. The influence of element content onα₂ phase showed there were high nucleation ratio and growing in Al-rich alloys in the same aging temperature. When the Al content was low,α₂ phase observation was relatively difficult, with increasing aging time, theα₂ phase particles'precipitation was fine and dispersion.In near-αhigh-temperature titanium alloys, the precipitation characteristics ofα₂ phase under aging conditions are predominantly determined by aging temperature. Aging temperature changes lead to significant changes in the size and distribution ofα₂ phase, but aging time mainly influent the size ofα₂ phase particles. Before theα₂ phase completely precipitation, the extension of aging time would resultα₂ phase monotonous growing up. Observation ofα₂ phase on dislocation showed that the low-temperature aging it was not preferential precipitation only slightly gathered. With the prolongation of the aging time, the higher Al and Si elements content, the more obvious the tensile strength increased. And the reduction of area decreased with the improvement of the Al and Si elements content. After blank thermal exposure, the tensile strength increased but the plastic reduced after long-term aging, especially for the alloys with high Al and Si elements content. This phenomenon may be interpreted by the transition between the strengthening and the embrittlement for the thermal stability of theα2 phase and silicide.The endurance and creep property improved obviously with the increase of the Al and Si elements content. The effect of Si elements on the endurance and creep property was bigger than that of Al.There were not any slicide precipitated in theα+βsolution-treated specimens by air cooling. After the following aging treatment at 650℃and 700℃, however, the slicide were observed and the more the Si elements content, the more the numbers and growing speed of the slicide. Sometimes there were very big and aggregated slicide in the specimens. The slicide grew fastly with the improvement of the aging time in the Si-rich alloys. It illuminated that the precipitation of the slicide depended on the aging time strongly when the Si elements content were bigger. There were not slicide precipitated in the solution-treated specimens until the aging treatment, which indicated that the slicide were resulted by the aging treatment.