Study On The Preparation And Microstructure Evolution Of Powder Metallurgy Ti-22Al-25Nb Alloy

The Ti₂AlNb-based alloy containing an O phase?orthogonal structure?,?₂phase?hexagonal closepacked structure?,and B2/?phase has a low relative density,a high specific strength,and a good overall high temperature performance.It is considered to be the most promising aerospace high temperature material after Ni-based superalloys,so its application in aerospace is also becoming more and more extensive.The conventional Ti₂AlNb-based alloy is prepared by a melt casting method.Due to the large difference in melting point and chemical activity of each element,the prepared alloy composition and structure are not uniform and the mechanical properties are poor.In addition,Ti₂AlNb-based alloys are brittle alloys,and their poor room temperature ductility and toughness also severely restrict the application of the alloy in a wider range of fields.Therefore,how to further improve the room temperature plasticity,toughness and improve the comprehensive mechanical properties of the alloy has become a problem that researchers have to solve.In this paper,solid solution powder and prealloyed powder are used as the initial powder,respectively.Preparation of high performance powder metallurgy Ti-22Al-25Nb?at.%?alloy bulk material by spark plasma sintering?SPS?process.The effects of mechanical alloying process parameters on the morphology,particle size,crystallite size,solid solubility and strengthening mechanism of solid solution powder are revealed.The effects of sintering process parameters and heat treatment process parameters on microstructure,properties and phase transformation of sintered alloys were systematically investigated.Ti-22Al-25Nb solid solution powder and nanocrystalline Ti2AlNb-Al2O3 composite powder were prepared by mechanical alloying.The evolution laws of morphology,particle size and microcrystalline size of mixed powders were obtained.The effects of alumina and stearic acid on the morphology,particle size,microcrystalline size and solid solution mechanism of mixed powders were studied.The strengthening mechanism of mixed powders in MA process was analyzed,and the hardness of mixed powders was tested.The 2Al-25Nb alloy was prepared by SPS process using mechanical alloyed Ti-22Al-25Nb solid solution powder as initial material.It was found that the sintered alloys mainly contain B2 and O phases.The needle-like O phases distribute in the equiaxed B2 matrix,and the precipitation of O phases increases with the increase of temperature.Compression tests show that the sintered alloy at 1250 C/90MPa/10min has the best compressive mechanical properties,and its YS,US and EL are 1100 MPa,1210MPa and 21%respectively.Using the pre-alloyed powder as the starting material,the powder metallurgy Ti-22Al-25Nb alloy bulk material was prepared by SPS process at 1000?/80MPa/10min.The solid solution treatment parameters were systematically studied on the sintered alloy.The results show that the Beck equation and the Hillert equation can well describe the growth of the average grain size of the B2 phase.Microstructure,phase composition,phase content,phase morphology are sensitive to solution temperature and time.The solution treated at 940?/10 min has the best comprehensive tensile mechanical properties,and its YS,TS and EL are 867.8 MPa,949.5 MPa and 7.4%,respectively.After solution treatment at 1030?,a phase change reaction occurs at the grain boundary to form a layered structure,which increases the brittleness of the material.