Effect Of Rare Earth Element Y-doping On Microstructure And Mechanical Properties Of Tial-based Alloys Manufactured By Powder Metallurgy

TiAl intermetallic compound is a class of potential high-temperature structural materials and its application temperature reachs to 900℃, it has low density, high hardness, high intensity and some other excellent properties, it is a good substitute for high temperature structure materials in the aerospace and engine field which has good studying prospects. At present, the TiAl based alloy main preparation method is casting method, but it has some defects, such as shrinkage, dispersed porosity and composition segregation etc., the material prepared by means of powder metallurgy can avoid the generation of such defects effectively.In this paper, pure Al powder 、TiH₂ powder and YH₂ powder were used to prepare （Ti-45Al-xY x=0,0.3,0.8） （at.%） alloy by vacuum hot pressing sintering method. The influences of vacuum hot pressing sintering process and Y content on tissues （phase and microstructure） and mechanical properties （hardness, compression and three point bending） of the alloy were systematically analyzed.It was found that the microstructure of Ti-45Al-xY （x=0） alloy was near lamellar structure （NL）, which was mainly composed of a large number of γ+α2 lamellar crystal and equal axis y phase which distributed along the lamellar crystal boundary. Ti-45Al-xY （x=0.3, 0.8） alloy was also near lamellar structure （NL）, which was mainly composed of a large number of γ+α2 lamellar crystal and equal axis y phase which distributed along the lamellar crystal boundary, and some white YAl₂ and Y₂O₃ phases distributed at the grain boundary and internal layers of lamellar crystal. The holding temperature and holding time of the alloy had a certain influnce on the microstructure of the alloy, but the influnce of pressure on the strcture was not obvious.When the holding temperature was low, the holding time was relatively short, the holding temperature increasing and the holding time extending can make the equiaxed axis y phase reduced, and can make γ+α2 lamellar crystal increased, but the higher holding temperature and longer holding time will make the grain growth. So Y-doping hindered the growth of grain.Regardless of whether the alloy containing Y, the density of the alloy increased with increasing of the holding temperature, prolong of holding time and increasing of the pressure. The properties of the alloy without Y-doping were controlled by the following factors:1) the grain size,2) the proportion of the phase and the uniformity of tissue of the alloy.When the sintering temperature was low, the heating time was relatively short, the main control factors of the alloy properties were the proportion of the phase and the uniformity of tissue; when the sintering temperature was high, the holding time prolonged, the main control factors of aolly properties was grain size. The properties of the alloy were improved with the increase of holding pressure.The optimum process was as follows:the holding temperature was 1400℃ with 90min holding time under 42MPa holding pressure, and the heating rate and cooling rate during the sintering process was controlled.In addition to the influence of the grain size, the proportion of the phase and the uniformity of tissue, the proporties of Y-doping alloy were influenced by the formation of YAl₂ and Y₂O₃ phase. Y-doping can refine the grain and internal layers of lamellar crystal, and also can obsorb the O to make the alloy pure. Because the YAI₂ and Y₂O₃ phase can pinned the dislocation, and prevent the lateral grouth of the internal layers of lamellar crystal, Y can rob O in the alloy, thus Y can refine the grain size and can make structure of the alloy pure. With the addition of Y, the grain size can be refined to 40μm, and the inner layers of lamellar crystal were refined to 1μm. With the increase of Y content, the grains and the inner layers of lamellar crystal were refined, which made the properties of the alloy improved. When the Y content was 0.8%, the alloy had the best performance.