| Currently, gamma titanium aluminides based alloys, with their low density, high specific strength and stiffness, high corrosion and oxidation resistance at high temperature, make them one of the most prospering high temperature structure materials to replace heavier nickel-based superalloys in the applications of aerospace and automotive components such as nozzles, divergent flaps, compressor blades, etc., which attracts a lot of researches’and research institutions’attention. In this paper, a novel powder metallurgy technique, namely spark plasma sintering, was successfully used to prepare refined-grain TiAl alloys with a little amount of high-temperature retained β phase particles through prealloyed TiAl-based powders produce by plasma rotating electrode processing (PREP); the process of PREP, sintering and densification mechanisms and microstructure and mechanical properties of as-SPSed bulk samples were investigated systematically. The following results had been obtained:1) Prealloyed Ti-47Al-2Cr-2Nb-0.2W powders produced by PREP were at narrow size distribution with mean particle size d50being85μm; The average contents of impurities O and N were550ppm and40ppm, respectively, and the content of O was increasing with decline of the particle size, but remained less than1000ppm, while N stayed unchanged; The sphericity started reducing when the mean particle size surpassed150μm, and the surface became rough as well. Regress analysis showed that the relation between diameter of electrode bar and rotating speed was: d=(12000/n) D-1/2-140. Further investigation indicated that inner microstructure of the particle had changed from sub-microcrystalline to equiaxed cellular crystalline as the particle sized increased; The main phase of the particle was a2with minor y phase, and the content of y declined with particle size decreasing, resulting in a2/y ratio increasing, and B2phase was observed at the same time, which was mainly because of the higher cooling rate during the PERP; Dendritic segregation appeared between dendritic arms, resulting from the difference of distribution modulus at the interface between solid-liquid during cooling process. 2) Influences of sintering temperature and pressure on microstructure and mechanical properties during SPS had been studied. During SPS, we found that relative density of SPSed TiAl bulk samples was more than99.6%when the sintering temperature was higher than1500℃; further investigation revealed that increasing the pressure could enhance the relative density of the loose density and decrease the solidification time at initial stage of SPS, but had little impact on the microstructure. The process of α2â†'γ phase transformation completed when sintering temperature closed to900℃, and the main phase in the bulk was y, with minor a2phase. When the sintering temperature was higher than1200℃, a small amount of B2phased were also discovered, which was caused by higher cooling rate during SPS. Microstructure of the SPSed bulk samples primarily depended on sintering temperature, with duplex phase at1100℃, near lamellar at1200℃and full lamellar at1250℃. The highest compressive fracture strength2367MPa, elongation2.25%and tensile fracture strength600MPa were achieved at1100℃sintering temperature. The highest Vickers hardness at the same temperature was516Hv. The tensile test results demonstrated that the fracture mode of the SPSed TiAl bulk was brittle mode, and superplastic behavior appeared when testing temperature was at800℃, with tensile strength435MPa and elongation45%, respectively.3) Identification of the microstructure mechanisms during SPSed TiAl densification had been studied systematically. Through sintering at three different pressures and six different sintering temperatures for5min, it was found that a SPS circle for TiAl bulk material was consist of four stages:initial pressure of machine, formation of sintering necklace, the growth of necklace and high-temperature creep stage. Formed pressure had no influence on the densification. Densification took place between900℃and1200℃via plastic deformation of the powders. TEM observation showed that the repartition of the plastic deformation was correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occurred. At1250℃, a2phase, y phase and retained P phase followed the crystalline oriental relationship as below:(0001)a2//{111}γ//{110}β,<1120>α2//<110>γ//<111>β And at above1250℃, deformation induced phase transformation and deformation twins occurred.4) A TiAl-Nb composite had been synthesized by SPS from pre-alloyed TiAl powders and elemental Nb powders at the molar ratio of90:10and95:5, and the as-prepared samples were mainly constituted of y phase, O phase, niobium solid solution(Nbss) phase and B2phase. The fracture toughness was about28.7MPa-m-1/2. The ductile phase played an important role in absorbing the fracture energy in front of the cracks and B2phase could branch the propagation of the cracks. It was discovered that sintering temperature could be decreased by adding Nb powders and relative density99.2%was obtained at1150℃. The microhardness of each phase of the composite was tested. |
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