Study On Grain Refinement Of Powder Metallurgy TiAl Based Alloy And Its Effect

It is the most important developing prospect that microstructures of TiAl based alloys are homogeneously refined to improve their ductility at room temperature and to balance their comprehensive mechanical properties. Based on the grain refinement of TiAl based alloys elaborated by powder metallurgies, the preparation and the phase evolution of TiAl alloy nanoparticles, the boron effects and the deformation mechanism of near y microstructures in boron-containing TiAl alloys densified by Spark Plasma Sintering (SPS) have been investigated in this dissertation. Main conclusions include:TiAl alloy nanoparticles with average diameter less than100nm and various phases were synthesized by the improved flow-levitation method. Their phase compositions were adjusted by the evaporating temperature of the mixed metallic droplet. With the decreasing evaporating temperature of droplet from2400℃, pure α2-Ti3Al phase of TiAl alloy nanoparticles is gradually transformed into the mixed phase including γ-TiAl phase, and the content of α2-Ti3Al phase correspondingly decreases. When the evaporating temperature decreases to2180℃,ε(h)-TiAl3phase forms accompanying with α2-Ti3Al and γ-TiAl phases. When the evaporating temperature decreases further to2150℃,α2-Ti3Al phase disappears firstly and the content of γ-TiAl phase decreases. When the evaporating temperature decreases to2120℃, γ-TiAl phase also disappears and more ε(h)-TiAl3phase is crystallized.Based on the theory of Gibbs free energy, the probable alloying mechanism was deduced according to the physical principle of the flow-levitation method and some experimental results. The evolution of phase composition of TiAl alloy nanoparticles was correspondingly disclosed. During the cooling process, the mixture of Ti and Al small clusters is formed from the mixed metallic vapor. Then the alloying reaction is possible to proceed between the small clusters of Ti and Al, resulting in the formation of intermetallic clusters. At last, nanoparticles with various intermetallic phases form because of the further cooling. The phase composition of nanoparticles is thermodynamically determined by the instantaneous temperature and the amount of Ti and Al small clusters.The phase transformation of TiAl alloy nanoparticles with pure phase and mixed phase was investigated by vacuum annealing. More γ-TiAl phase is transformed from α2-Ti3Al phase with the increasing of annealing temperature and time. And the grain size of two phases increases accordingly. This phase transformation is promoted after nanoparticles are compressed into a bulk by vacuum pressing. The nanocrystalline alloy with pure γ-TiAl phase is obtained after the bulk compressed from nanoparticles with mixed phase is vacuum-annealed.The refinement of microstructures was compared at various sintering temperatures between boron-free and boron-containing TiAl based alloys elaborated by SPS. The average grain size of y grains in near y microstructure of boron-containing alloys is less than that of boron-free alloys sintered at low temperatures. At the same range of sintering temperatures (1312℃～1365℃), microstructures of boron-free alloys are transformed from the duplex and the near lamellar to the fully lamellar, but the duplex microstructure is still kept for boron-containing alloys. So there is a wide range of SPS processing conditions for the similar duplex microstructure of boron-containing alloys.Based on the analysis of characteristics of borides, including type, shape, size, and their evolutions with sintering temperatures, and on the TEM characterization of quasi-lamellar microstructures, the grain refinement of boron was disclosed in TiAl alloys. The pinning of grain boundaries on borides (TiB2) limits the growth of parent a grains at sintering temperatures higher than a transus and leads to the final grain size of TiAl alloys. During the cooling, y lamellae are heterogeneously nucleated on borides situated mainly inside a grains, which reduces the undercooling and increases the nucleation temperature of y lamellae. The quasi-lamellar microstructure is formed with wider y lamellae.The tensile mechanical properties at room temperature were measured and compared for boron-free and boron-containing TiAl alloys. For near y microstructures, yield stresses of boron-containing alloys are higher than those of boron-free alloys, but boron-containing alloys suffer from poor ductility. Boron-containing alloys with the quasi-lamellar microstructure exhibits interesting strength of468MPa and high ductility of1.54%. But the fully lamellar microstructure with large grains in boron-free alloys shows low strength and low ductility.The deformation mechanism and the size effect of near y microstructures in SPS-TiAl alloys were investigated according to the characteristic of dislocations, the distribution of Schmid factors, the crossing of grain boundaries, and to the quantitative calculation of the Hall-Petch effect. The deformation in near y microstructures is due to the glide of ordinary dislocations and to the twinning deformation. Various boundary crossings show that the activation of deformation modes is more correlated to their nucleation at boundaries than to the orientation of the macroscopic applied stress. The Hall-Petch effect is consistent with the dislocation pile-up model which is probably associated to the crossing mechanism of boundaries and selects the activated systems with a partly discrepancy with the Schmid law. And this effect is more resulting from Shockely dislocations moving in twins than from ordinary dislocations according to the calculation of the length of dislocation pile-ups.