Precipitation Behavior Of Orthorhombic Phase In The Microstructure Of W-modified Ti₂AlNb-based Alloy Prepared By Powder Metallurgy

Ti₂AlNb-based alloy with comprehensive high temperature performance has become one of the research hotpots in the group of high-temperature structural materials,and has been regarded as a potential candidate for aerospace application in recent years.Compared with the conventional preparation processes such as rolling and forging,powder metallurgy is an effective technique to obtain the Ti₂AlNb-based alloy with a controllable and desirable shape.This on the other hand reduces a further cutting process,which avoids the complicated phase transformation during hot working.Meanwhile,the addition of alloying element such as W element brings more possibilities for the regulation of microstructure and properties of the Ti₂AlNb-based alloy.Therefore,studying the effects of powder sintering and post heat treatment on the microstructure evolution and properties of Ti₂AlNb-based alloys has guiding significance for accelerating the industrialization process of Ti₂AlNb-based alloys.In view of this,the powder metallurgy method is used in this thesis to prepare the alloy with the nominal composition of Ti-22Al-23.4Nb-1.6W in different phase regions.We focused on the precipitation behavior and phase composition evolution of the O phase during the cooling process.We also explored the effect of ageing process on the precipitation behavior of O phase and the properties of Ti-22Al-23.4Nb-1.6W,in order to provide guidance for microstructural control.The conclusions can be summarized as follows:?1?W addition accelerated the solutions of?₂ and O phases during the high-temperature holding,and a Widmannst?tten B2+O structure,which contributed to the properties,was induced by furnace cooling from all the phase regions.The precipitation of the O phase in the Ti-22Al-23.4Nb-1.6W alloy which was solid soluted in the B2 single-phase region and cooled to room temperature was achieved by the reactions of B2_matrix?B2_lath+O_primary and B2_lath?B2+O_secondary.?2?W alloying refined the Widmannst?tten structure only when the alloys were solution-treated and then cooled from the single B2 phase.However,the Widmannst?tten structure can be hardly refined when alloys are solution-treated in B2+O and?₂+B2+O phase regions.?3?The morphology of O phase in the W-modified Ti₂AlNb-based alloys was significantly affected by the aging processes.As the aging temperature increased,the B2/O structure evolved from B2+O colonies to Widmannst?tten structure,followed by a“disordering to ordering”procedure.An extension of the aging time resulted in coarsening and bending of the O-phase laths.?4?Compared with the W-free Ti₂AlNb-based alloy,the hardness of W-modified Ti₂AlNb-based alloys has been significantly improved,reaching about 750 HV.However,the hardness of the alloy decreases with the increase of aging time.?5?A herringbone Widmannst?tten B2+O structure was constructed in the alloys aged in the?₂+B2+O phase region,which is related to the diffusion of W and the substitution of W for Nb in the lattice of B2 or O during the reaction of B2?B2+O.