| Aluminum(Al)is an important alloying element in ?-Ti alloys and ?+? Ti alloys.In Ti alloys,Al element disturbs the lattice stability and c/a ratio of HCP-? matrix,thus influencing the deformation mechanism and consequently the mechanical properties.Also,Al element will promote the formation of ?2(Ti3Al),which is dependent on the Al contents and the heat treatment.Previous studies have shown that ?2 phase has a significant effect on the crack related to the mechanical properties(fracture toughness and fatigue crack growth rate)of Ti alloys.Thus,the investigation of ?2 phase and its influence on deforming mechanism(slipping and twinning)will help optimize strength and toughness properties.Recently,with the development of damage-tolerant concept and fatigue/strength-based criteria,increasing demands have been put forward for the Ti alloys which have higher fracture toughness and resistance to fatigue crack growth(FCG).Therefore,the investigation of strengthening and toughening mechanism of Ti alloys,especially the Al alloying effect,is of considerable attention in recent years.The main themes of this dissertation are to determine how different Al contents and ordering can affect the mechanical properties of binary Ti-xAl alloys.To fulfill these objectives the binary Ti-xAl(x=0,2,4,6,8 wt.%)alloys were used to study the influence of Al content,heat-treatment on precipitation behavior of ?2 phase and to understand the influence of Al content and ordering on the strength,fracture toughness and fatigue crack growth.This research will provide a theoretical basis for the accurate control of alloys composition and the optimization of mechanical performance.The study of the influence of Al content on the tensile properties and fracture toughness of binary Ti-xAl alloys revealed that the strength and the fracture toughness of these alloys significantly increased while the ductility decreased with increasing Al content.The strengthening effect was interpreted as a result of solid solution strengthening caused by the increase of the critical resolved shear stress(CRSS)with increasing Al content in the absence of ordering,while in Ti-8Al alloy the precipitation of ?2 phase contributed to second phase strengthening.With increasing the Al content,the CRSS of prismatic slip system approaches to that of basal slip system,leading to the activation of both two slip systems under stress and then higher dislocation density.This should benefit the fracture toughness because dislocations help to release the stress concentration around the crack tip plastic zone(CTPZ)and then slow down the crack propagation rate.A pronounced drop in the fracture toughness and the ductility was observed in Ti-8Al alloy relative to Ti-6Al alloy as a result of the precipitation of ?2.The increase of Al content reduced the twinning activity in CTPZ,which slightly decreased the CTPZ size and obviously decreased the ductility.The investigations result of the precipitation behaviour of ?2 phase in Ti-xAl binary alloys revealed nano-scale ordered ?2 precipitates and the size and volume fraction of?2 precipitates increased in Ti-8Al(wt.%)as aging time prolonged.Meanwhile,the results showed tiny size clusters in Ti-6Al(wt.%)alloy which supposed to give evidence of the initial ordering process of ?2 precipitates.In Ti-8Al alloy,it was found that the increase in the volume fraction of ?2 enhanced the alloy strength,meanwhile,induced the long-range planar slipping and inhibited the twinning activity of HCP-? matrix.This could intensify strain localization and decrease the size of CTPZ,leading to the reduction of ductility and fracture toughness.The large-scale planar slipping and high strain localization in large a colony due to high volume fraction of ?2 precipitates promote crack to initiate and propagate along the slipping bands,inducing smooth crack path and low fracture toughness.The effect of Al content on FCG of binary Ti-xAl alloys showed that the increase of Al content reduces the FCG rate.The enhanced resistance to Ti-xAl propagation is related to the increased CRSS for dislocation slipping that acts as crack initiation sites,basing on solution strengthening with the addition of Al.Furthermore,roughness-induced crack closure with increasing the Al content also contributes to the decrease of FCG rate.With higher Al content,the early fatigue crack propagation stage will be enlarged due to the decrease of CTPZ,leading to crack deflection and the reduction of FCG rate.Moreover,the effect of ?2 precipitates on FCG of Ti-8Al revealed that the increment of ?2 precipitates size and volume fraction after aging will result in a significant increase of FCG rate.The precipitation of ?2 resulting in the activation of large-scale slipping in relatively larger colonies,which promotes the crack to propagate along the large-scale slipping that induces smoother crack path and higher FCG rate.The precipitation of ?2 intensifying the brittle fracture characteristics,which significantly reduced the fracture toughness and fatigue crack growth resistance of Ti-xAl binary alloys. |
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