Lamellar Microstructure Of Ti-22Al-25Nb Alloy And The Quasi Cleavage Fracture

A Ti₂AlNb based alloy has higher strength and higher creep resistance over common titanium alloys,as well as higher ductility,higher toughness and higher workability comparing with Ti-Al based alloys.Ti₂Al Nb based alloys have attracted much attentions all over the world due to the excellent properties above.However,the intrinsic brittleness of an intermetallic makes Ti₂Al Nb based alloys suffer from insufficient toughness.It is urgent to increase the toughness of a Ti₂Al Nb based alloy via innovative thermal mechanical processing technologys while considering applications in aerospace field.A so called bimodal size lamellar O phase microstructure?BSLO microstructure here after?which gets outstanding combination of high strength,high creep resistance and good toughness,has been obtained by NWPU together with CIRSI by a novel B2-forging technology.In order to adjust BSLO microstructures during the thermo-mechanical processing to getting higher fracture toughness,Ti-22Al-25Nb?at.%?alloy is employed in the studying of this dissertation.The studying work involves the deformation in the B2 phase region,the O phase precipitation in the deformed microstructures,the quasi cleavage crack mechanisms and the fracture toughness of BSLO microstructures.Power low type Arrhenius constitutive relation of the deformation at B2 phase region is established using Zener-Hollomon parameter.The verification indicates that this constitutive equation could accurately predict the deformation in B2 phase region.It is found that dynamic recover and dynamic recrystallization are the dominative deformation mechanisms.Strip structures with high deformation dislocation densities priorly form at B2 grain boundaries.Then dynamic recovery occoures in the strips and the deformation dislocations form subgrains.The migration of the subgrainboundaries makes the subgrains develop into recrytallization grains with high angle grain boundaries.The weak dynamic recovery during the deformation at high strain rate results in the high dislocation density in the deformed microstructure.Moreover,dislocations in some localized area are regularly arranged and the stress field of these dislocations has strong anisotropy.The interaction between the anisotropy and the transformation strain lead to the strong variant selection of O phase precipitates.The selected O variants develop in to parallel O phase lamellae and form intragranular O phase colony structures at last.The strong dynamic recovery during the deformation at low strain rate results in the low dislocation density.The dislocations are rearranged and the anisotropy of the dislocation stress field is signicicantly reduced.Thus no strong O phase variant selection occurs and no intragranular O phase colony structure forms.The interaction energy between the transformation strain of O precipitation and the stress field of prior deformation dislocations are calculated using the established crystal-mathematical model.The calculating results suggest that different O variants have different minimum interaction energy in the stress field of a dislocation.The stress field promotes the precipitations of O variants with lower minimum interaction energys,and inhibits the O variants with higher minimum interaction energys at the sample time.As for the O precipitates on B2 grain boundaries,conditions are depending on the orientation characteristics of the boundaries.The O variant that keeps the specific orientation relation with one of the adjacent B2 grains can be selected at general high angle boundaries,while the O variant that keeps near specific orientation relation with both the adjacent B2grains can be selected at low angle boundaries.In the special case that the B2 grains adjacent to a high angle boundary have a orientation with a common{110}B2 plane and a common<111>B2//<1 11>B2 direction,the two O variants that have?001?O planes parallel to the common{110}B2 plane can be selected at the same time.The two variants keep orientation relation with the two B2 grains respectively and have a orientation with<110]O//<1 10]O at the same time.The orientation relation between the two O variants is the so-called poly-twins.The fracture mechanism at room temperature of the BSLO microstructure is quasi-cleavage.Cleavage of B2 phase dominates the fractures.The cleavage cracks grow along the cleavage planes of B2 phase.At the crack tips,micro cracks initiates at lamellar O phases and mini facets form on broken O phases.The micro cracks are blunted at adjacent B2matrix,which results in strip shallow dimples.The cleavage planes of B2 phase are measured to be{100}planes and{110}planes.The change of cleavage planes can achieve the deflection of the crack at either high angle B2 boundaries or low angle B2 boundaries.Cleavage factor f _C is novlelly defined to tell if a crystal plane is the most favorable cleavage plane.It is found that B2 grains cleavage generally along a plane with high f _C but not always along the plane with the highest f _C since the cleavage of a grain is also affected by its adjacent grains.BSLO microstructures have excellent fracture toughness.The value of K_IC at room temperature gets 32.9MPa m and the value at 650?gets 65.4MPa m.The fracture toughness at room temperature increases with incrasing valome fraction of B2 phase.Intrinsic crack resistance provides the primary fracture toughness at room temperature while the extrinsic crack resistantce provies only 7?8%.The fracture mechanism transforms from cleavage into dimple fracture when the testing temperature increases from room temperature to 650?.The distortion of the crack propagation path increases significantly at such a high temperature and the extrinsic contribution to the fracture toughness increase into?20%as well.A mathematic model is build to calculate the heights of I type crack tip zones in BSLO microstructures.It is found that the height of a BSLO microstructure at 650?is much higher than at 20??RT?,which leads to the much higher intrinsic compounent of the fracture toughness at 650?comparing with that at 20??RT?.