Polysynthetic twinned(PST)TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification.In this paper,PST single...Polysynthetic twinned(PST)TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification.In this paper,PST single crystals with different phase ratios are obtained by annealing at specific temperatures and holding times.The results show that the diffusion rates of Ti and Al elements at various temperatures directly trigger and propel the surface recrystallization and variation in the internal phase ratio.When the temperature is lower than 1448 K,the diffusion rate of Ti is obviously higher than that of Al,which causes one denseα_(2)recrystallized layer to form on the surface of TiAl single crystals.Meanwhile,as more Ti elements migrate to the surface,theα_(2)phase ratio inside the TiAl single crystal thereby decreases.When the temperature exceeds 1448 K,the diffusion rate of Al gradually reverses to exceed that of Ti,which forms the surface sandwiched recrystallization dominated byγphase and simultaneously increasesα_(2)phase ratio inside the TiAl single crystal.The variation in the two-phase ratio directly induces a significant change in the lamellae thickness,which exhibits different tensile behaviors of PST-TiAl single crystal.When theα_(2)phase content is less than 20%,widerγlamellae make it easier for dislocations to be activated within its lamellae and continuously move across theγ/α_(2)interfaces,thereby obtaining better tensile plasticity.As theα_(2)phase content exceeds 30%,finerγlamellae inhibit the dislocation initiation,resulting in the fracture occurrence of TiAl single crystal before yielding.No matter how the phase ratio changes,the crack preferentially initiates withinα_(2)lamellae.However,the crack propagation follows different paths based on variousγlamella thicknesses.The fracture mode of PST-TiAl single crystal also changes from shear fracture along slip bands within theγlamella to brittle fracture along the{1¯100}planes withinα_(2)lamella.展开更多
Creep behavior of γ-TiAl polysynthetically twinned (PST) crystals has been investigated at temperature between 700 and 800℃. The results show that the creep behavior of the crystals is strongly dependent on the angl...Creep behavior of γ-TiAl polysynthetically twinned (PST) crystals has been investigated at temperature between 700 and 800℃. The results show that the creep behavior of the crystals is strongly dependent on the angle between the lamellar boundaries and loading axis. The samples with the angle φ=45 deg. have the lowest creep resistance, while the samples with φ=0 deg., rather than φ=90 deg.,show the strongest creep resistance. The stress exponent and creep activation energy for the power law creep vary with the orientations of samples, which indicates that the creep mechanisms of the samples with different angle φ are quite different.The deformation substructure has been examined by transmission electron microscopy (TEM),which shows that both gliding, perhaps also climbing, of dislocations and twinning contribute to the creep deformation with some particular observation in the samples with φ=90 deg. in which rotation of the γ plates across a true twin boundary was observed, which indicates the deformation mechanism of the samples is different from the samples in other orientations.展开更多
The fracture behavior and mechanism of PST crystals of a Ti 49%(mole fraction)Al alloy have been studied by using in situ straining and micromechanical calculation. The three dimensional micromechanical model represen...The fracture behavior and mechanism of PST crystals of a Ti 49%(mole fraction)Al alloy have been studied by using in situ straining and micromechanical calculation. The three dimensional micromechanical model representing the structure of PST crystal has been built, and the stress distribution ahead of the sharp and blunt crack tips either parallel to lamellar interface or perpendicular to the lamellae has been calculated by using finite element method based on linear elasticity of PST crystals. The experimental results show that the fracture behaviors and mechanisms are strongly dependent on the angle of loading axis to the lamellae. The calculation indicates that nucleation and propagation of microcrack along the interfaces are controlled by the normal stress and translamellar microcrack is controlled by shear stress ahead of crack tip.展开更多
The influence of the lamellar orientation with respect to the loading axis on the creep behavior of Ti 48Al PST crystals was investigated. The results indicated that the creep behavior of PST crystals is strongly depe...The influence of the lamellar orientation with respect to the loading axis on the creep behavior of Ti 48Al PST crystals was investigated. The results indicated that the creep behavior of PST crystals is strongly dependent on the angle between lamellar boundary and loading axis, and the PST crystals yield high stress exponents for power law creep under the test conditions, which may be associated with lamellar interface and their fine spacing; at 800 ℃ and the 350 MPa stress level, the onset of tertiary creep regime is earlier in the sample with =90° compared with that in the sample with =0°, which may be caused by spheriodization during creep.展开更多
The behaviour of 1/2〈112] superlattice dislocations in polysynthetically twinned (PST) crystals of TiAl at elevated temperatures have been investigated by transmission electron microscopy (TEM). The results showed th...The behaviour of 1/2〈112] superlattice dislocations in polysynthetically twinned (PST) crystals of TiAl at elevated temperatures have been investigated by transmission electron microscopy (TEM). The results showed that 1/2〈112] superlattice dislocations play an important part in the deformation. At room temperature, 1/2〈112] superlattice dislocations can be activated easily but are often dissociated into partials trailed by faulted dipoles with super extrinsic stacking faults (SESFs) and difficult to move. In situ heating observations on the faulted dipoles showed that annihilation of the partials bounding the SESFs of the faulted dipoles occurred and resulted in discontinuous stacking faults along the direction of the partials at about 500 ℃. The faulted dipoles disappeared completely at about 600 ℃. The temperature range of annihilation coincided with that of brittle to ductile transition of TiAl PST crystals reported by Inui et al (1995). At 850 ℃, they are often dissociated into partials including super intrinsic stacking faults (SISFs) and can move easily. The experimental observations suggest that the behaviors of 1/2〈112] superlattice dislocations control the phenomenon of brittle to ductile transition of PST crystals of TiAl alloy.展开更多
基金supported by the National Natural Science Foundation of China(NSFC)under Grant Nos.52288102,52322101,92163215,52174364,52101143,U23A20542the Fundamental Research Funds for the Central Universities under Grant No.30922010202+1 种基金the 100 Talents Plan of Hebei Province under Grant No.E2020100005the Natural Science Foundation of Hebei Province under Grant No.E2022203109.
文摘Polysynthetic twinned(PST)TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification.In this paper,PST single crystals with different phase ratios are obtained by annealing at specific temperatures and holding times.The results show that the diffusion rates of Ti and Al elements at various temperatures directly trigger and propel the surface recrystallization and variation in the internal phase ratio.When the temperature is lower than 1448 K,the diffusion rate of Ti is obviously higher than that of Al,which causes one denseα_(2)recrystallized layer to form on the surface of TiAl single crystals.Meanwhile,as more Ti elements migrate to the surface,theα_(2)phase ratio inside the TiAl single crystal thereby decreases.When the temperature exceeds 1448 K,the diffusion rate of Al gradually reverses to exceed that of Ti,which forms the surface sandwiched recrystallization dominated byγphase and simultaneously increasesα_(2)phase ratio inside the TiAl single crystal.The variation in the two-phase ratio directly induces a significant change in the lamellae thickness,which exhibits different tensile behaviors of PST-TiAl single crystal.When theα_(2)phase content is less than 20%,widerγlamellae make it easier for dislocations to be activated within its lamellae and continuously move across theγ/α_(2)interfaces,thereby obtaining better tensile plasticity.As theα_(2)phase content exceeds 30%,finerγlamellae inhibit the dislocation initiation,resulting in the fracture occurrence of TiAl single crystal before yielding.No matter how the phase ratio changes,the crack preferentially initiates withinα_(2)lamellae.However,the crack propagation follows different paths based on variousγlamella thicknesses.The fracture mode of PST-TiAl single crystal also changes from shear fracture along slip bands within theγlamella to brittle fracture along the{1¯100}planes withinα_(2)lamella.
文摘Creep behavior of γ-TiAl polysynthetically twinned (PST) crystals has been investigated at temperature between 700 and 800℃. The results show that the creep behavior of the crystals is strongly dependent on the angle between the lamellar boundaries and loading axis. The samples with the angle φ=45 deg. have the lowest creep resistance, while the samples with φ=0 deg., rather than φ=90 deg.,show the strongest creep resistance. The stress exponent and creep activation energy for the power law creep vary with the orientations of samples, which indicates that the creep mechanisms of the samples with different angle φ are quite different.The deformation substructure has been examined by transmission electron microscopy (TEM),which shows that both gliding, perhaps also climbing, of dislocations and twinning contribute to the creep deformation with some particular observation in the samples with φ=90 deg. in which rotation of the γ plates across a true twin boundary was observed, which indicates the deformation mechanism of the samples is different from the samples in other orientations.
文摘The fracture behavior and mechanism of PST crystals of a Ti 49%(mole fraction)Al alloy have been studied by using in situ straining and micromechanical calculation. The three dimensional micromechanical model representing the structure of PST crystal has been built, and the stress distribution ahead of the sharp and blunt crack tips either parallel to lamellar interface or perpendicular to the lamellae has been calculated by using finite element method based on linear elasticity of PST crystals. The experimental results show that the fracture behaviors and mechanisms are strongly dependent on the angle of loading axis to the lamellae. The calculation indicates that nucleation and propagation of microcrack along the interfaces are controlled by the normal stress and translamellar microcrack is controlled by shear stress ahead of crack tip.
文摘The influence of the lamellar orientation with respect to the loading axis on the creep behavior of Ti 48Al PST crystals was investigated. The results indicated that the creep behavior of PST crystals is strongly dependent on the angle between lamellar boundary and loading axis, and the PST crystals yield high stress exponents for power law creep under the test conditions, which may be associated with lamellar interface and their fine spacing; at 800 ℃ and the 350 MPa stress level, the onset of tertiary creep regime is earlier in the sample with =90° compared with that in the sample with =0°, which may be caused by spheriodization during creep.
文摘The behaviour of 1/2〈112] superlattice dislocations in polysynthetically twinned (PST) crystals of TiAl at elevated temperatures have been investigated by transmission electron microscopy (TEM). The results showed that 1/2〈112] superlattice dislocations play an important part in the deformation. At room temperature, 1/2〈112] superlattice dislocations can be activated easily but are often dissociated into partials trailed by faulted dipoles with super extrinsic stacking faults (SESFs) and difficult to move. In situ heating observations on the faulted dipoles showed that annihilation of the partials bounding the SESFs of the faulted dipoles occurred and resulted in discontinuous stacking faults along the direction of the partials at about 500 ℃. The faulted dipoles disappeared completely at about 600 ℃. The temperature range of annihilation coincided with that of brittle to ductile transition of TiAl PST crystals reported by Inui et al (1995). At 850 ℃, they are often dissociated into partials including super intrinsic stacking faults (SISFs) and can move easily. The experimental observations suggest that the behaviors of 1/2〈112] superlattice dislocations control the phenomenon of brittle to ductile transition of PST crystals of TiAl alloy.
