The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are ...The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.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
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 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
文摘The effect of lamellar orientation on crack paths in PST crystals of y-TiAl based alloys was investigated by in-situ SEM technique. The results indicate that the crack paths in PST crystals of y-TiAl based alloys are strongly dependent on lamellar orientation ofPST crystals, and the differently oriented PST crystals show different nucleation and propagation mechanisms of crack, resulting in different levels of fracture toughness.
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 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.
