A Ti-45at.%Al alloy which was solution-treated at 1350℃ for 30 minutes and quenched in water is employed to explore mechanisms of α_2→γphase transformation. The ageing process of the quenched alloy has been in si...A Ti-45at.%Al alloy which was solution-treated at 1350℃ for 30 minutes and quenched in water is employed to explore mechanisms of α_2→γphase transformation. The ageing process of the quenched alloy has been in situ studied it has been found that the γ lamellae can precipitate in the α_2 matrix by two mechanisms. When The alloy is aged at 700℃, the stacking faults pre-existing in the α_2 matrix start to grow and more are generated.With the increase of ageing time and ageing temperature the density of stacking faults is increased and the γ lamellae then precipitate. This isα diffusion-controlled process. Alternatively, the γ lamellae may be formed from the α_2 matrix by 1/3(1100) shearing on the (0001) plane. The latter mechanism implies that the strain-induced transformation may occur, which is confirmed by deformation of the quenched alloy.展开更多
The phase transformation induced by deformation in TiAl(Cr) intermetallics at room temperature has been studied by means of TEM and HREM observations and nanoprobe analysis. The results showed that, twinning is a prim...The phase transformation induced by deformation in TiAl(Cr) intermetallics at room temperature has been studied by means of TEM and HREM observations and nanoprobe analysis. The results showed that, twinning is a primary deformation mechanism at room temperature, when plastic deformation is small twinning takes place on {111} planes and when plastic deformation is great two sets twins takes place usually from intersection region, where the atomic structure changes to hexagonal structure with the composition of Ti 55.7 2Al 42.2 8Cr 1.7 .展开更多
The influence of heat treatment and of thermomechanical processing on the structure and properties of a range of TiAl based alloys has been assessed and in agreement with other reports it has been found that increased...The influence of heat treatment and of thermomechanical processing on the structure and properties of a range of TiAl based alloys has been assessed and in agreement with other reports it has been found that increased refinement of the microstructure leads to improved mechanical strength at room temperature, both for the lamellar and the duplex structures. In the case of alloys cooled rapidly from the alpha phase field the increased refinement in lamellar spacing leads to significant increases in room temperature strength but thermomechanical processing can lead to far greater increases. The origin of this increase in strength in samples with a lamellar structure has been assessed in terms of the ability of dislocations to cross gamma/gamma and gamma/alpha 2 lamellar interfaces. It was concluded that the alpha 2 gamma interfaces and the alpha itself are important factors in strengthening the lamellar alloys. The stability of the various structures developed either by appropriate heat treatments or by thermomechanical processing has been investigated by exposing samples for a range of times at temperatures between 700 and 1 000 ℃. It has been found that the yield strength and the ultimate tensile strength generally decreased by about 20% during high temperature exposure at 700 ℃ for 3 000 h. The detailed behaviour on exposure at 700 ℃ has been found to be a function of alloy composition, with complex precipitates being formed in some alloys, but in all cases the amount of alpha 2 decreased with increased heat treatment time. It has been found that during exposure the alpha 2 lamellae decomposed to gamma phase by a mechanism that can involve the formation of thin gamma lamellae within the original alpha 2 lamellae.展开更多
文摘A Ti-45at.%Al alloy which was solution-treated at 1350℃ for 30 minutes and quenched in water is employed to explore mechanisms of α_2→γphase transformation. The ageing process of the quenched alloy has been in situ studied it has been found that the γ lamellae can precipitate in the α_2 matrix by two mechanisms. When The alloy is aged at 700℃, the stacking faults pre-existing in the α_2 matrix start to grow and more are generated.With the increase of ageing time and ageing temperature the density of stacking faults is increased and the γ lamellae then precipitate. This isα diffusion-controlled process. Alternatively, the γ lamellae may be formed from the α_2 matrix by 1/3(1100) shearing on the (0001) plane. The latter mechanism implies that the strain-induced transformation may occur, which is confirmed by deformation of the quenched alloy.
文摘The phase transformation induced by deformation in TiAl(Cr) intermetallics at room temperature has been studied by means of TEM and HREM observations and nanoprobe analysis. The results showed that, twinning is a primary deformation mechanism at room temperature, when plastic deformation is small twinning takes place on {111} planes and when plastic deformation is great two sets twins takes place usually from intersection region, where the atomic structure changes to hexagonal structure with the composition of Ti 55.7 2Al 42.2 8Cr 1.7 .
文摘The influence of heat treatment and of thermomechanical processing on the structure and properties of a range of TiAl based alloys has been assessed and in agreement with other reports it has been found that increased refinement of the microstructure leads to improved mechanical strength at room temperature, both for the lamellar and the duplex structures. In the case of alloys cooled rapidly from the alpha phase field the increased refinement in lamellar spacing leads to significant increases in room temperature strength but thermomechanical processing can lead to far greater increases. The origin of this increase in strength in samples with a lamellar structure has been assessed in terms of the ability of dislocations to cross gamma/gamma and gamma/alpha 2 lamellar interfaces. It was concluded that the alpha 2 gamma interfaces and the alpha itself are important factors in strengthening the lamellar alloys. The stability of the various structures developed either by appropriate heat treatments or by thermomechanical processing has been investigated by exposing samples for a range of times at temperatures between 700 and 1 000 ℃. It has been found that the yield strength and the ultimate tensile strength generally decreased by about 20% during high temperature exposure at 700 ℃ for 3 000 h. The detailed behaviour on exposure at 700 ℃ has been found to be a function of alloy composition, with complex precipitates being formed in some alloys, but in all cases the amount of alpha 2 decreased with increased heat treatment time. It has been found that during exposure the alpha 2 lamellae decomposed to gamma phase by a mechanism that can involve the formation of thin gamma lamellae within the original alpha 2 lamellae.
