| a titanium alloys possess characteristics such as high specific strength, low thermal conductivity, good compatibility to medium and good resistant to hydrogen embrittlement, which makes a titanium alloy suitable for the use in liquid fuel tank and tube at cryogenic temperature. The two near a titanium alloys CT20 andCT20A, developed by NIN, are for application of structural parts at cryogenic temperature. It is meaningful toe arry o ut s tudy one ffect o f microstructure and interstitial c ontent on cryogenic properties of these two near a titanium alloys and will promote the application of newly developed alloys in cryogenic engineering.In order to accomplish the main targets, the following jobs were done: the annealing of CT20 alloy tube at 6 different temperatures was carried out and the specimens with different microstructure were tested at room temperature (RT) and 20K. The tensile fractures were analyzed by means of SEM and TEM. The C T20A alloy specimens with 3 kind of Oxygen Equivalent (Oeq) were prepared, and the tensile properties at RT and 20K, the impact toughness at RT and 77K were tested. SEM and TEM observation were carried out on CT20A alloy too. At last, the deformation mechanism at cryogenic temperature was probed on the basis of the work done.The studying results indicated that:Three typical microstructures were abtained when CT20 alloy were annealed under different temperature, that is, equiaxed microstructure (annealed at T1, T2 and T3), bimodal microstructure (annealed at T4 and T5) and lamellar microstructure (annealed at T6). The effect of microstructure on RT tensile properties was weak, while at 20K was notable. At 20K, the strength of CT20 alloy with various microstructuresincreased while elongation obviously dropped down, and the lamellar microstructure had the best elongation. The fracture of CT20 alloy was of plastic dimples character at RT, while some holes and cleavage appeared at 20K, showing some brittle feature. When CT20 alloy was tensioned at 20K, the main deformation mechanism in equiaxed microstructure was the slip of dislocations, and in bimodal microstructure the slip of dislocations and a small amount of twinning and in lamellar microstructure a large amount of twinning. Corresponding to the amount rising of twins, the ductility at cryogenic temperature of CT20 alloy increased.The changing of interstitial content (Oeq varies from 0.15% to 0.23%) had no obvious influence on the microstructure of CT20A alloy, while the properties at 20K were strongly influenced. With interstitial content increasing, the strength of CT20A alloy increased while the elongation dropped down. The alloy with low interstitial content had better ductility and toughness. The fracture of CT20A alloy, whose Oeq is 0.15%, was occupied by dimples, showing good ductility; while the distribution and size of dimples in the fracture of alloy with Oeq 0.23% were not uniform and localized quasi-cleavage feature was found, indicting bad ductility and toughness. With Oeq increasing, the impact toughness of CT20A alloy decreased rapidly, especially at 77K. Quite notable {11 22 } twins with the appearance of strip piercing through lamellar colony were found in the tensile microstructure of CT20A alloy by TEM analyzing, while no deformation twinning was found in the microstructure of alloy with high Oeq.The deformation of near a titanium alloy at RT is controlled by dislocation slip while at 20K is under the mutual control of dislocation slip and twinning. In this investigation, three types of twins were observed in the deformation microstructures of CT20 alloy at cryogenic temperature. That is, twins of {11 2~l }, {10 T2 } and {li 22 } type. While only {11 2~2 } type twin was found in CT20A alloy. The role that twinning plays in the deformation of near a titanium alloy at cryogenic temperature can be divided into two parts, that is, direct part and indirect part. The direct part is, twinning itself can bring some deformation, which is beneficial to the improvement of plasticity. While this con...
|
PDF Full Text Request