Research Of CuAlBe Shape Memory Alloy Pipe Joints

SMA pipe joints always serve at minus temperature. When the service temperature is close to the Ms temperature of the pipe joints, the pipe joints will expand due to the two way memory effect, resulting in relaxation of the connection of the pipe joints. Therefore it is necessary to design the Ms temperature of the alloy lower than the service temperature by about 10℃.In our previous work, the CuAlBe pipe joints with columnar grain structure prepared by heated mold continuous casting process which have a Ms temperature about 0℃ are able to be expanded by 11%, so as to increase the As temperature to 70℃～80℃, which means the transformation hysteresis reaching to 70℃～80℃ and can be stored at room temperature. In this study, CuAlBe pipe joints with Ms temperature of -30℃ will be expanded to study the effects of expansion rate, expansion temperature, aging on the shape recovery rate and hysteresis. The possibility of storage at room temperature will also be evaluated.The compositions of the CuAlBe pipe joints are of 11.7wt%Al,0.5wt%Be and balance of Cu, with Ms -28℃ and As -20℃. Tubes with OD 8mm, ID 5.4mm and plate with 50mm in width 1.5mm in thickness were continuously cast by heated mold continuous casting process to obtain columnar grain structure.Tensile specimens were cut from plate in direction perpendicular to the casting direction. Tensile tests were conducted to simulate the stress status of the pipe joints expansion. Deformation of 10 grains were in situ observed and measured. Individual tensile-strain curve of the 10 grains were obtained to study the anisotropic in martensitic transformation and elastic modulus.The cast tube were cut in definite length as blanks of pipe joints. After solid solution, hole machining, the pipe joints were expanded by 9%、10%、11、at -50℃ and room temperature respectively. Elastic recovery and memory recovery were measured. Ms and As temperature before and after expansion were determined by DSC. XRD were conducted to determine the phase structure before and after expansion.In order to study the effects of aging on transformation hysteresis, pipe joints were aged at 250℃ for 10h, then expanded at room temperature by 9%、10%、11%, followed by Ms, As measurement and XRD analysis.Results of in situ tensile indicate that strong anisotropic exists in pipe joints. Tensile curves of individual grains can be divided into superelastic type and shape memory type. In shape memory type grains, P’or y’martensite is induced respectively according to the grain orientation, which lead to different degrees of martensite stabilization. Shape memory type grains are advantageous to pipe joints due to their larger strain, wider hysteresis. Whereas superelastic type grains are disadvantageous to pipe joints due to their entirely recovery after expansion, which both increases elastic recovery and contributes no effect to hysteresis.The expansion results show that the maximum expansion rate of the pipe joints is as high as 11%. With the increase of expansion rate, both elastic recovery and memory recovery decrease. Elastic recoveries with 11% expansion rate at -50℃ and room temperature are 26.9% and 30.8% respectively, while memory recoveries,88.9% and 87.5% respectively. The XRD results also show that with the increase of expansion rate, the amount diffraction peaks of β’and intensity of the peaks increase in the XRD spectrum. These indicate that with increase of expansion rate, martensite become more stable. Transformation hysteresis of -50℃ expansion and room temperature expansion are 78℃ and 67℃ respectively.Two separated endothermic peaks appear at the DSC curves after expansion both at-50℃ and at room temperature. The peak at low temperature side is narrow and high, with temperature range similar to that before expansion. The peak at high temperature side is spread and low, with temperature range similar to that of pipe joint with Ms 0℃. The low temperature side peaks are produced from the superelastic type grains, while the high temperature side peaks are produced from the memory type grains.The elastic recovery of pipe joints aged at 250℃ for 10h and expansion rate at 11% is 24.8%, and memory recovery is 79.1%. Both results are lower than that without aging. Aging causes both As and Ms temperature increase. The reason is that aging results in ordering of β phase, which decreases the elastic deformation energy of martensite, leading to the increase of As temperature. On the other hand, ordering of βphase facilitates the transformation of P phase to martensite, leading to the increase of Ms temperature.The expansion results indicate that expanding at -50℃, the lowest elastic recovery and highest memory recovery can be obtained.Pipe joint with ID5.76mm is expanded by 11% at -50℃ and then stored at room temperature, the ID can maintain at 6.242mm. By heating to 200℃, the ID is recovered to 5.814mm. For connection of OD.6mm tubes, this joint can provide sufficient clearance and interference amount. Although the transformation hysteresis is relatively narrow, leading to small amount of shrinkage of the ID when stored at room temperature, the expansion rate is large enough to offset this shrinkage. Examination of tube connection shows that the sealing performance is excellent.In summary, pipe joints of Cu-11.7wt%Al-0.5wt%Be, being lower Ms temperature, can satisfy the requirements of minus temperature service and storage at room temperature with expanding at -50℃ by 11% expansion rate.