Study On Hot Deformation Behavior And Powder Metallurgy Process Of NiTiNb Alloy Based On Preparation Of Pipe-joint

NiTiNb shape memory alloy with wide hysteresis effect is the best and most promising pipe-joint material at present.However,the processing and molding of NiTiNb alloy are difficult,and large size pipe-joint(inner diameter?30mm)is difficult to prepare,which greatly limits the application of the alloy.Especially in the field of manned space flight and deep space self-assembly,the development and application of high performance and large size pipe-joint are particularly urgent.In this paper,for the shortcomings of processing and molding,the hot deformation behavior of forged NiTiNb alloy under high temperature compression was studied,the activation energy was calculated and the constitutive equation was obtained.For the preparation of large size pipe-joint of the alloy,hot isostatic pressing is proposed to solve this problem.The process of hot isostatic pressing technology(including ball milling,capsule making and hot isostatic pressing)of NiTiNb alloy has been preliminarily explored.The microstructure and properties of HIPed NiTiNb alloy were investigated and compared with the forged Ni TiNb alloy.The main conclusions can be drawn as the following:(1)In the process of hot compression,the forged Ni TiNb alloy will show obvious yield phenomenon.Calculation shows that the activation energy of the alloy is198.004kJ/mol.The constitutive equation of it in hyperbolic-sine function can be expressed as:(2)The peak stress decreases with the strain rate and increases with the decreases of deformation temperature during the hot compression.NiTi matrix phase,?-Nb phase and a small amount of(Ti,Nb)2Ni phase exist in its microstructure,which content and distribution will affect the properties of the alloy,when the NiTi phase increases and the?-Nb phase decreases,it is beneficial to improve the strength but may reduce the plasticity of the alloy,(Ti,Nb)2Ni hard brittle phase is detrimental to the performance of the alloy.(3)The powders of Ni,Ti and Nb were refined and mixed evenly by ball milling with parameters as 8:1(ball to powder weight ratio),200r/min(rotational speed)and 2h(milling time).Then,NiTiNb alloy was prepared by hot isostatic pressing with parameters as 1200?,150 MPa,5h.The optimum milling and hot isostatic pressingparameters are still to be explored.(4)Compared with the forged NiTiNb alloy,the hardness of HIPed NiTiNb alloy is obviously improved.However,the alloy atoms do not diffuse sufficiently,which results in the NiTi matrix linked together and the distribution of ?-Nb phase is not uniform in the microstructure,which will affect the performance of NiTiNb alloy seriously.