With excellent shape memory effect,superelasticity and many other properties,NiTi based alloys have been widely used in fields such as aviation/ aerospace,nuclear power and military industry. For engineering applications,NiTi based alloys are also one of the most important sorts from main shape memory alloys. Many research works have been conducted to improve the comprehensive performance of NiTi based shape memory alloys. With those works,microstructures,constituent phases,phase transformation behavior,shape memory ability and mechanical property of the alloys have been recognized progressively. Moreover,further efforts are also continuously carried out to improve the properties of NiTi based alloys. Equal channel angular pressing(ECAP)is one kind of severe plastic deformation(SPD)process which can produce three-dimensional bulk materials with fine grains very efficient. ECAP is one of the best ways for industrial application among different SPD methods. This technique has been applied with many metallic materials and now people try to use it within the field from traditional structural materials to functional materials. Therefore,more and more works have been reported with ECAPed NiTi based shape memory alloys,including NiTi dual alloys and several NiTi based ternary alloys. From the investigations we can draw the conclusion that ECAP can not only increase the mechanical property,but also increase many other properties such as shape memory effect and wear resistance ability,of the NiTi based alloys. With the existing reports, ECAP is very efficient at grain refinement and usually a grain size of 300~600 nm can be obtained. Compared with normal grain refining methods,ECAP process can obtain ultrafine grains which with high angle grain boundaries,not subgrains which with low angle grain boundaries. Beside grain refining,ECAP can also introduce large number of dislocations. Many works have reported the remarkable strengthening effect which caused by the dense dislocations in different kind of NiTi based alloys. ECAP is not always beneficial for the plasticity of the material. However,this can be adjusted by post deformation annealing(PDA)treatment. By appropriate PDA, not only the plasticity can be improved,but also the high strength can be preserved. The influences of heating temperature,holding time and cooling rate on microstructure and phase transformation behavior of NiTi based alloys are widely studied. Similarly,PDA is also beneficial to the recoverable strain of NiTi based alloy which treated by ECAP process. Consequently,shape memory property of the alloy can increase obviously after heat treatment. Since grain size can be reduced obviously by ECAP,yield strength of the alloy and critical shear stress of phase transformation can be improved. This increases the driving force of phase transformation in NiTi based alloys. As a result,the extrusion process will decrease the characteristic temperature of phase transformation. The superelasticity of NiTi based alloys can also be improved by increasing the recoverable strain after ECAP process. Besides,some works also showed that ECAP was beneficial for the wear resistance ability and corrosion resistance property of NiTi based alloys. For the advance of wear resistance ability,it was mostly affected by the outstanding superelasticity of the alloy. Good superelasticity could give better adaptive deformation of the alloy,thus might decrease the contact stress by increasing contact area during friction process. By refining the grains, density and volume of grain boundary were increased. More Ti atoms could diffuse through those‘channels’to form TiO2 protective film at the surface of NiTi based alloy,thereby corrosion resistance property was improved. With the advantages of ECAP method,it also should be noticed that the use of ECAP in shape memory alloys was still not sufficient compared with traditional structural materials. There were still some further works needed to be carried out in the near future. For example,most former ECAP works were aimed at NiTi dual alloys,works on NiTi based ternary alloys such as NiTiNb(wide hysteresis shape memory alloy),NiTiCu(narrow hysteresis shape memory alloy),NiTiFe(low temperature shape memory alloy),NiTiHf(high temperature shape memory alloy)were very limited. Furthermore,the evolution of microstructure during warm ECAP process should be very complex since the participation of precipitation and dynamic recovery. Accurate characterization and quantificational analysis of the microstructure during ECAP process was still not enough. Moreover,the relationship between ECAP parameters,microstructure and functional properties of NiTi based shape memory alloys also needed to be studied more deeply. Investigations about NiTi based shape memory alloys which treated with ECAP technique in recent years were reviewed in this paper. Usual ECAP parameters,and the effect of this technique on microstructure,mechanical property,phase transformation behavior and shape memory effect of the alloy were also summarized. Advantages and disadvantages of NiTi based alloys treated with ECAP were analyzed. The development and optimization direction of this technique in the future were also discussed. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.
