Microstructure and Mechanical Properties of NiTi Alloy with Thermo-Mechanical Treatment AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY24110013

Chinese Library Classification Number:TG146.2+1

Citation Information:

NiTi alloys have been widely used in various fields such as healthcare，aerospace，consumer electronics，and robotics due to their excellent shape memory effect，superelasticity，corrosion resistance，biocompatibility，good electrical and thermal conductivity. Near-equiatomic NiTi alloys are those in which the atomic percent of nickel and titanium is close to 1∶1. The nickel content is typically between 50% and 51% （atom fraction），while the titanium content is between 49% and 50% （atom fraction）. This specific composition ratio is essential for the formation of a stable austenitic phase and subsequent martensitic transformation，which is the basis for the superelasticity and shape memory effects of NiTi shape memory alloys. The excellent properties of near-equiatomic NiTi alloys are mainly due to the complex internal martensitic transformation process. In order to meet the specific requirements for material properties in different application scenarios，it is crucial to have a deep understanding and precise control of the microstructure of NiTi alloys. Appropriate heat treatment processes can significantly alter the phase composition and distribution of alloys，affecting the functionality and mechanical properties of the final product. For near-equiatomic NiTi alloys，the martensitic transformation （M_s） temperature is strongly dependent on the nickel content. For every 0.1% （atom fraction） increase in nickel，M_s temperature decreases by more than 10 ℃. Although there is a lot of literature on how various heat treatment schemes can improve the overall properties of near-equiatomic NiTi alloys，there is still a lack of research on the extremely-near-equiatomic NiTi alloys such as Ni_50.1Ti_49.9 alloy. In this paper，the effects of solid solution treatment and annealing treatment on the microstructure and mechanical properties of a near-equiatomic Ni_50.1Ti_49.9 alloy in the cold-rolled state were investigated. In addition，microscopic analysis techniques （e.g. X-ray diffraction （XRD），scanning electron microscope （SEM），electron backscatter diffraction （EBSD），etc.） were used to reveal the intrinsic relationship between microstructure and macroscopic mechanical properties，providing a theoretical basis and technical support for the subsequent development of new high performance NiTi based materials. The results showed that the rolled Ni_50.1Ti_49.9 alloy was mainly composed of B19' martensite，while the content of B2 austenite and B19' martensite in annealed and solid solution alloys was about 1∶1. A large number of martensite laths and a small amount of Ti₂Ni precipitates were found in the alloys. The thermally induced martensite in the solid solution alloy was criss-crossed and uniformly distributed within the austenite grains，with no obvious grain orientation. Differential scanning calorimetry （DSC） curve analysis showed that the phase transformation points of Ni_50.1Ti_49.9 alloy was near room temperature. The presence of dislocations and twinning in the rolled state alloy hinders the thermally induced martensitic transformation and reverse transformation，and its heat absorption and release peaks were relatively flat. After a short period of annealing，the absorption and exothermic peaks of the sample became obvious，and the onset and end temperatures of martensitic transformation and reverse transformation could be clearly distinguished. The solid solution alloy exhibited a two-step transformation process during the cooling stage，i.e.，B2→R→B19'，which was attributed to the stress field around Ti₂Ni precipitated phase suppressing the usual one-step martensitic transformation. The mechanical property tests showed that the rolled Ni_50.1Ti_49.9 alloy had excellent mechanical properties along the rolling direction，with a tensile strength of 1246.7 MPa and an elongation of 17.9%. This excellent performance was due to the texture formed during the rolling process，which optimized grain orientation and promotes effective stress transfer. The tensile curve of the rolled Ni_50.1Ti_49.9 alloy could be roughly divided into two stages：the first stage was the near-linear deformation caused by the intrinsic elastic deformation of the sample and the slow continuous stress-induced martensitic transformation （SIMT）；the second stage was mainly plastic deformation，which might be accompanied by a small amount of SIMT. The annealed state sample exhibited a typical "double yielding" phenomenon during the tensile process. The tensile curve of the annealed state alloy could be roughly divided into four stages：the first stage was the elastic deformation stage；the second stage was caused by the SIMT of B2 austenite，reorientation of self-accommodating of B19' martensitic variant together with a small amount of plastic deformation；the third stage mainly included elastic deformation of martensite，SIMT of residual B2 phase，reorientation of martensite variants previously in unfavorable orientation，and plastic deformation；the fourth stage was the plastic deformation. The strength and strain of the solid solution sample decreased significantly，but exhibited a significant shape memory effect. The tensile curve of the solid solution Ni_50.1Ti_49.9 alloy could be roughly divided into three stages，similar to the deformation mechanism of the first，second，and third stages of the annealed state alloy. After unloading at 4.5% strain，the residual strain could be fully recovered under brief thermal stimulation.