Microstructure And Martensitic Transformation Of TiNiHfSc High Temperature Shape Memory Alloys

NiTiHf alloys have been regarded as the most promising candidates for practical applications.In order to improve the properties of TiNiHf high temperature shape memory alloy,the fourth element was added to the alloy by alloying.In present work,the microstructure,martensitic transformation behavior and mechanical properties of Ni-lean Ti_36-xNi₄₉Hf₁₅Sc_x?x=0,0.3,0.5,0.7,1,2at.%?alloys,Ni-rich Ti_29.7-xNi_50.3Hf₂₀Sc_x?x=0,0.3,0.5,0.7,1,2at.%?alloys and aged Ti_29.7-xNi_50.3Hf₂₀Sc_x?x=0,0.3,0.5,0.7,1,2at.%?alloys were studied by scanning electron microscope?SEM?,X-ray diffraction?XRD?analysis,Electron Probe Micro-analyzer?EPMA?,transmission electron microscope?TEM?,differential scanning calorimetry?DSC?and microhardness testing.The influencing mechanism was analyzed.Microstructure of Ti₃₆ Ni₄₉Hf₁₅ alloy consists of B19?martensite matrix and?Ti,Hf?₂Ni phase at room temperature.After addition of Sc,a new second phase Sc2O3 phase appeared,and the volume fraction of this phase increased with the Sc content increasing.The addition of Sc doesn't change the type of martensite,and the martensite of Ti_36-x Ni₄₉Hf₁₅Sc_x alloys shows the lath-like plates which are mainly characterized by?001?compound twin substructure and related to?011?type I twinning mode.Martensitic transformation of Ti_36-x Ni₄₉Hf₁₅Sc_x alloys is characterized by B2?B19?single-step transformation.The characteristic temperatures decreased with Sc content increasing.The proper addition of Sc is beneficial to improving the thermal cycling stability of martensitic transformation for Ti_36-xNi₄₉Hf₁₅Sc_x alloys.It can attribute to the addition of Sc strengthened the matrix and improved the crystallographic compatibility between martensite and partent phase of alloys.When the Sc content is 0.5at.%,the thermal cycling stability of alloys is the best.After 10 cycles,M_p only decreased 4.7?.Both solution-treated and aged Ti_29.7-xNi_50.3Hf₂₀Sc_x alloys mainly consist of B19?martensite matrix and Ti2Ni4O phase at room temperature.With the Sc content increasing,Sc2O3 phase appeared and the volume fraction of Ti2Ni4O decreased gradually to disappear.Two different types of martensite variants are observed in both solution-treated and aged alloys,which are?001?compound twin and?011?type I twinning.Between two martensite variants were mainly twin related with?011?type I mode.Martensitic transformation of Ti29.7-_xNi_50.3Hf₂₀Sc_x alloys is characterized by B2?B19?single-step transformation in both two states,and the characteristic temperatures increased at first,and then decreased with Sc content increasing.Aged Ti_29.2Ni_50.3Hf₂₀Sc_0.5 alloy shows the best thermal cycling stability in all Ni-rich Ti29.7-x Ni50.3Hf20Scx.The nanosized H-phase precipitates presented in the aged Ti_29.7-xNi_50.3Hf₂₀Sc_x alloys.The addition of Sc changes the content of Ni in the alloy matrix,and the amount of H phase increases first and then decreases with the increase of Sc content,while the size of H phase increases with the increase of Sc content.The characteristic temperatures of aged alloys are higher than those of solution-treated alloys,because that the formation of H-phase precipitates depleted Ni from the matrix.The microhardness of alloys increased with the precipitation of fine H phase.The addition of Sc changes the amount and the size of precipitation,resulting in different precipitation strengthening effects,which makes the microhardness of the alloy decreased first and then increased with the increase content of Sc.The aged Ti_29.7Ni_50.3Hf₂₀ alloy shows the highest microhardness.