Study On Microstructure And Properties Of TiNiFeV Low Temperature Shape Memory Alloys

Low temperature shape memory alloys are widely used in aircraft pipe joints because of low transformation temperatures,good mechanical properties and shape memory effects.There are two kinds of low temperature shape memory alloys widely applied:Cu-Al-Mn based and Ti-Ni-Fe based.Although the price of Cu-Al-Mn based alloys is relatively low,sometime they cannot meet the requirments of mechanical properters,because of poor machinability and high order as well as strong anisotropy of formed microstructure.Ti-Ni-Fe alloys can be used as an ideal structural material attributed to the good mechanical properties,such as excellent corrosion resistance and wear resistance.Since the M_S transformation temperatures of Ti-Ni-Fe alloys are low,as the aircraft pipe joint,it will not loosen due to martensitic transformation even at low temperature.So the aircraft pipe joints fabricated by Ti-Ni-Fe shape memory alloys have been widely used for hydraulic system pipelines.To further improve aircraft performance,higher requirements for mechanical properties of pipe joints are needed.Improving the mechanical properties while maintaining the existing transformation temperatures has become an important research direction for low temperature memory alloys.Research on Ti-Ni-Fe based low temperature shape memory alloys is mainly focused on the structure and properties of Ti-Ni-Fe ternary alloy.Few investigations have been carried out for quarternary alloys.In the present work,by adding V elements(Ti_xNi_100-x,Ti_97-x-yNi_xFe₃V_y（x=46,47,48;y=0,0.5,1,1.5,2,2.5,3）)to Ti-Ni-Fe-V ternary alloy,the effects of Ni content and V content on the microstructure and properties of Ti-Ni-Fe-V quaternary alloys and the influences of homogenization heat treatment on the microstructure and properties of Ti-Ni-Fe-V quaternary alloy are investigate.The microstructure of Ti-Ni-Fe alloy consists of ductitle primary phase Ti Ni（matrix）and hard-brittle second phase Ti₂Ni.The atom Fe replaces the atom Ni of TiNiphase and the substitutional solid solution is formed.The microstructure of Ti-Ni-Fe-V alloy also consists of primary phase TiNiand second phase Ti₂Ni.The atom V preferentially resolves into the second phase to form solid solution.With the increase of Ni concentration,the fraction of primary phase increases.With the increase of V concentration,the dendrite structure gradually disappers and the second phase presents a dispersed-distribution pattern.After homogenization of heat treatment,no new phase is formed and the microstructure is refined in some degree.The hardness of Ti-Ni-Fe-V alloy increases with the increase of Ni content.It first increases and then decreases with the increase of V content.The Ti_47.5Ni₄₈Fe₃V_1.5 has the highest value of hardness,849.97hv0.2.On the contrary,after homogenization treatment,the hardness of the alloy decreases in different degrees.The compressive strength of Ti-Ni-Fe-V alloy increases with the increase of Ni content.With the increase of V content,the compressive strength increases first and then decreases.The maximum compressive strength is of 1066.7MPa after heat treatment.The transformation temperature of Ti-Ni-Fe-V alloy decreases with the increase of Ni content.There is a two-stage transformation process（B2-R-B19’）in Ti₄₉Ni₄₆Fe₃V₂ alloy.The single-stage transformation（B2-R）can be observed in Ti₄₈Ni₄₇Fe₃V₂alloy.The transformation temperature of Ti₄₇Ni₄₈Fe₃V₂ alloy is too low and cannot be captured,so the transformation is not observed.With the increase of V content,the transformation temperature of the alloy increases.Ti-Ni-Fe-V alloy with Ni content of 48%and V content of 1.5%～2.5%has better comprehensive properties.