Microstructure And Friction And Wear Characteristics Of NiTiHf Alloys

Ni-rich NiTi alloy has become a potential material for important parts such as bearings due to its high hardness,high strength,good wear resistance and corrosion resistance.In order to further improve the properties of NiTi alloy,Hf element was added.In present work,microstructure,microhardness and wear properties of Ni₅₅Ti_45-xHf_x(x=0,1,2,3,4,5 at.%)alloys were studied by optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)analysis,X-ray photoelectron spectroscopy(XPS)analysis,microhardness,mechanical compressive properties and friction and wear characteristics testing methods.The effect of Hf element and aging treatment on microstructure,hardness,friction and wear and other mechanical properties of NiTiHf alloy was discussed.At room temperature,the microstructure of Ni₅₅Ti_45-xHf_x(at.%)alloy after solution treatment and aging treatment consists of NiTi matrix phase,Ni₄Ti₃ phase and a small amount of Ni₃Ti phase.The aging treatment at 400?is beneficial to the precipitation of Ni₄Ti₃ phase in the alloy.Compared with the solution-treated alloy,the aged alloy shows the larger volume fraction of Ni₄Ti₃ phase.With the addition of a small amount of Hf element,fine H phase was formed,which was beneficial to obtaining finer Ni₄Ti₃ phase.The morphology of Ni₄Ti₃precipitates in Ni₅₅Ti₄₅ alloy and Ni₅₅Ti₄₀Hf₅ alloy are obviously different.The precipitates of Ni₅₅Ti₄₅ alloy are elongated,and those of Ni₅₅Ti₄₀Hf₅ alloy are spherical.With the increase of Hf content,microhardness of Ni₅₅Ti_45-xHf_x(at.%)alloy gradually increases;With the increase of ageing time,microhardness of Ni₅₅Ti_45-xHf_x(at.%)alloy increases first and then decreases.Among them,Ni₅₅Ti₄₀Hf₅ alloy aged at 400?for 10h has the largest microhardness,about 693HV0.3.The solution treated and aged of NiTiHf alloys have poor plasticity and larger compressive strength.The addition of Hf element can further increase the elastic modulus of the alloy.For friction and wear at room temperature,the coefficient of friction of Si₃N₄ ceramic balls and alloys is smaller than that of GCr15 steel balls,showing typical adhesive wear and abrasive wear characteristics.The alloy can increase the hardness of the alloy through the addition of Hf element and aging treatment,and then improve the wear resistance.Among them,the friction coefficient of the aged Ni₅₅Ti₄₀Hf₅alloy is the smallest,about 0.3.The alloys have a lower coefficient of friction when the friction and wear tests were carried out at 400?.This is mainly because the oxide layer on the surface of the alloy plays a lubricating effect,which can reduce the degree of wear,showing typical abrasive wear and oxidative wear characteristics.