Growth Mechanism And Properties Of Dy Alloyed NiAl-V Eutectic Composites During Directional Solidification Under High Temperature Gradien

Intermetallic compound NiAl has many excellent properties such as high melting point,high thermal conductivity,oxidation resistance and corrosion resistance,which is expected to become a new generation of high temperature materials.However,NiAl alloy has poor room temperature toughness and high temperature strength,which limits its industrial application.NiAl matrix composites were prepared by alloying and directional solidification technology to improve the plasticity,toughness and high temperature strength.In this paper,NiAl-39V-x Dy(x=0,0.05 wt%)eutectic alloy and NiAl-43V-x Dy(x=0,0.05wt%)hypereutectic alloy were prepared by high temperature gradient directional solidification(LMC,G_L=310 K/cm).The effects of growth rate on solid-liquid interface morphology,microstructure evolution and alloy properties of NiAl-39V-x Dy(x=0,0.05 wt%)eutectic alloy and hypereutectic alloy were studied.At the same time,the effects of rare earth element Dy(0.05 wt%)on directional solidification microstructure and properties of NiAl-V series alloy were studied.The strengthening mechanism was analyzed.The following conclusions were obtained:The directionally solidified NiAl-39V(Dy)eutectic alloy is composed of lamellar eutectic structure(NiAl+V)in the growth rate range of 6?150?m/s.The morphology of solid-liquid interface changed from 6?m/s flat interface to 10?150?m/s cellular interface,and the corresponding cross-sectional steady-state microstructure changed from planar lamellar eutectic to cellular lamellar eutectic.The growth rate(10?150?m/s)increased,the size of eutectic cell decreased,and the spacing of lamellar eutectic decreased gradually.The intracellular central rod-like V eutectic structure evolved into lamellar eutectic.The solidification structure of directional solidification NiAl-43V(Dy)hypereutectic alloy is composed of primary V dendrite and cellular eutectic structure in the growth rate range of 6?150?m/s.The solid-liquid interface morphology of the alloy is cell-branch coexistence interface.With the increase of growth rate(6?150?m/s),the solidification structure is refined,the primary dendrite spacing decreases,and the integral number of primary V dendrite increases.At the same growth rate,a small amount of Dy will refine the NiAl-V eutectic lamellar spacing in the NiAl-39V eutectic alloy.Dy element has little effect on the eutectic cell size at the growth rate of 10?60?m/s,which is about 1.42?m.When the growth rate is 150?m/s,the eutectic cell size decreases about 13.24?m.At the same growth rate,a small amount of Dy can reduce the undercooling of dendrite tip in NiAl-43V hypereutectic alloy,reduce the driving force of dendrite growth,inhibit the growth of primary V phase,reduce the integral number of dendrite by about 0.1%?2.8%,and increase the primary dendrite spacing by about 44.27?92.46?m.The hardness(Hv0.5)of directionally solidified NiAl-39V(Dy)eutectic alloy with flat interface growth is 455.9(0.05Dy)and 434.3(0Dy).When the cell interface is grown,the hardness of the alloy increases with the growth rate,from 470.45(0.05Dy)and 451.5(0Dy)of10?m/s to 505.85(0.05Dy)and 482.35(0Dy)of 150?m/s.Microamount of Dy increases the hardness of eutectic structure and cell boundary region growing at flat or cellular interface.The hardness of NiAl-43V(Dy)hypereutectic alloy grown at the cell-branch interface increases with the growth rate,from 432.95(0.05Dy)and 438.8(0Dy)at 6?m/s to 501.7(0.05Dy)and 538.3(0Dy)at 150?m/s;Adding trace Dy element into hypereutectic alloy will enrich between dendrites and reduce the hardness of alloy at room temperature.The peak stress of high temperature compression of eutectic and hypereutectic alloys at the growth rate of 6?150?m/s was all obtained at the low growth rate of?=6?m/s,which were 399.46 MPa(0.05Dy)and365.51 MPa(0Dy)for eutectic alloy,370.91 MPa(0.05Dy)and 364.71 MPa(0Dy)for hypereutectic alloy,respectively.Adding trace Dy element at the same growth rate can play the role of fine grain strengthening,second phase strengthening and solid solution strengthening,improve the peak point of high temperature compressive stress and increase the mechanical properties of the alloy.