Effect Of Cr And Cr₃C₂ Addition On Properties And Microstructure Of Ni₃Al-bonded TiC-based Cermets

Many attention has been attracted by intermetallic compound Ni₃Al,due to its high hardness,high melting point,outstanding oxidation resistance and corrosion resistance,and the improvement of yield strength with temperature increasing below 950?.In recent years,more and more researches have been carried on the use of Ni₃Al as binder of cemented carbide and cermets for domestic and overseas.In this article,Ni and Al powders were used in formation of Ni₃Al powder by mechanical alloying,and then Ni₃Al-TiC-based cermets were prepared by traditional powder metallurgy route.Finally,microstructure and properties of Ni₃Al-bonded TiC-based cermets were performed by X-ray diffraction?XRD?,environmental scanning electron microscope?ESEM?with energy dispersive spectrometer?EDS?,etc.The observations in SEM-BSE electron mode shows that,cermets were consists of Ni₃Al binder and hard particles of TiC which hada black core-white rim structure.With Cr or Cr₃C₂ content increasing,the white rim became complete and got thicker gradually,as well as the rim size was firstly rose and then declined.SEM-BSE/EDS analysis indicated that,the core was mainly TiC and the rim was mainly solid solution of?Ti,W,Mo,Cr?C while the binders were primarily Ni₃Al in which a spot of Cr,Mo and other elements were dissolved.It is showed that hardness and TRS of cermets were increased firstly and then reduced with WC content increasing,and the fracture toughness also has a similar trend.It is that the addition of Cr promoted the formation of the ring phase,which can restrain the process of dissolution-precipitation,so the grain refinement of cermets is promoted.Nevertheless,the increase in excess of the solid solubility of cermets could raise composition inhomogeneity and reduce the melting point,which in turn promote the growth of crmets grains,so the two factors work together.The effect of Cr₃C₂ addition on the strength is similar to that of Cr,but there is a transient increase in fracture toughness,which is related to the passivation of holes resulted from the increase of carbon content in Cr₃C₂.Ni₃Al-bonded TiC-based cermets with Cr coetent of 1.0wt.%has a comprehensive mechanical properties after sintering at 1485?,and its TRS,Rockwell hardness and HV₃₀reach to 1547MPa,88.6HRA,12.3 MPa.m^1/2,respectively.Compare the polarization curves of Ni₃Al-bonded TiC-based cermets after the addition of Cr and Cr₃C₂,it was found that there were two passivation regions in which the first passivation regions was a ture passivation regions and the second with a large current density was a pseudo passivation regions.The corrosion current density was in a straight line between small addition of Cr or Cr₃C₂,almost no difference,but after the addition of Cr₃C₂ passivation current is lower than the addition of Cr,so the corrosion resistance is better.