The Thermal Stability Of NiAl-28Cr-6Mo Eutectic Alloy

NiAl-28Cr-6Mo eutectic alloy shall be deemed to be the best one in many NiAl based superalloys for its fine combination properties,but its lack of high temperature strength and the poor thermal stability are still restricting the development and application of the alloy.Therefore,iNiAl-28Cr-6Mo(at.%)alloy was prepared by vacuum induction melting.And secondly,the alloy was directional solidificated with different withdrawal rate(2.5-160?m/s).Finally,the thermal stability and instability mechanism of NiAl-28Cr-6Mo alloy were been studied,the research conclusion is as follows:In the low withdrawal rate(2.5-10?m/s),it was found that solid-liquid interface morphology is flat interface,and transversal surface microstructure is made up of parallel black NiAl phase and gray Cr(Mo)phase.In the high withdrawal rate(15-160?m/s),it was found that solid-liquid interface morphology is cellular interface or branch interface,and transversal surface microstructure is made up of varies of eutectic alloy cells.It is found that the lamellar spacing decreases gradually with the increase of the withdrawal rate,and the withdrawal rate is inversely proportional to the interlamellar spacing.NiAl-28Cr-6Mo eutectic alloy which possessed different interlamellar spacing was been heat treatment at 1150?,then the effect of interlamellar spacing on the thermal stability of the eutectic alloy was studied.It turns out the thermal stability of alloy is better when the withdrawal rate is 2.5?m/s and interlamellar spacing is 3.69?m.When the withdrawal rate was increased to 160?m/s and interlamellar spacing is 0.57?m,the thermal stability of alloy was worst.Under the condition of shallow corrosion,the thermal instability of the microstructure is mainly composed of lamellar inclusion and spheroidization.But after further study of deep corrosion,it was found that the real situation of the microstructure changes were partial dissolution and cylinder.Besides,it is concluded that the mechanism of the thermal instability of the alloy is termination migration mechanism,lamellar coarsening mechanism and lamellar cylinder mechanism.NiAl-28Cr-6Mo eutectic alloy which possessed different interlamellar spacing was been heat treatment at 950-1100?,then the effect of temperature on the thermal stability of the eutectic alloy was studied.The results found that the alloy in the flat interface rate(2.5-7.5?m/s),temperature has little effect on the thermal stability of microstructure and,only when the temperature gradually increased,partial dissolution occurs in the organization.This change was caused by terminal migration mechanism.In the cellular interface rate(15-160?m/s),the effect of temperature on thermal stability of microstructure is evidently.With increasing heat treatment temperature,the lamellar microstructure was gradually coarsening and cylindrical.For lamella microstructure was acted by termination migration mechanism,coarsening mechanism and cylinder mechanism.Therefore,in this paper,it is found that the main factors that affect the thermal stability of NiAl-28Cr-6Mo eutectic alloy were lamellar spacing and temperature.The thermal stability of the alloy decreases with the increase of the withdrawal rate(the decrease of the lamellar spacing)and the increase of the heat treatment temperature.