Study On Microstructure And Thermal Stability Of Directionally Solidified NiAl-V Eutectic Alloy

NiAlV alloys are gradually expected to become a new generation of NiAlbased superalloy materials due to their unique excellent properties.However,the application and development of NiAlV alloys in the industrial field are restricted due to the poor hightemperature strength.On this basis,NiAlbased eutectic alloys with different V contents were prepared by vacuum induction melting furnace in this thesis: NiAl32 V pseudo-binary hypoeutectic alloy,NiAl39 V pseudo-binary eutectic alloy and NiAl43 V pseudo-binary hypereutectic alloy as raw alloys.Alloy samples with different withdrawal rates were prepared by liquid cooling directional solidification equipment,and the thermal stability and instability mechanism of the alloy microstructure were studied after high temperature heat treatment.The conclusions are as follows:After heat treatment at 900 °C for directional solidified NiAl32 V hypoeutectic alloy,the proportion of primary phase area of all samples changed slightly with the increase of heat treatment time,and the thermal stability of primary phase are good as a whole.The thickness of Ni Al eutectic lamellae of all samples increased with the increase of heat treatment time,and obvious coarsening and spheroidization occurred after long-term heat treatment.After high temperature heat treatment at different temperatures for 5 h(900°C,1000°C,1100°C),the primary phase area of directional solidification NiAl32 V alloy did not change significantly,and its thermal stability is good.However,the Ni Al eutectic lamellae appears coarsening and spheroidization,and its 5h high temperature thermal stability is general.The grain size of directionally solidified NiAl39 V eutectic alloy has no change after high temperature heat treatment at 900 °C for 100 h.The coarsening and spheroidization of eutectic lamellae are relatively obvious in the sample withdrawing rate v=6μm/s,and the thermal stability of eutectic layer is poor.If the aspect ratio of Ni Al eutectic lamellae is greater than 3,the dominant instability mechanism is boundary splitting mechanism.When the aspect ratio is less than 3,the dominant mechanism of mass transport is direct cylinderization mechanism.With the increase of withdrawal rates,the Ni Al eutectic lamellae also coarsens in varying degrees,and its thermal stability is general.The theoretical calculation shows that the dominant instability mechanism is boundary splitting mechanism and terminal migration mechanism.With the time and temperature of heat treatment increased,the Ni Al eutectic lamellae of all withdrawal rate samples are obviously coarsened.After heat treatment at 900°C for directional solidified NiAl43 V hypereutectic alloy,the thermal stability of primary phase of all withdrawal rate samples is good,and the thickness of Ni Al eutectic lamellae increases with the increase of heat treatment time.The coarsening degree of the lamellae is in direct proportion to the withdrawal rate,and the thermal stability of Ni Al eutectic lamellae is general.After 5h high temperature heat treatment(1000°C,1100°C),the Ni Al eutectic lamellae of all withdrawing rate samples have coarsening phenomenon,and the 5h high temperature thermal stability is general.