Microstructure Evolution and Microhardness of Directionally Solidified Nb30Ti35Co35 Alloy
In the last few decades,the solidification theory of binary alloys has been built,for example,model of dendritic and eutec-tic growths.Nevertheless,the solidification theory of ternary alloys has not been established due to the solute redistribution and mutual coupling of multicomponent during the solidification.Up to now,the solidification theory of ternary and multicomponent alloys has be-come a hot research topic in the field of materials.Especially,the essential relationship between solidification structure and mechani-cal properties is an urgent problem to research.Based on this,the microstructure evolution and microhardness of directional solidifica-tion Nb30Ti35Co35 eutectic alloy(measured Nb31Ti34Co35)were studied in detail.The columnar master alloy was prepared by using a wa-ter-cooled copper crucible,and then it was cut into rodlike specimens with a diameter of 3 mm and a height of 120 mm.When the sam-ple was heated to 1200 ℃ for 30 min,the directional solidification experiment was carried out at different drawing rates(v=3,5,15,30,70 μm·s-1)by using the Bridgman technique.Other measuring equipments such as SEM,EDS and XRD were used to analyze the microstructure,phases and their distribution.Vickers hardness(HV0.3)under different drawing rates was aslo measured by a digital microhardness meter.The research results show that,except for a very small amount of primary α-Nb,the as-cast alloy consists almost entirely of eutectic(α-Nb+TiCo)structure.Similarly,the directional solidification alloy also contained the same structure type(ex-cepted a small amount of Ti2Co),i.e.,primary α-Nb and eutectic(α-Nb+TiCo).The sample after directional solidification was com-posed of the initial-growth interface,steady-state growth interface and quenching interface.For the initial-growth interface,there was a thermal transition zone at the initial-growth interface of directional solidification.At low drawing rates(3 and 5 μm·s-1),the micro-structures between the thermal transition zone and the directional solidification zone were connected by a band-like eutectic.In this sit-uation,the interface with the straight shape was clearly visible.When the drawing rate continued to increase,the directional solidifica-tion microstructure in the thermal transition zone were intertwined,and the eutectic interface gradually disappeared.At the same time,the spherical white primary phase α-Nb phase gradually became a small spherical shape,which implied that as the drawing rate in-creased,the interface correlation gradually became better.For the steady-state growthinterface at low drawing rates(3 and 5 μm·s-1),the eutectic structure grew parallel to the drawing direction.With the gradual increase of the pulling rate,the primary α-Nb successive-ly underwent"spherical → cluster → dendritic".Correspondingly,the eutectic structure in the steady-state growth zone was gradually coarsening,but the eutectic arrangement was irregular and the phase spacing was obviously larger.The quenching interface was unsta-ble and underwent the transformation from the cellular interface to the dendritic interface in turn.In addition,compared with the con-ventional as-cast alloy,the microhardness of the alloy after directional solidification increased significantly and the values increased significantly with the decrease of the drawing rate.The relationship between these two parameters was H=222.4+v-04328+319.8(H was microhardness).The alloy at drawing rate of 3 μm·s-1 had a highest microhardness value,which was HV03 462.This value was about 1.5 times that of the as-cast counterpart.The increase in hardness after directional solidification was mainly related to the anisotropy of α-Nb phase and TiCo phase in the eutectic structure and the content of primary α-Nb.In short,the results indicated that the micro-structure of Nb-Ti-Co alloy could be controlled by directional solidification,and their mechanical properties could be improved effec-tively by using special preparation technology.
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