Type and Size of V-Containing Phases during Preparation of Al-4％V Master Alloys with Cooling Rates
Vanadium(V)is an important trace element that influences the microstructures and properties of aluminum alloys.To avoid mass loss at high temperatures,V is usually added to aluminum alloys by Al-based master alloys.However,different V-contain-ing phases were present in Al-based master alloys,resulting in quite different effects and evolution in their subsequent addition pro-cess.The formation of V-containing constituents in Al-based Al-V master alloys is closely related to the preparation process,so the ef-fects of process parameters(cooling rate,melting temperature,etc.)on the V-containing phases in Al-based Al-V master alloys need to be studied.Therefore,in this work,by using a wedge-shaped water-cooling copper mold,graphite mold and refractory mold,Al-4％V master alloys with different cooling rates were produced at 1150 ℃.In addition to the samples being cut from the center of ingots prepared by the graphite mold and refractory mold,more samples were cut from different positions of the wedge-shaped ingot,which represented different cooling rates.The cooling curves inside the cavities of these molds were obtained to determine the mean solidifica-tion cooling rate(vs).Scanning electron microscopy(SEM)equipped with an energy dispersive spectrometry detector(EDS)and transmission electron microscopy(TEM)were used to determine the types of V-containing phases.A 3-dimentional(3D)laser scan-ning microscope was used to observe the microstructures,and then the area fraction,mean area and the variation in size of different V-containing phases were counted quantitatively.As a result,the effects of vs,ranging from 2.4～1030.8 ℃·s-1,on the proportion,mean size and size distribution of V-containing phases during the preparation of Al-4％V master alloy were quantitatively investigated by the contrast of microstructures,and the formation of V-containing phases was analyzed based on thermodynamics and dynamics.The re-sults showed that the proportion of Al10V phases decreased dramatically to～0 with vs increasing at first,which meant that the increase in vs inhibited the formation of Al10V phase.The minimum value of vs to constrain the formation of Al10V phase completely was 120.4 ℃·s-1,and inhibition occurred continuously at 120.4～308.5 ℃·s-1,resulting in the availability of Al3V phases only.However,Al10V phase would form again when vs was higher than 308.5 ℃·s-1,and its proportion increased with vs increasing.The change in Al3V phases in the area fraction was almost the opposite to that of Al10V phases with increasing vs,and the maximum area fraction(14.56％)of Al3V phases was present at vs value of 224.5 ℃·s-1.Once vs was higher than 308.5 ℃·s-1,the area fraction of Al3V de-creased significantly,but that of Al10V increased dramatically with increasing vs,and Al3V phase almost disappeared when vs was high-er than 698.4 ℃·s-1.The reason should be that the metastable Al3V phase did not have enough time to form from 985 to 670 ℃ owing to the large solidification cooling rate,and then leading to the supersaturation state of the melt.As a result,Al10V phase,as the stable phase,should be precipitated largely and quickly below 670 ℃ according to Al-V binary equilibrium phase diagram.Additionally,the size of Al10V phase was quite small at this moment due to the largevs.vs influenced the sizes of both Al3V and Al10V phases obviously as well,which meant that both Al3V and Al10V phases could be refined by the increase of vs.When vs was lower than 54.0 ℃·s-1,the sizes of both Al3V and Al10V phases were quite large,and the maximum mean area of Al10V and Al3V phase was as large as 409.5 and 208.0 μm2,respectively.The refining effect of increasing vs on Al3V phase was larger than that for Al10V phase at this moment(vs≤54.0 ℃·s-1).However,when vs increased to higher than 54.0 ℃·s-1,the refining effect of increasing vs on Al3V phase decreased significantly,but the increase in vs could refine Al10V phase largely instead.The minimum mean size of Al10V phase was as small as 4.1 μm2.When both the Al10V phases and Al3V phases are available in the ingot,the size of the Al10V phase is much larger than that of the Al3V phase.Un-der every condition of vs,the distribution of both Al10V phase and Al3V phase in size corresponded to the logarithmic normal distribu-tion function.The effect of vs on the fluctuation of Al10V phase in size was larger than that of Al3V phase.The size variation of Al10V grew large with the decrease in vs,but the size fluctuation of Al3V phase increased first and then decreased with the increase in vs,lead-ing to the largest fluctuation for Al3V phase in size at the solidification cooling rate of 120.4 ℃·s-1.
master alloyV-containing phasesolidificationcooling ratesize distribution
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