Study On Co-Al-W Directional Solidification Structure Of New Type Co-based Superalloy Under Magnetic Field

In the past decade,the high temperature stable ?' phase Co3(Al,W)found in Co-Al-W-based superalloys has become a hot research topic in the field of superalloys.In this paper,Co-8.8Al-9.8W and Co-8.8Al-9.8W-2Ta alloys has been investigated under transversal stability magnetic field and longitudinal magnetic field in directional solidification.To study the changes on the macrostructure of the alloy,the solid/liquid interface shape and macro-segregation have been investigated.The longitudinal magnetic field deeply influenced the interface shape and solidification structure of Co-8.8Al-9.8W alloy in directional solidification.At a low growing speed(R?10um/s),after applying the longitudinal magnetic field.,the solid-liquid interface becomes smoother and the dendrite length turns thicker and the number is reduced with the increase of longitudinal magnetic field intensity(B?1T).When the longitudinal magnetic field intensity continues to increase,the dendrite length turns thinner and the number increase.Moreover.When the longitudinal magnetic field intensity is small,the solute distribution has been changed by thermal electromagnetic at the forefront,secondary dendrite developed.When the longitudinal magnetic field intensity is large enough,the thermoelectric magnetic force can twist off the dendrite.In the process of directional solidification of Co-8.8Al-9.8W-2Ta alloy,the dendrites are deformed from the steady growth of the columnar crystals with the increase of the magnetic field strength at the low drawing rate.The thermo-electromagnetic convection causes the dendrites to break.This is because the added Ta element is a positive segregation element,exacerbating the segregation behavior during the solidification of the alloy,so the solid-liquid interface is different from the ternary Co-Al-W alloy.When the drawing rate is larger(R?50um/s),the magnetic field has little effect on the solid-liquid interface of the two alloys.The primary dendritic spacing is consistent with the law of low pulling speed,and increases with the increase of the magnetic field after the smaller.For the segregation behavior in the longitudinal magnetic field,the two alloys are basically the same.The segregation behavior of the two alloys in the magnetic field is mainly based on the Al element,and it can be seen from the experimental results that the solute Al content decreases with the increase of the solid phase ratio Also increased.While the W element segregation,the magnetic field of its impact is small.The transverse steady magnetic field has different effects on the directional solidification structure and interface morphology of Co-8.8Al-9.8W alloy.In the process of directional solidification of Co-8.8Al-9.8W alloy,a transverse steady magnetic field was applied and it was found that in the case of low tensile speed(R = 5um/s),the solid-liquid interface was recessed to the right side of the sample and produces solute Al on the right side.Besides the patchy tissue appears on the right side of the sample.This should be attributed to the solute migration caused by the thermoelectric magnetic convenction(TEMC)on the specimen scales and the dendritic scales induced by the magnetic field during the directional solidification,and the effect of the TEMC is different in each scale.The effect of transverse magnetic field on the primary dendrite spacing during directional solidification is further analyzed.It is found that the increase of magnetic field intensity can reduce the primary dendrite spacing.When the casting speed is increased to 50um/s,the magnetic field of the alloy solid-liquid interface is almost no effect,the primary dendrite spacing with increasing magnetic field strength has not been refined,this is because the pumping speed is faster,the transverse magnetic field caused by the thermal convection between the dendrite TEMC action time becomes shorter,so the effect of low.