Study On Structure Evolution Of Fe-based Peritectic Alloy During Sub-rapid Solidification

It is well known that peritectic alloys are widely used for engineering materials.However, the structure evolution under near-rapid solidification process remains to befurther research. In this dissertation, the suction casting method was used to prepareFe-Mn and Fe-Ni peritectic alloys strips for near-rapid solidification condition. Thestructure and phase formation within the alloy strips were analyzed by means ofoptical microscope （OM）, X-ray diffraction （XRD）, transmission electron microscope（TEM）. In addition, the composition distribution was characterized by using energydispersion spectrometer （EDS）. The purpose of this work was to study the effects ofperitectic reaction and solute on structural characterization of Fe-based peritecticalloys, which can meet the needs of industrialization development and highperformance engineering materials.The solidified microstructure of Fe-（11,13,15）wt.%Mn and Fe-（2.6,4.2,4.7,7.9）wt.%Ni strips consists of two regions at least: chilled layer, columnar zone. Thehyper-peritectic alloy of Fe-13Mn and Fe-4.7Ni show a number of equiaxed grains,while other strips, show no or little equiaxed zone. It is ascribed to the phasecompetition between and γ during the solidification process would lead to core forequiaxed grains nucleation.The relationship between primary dendrite spacing and cooling rate of form,λ=A （G_L·ν）^-1/3, also performs adequately in Fe-Mn and Fe-Ni strips. The solutedistribution shows the central area of micro-segregation is heavy than the near-surfaceregion for Fe-Mn and Fe-Ni alloy strips. Due to the primary phase of transformed toγ, Fe-15Mn strip and Fe-7.9Ni strip show less micro-segregation.The phase constitution of Fe-Mn alloy strips were analyzed by XRD. Only’-martensite （body center cubic） phase was observed in the strip when manganesecontent is11wt.%. As the the manganese content increases to13percent,ε-martensite（face center cubic） emerged in the central area of strip, which is more obvious than inthe near-surface region. Moreover, theε-martensite increased with the manganese content increases to15percent.Phase microstructure and morphology in Fe-13Mn alloy strip was investigatedby TEM. It can be seen that the mounts of dislocations in the near-surface region islarger than in the central area. The average width of a’-martensite is about157nm.On the other hand, the average width of’-martensite is wider in the central areawhich is about211nm. The width ofε-martensite in this area is about3.0nm¹0.8nm,with average width of5nm.The solidification structure evolutions of Fe-4.7Ni under the near rapidsolidification conditions was analyzed with dendrite growth theories. The resultsshowed that γ is easy to precipite when growth velocity exceed1m/s. On the otherhand, is easy to exhibit with a lower dendrite growth rate.