The Effect Of Variable Withdrawing Speed On Microstructure And Mechanical Properties For DZ125Alloy

By using different withdrawing rate to prepare DZ125alloy, we determined the creepproperties of the alloy and the strain rate during steady state creep after the standard heattreatment, calculated the creep activation energy and stress exponent of alloy at differentwithdrawing rate within a certain temperature range.The results shown that as the withdrawing rate increased, the primary dendrite spacing onthe cross section vertical to the direction of crystal grows in the alloy decreased gradually,secondary dendrite spacing on the longitudinal section parallel to the direction of crystal growsgradually increased.Determination of the distribution of different elements show that the segregationcoefficient of W, an interdendritic segregation element, decreased with the withdrawing rateincreasing; the segregation coefficient of Co, dendrite segregation element, fluctuated around acertain value with small fluctuation as the withdrawing rate increased; the segregationcoefficient of Hf, an interdendritic segregation element, reduced with the withdrawing rateincreasing.Withdrawing rate had no influence on the phase composition in the alloy, but wouldseriously impact alloy microstructure, as the withdrawing rate increased, the carbide in thealloy transited from large sheet into granule. As the withdrawing rate increased, γ’ phase in thecast alloy gradually became smaller, while the number of the eutectic phases and eutecticphase size in the alloy did not changes much. When the withdrawing rate was7mm/min, therewas significantly large microporosity in the alloy.The microstructure observation shown that carbide size in the as-cast alloy was larger,after the standard heat treatment, large sheet carbides in alloy would become massive withravines, carbide partially dissolved, and the remaining transited into granular, the granularcarbide distributed discontinuously along the grain boundary, increasing grain boundarysliding resistance, impeding dislocation movement, suppressing the grain boundary sliding,therefore improving the alloy creep strength.During high temperature creep, the alloy possessed variable strain rate and creep life atdifferent withdrawing rate. Under test temperature and stress conditions, the creep activationenergy of the alloy, at different withdrawing rate of2mm/min、4mm/min、6mm/min and 7mm/min range at980℃~1000℃, would be76.39kJ/mol,96.74kJ/mol,114.61kJ/mol and98.23kJ/mol, respectively.