| Nickel-base C-276 alloy is a solid solution strengthening corrosion-resistant alloy belonging to Ni-Cr-Mo system.C-276 alloy is used as a key structural material applied in the nuclear power system due to its outstanding combination of corrosion resistance and mechanical properties,which has been paid more and more attention.This paper studied the solution treatment processes,hot deformation characteristic,intermediate and low temperature mechanical behavior as well as microstructure evolution of this alloy.The results obtained by solution treatment and room temperature mechanical property testing,indicated that the precipitated particles can be dissolved fast and austenite grain growth rate increased significantly at solution temperatures higher than 1150 °C.It was found that the evolution of precipitated particles has much more significant effect on the tensile properties compared to that of grain growth.The mathematical relationships between tensile properties and grain size of the alloy can be described well by the Hall–Petch relation.The Hall–Petch strengthening coefficient for yield strength of the C-276 alloy was 18.35 MPa mm1/2.Based on hot compression tests and dynamic materials model,processing maps at different true strains were established for the solutionized and aged C-276 alloy.The optimum hot deformation parameters at true strain 0.9 for both alloys,which were obtained by combining the processing maps with microstructural observation,were 1000~1220 °C/0.07~2.2 s-1 and 1025~1220 °C/0.07~2.5 s-1.The samples compressed in these two optimum hot working windows exhibited higher efficiencies of power dissipation,and dynamic recrystallization(DRX)grains have relatively lower grain average misorientation(GAM).Aging pre-treatment increased the DRX degree and efficiencies of power dissipation during hot deformation,and eliminated the instable domain in the processing maps at the strains of 20%~40%.It was revealed that discontinuous dynamic recrystallization is the dominant DRX mechanism for the C-276 alloy.When the hot deformation was performed at temperature of 1150 °C and strain rate of 0.1 s-1,both alloys have the largest number fraction of ?3 boundaries in comparison with that of other deformation conditions,and the corresponding DRX microstructures have the lowest average GAM.Dynamic strain aging(DSA)manifested in the form of various types of serrations was noticed in the C-276 alloy during tensile deformation at intermediate and low temperature.The average activation energy for serrated flow of the studied alloy was calculated as 132 k J/mol,which was in line with migration of Mo through pipe diffusion in a Ni matrix.The results obtained by theoretical analysis and experimental characterization indicated that the segregation of Mo atom to mobile dislocations is mainly responsible for the DSA at the temperatures tested.The C-276 alloy showed the typical intermediate temperature embrittlement(ITE),and exhibited a minimum elongation at 700 °C.It was found that decreasing strain rate reduced the lower limit of temperature,at which intergranular brittle failure occurred,as well as expanded the brittle temperature spectrum of the alloy.Negative strain rate sensitivity(m)was observed in the DSA regime.The occurrence of DRX could significantly increase the values of m during tensile deformation.It is found that the ITE of the C-276 alloy is attributed to the grain boundary segregation of sulfur and other impurities.The presence of large amounts of precipitated particles resulting from the aging pre-treatment significantly weakened serrated flow in the sensitive temperature spectrum of precipitation in this alloy,and could effectively eliminate the ITE of the C-276 alloy. |
PDF Full Text Request