The fracture toughness of SA508-III steel was studied in the temperature range from room temperature to 320°C using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness(JQ) of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of JQ were 517.4 k J/m2 at 25°C and 304.5 k J/m2 at 180°C, respectively. Dynamic strain aging(DSA) was also observed to occur when the temperature exceeded 260°C with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260°C. Thus, the deformation resistance of the steel was improved and the JQ was enhanced.
Electric field treatment(EFT)was applied on GH4169 alloy during aging at 500-800 ? C to investigate the microstructure and property variation of the alloy under the action of EFT.The results demonstrate that the shortdistance di?usion of Al,Ti,and Nb atoms can be accelerated by EFT,which results in the coarsening of γ and γ phases.Meanwhile,lattice distortion can be caused by the segregation of Fe and Cr atoms,owing to the vacancy flows migrating toward the charged surfaces of the alloy.Therefore,the alloy is hardened by the application of EFT,even if the strength of the alloy is partly reduced,which is caused by precipitation coarsening.
Yao WangLei WangYang LiuXiu SongBei-jiang ZhangJin-hui Du
The coarsening behaviors of γ' and γ'' phases in GH4169 alloy aged at 1023 and 1073 K with electric feld treatment(EFT) were investigated by transmission electron microscopy(TEM) and positron annihilation lifetime spectroscopy(PALS).It is demonstrated that precipitation coarsening occurs,and the growth activation energies of γ' and γ'' phases can be decreased to 115.6 and 198.1 kJ·mol-1,respectively,by applying the electric feld.The formation of a large number of vacancies in the matrix is induced by EFT.Due to the occurrence of vacancy migration,the difusion coefcients of Al and Nb atoms are increased to be 1.6-5.0 times larger than those without EFT at 1023 or 1073 K.Furthermore,the formation of vacancy clusters is promoted by EFT,and the increase in strain energy for the coarsening of γ' and γ'' phases can be counterbalanced by the formation of vacancy clusters.
Lei WangYao WangYang LiuXiu SongXu-dong LüBei-jiang Zhang
SA508-Ⅲ steel was charged with different hydrogen(H) contents using a high-pressure thermal charging method to study the effects of H content on the tensile properties and evaluate the H embrittlement behavior of the steel. The results indicate that the ultimate tensile strength remains nearly unchanged with the addition of H. In contrast, the yielding strength slightly increases, and the elongation significantly decreases with increasing H content, especially at concentrations exceeding 5.6 × 10-6. On the basis of fractographic analysis, it is clear that the addition of H changes the fracture mode from microvoid coalescence to a mixture of river patterns and dimples. Carbides are strong traps for H; thus, the H atoms easily migrate in the form of Cottrell atmosphere toward the carbides following moving dislocations during tensile deformation. In addition, stress-induced H atoms accumulate at the interface between carbides and the matrix after necking under three-dimensional stress, which weakens the interfacial bonding force. Consequently, when the local H concentration reaches a critical value, microcracks occur at the interface, resulting in fracture.
