Dynamic recrystallization behavior and coincidence site lattice evolution in thermal deformation of 316H stainless steel used in nuclear systems

The hot deformation behavior of 316H stainless steel used in the 4th-generation nuclear systems was investigated by thermal compression tests at 1000–1150 C and 0.01–10 s^(-1).It was found that true stress firstly increased and then decreased with the increasing strain rate with a threshold of 1 s^(-1).Electron backscatter diffraction was used to analyze the microstructure evolution.Discontinuous dynamic recrystallization(DDRX)was the dominant dynamic recrystallization(DRX)mechanism,while continuous dynamic recrystallization(CDRX)was the supplementary one.DDRX happened before CDRX and provided additional nucleation sites for the latter.Twin grain boundaries(R3)appeared in DRX grains due to growth accidents.As the length fraction of R3 increased,the coincidence site lattice(CSL)boundary transition began to occur,forming R9 and R27.After the occurrence of full DRX,the growth and annexation of DRX grains were easy to be promoted,in which progress both equiaxed grains and CSL boundaries disappeared.The ideal deformation microstructure with fine and uniform DRX grains,which was accompanied by a high length fraction of CSL boundaries,appeared at 1000℃–0.01 s^(-1),1050℃–0.01–0.1 s^(-1),1100℃–0.1–1 s^(-1) and 1150℃–1–10 s^(-1).That is,the deformation conditions mentioned above were the preferable thermal forming parameters for 316H stainless steel in actual productions.

Temperature dependence in tensile properties and deformation behavior of GH4169 alloy

The effect of temperature on the tensile properties and deformation mechanism of GH4169 alloy has been systematically studied over a wide range of room temperature(RT)to 1000℃.The results indicate that the stress–strain curve of the alloy shows serrations at 200–600℃,and the character of the serrations changes from type A to type B and then to type C at different temperatures.The ultimate tensile strength of the alloy decreases gradually from RT to 600℃.The yield strength decreases slowly from RT to 700℃ but decreases rapidly above 800℃.Transmission electron microscopy analysis relieves that the primary deformation mechanism of the alloy below 500℃ is Orowan bypass mechanism.At temperatures between 600 and 700℃,the coordinated deformation of twins and cross-slip of dislocations are activated.The transformation of\upgamma^{\prime\prime}phase toδphase above 650℃ will decrease the strength.The primary deformation mechanism above 800℃ transforms into the repeated shearing of\upgamma^{\prime\prime}by dislocations to form multiple stacking faults.Recrystallized grains were observed above 800℃,and continuous dynamic recrystallization and discontinuous dynamic recrystallization were observed.The stress concentration caused by Nb-rich carbides is the cause of intracrystalline crack nucleation.At 700℃,grain boundary crack sprouting is caused by the combined effect of slip band impact on grain boundaries and grain boundary dislocation plugging.The relationship between the serrated flow behavior and the deformation mechanism has been discussed based on the experimental results.