Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： Low cycle fatigue tests for different strain ranges were conducted for 316L stainless steel at 600℃ under uniaxial loading. The results show that the dynamic strain aging (DSA) can be observed for three strain ranges. The concept of stress drop has been introduced to characterize the degree of serrated yielding for different strain ranges. The difference in the serrated yielding amount for different strain ranges can be attributed to the different interactions between solute atoms and dislocations. The maximum stress drop is related to the cyclic hardening or cyclic softening. DSA is related to the number of cycles. The material presents the obvious DSA for a few cycles and then followed by weak serrated yielding, even disappearing. However, the serrated yielding can be observed again before fatigue failure. The difference of serrated yielding can be attributed to the types of atom atmospheres at different cycles. Atype serrated wave was observed for smaller strain range, however, types of A, B, A+B, C, and B+C serrated wave can be found for different cycles and different phases in one cycle for larger strain range. Finally, the crack source region and crack propagation region of the fatigue fracture were observed by SEM.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Ni-based single crystal (SC) superalloys have been widely used to produce turbine blades of aeroengines, but under the action of centrifugal force, creep damage is still the main failure mode. In service, the blades experience multiple cycles of various conditions of high temperatures, low stresses and intermediate temperatures, high stresses, and due to effective and efficient means of cooling and insulating the blades during operation, the actual temperature the blades bear can be smaller than the working temperature at the hot ends of aeroengines, so the systematical study on the creep behavior of SC superalloys at intermediate temperatures, high stresses is significant. It is generally considered that dislocations cutting g′ phase is the main deformation mechanism of SC alloys at intermediate temperatures, high stresses, and dislocations cutting into g′ phase can be decomposed into different configurations for different alloy systems, even under similar conditions. Moreover, large amount of dislocations cutting into g′ phase means the degradation of creep performance of the alloys, so it is significant to study the cutting modes of dislocations. In this work, by means of creep tests, TEM observations and diffraction contrast analysis of dislocations, the deformation mechanisms of a Ni-based SC superalloy during steady-state creep at intermediate temperatures, high stresses are studied. Results show that, under the conditions of 760 ℃, 760 MPa and 800 ℃, 650 MPa, dislocations cutting into g′ phase are decomposed to form partial dislocations plus superlattice intrinsic stacking faults (SISF). Thereinto, the leading a/3<112> super Shockley partial dislocations cut into g′ precipitates, while the dragging a/6<112> Shockley partial dislocations remain at g′/g interfaces, and between them there exists SISF. Additionally, super dislocations shearing into g′ phase can cross slip from {111} to {100} crystal planes to form Kear-Wilsdorf (K-W) locks with non-plane dislocation core structure, which can inhibit the slip and cross slip of dislocations to enhance the creep strength of the alloy. At 850 ℃, 500 MPa, stacking faults disappear in the alloy, and some a<110> super dislocations cutting into g′ rafts can be decomposed to form the configuration of two partial dislocations with Burgers vector of a/2<110> plus antiphase boundary (APB), and K-W locks are released for high-temperature thermal activation results in the cross slip of dislocations from cubic slip systems to octahedral ones.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Single crystal nickel-based superalloys mainly consisting of g and g ' phases are widely used in turbine blades for their excellent mechanical performances, in particular creep resistance at high temperatures. The creep properties of single crystal superalloys are inherently anisotropic. Although commercial single crystal superalloys are all along <001> orientation, it is difficult to ensure the exact orientation during production, and in service the applied centrifugal force on blades unavoidably deviates from <001> orientation, so the study on non-<001> oriented single crystal superalloys is significant on improving the creep theory of superalloys. Compared with the <001> oriented single crystal superalloys, the <011> oriented ones generally display poor creep resistance. Through pre-rafting treatment of g ' phase in <001> oriented single crystal superalloys, the creep resistance of the alloys are expected to be enhanced by changing the deformation mechanism, especially by hindering dislocation movements, but the effect of the treatment on <011> oriented alloys is still not clear. For this reason, by means of pre-compression, creep tests and microstructure observation, the effect of the pre-rafting of g ' phase on creep behavior of a [011] oriented single crystal nickel-based superalloy at 1040 ℃, 137 MPa is studied. Results show that after pre-compression at 1040 ℃, 180 MPa along [100] orientation for 38 h, the g ' phase in the [011] oriented alloy has transformed into P-type rafted structure parallel to the compressive stress axis. Different strain energy density distribution and lattice strain on crystal planes during pre-compression are the main reasons for the rafting behavior of g ' phase. Dislocations slipping in g matrix channels is the main deformation mechanism of the [011] oriented alloy with and without pre-compression. Thereinto, many dislocations-climbing movements occur in the pre-compressed one. The lateral pre-compression remarkably improves the creep strength of the alloy at 1040 ℃, 137 MPa. The elimination of gable channels, relatively narrow roof channels during steady-state creep stage, the formation of micro-bottleneck-like channels and labyrinth-like microstructures due to the lateral growth of g ' rafts, and the more effectively activated slip systems of dislocations are beneficial to impeding dislocation movements and responsible for the better creep resistance of the pre-compressed alloy.
Hits
Hits
Downloads
Comment