Creep Behaviors And Effect Factors Of A Single Crystal Nickel-Based Superalloy With Various Orientations

In this dissertation, by means of creep tests and microstructure observations, the creep behaviors and effect factors of a single crystal nickel-based superalloy with various orientations are studied. By means of lateral pre-compressive treatment, the effect of γ’-phase pre-rafting on creep behavior of the alloy is studied. And, by means of configuration observations and diffraction contrast analysis of dislocations, the deformation mechanism of the [001]-oriented alloy during creep at intermediate temperatures is studied. The main conclusions can be drawn as followings.After full heat treatment, the microstructure of the alloy with various orientations is cuboidal γ’phase embedded coherently in γ channels and arranged regularly along<100> orientations. Thereinto, in the [011]-oriented alloy, there exist "roof" channels (γr(001) and γr(010)) arranged at 45° angle relative to the [011] orientation, and "gable" channel γg(100) parallel to the [011] orientation. While in the [111]-oriented alloy, there exist "semi-roof" channels γsr(001), γsr(010) and γsr(100) at about 55° angle relative to the [111] orientation. From room temperature to 1040℃, the misfit between the γ’/γ phases in the alloy is negative, and as temperature rises, the misfit increases.During steady-state creep at 1040℃/137 MPa, the γ’phase in the [001]-oriented alloy is transformed into mesh-like lamellar rafts perpendicular to the stress axis. The γ matrix phase fills in between the γ’rafts. In the [011]-oriented alloy, the γ’phase is transformed into one-dimensional stripe-like rafted structure along [001] direction. The γr(001) channel disappears, while the γr(010) and γg(100) channels are retained. In the [111]-oriented alloy, the γ’ phase is transformed into two-dimensional lamellar structure parallel to (010) crystal plane, and only the γsr(010) channel is retained. During high-tempreture creep under applied stresses, the lattice expanding and contracting on different crystal planes are important reasons for the configuration evolution of γ’phase. During steady-state creep at 1040℃/137 MPa, the deformation mechanisms for the alloy with various orientations are dislocations moving in γ matrix channels. Thereinto, dislocation climb is the main deformation mechanism of the [001]-oriented alloy, dislocation slip in the γr(010) channel is the main deformation mechanism of the [011]-oriented one, and dislocation slip and cross slip in the γsr(010) channel is the main deformation mechanism of the [111]-oriented one.During steady-state creep at high temperatures, the sequence of the internal friction stresses of the alloy with various orientations is σi[001]>σi[111]>σi[011].The creep resistance of the alloy is [001]> [111]> [011]. Thereinto, the effective creep activation energy of the [001]-oriented alloy is Qe[001]=281.32 kJ/mol, indicating that dislocation climb accompanied by element diffusion is the main deformation mechanism of the alloy during steady-state creep. The mesh-like lamellar γ’ rafts parallel to (001) crystal plane formed in the alloy possessing big resistance for dislocation movements is the main reason for the alloy having the best creep resistance. The effective creep activation energies of the alloy with [011] and [111] orientations are Qe[011]-146.87 kJ/mol and Qe[111]=182.61 kJ/mol. Thereinto, the small resistance for dislocation slip and cross slip in y matrix channels is the main reason for the alloy with the two orientations has poor creep resistance.During steady-state creep of the [001]-oriented alloy at 760℃/760 MPa and 800℃/650 MPa, dislocations shearing into γ’ phase can be decomposed to form the configuration of "a/3<112> partials+ superlattice intrinsic stacking faults (SISF)+a/6<112> partials". Thereinto, a/3<112> partials expand into the γ’phase, while a/6<112> partials are retained at γ’/γ interfaces, and SISF locates between the above two partials. It is determined that at 800℃ the stacking fault energy of the alloy is 89.9 mJ/m2, and the threshold stress for a/3<112> partial dislocations shearing into γ’ precipitates at this temperature is 650 MPa.During creep of the [001]-oriented alloy at 760℃ and 800℃, super dislocations shearing into γ’ phase can cross slip from{111} to{100} crystal planes to form K-W locks with non-plane dislocation core structure, which is one of the main reasons for the good creep strength of the alloy under the conditions. While at 850℃/500 MPa, some a<110> super dislocations shearing into γ’ rafts can be decomposed to form the configuration of "(a/2)<110> partials+ antiphase boundary (APB)", and high-temperature thermal activation makes the dislocations in K-W locks re-activated and re-cross-slip to{111} plane, which is the main reason for the disappearance of K-W locks at 850℃.For the [001]-oriented alloy, after being pre-compressed for 38 h along [100] orientation at 1040℃/180 MPa, the cuboidal γ’phase has been transformed into stripe-like rafts parallel to the [100] orientation. Dislocation slip and climb in y matrix channels are the main deformation mechanism for the alloys with and without pre-compression during steady-state creep at 980℃/200 MPa. Compared to the alloy without pre-compression, the microstructure of the pre-compressed alloy has low resistance for dislocation movements, which is the main reason for the pre-compressive treatment decreasing the creep resistance of the [001]-oriented alloy.After being pre-compressed for 38 h along [100] orientation at 1040℃/180 MPa, the γ’ phase in the [011]-oriented alloy has been transformed into stripe-like rafted structure along [100] orientation, which remarkably improves the creep strength of the [011]-oriented alloy. Thereinto, the creep lives of the alloy at 1040℃/137 MPa and 850℃/400 MPa are enhanced by about 123% and 15 times respectively. In the pre-compressed alloy, micro-bottleneck-like channels between γ’ rafts and labyrinth-like microstructures form, which increase the resistance for dislocation movements, and are the main reasons for the pre-compression enhancing the creep resistance of the alloy. Compared to the alloy without pre-compression, in dendrite/inter-dendrite regions of the pre-compressed alloy, the γ’ phase has bigger size, and the y matrix channels have smaller size, which increase the distance of dislocation climb and decrease the possibility of dislocation bowing out. Moreover, more slip systems activated in the pre-compressed alloy during creep result in more obvious strain hardening effect, which is also the important reason for the alloy possessing better creep resistance.