Tensile Properties of DD15 Single Crystal Superalloy at Different Temperature

DD15 single crystal nickel-based alloy was prepared by a directional solidification induction heating furnace and its tensile properties were investigated at the 23°C, 400°C, 800 °C, 1100°C and 1140°C. The tensile fracture mechanism of the alloy at different temperature was investigated. Results indicate that tensile strength of the alloy reduces first, increases afterward, and decrease sharply at last as the test temperature rises, while the tensile plasticity exhibit the reverse change trend. The extension strength of the alloy is superior to that of EPM-102 alloy. At 23°C, 400°C and 800°C, the tensile fracture mode of the alloy is quasi-cleavage mechanism, and at 1100°C and 1140°C that is dimple model. As tensile fracture at 23°C, 400°C and 800°C, the γ′ precipitate nearly keeps cube shape, while that becomes rectangular after tensile fracture at 1100°C and 1140°C. The APB and stacking fault can be seen in the γ′ phase at 23°C, 400°C and 800°C. There is dislocation networks at the γ/γ′ interface and the deformation characteristic is dislocation by-passing mechanism at 1100°C and 1140°C.


Introduction
Due to the excellent comprehensive properties of Nickel-based single crystal alloys at high temperature, they have become the main preparation materials for advanced aero engine turbine blades [1]   .Tensile properties are regarded as a important index to characterize the overall performance of the alloy and could provide possible prediction for creep and fatigue properties [2] .The γ′ morphology and content, crystal direction deviation of the alloys, service temperature and strain rate have a great influence on their tensile properties [3][4] .There have some differences in the tensile behavior, microstructure evolution and dislocation configuration with different single crystal superalloys [2,[5][6] .The tensile behavior of low generation superalloys had been investigated in recent years [7][8][9][10][11][12] .However, there are relatively less systematic study about tensile properties at different temperature for the fourth generation single crystal alloy [5,6] .DD15 alloy studied was invented by BIAM [13][14] .In order to improve development and application of the alloy, the tensile properties tensile fracture mechanism were investigated at 23℃, 400℃, 800℃, 1100℃and 1140℃.

Experimental procedure
The element content dimension of DD15 alloy are shown in Table 1.The size of single crystal superalloy sample is Φ15mm×160mm.Samples were prepared using crystal selection ways in the high temperature gradient directional solidification furnace.The crystal direction deviation of specimens is determined using X-ray instrument.Their deviations were kept within 15 degree of [001] direction.All samples were treated with a solid solution(1335℃/6h, AC) and two-stage aging (1140℃/4h, Air Cool + 870℃/20h, Air Cool).Then the specimen with the size of Φ12mm×71mm for tensile experiment was processed.The tensile properties were tests at 23℃, 400℃, 800℃, 1100℃ and 1140℃ in air.Average values were measured three times in each temperature condition.The appearance of fracture of the alloy was observed under scanning electron microscope.The tensile fracture dislocation microstructure was observed using transmission electron microscopy.

Tensile properties
The tensile strength and plasticity at different temperatures of DD15 alloy is demonstrated in Fig. 1.It is shown that tensile strength demonstrates a complicated trend with rising temperature.The yield and ultimate strength of the alloy all decrease first, rise afterward, and reduce sharply at last with rising temperature.Compared changes in extension strength with test temperature rises, the plasticity almost exhibit reverse change trend.DD15 alloy has relatively high yield strength, maximum ultimate strength and minimum plasticity at intermediate temperature.It shows that the alloy has almost the same abnormal yield characteristic with other single crystal alloys [8,9] .he approximate value of yield and ultimate strength of EPM-102 alloy at 1093℃ obtained from the Fig. 2. The tensile strength comparison of DD15 alloy and EPM-102 alloy is shown in Table 2.The tensile strength of DD15 alloy are superior to that of EPM-102 alloy.The main reason why the two alloys have different tensile properties is that they have different alloying element content, especially the solid solution element content is different.3.2.The fracture appearance Fig. 3 exhibits the fracture surface of DD15 alloy after tensile at different temperature.At 23℃ and 400 ℃ , both of fracture surfaces are macroscopically elliptical shape and there are a number of irregular fluctuation at the fracture.There are many river patterns at the fracture surface tensile at 23.However, there are a lot of cleavage plane and cleavage step at the fracture surface tensile at 400℃.At 800 ℃ , there is almost one plane for the fracture surface.The angle between the plane and [001] orientation is about 45°, indicating that crack spread on the {111} planes.Some river patterns can be seen on the fracture plane.So the tensile fracture mechanism at 23℃, 400℃ and 800℃ all illustrates qusi-cleavage fracture.Easy to operate slip system of alloy is {111} <110> at low or moderate temperatures because DD15 alloy has a face-centered cubic structure.So the crack is growing at {111} crystal face.It has been reported that single crystal superalloy may exhibit quasi-cleavage rupture mechanism at low and moderate temperatures [10] .The main feature of the fracture surfaces tensile at 1100℃ and 1140℃ is dimples and it demonstrates a ductile fracture (Fig. 3(g) ~(j)).There are small microporosities at center of tetragonum dimples.The microporosity is location of the crack origin and caused the fracture to occur.Therefore, the extension rupture of DD15 alloy at 1100℃ and 1140℃ all illustrates dimple model.

γ/γ′ microstructures
Fig. 4(a) shows the microstructure of the sample after complete thermal treatment.It can be seen that cubic γ′ precipitate inserted coherent from γ phase.The γ′ phase size is about 200nm~400nm.Its volume content is about 60%~70%.The microstructure 5mm from rupture surface on the longitudinal cross-section of the sample ruptured at various temperature was observed by SEM.It shows that the γ′ phase morphology have a relatively small variation and almost remain to keep cubical shape rupture under 23 ℃ , 400 ℃ and 800 ℃ .The vertical γ phase channel turns thinner and horizontal γ phase channel turns thicker at 980℃ and 1100℃.The γ′ phase extended in the tensile stress orientation.The γ′ precipitates are no longer cubic and some of them become a rectangle or an oval.As tensile temperature rises, the γ phase breadth increases.The γ′ phase was longitudinally merged, but did not form rafts as in other single crystal superalloys containing 3%Re [2] .It indicates that DD15 alloy has very strong resistance to deformation under elevated temperature.

Dislocation configuration
The Fig. 5 shows the dislocation microstructure of the sample deformed at various temperature.The initial dislocation density is extremely low and almost no dislocation is observed for undeformed sample (Fig. 5(a)).Fig. 5(b) illustrates the dislocation morphology after deformed at 23 ℃ .The morphology of the γ′ particles is still maintained.There is no slip bands observed.So the plastic deformation takes place quite homogeneously.A lot of dislocations can be seen in the γ phase and γ′ particles.The mainly deformation characteristics is cutting γ′ precipitate with a/2〈110〉dislocation pairs (blue short arrows) limited to octahedral planes [16] .In Wang et al. work [6] about fourth generation single crystal alloy, however, the stacking faults defects rather than parallel slip lines were observed at room temperature.The dislocation of the sample fracture at 400℃ (Fig. 5(c)) is obviously resemblance with that ruptured at 23 ℃ , the distinction may be that there are considerably less dislocations and the dislocation separation distance is relatively far.It is verified that most of dislocations watched under lower temperatures can be a/2〈110〉{111}.Dislocations cut the γ′ phase with compactly spaced pairs for the sake of least value of APB (green short arrows) area produced from a/2〈110〉displacement of the superlattice.At 800 ℃, the dislocation is inhomogeneous as shown in the Fig. 5(d) .A dense dislocation forms in the γ matrix access and there are a lot of stacking faults (red short arrows) along two orientation, a small number singe dislocation and dislocation pairs in the γ′ phase.It is formed by the γ phase dislocation dissociation with the subsequent reaction: a/2〈110〉→a/3〈112〉+ a/6〈112〉 [8] .The a/3〈112〉partial cuts into the γ′ phase, producing a SISF, but the a/6〈112〉dislocation maintains at interface, fixed with APB which generate if it passed into γ′ phase [17] .This morphology can be often seen in the creep rupture sample at moderate temperature [18] .Compared with the cutting γ′ phase at the lower and moderate temperatures, the morphology at high temperatures is formed mainly by dislocations movement surrounding the γ′ phase.This demonstrates that the deformation mechanism has been converted to by-passing from cutting.There are inerratic dislocation networks in γ/γ′ interfacial plane because of different slip dislocation reaction.The dislocation network may be nearly observed in every γ′ particles.These dislocation networks are very stable and could strongly impede the dislocation from shearing γ′ precipitates in the later tensile deformation.Moreover, the morphology of the γ′ phase is no longer cubic.

Conclusions 1)
The tensile strength reduces first, increases afterward, and decrease sharply at last as the temperature increase, while tensile plasticity exhibit the reverse change trend.The tensile strength of DD15 alloy is superior to that of EPM-102 alloy.
2) At 23℃, 400℃ and 800℃, the tensile fracture model of the alloy is quasi-cleavage mechanism, and that is dimple model at 1100℃ and 1140℃.
4) The APB and stacking fault can be seen in the γ′ phase at 23 ℃ , 400 ℃ and 800 ℃ .There is dislocation networks at the γ/γ′ interface and the deformation characteristic is dislocation by-passing mechanism at 1100℃ and 1140℃.

Fig. 1
Fig.1 Variation of tensile properties of DD15 alloy with temperatures (a) ultimate strength; (b) yield strength; (c) contraction of area; (d) elongation

Fig. 5
Fig.5 Dislocation morphology of samples fracture at different temperature (a) undeformed; (b) 23℃; (c) 400℃; (d) 800℃; (e) 1100℃; (f) 1140℃ Fig.5(e) and 4(f) illustrates the dislocation microstructure of the sample tensile at 1100℃ and 1140℃.Compared with the cutting γ′ phase at the lower and moderate temperatures, the morphology at high temperatures is formed mainly by dislocations movement surrounding the γ′ phase.This demonstrates that the deformation mechanism has been converted to by-passing from cutting.There are inerratic dislocation networks in γ/γ′ interfacial plane because of different slip dislocation reaction.The dislocation network may be nearly observed in every γ′ particles.These dislocation networks are very stable and could strongly impede the dislocation from shearing γ′ precipitates in the later tensile deformation.Moreover, the morphology of the γ′ phase is no longer cubic.