Segregation behavior of alloying elements in different oriented single crystal nickel based superalloys

doi:10.1016/j.matlet.2009.09.035

Materials Letters

Volume 63, Issue 30, 31 December 2009, Pages 2635-2638

https://doi.org/10.1016/j.matlet.2009.09.035 Get rights and content

Abstract

The micro-segregation behavior of different oriented single crystal nickel based superalloys in directional solidification has been explored. The results show that the segregation level in the dendrite structure has relationship with the crystal orientation. In general, with increasing deviation angle from the <001> orientation, the segregation coefficients of Al, Ti and Ta increase, and the segregation coefficients of W and Co decrease. The segregation degree of Al, Ta, W and Co increases with the misorientation in the <011> oriented crystals, except for Ti. Segregation of Mo and Cr is less affected by the orientation variation. Different misorientations also lead to different segregation profiles. The crystal with smaller deviation angle from the <001> orientation has a shorter segregation distance.

Introduction

Nickel based single crystal superalloys are typically used in turbine blades where high temperature strength and creep resistance are required. The composition of the superalloy is so complex that there are more than seven alloying elements in the first generation superalloys. With alloy development, more refractory elements are added into the new superalloys, e.g. Re and Ru [1], [2], [3]. Each of these additions can affect the as-cast microstructure and performance. However, after further alloying elements were added, the segregation in the casting is more severe and the segregation profiles are much more complex [4], [5].

One of the inherent characteristics during directional solidification is the solute redistribution between the solid and the liquid. It can be described by segregation coefficient k′, defined as the ratio of the concentration of the elements in the dendrite core to that in the interdendritic region [6], [7]. The segregation coefficient depends on processing parameters and growth orientations. Previous works have found that the cooling rate has great influence on the segregation coefficient [8], [9]. Nickel based single crystal superalloys are face centered cubic metals with a significant anisotropy. Recent research by Ma and Grafe [10] found that the solution distribution of alloying elements along the <001> and the <011> direction across a dendritic cell is different. The crystal orientation would affect the degree and the kind of segregation due to the crystal growth anisotropy. Dendrites are the common microstructures of the blade alloys in engineering, so the solution distribution along different oriented dendritic crystals even with non-<001> orientations needs to be clarified.

In the present work, the segregation distribution behavior of alloying elements across dendrites with different crystallographic orientations was measured to indicate the influence of crystal orientation on the segregation behavior. The segregation profiles of different elements between adjacent dendrite cores in different orientation crystals were also investigated.

Section snippets

Experimental

The experiments were performed on a high temperature gradient directional solidification furnace. Single crystal superalloy AM3 with composition of Ni–7.82Cr–5.34Co–2.25Mo–4.88W–6.02Al–1.94Ti–3.49Ta–0.006C (wt.%) was used. Cylindrical seeds with different orientations were predetermined to obtain desired orientated crystals. The relationship of crystallographic orientation and the cylindrical direction in the seed was shown in a schematic diagram (Fig. 1). The withdrawal velocity was kept at a

Solute segregation in off-axis <001> oriented crystals

Fig. 2 shows the influence of the crystal orientations on elements segregation behavior. The misorientations (Φ) between the <001> direction and the cylinder axis of the crystals selected for evaluations are 4°, 8°, 11° and 13°. Among the elements that partition to the interdendritic region, Ti exhibited the strongest segregation degree. With an increased misorientation, the tendency of Al segregation to the interdendritic region decreased, but the segregation of Ta and Ti did not show monotone

Conclusions

The effect of orientation deviation on the segregation degree of alloying element and segregation profiles was investigated. Orientations distant from the <001> and <011> orientation had different segregation coefficients. An increase in the misorientation between the <001> direction and cylinder axis decreased the segregation levels of Al, Ta, Ti, W and Co. Increasing the deviation angle from the <011> direction, resulted in an increase in segregation of Al, Ta, W and Co and a decrease in

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant No. 50771081).

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Segregation behavior of alloying elements in different oriented single crystal nickel based superalloys

Abstract

Introduction

Section snippets

Experimental

Solute segregation in off-axis <001> oriented crystals

Conclusions

Acknowledgment

Acta Mater

Mater Lett

Mater Sci Eng, A Struct Mater: Prop Microstruct Process

Grain defect formation during directional solidification of nickel base single crystals

The microstructure of superalloys

JOM