Analysis of high angle boundaries in directionally solidified

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Analysis of high angle boundaries in directionallysolidified turbine blade made of CMSX-4regsuperalloyTo cite this article B Chmiela and M Sozaska 2011 IOP Conf Ser Mater Sci Eng 22 012008

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Analysis of high angle boundaries in directionally solidified

turbine blade made of CMSX-4reg superalloy

B Chmiela and M Sozańska

Department of Materials Science and Metallurgy Silesian University of Technology

40-019 Katowice Poland

E-mail bartoszchmielapolslpl

Abstract High angle boundary (HAB) and low angle boundary (LAB) are casting defects of

single crystal (SX) and directionally solidified (DS) turbine blades and decrease the lifetime of

these blades during service During directional solidification primary dendrite arms grow in

the opposite direction of the thermal gradient direction and perpendicular to the mushy zone

interface When this interface is not flat primary dendrite arms growing in various areas of the

mushy zone are characterized by different growth directions Then after the primary dendrite

tips contact each other LABs or HABs form (depending on the angle between the directions of

the primary dendrite arms) This paper presents characterization studies of HABs in a DS

turbine blade made of CMSX-4reg superalloy The blade was characterized by three columnar

grains with HABs Qualitative and quantitative analyses of the HABs using electron

backscatter diffraction in a scanning electron microscope (SEM-EBSD) were carried out The

EBSD technique helped to determine the crystallographic orientation of the grains near the

HAB misorientation angles between grains (and inside each grain) and the angle of deviation

between the [001] direction and the blade axis

1 Introduction

Modern aero engines have been improved based on increasing the thrust and thermal efficiency In

order to accomplish this improvement it is necessary to increase the operating temperature in the

combustion chamber Therefore single crystal (SX) and directionally solidified (DS) superalloy

turbine blades have been developed to replace the commonly used polycrystalline equiaxed turbine

blades [1 2]

Modern SX and DS turbine blades are produced by the Bridgman bottom seed method The

preferred crystallographic orientation of the seed is usually [001] In practise a deviation between the

blade axis and the [001] direction of up to 12deg is acceptable But to achieve the best mechanical

properties (especially fatigue strength under variable load conditions) the angle between the growth

direction and the [001] direction must be as small as possible [1 2] Furthermore in DS blades the

misorientation between neighbouring columnar grains should not exceed 15deg to avoid a high angle

boundary (HAB) Thus it is necessary to accurately monitor the crystallographic orientation during

directional solidification Unfortunately for many reasons many disturbances in the solidification

front take place and dendrites in different areas of the casting grow in different directions Then after

the primary dendrite tips contact each other low angle boundaries (LABs) or HABs form (depending

on the angle between the primary dendrite arm directions) [3] Incorrect orientation of the seed (or

heterogeneous nucleation by foreign particles [4]) can promote the growth of a stray grain thus

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf Series Materials Science and Engineering 22 (2011) 012008 doi1010881757-899X221012008

Published under licence by IOP Publishing Ltd 1

forming a HAB with the SX or causing too high a misorientation among the columnar grains in the DS

castings Because these defects have a negative effect on the mechanical properties of the SX and DS

blades they should be characterized in detail to understand the formation mechanisms and improve

the solidification process

This paper presents an analysis of HABs in an experimental turbine blade made of CMSX-4reg

nickel-base superalloy Qualitative and quantitative analyses of the many aspects of crystallographic

orientation were conducted using electron backscatter diffraction (EBSD) in a scanning electron

microscope (SEM)

2 Material and experimental procedure

An experimental DS turbine blade made of CMSX-4reg superalloy was used in this study The chemical

composition of the alloy is shown in table 1 The directional solidification process was performed in a

Bridgman-type furnace Three differently oriented seeds were used in the process

Table 1 Chemical composition of CMSX-4reg superalloy

Element Ni Cr Co Mo W Ta Ti Al Hf Re

Concentration (wt ) bal 65 9 06 6 65 1 56 01 3

The withdrawal rate was 000005 m s-1

The blade surface was etched in a solution of 14 cm3 HCl

21 cm3 H2O and 8 g FeCl3 to reveal columnar grains

Investigations of the microstructure and crystallographic orientation were performed on the one

cross-section of the upper part of the blade because the highest deviation from the preferred

orientation usually is found in this part [5] The blade was cut perpendicular to the main axis and the

obtained specimen was ground and polished according to a procedure that was slightly modified from

one described earlier [6 7] (diamond suspension grit sizes of 9 μm 3 μm 1 μm and 025 μm

005 μm alumina suspension) The specimen was etched in a solution of 100 cm3 HCl 100 cm

3 HNO3

100 cm3 H2O and 3 g MoO3 for metallographic examination A macrostructural examination of the

blade surface was performed using a stereoscopic microscope (Olympus SZX-9) The microstructure

and crystallographic orientation were characterized using an SEM (Hitachi S-3400N) equipped with

an energy dispersive spectrometer (EDS) (Thermo NORAN (System Six)) and electron backscatter

diffraction detector (INCA HKL Nordlys II (Channel 5)) For orientation analysis the specimen was

additionally vibratory polished (alumina suspension 005 μm grit size) for 5 hours to obtain as low

surface roughness as possible

3 Results and discussion

Figure 1 shows the blade surface with the columnar grains revealed Some deviation of the dendrite

growth direction from the heat flow direction is visible

Figure 1 Columnar grains on the surface of the turbine blade


2

The dendritic structure on the cross-section of the blade is shown in figure 2 The asymmetry of the

secondary dendrite arms is a result of the deviation of the dendrite growth direction from the heat flow

direction the heat flow disturbances and the thermal gradient The deviation of the primary dendrite

arms is mainly a result of the initial seed orientation [8] and causes the overgrowth of secondary arms

on one side of the primary arms [9]

Figure 2 Dendritic structure on the cross-section of the blade

An SEM micrograph of the cross-section reveals the grain boundary area and the different

orientations of the columnar grains Crystallographic orientation analysis using EBSD allowed

detailed characterization of the HAB Qualitative evaluation of the orientation was performed by

comparing the Kikuchi patterns of both columnar grains Figure 3 shows two different Kikuchi

patterns originating from the two columnar grains

Figure 3 Kikuchi patterns of two columnar grains near the high angle boundary

(a) pattern from grain 1and (b) pattern from grain 2

Crystallographic orientation maps were taken in the area surrounding the HAB (figure 4)

Orientation maps reveal the grain boundary shape which is not clearly visible on the SEM

micrograph Different colours assigned to the grains correspond to different crystallographic

orientations


3

Figure 4 Orientation maps of the high angle boundary area (a) map with Euler contrast and (b) map

with inverse pole figures with gray scale contrast (black line - high angle boundary)

An orientation map with inverse pole figures (IPFs) with gray scale contrast (figure 4b) aids in the

semi quantitative evaluation of crystallographic orientations Figure 4b shows that both columnar

grains deviate from the preferred [001] orientation Using the saved quantitative orientation data from

each point on the orientation map misorientation profiles in the grain boundary area and inside each

grain were determined Misorientation profiles are plots of the misorientation angles as a function of

the distance along a line (chosen arbitrarily) The misorientation describes the orientation difference

between grains (or micro areas in general) by rotating their crystal coordinate systems into

coincidence The misorientation profile in the grain boundary area which has a misorientation of

about 40deg directly shows the presence of a HAB - figure 5

Figure 5 Misorientation profile in the HAB area

However the misorientation profile inside each columnar grain reveals very small differences in

orientation that do not exceed 1degmdashfigure 6 These results indicate that columnar grains are single


4

crystals with a mosaic structure In this mosaic structure the lower misorientation values of mosaic

blocks (01degndash 03deg) are associated with the dendrite interiors but higher misorientation values are

associated with dendritendashinterdendritic area boundaries [10]

Figure 6 Misorientation profiles inside columnar grains (a) grain 1 and (b) grain 2

Quantitative evaluation of the orientation in relation to the cross-section plane was performed

based on pole figures (PFs) and inverse pole figures (IPFs) PFs and IPFs were determined for each

columnar grain (figure 7 and figure 8) and for the HAB area (figure 9)

Figure 7 Pole figures and inverse pole figures

for grain 1 Figure 8 Pole figures and inverse pole figures for

grain 2

PFs 100 110 and 111 directly reveal that the angle of deviation between the [001] direction

and the blade growth direction is similar for both grains Table 2 presents the values of the angles

between the [001] direction and the blade growth direction for each grain

Table 2 Angles between the [001] direction and the blade axis

Grain Angle between [001]

direction and blade axis (deg)

1 97

2 102

The rotation angle between the [100] directions of both grains is 425deg The obtained results show that

the columnar grains are rotated relative to each other with a simultaneous deviation from the [001]


5

orientation One should note that the deviation of the primary dendrite arms is quite large (about 10deg)

near the maximum acceptable value

Figure 9 Pole figures and inverse pole figures for the HAB area (points in squares are related to

grain 2)

4 Conclusions

A high angle boundary (HAB) is one example of the many casting defects in single crystal (SX) and

directionally solidified (DS) turbine blades HABs strongly deteriorate the mechanical properties of

the blade especially in the case of a stray grain in the SX blade Moreover the deterioration of the

mechanical properties is greater while the slope of the grain boundary plane in relation to the crystal

growth direction increases [11] The investigation shows that employing the correct seeding process

and maintaining heat flow control are extremely important because if they are not optimized

deviations that are too high from the preferred orientation occurmdasheg HABs form

Control examinations involving the evaluation of the superalloy turbine blade microstructure and

orientation should be standard These techniques offer electron backscatter diffraction on the scanning

electron microscope which allows the evaluation of even small misorientations in micro areas

Acknowledgements

Financial support of Structural Funds in the Operational Programme-Innovative Economy (IE OP)

financed from the European Regional Development Fund-Project No POIG010102-00-01508 is

gratefully acknowledged


6

References

[1] Reed R C 2006 The Superalloys Fundamentals and Applications (Cambridge University

Press)[2] Durand-Charre M 1997 The Microstructure of Superalloys (Amsterdam Overseas

Publishers Association)

[3] Yu K-O (Oscar) et al 2002 Modelling for Casting and Solidification Processing (Marcel

Dekker Inc)

[4] Ford D A and Wallbank J 1998 Int J Cast Metals Res 11 23

[5] Onyszko A Bogdanowicz W Nowotnik A Kubiak K and Sieniawski J 2010 Inż Mater 3 629

[6] Szczotok A and Sozańska M 2009 Prakt Metall 46 1

[7] Szczotok A Chmiela B and Sozańska M 2010 Inż Mater 3 695

[8] Esaka H Daimon H Natsuma Y Ohsasa K and Tamura M 2002 Mater Trans JIM 43 1312

[9] Zhao X Liu L Yu Z Zhang W and Fu H 2010 Mat Charact 61 7

[10] Bruumlckner U Epishin A and Link T 1997 Acta Mater 45 5223

[11] Chen Q Z Jones C N and Knowles D M 2004 Mat Sci Eng A 385 402


7

Analysis of high angle boundaries in directionally solidified

turbine blade made of CMSX-4reg superalloy

B Chmiela and M Sozańska

Department of Materials Science and Metallurgy Silesian University of Technology

40-019 Katowice Poland

E-mail bartoszchmielapolslpl

Abstract High angle boundary (HAB) and low angle boundary (LAB) are casting defects of

single crystal (SX) and directionally solidified (DS) turbine blades and decrease the lifetime of

these blades during service During directional solidification primary dendrite arms grow in

the opposite direction of the thermal gradient direction and perpendicular to the mushy zone

interface When this interface is not flat primary dendrite arms growing in various areas of the

mushy zone are characterized by different growth directions Then after the primary dendrite

tips contact each other LABs or HABs form (depending on the angle between the directions of

the primary dendrite arms) This paper presents characterization studies of HABs in a DS

turbine blade made of CMSX-4reg superalloy The blade was characterized by three columnar

grains with HABs Qualitative and quantitative analyses of the HABs using electron

backscatter diffraction in a scanning electron microscope (SEM-EBSD) were carried out The

EBSD technique helped to determine the crystallographic orientation of the grains near the

HAB misorientation angles between grains (and inside each grain) and the angle of deviation

between the [001] direction and the blade axis

1 Introduction

Modern aero engines have been improved based on increasing the thrust and thermal efficiency In

order to accomplish this improvement it is necessary to increase the operating temperature in the

combustion chamber Therefore single crystal (SX) and directionally solidified (DS) superalloy

turbine blades have been developed to replace the commonly used polycrystalline equiaxed turbine

blades [1 2]

Modern SX and DS turbine blades are produced by the Bridgman bottom seed method The

preferred crystallographic orientation of the seed is usually [001] In practise a deviation between the

blade axis and the [001] direction of up to 12deg is acceptable But to achieve the best mechanical

properties (especially fatigue strength under variable load conditions) the angle between the growth

direction and the [001] direction must be as small as possible [1 2] Furthermore in DS blades the

misorientation between neighbouring columnar grains should not exceed 15deg to avoid a high angle

boundary (HAB) Thus it is necessary to accurately monitor the crystallographic orientation during

directional solidification Unfortunately for many reasons many disturbances in the solidification

front take place and dendrites in different areas of the casting grow in different directions Then after

the primary dendrite tips contact each other low angle boundaries (LABs) or HABs form (depending

on the angle between the primary dendrite arm directions) [3] Incorrect orientation of the seed (or

heterogeneous nucleation by foreign particles [4]) can promote the growth of a stray grain thus


Published under licence by IOP Publishing Ltd 1








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Analysis of high angle boundaries in directionally solidified

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