“Analysis of surface scale on the

Ni-based superalloy CMSX-10N”

Presented by:

Scott Simmonds

Department of Engineering, University of Leicester, LE1 7RH, UK

Introduction

Ni based superalloys

Investment casting and directional solidification

Surface Scale

What is surface scale?

Surface characterisation techniques and compositional depth-profiling

Mould/metal separation

Mechanism of formation

Conclusions

Used in high temp applications e.g. turbine blades and nozzle guide vanes

Ni superalloys exhibit high temp strength and creep resistance, up to ~80% of Tmelt

Greater stability and resistance to thermal shock/fatigue at temperatures >800°C

Introduction of single crystal components has further improved these properties by

completely removing grain boundaries.

Ni based superalloys in gas turbine aero engines

Rolls-Royce Trent 800 High-Bypass Turbofan [courtesy of Rolls-Royce][2]

[1] Man Hoi Wong “Single Crystalline Turbine Blades” MSE 407 Case Study, Nov 2003

[2] © Copyright Rolls-Royce plc 2011. All rights reserved. This copyright work has been reproduced with the kind permission of Rolls-Royce plc

Comparison of material strength vs. temp

for typical alloy systems [1]

Temperature

Nickel Alloy

Titanium Alloy

Steel AlloyAluminium Alloy

Specific

Strength

Specific Strength

Turbine section

Ni Superalloys

1) Assembly 2) Investing 3) Stuccoing 4) De-waxing 5) Firing 6) Pouring 7) Knockout 8) Finishing

© Copyright Rolls-Royce plc 2011. All rights reserved. This copyright work has been reproduced with the kind permission of Rolls-Royce plc

Investment Casting –The Lost Wax Process

H. Dai “A Study of Solidification Structure Evolution during Investment Casting of Ni-based Superalloy for Aero-Engine Turbine Blades” PhD Thesis, University

of Leicester, 2008.

Directional Solidification

Alignment of grain boundaries with the stress axis

increases creep rupture life and ductility

Furnace under vacuum to prevent reactions between molten alloy & atm

Melted „charge‟ of alloy poured into preheated mould

Solidification of equiaxed grains begins on chill plate at bottom

Mould/chill plate lowered slowly from the furnace hot-zone (10-30cm/hr)

Vertical temp gradient produced grains grow vertically upwards

Competitive growth favours grains whose orientation matches the

thermal gradient produces long, vertical grains

Single Crystal (SX) Turbine Blades

„Pig-tail‟ spiral grain selector or „seed‟ used to select a grain orientation

Allows only a single grain of the desired orientation to enter the mould

Surface Scale

© Copyright Rolls-Royce plc 2011. All rights reserved. This copyright work has been reproduced with the kind permission of Rolls-Royce plc

Common defect on SX Ni-base superalloy components

Surface defect identifiable post-cast & etch as „fish-scaled‟ area

of discolouration with an associated texture

typically found on upper, convex section of aerofoil

becomes a darker grey after solution heat-treatment

Obscures visual inspection prior to heat-treatment

hiding potential grain defects beneath

Additional work (grinding/blasting) to remove before re-inspection

changes to aerofoil geometry, recrystallisation, cost

Surface Scale

© Copyright Rolls-Royce plc 2011. All rights reserved. This copyright work has been reproduced with the kind permission of Rolls-Royce plc

Previous studies have shown:

Formation sensitive to blade configuration, geometry and casting parameters.

Increasing furnace withdrawal rate increases extent of surface scale.

Scale always forms on blade sections last to solidify.

Spatial location dependant on blade geometry and orientation.

No mechanism of formation suggested…

FRE103577 IP Blades

Surface Characterisation of CMSX-10N Flat, rectangular bars were cast under normal conditions

Test samples cut using EDM from scaled and unscaled areas

SEM, Auger electron and X-ray photoelectron spectroscopy

techniques used to characterise the sample surfaces

Surface

Scale

Scaled

region

„smooth‟ Unscaled region

covered in adhered

ceramic particles

Distinct border

between two regions

Surface Characterisation of CMSX-10N X-ray photoelectron spectroscopy (XPS) analysis of scaled and unscaled surfaces

Scaled

Unscaled (as-cast)

Unscaled (Ar etch)

Ni (2p)

Zr (3d)

Al (2p)

Re, Ta & W

Unscaled (as-cast)

Unscaled (Ar etch)

Scaled sample Unscaled sample

Auger electron spectroscopy (AES)

1200

Scale thickness is sub-micron AES gives accurate analysis at these depths

Depth-wise compositional analysis performed of scaled & unscaled regions

Scaled areas covered with thin oxide layer (~800nm) – Ni oxides predominant

Unscaled areas covered with Al2O3 layer – thicker than scale oxides

AES and XPS data agree with each othe

C peak is contamination from the furnace heating elements (graphite)

Two distinct oxides formed in scaled and un-scaled areas A mould/metal reaction cannot solely

account for scale formation as this would produce a common oxide everywhere

Mould/metal separation

Shown by Brewster et al [1] that a 0.5-1.5µm Al2O3 layer forms

over entire blade surface during initial stages of casting

reaction between liquid metal and solid mould prime-coat

Differential thermal contraction occurs during cooling

some areas of solidified blade separate from mould

Al2O3 is stripped from these areas & remains on mould wall

Exposes “bare” metal to casting atmosphere

further oxidation of metal surface results in surface scale

Al2O3 layer is retained on the blade surface in unscaled areas

surface remains “protected” and scale can‟t form

[1] G. Brewster; N. D‟Souza; K. S. Ryder; S. Simmnds; H. B. Dong. Mechanism for Formation of Surface Scale during Directional Solidification of Ni-base

Superalloys. Submitted to Met Trans A for publication Jan 2012.

Gap widths of ~50μm predicted

using ProCASTTM modeling

software

Surface scale – Mechanism of formation

RR3010

Face-coat

particles

adhered to

unscaled

regions

Al2O3 on face-coat

opposite scaled

region

Formation of surface scale has been presented

Surface scale constitutes the oxides that form on the surface regions that have detached

from the mould wall.

The scale on CMSX-10N components is predominately Ni rich oxide (likely to be NiO).

In the un-scaled regions, where the metal has not detached from the mould, the Al2O3

layer formed via the mould-metal reaction remains adhered to the solid metal.

Conclusions

Questions?

Thanks to: