Study of the Mechanisms of Initial Oxidation and of the Interaction with
Reactive Elements in the Halogen Effect for Ni-base Alloys
H.-E. Zschau and M. Schütze
e-mail: [email protected]
Funded by: DFG
Period: 01.07.2011 - 30.06.2013
Introduction
Introduction
Ni-coating
Spinel
Ni-base super alloys with Al-contents of less than 10 wt.-% are widely used in high
temperature technology due to their beneficial mechanical properties. In contrast to
this their oxidation behaviour is insufficient at temperatures above 1000°C.
Oxidation of Ni-base alloys in air at temperatures above 1000°C does not form a
dense protective Al2O3-scale on the surface, but rather a complex layer structure as
shown in fig. 1. The alumina scale is characterized by internal oxidation, i. e. it grows
into the metal showing a discontinuous structure.
Ni-coating
Turbine blade
TBC
Cr2O3
internal Al2O3
Al2O2 spalled
Nitrides
Al2O3 external - scale
Fig. 6: Left micrograph: Oxide scale of IN 738 (untreated) after oxidation
(1000h/1050°C/air). Right micrograph: The implanted sample (5 x 1016 F cm-2)
shows the change from internal to external oxidation of Al. Only few nitrides are
visible. The protective alumina scale formed can partly spall during cooling.
Combined
Combined treatment
treatment with
with FF and
and reactive
reactive elements
elements
Bond coat
Al2O3 (internal)
Alloy
Fig. 1: Oxide scale of IN738
after isothermal oxidation of
24h/1050°C/air.
Fig. 2: The turbine blades for aircraft engines and
gas fired power stations are manufactured with
an Al-rich bond coat and a TBC.
Oxidation protection can be achieved by the formation of a dense alumina scale.
Usually Ni-base alloys are covered with Al-rich coatings (bond coat).
State of the art (fig. 2).
Small additions of reactive elements (Y, Hf, Ce…) are beneficial for the adhesion of
protective alumina scales. An improved adhesion of the protective alumina scale
formed by the fluorine effect should be achieved by a combination of the fluorine
treatment and the reactive element effect. To find the optimum additions ion
implantation was used because of its high accuracy and reproducibility. The
implantations were performed at the 60 kV-implanter, whereas the non-destructive
analysis of F-and Y/Hf-profiles was carried out by using the PIGE/RBS-techniques at
the 2 MV Van de Graaff accelerator of the Institute of Nuclear Physics of Frankfurt
University. The implantations were planned by doing comprehensive Monte Carlo
simulations of the implantation profiles using the software T-DYN (fig. 7-10).
65
50
60
Alloy:
IN 738
First implantation:
Yttrium 110 keV 4e16
Second implantation: Fluorine 38 keV 2e17
50
45
Fluorine
Aluminium
Chrom
Cobalt
Nickel
Yttrium
40
35
30
25
20
15
10
Element concentration [at.%]
Formation of a “self-supporting” alumina scale:
Wagner´s oxidation theory: Formation of a dense alumina scale can be achieved, if
the Al-activity is sufficiently high.
Element concentration [at.%]
F-effect:
F-effect: change
change of
of the
the oxidation
oxidation mechanism
mechanism
IN 738
Yttrium 110 keV 4e16
Fluorine 38 keV 2e17
35
30
Fluorine (PIGE)
Fluorine (Simulation)
25
20
15
10
0
0
20
40
60
80
100
120
140
160
180
0
20
40
60
Depth [nm]
80
100
120
140
160
180
Depth [nm]
Fig. 7: Element profiles in the alloy IN 738 after Fig. 8: Comparison of the implanted Fimplantation of Y (4 x 1016 Y cm-2/110 keV) and profile (measured by using PIGE) and
F (2 x 1017 F cm-2/38 keV). Calc. by T-DYN.
the simulated (T-DYN) for data of fig. 7.
30
65
Alloy:
First implantation:
Second implantation:
55
50
IN 738
Hafnium 100 keV 4e16
Fluorine 20 keV 1e17
Fluorine
Aluminium
Chromium
Cobalt
Nickel
Hafnium
45
40
35
30
25
Alloy:
First implantation:
Second implantation:
25
Element concentration [at.%]
60
Element concentration [at.%]
In fig. 3 the possible reactions in a cavity at the oxide/metal interface are illustrated.
The Al-amount at the metal surface can be increased by a selective formation of
gaseous Al-fluorides in pores and microcracks and their migration towards the
surface. Due to the increasing oxygen partial pressure the Al-fluorides disintegrate
and the Al is oxidized forming a dense protective layer of Al2O3 (fig. 4-6).
40
5
5
0
„Artificial“ increase of the Al-amount by using the halogen effect.
This concept works in the case of TiAl-alloys. Fluorine is chosen due to the
good results obtained with TiAl.
Alloy:
First implantation:
Second implantation:
45
55
20
15
10
IN 738
Hafnium 100 keV 4e16
Fluorine 20 keV 1e17
20
Fluorine (PIGE)
Fluorine (Simulation)
15
10
5
5
0
0
0
20
40
60
80
100
120
140
160
180
0
20
40
60
80
100
120
140
160
180
Depth [nm]
Depth [nm]
Fig. 9: Element profiles in the alloy IN 738 after Fig. 10: Comparison of the implanted Fprofile (measured by using PIGE) and
implantation of Hf (4 x 1016 Hf cm-2/100 keV)
the simulated (T-DYN) for data of fig. 9.
and F (1017 F cm-2/20 keV). Calc. by T-DYN.
Fig. 3: Possible reactions for the formation of gaseous metal halides in a
cavity. The selective formation of
gaseous Al-halides is the key condition for the halogen effect.
TiO2
Fig. 4: The halogen effect achieves an artifical increase of the Al-activity and stimulates the formation of a dense protective
alumina scale.
Spinel
Ni-Coating
Ni-coating
Cr2O3
Al2O3 (internal)
Nitrides
The implantations of Y resp. Hf, followed by implantation of F show a distinct change
of the element profiles within the near-surface region of the alloy IN 738 as illustrated
in fig. 7 and 9. The measured F-depth profiles are in good agreement with the calculated F- implantation profiles (fig. 8 and 10). Based on these results first experiments
show an adherent protective alumina scale (fig. 11). Future work is planned to determine the optimum parameters for several reactive elements in combination with F.
Ni-coating
Al2O3 (external)
Fig. 5: Left micrograph: Oxide scale of IN 738 (untreated) after oxidation (24h/
1050°C/air). Right micrograph: The implanted sample (1017 F cm-2) after oxidation
(60h/1050°C/air) shows the change from internal to external oxidation of Al.
external Al2O3 - scale
external Al2O3 –
scale
crack
Fig. 11: Implanted sample of IN 738 after oxidation (60h/1050°C/air). Left
micrograph: 4 x 1016 Y cm-2 /110 keV and 1 x 1017 F cm-2 / 38 keV. Right
micrograph: 1 x 1016 Y cm-2 / 110 keV and 1 x 1017 F cm-2 / 38 keV.