High temperature oxidation protection of technical titanium and nickel alloys by combined aluminization and fluorination in a one‐step process
A. Donchev, M. C. Galetz
e‐mail: [email protected]
Funded by: BMWi via AiF
Period: 01.12.2015 ‐ 30.11.2017
Motivation
Titanium alloys cannot be used at elevated temperatures due to the high oxidation
rate of TiO2. Nickel alloys on the other hand are used as high temperature structural
materials. Usually a chromia layer is formed to protect those alloys from the
environmental attack, but this chromia layer has some disadvantages, e.g. in the
presence of water vapor. To overcome these problems alumina is the best choice. In
this project technical Ti and Ni alloys will be enriched in a surface zone with Al via the
pack cementation process to form an intermetallic diffusion layer. This layer serves as
an Al reservoir for the alumina formation via the parallel deposited fluorine. The
fluorine effect promotes the formation of a protective Al2O3 layer on the TiAl
diffusion zone (fig. 1).
Temperature: 600°C
Gas: Ar
M: Diffusion element
AX: Activator
Inert filler: Al2O3, SiO2
Naturally grown oxide ( 10 nm) T/t
Al2O3
F2
Substrate
Gas diffusion
M: Al
X: F or Cl
A: NH4 or Al
Fig. 1a
TiAl diffusion zone + F
MX(g) + A(l or g)
Al2O3 (≈1 µm)
TiAl diffusion zone + F
2 Ti(s) + 3 F2  2 TiF3(g)
2 Al(s) + F2  2 AlF(g)
X(g) + M(s)
Ni plating
Diffusion zone
Substrate
Substrate
Substrate
b)
a)
c)
Ni plating
Diffusion zone
Ni plating
Ni plating
Diffusion zone
Substrate
Diffusion zone
Substrate
d)
Substrate
e)
f)
AlF
Pore
MX(g)
M(s) + AX(s or l)
Ni plating
Diffusion zone
Ni plating
Mixed scale
Figure 2 a‐f: Light optical micrographs of several Ti specimens after cementation revealed
no diffusion layer with single AlF3 treatment (fig. 2a) while metallic Al or an intermetallic
Al‐containing alloy was needed to achieve Al enrichment (fig. 2b‐f).
a)
b)
4 AlF(g) + 3 O2  2 Al2O3(s) + 2 F2
Solid state diffusion
Fig. 1b
Ni plating
high
p TixFy << p AlxFy
Diffusion zone
Figure 1 a, b: Schemes of the powder pack process (a) and the fluorine effect
mechanism (b)
low
Substrate
Experimental
Specimens of different Ti alloys were treated via the powder pack cementation
process in Ar atmosphere. The samples were placed within the different powders so
that an “in‐pack” treatment was applied. The temperature was kept between 500°C
and 700°C. The samples were investigated after the pack by metallographic
preparation and light optical microscopy. Successful treatments were used to
evaluate the oxidation performance during exposure tests in air. These experiments
were performed in the same temperature range as the pack cementation.
100
90
80
70
60
50
40
30
20
10
0
Concentration [at.‐%]
c)
F
Ti
Al
0
5
10
15
Depth [µm]
Results
Conclusions
A combined powder pack cementation process of Al and F enrichment on Ti alloys in
a single step is possible. By carefully adjusting the parameters an Al‐rich diffusion
zone can be established which acts as a barrier against oxygen ingress and enhances
the oxidation resistance. Additionally fluorine is enriched at the surface to ensure the
fluorine effect. This positive effect promotes the formation of a protective alumina
layer.
Outlook
The treatment of technical Ni alloys will be performed next. With these alloys the
temperature range for an optimal Al inward diffusion will be evaluated first. It is
known that higher temperatures (usually > 800°C) are needed for the aluminization
of Ni‐based materials.
25
Figure 3 a‐c: EPMA analysis of a Ti specimen after combined Al/AlF3 cementation (1 wt.%
each); elemental distributions (a), BSE image (b), and depth profiles (c).
a)
b)
Ni plating
high
Alumina
Diffusion zone
Concentration [at.%]
low
Substrate
100
90
80
70
60
50
40
30
20
10
0
c)
F
Ti
O
Al
0
5
10
15
Depth [µm]
20
25
Figure 4 a‐c: EPMA analysis of a Ti specimen after optimized combined Al/AlF3
cementation (5 wt.% each) and oxidation at 600°C for 120h in air; elemental distributions
(a), BSE image (b), and depth profiles (c).
b)
a)
Mass change [mg/cm²]
The light optical microscopic analysis of the metallographic cross sections of several
Ti specimens after pack cementation reveal no diffusion layer with a single AlF3
treatment (fig. 2a). In contrast combinations of metallic Al powder with AlF3 (fig. 2d)
or NH4F (fig. 2b, e) are successful to develop 4‐8 µm thin Al diffusion coatings. Such
results are also achieved by the use of Cr‐44Al (fig. 2c) or Ti‐52Al (fig. 2f) together
with NH4F. The mixture of 1 wt.% AlF3 with 1 wt.% Al results in a roughly 8 µm thin
diffusion layer (fig. 3a, b) which consists of TiAl3 (fig. 3c). No oxygen is detected.
Unfortunately also only a minor amount of fluorine is found which is not sufficient to
get the fluorine effect to operate (less than 5 at.%). Optimization of the cementation
process by increasing the Al and NH4F content to 5 wt.% each also led to the
formation of an TiAl3 diffusion layer (fig. 4a, b). This layer was protective during
exposure at 600°C in air. A very thin oxide layer is found on the surface, followed by a
fluorine enrichment up to 10 at.% and the diffusion layer (fig. 4c). No oxygen is
detected in the diffusion layer and the subsurface zone of the substrate. This
indicates no environmental embrittlement. The Al‐rich diffusion layer acts as a
barrier against oxygen inward diffusion, serves as oxidation protection by forming an
alumina layer, and keeps the layer attached by forming an interdiffusion zone with a
gradual Al content (TiAl3, TiAl, Ti3Al) which hinders spallation of the scale. This also
works for technical alloys. In figure 5a the mass change data of untreated and treated
Ti6242 during thermocyclic exposure at 650°C in air are shown. The mass gain of the
untreated specimen is almost 0.8 mg/cm². SEM analysis reveals a roughly 4 µm thick
TiO2 scale after exposure (fig. 5b). The treated sample has much lower mass gain, the
Al‐rich diffusion layer is intact and a very thin (< 1 µm) alumina layer is found (fig. 5c).
20
1.00
Ni plating
TiO2
0.90
0.80
Substrate
0.70
0.60
0.50
0.40
c)
0.30
Ni plating
Alumina
0.20
Diffusion zone
0.10
0.00
0
50
100
150
200
Time [h]
250
300
350
Substrate
Figure 5: Mass change data (a) of untreated and treated Ti4262 samples during
thermocyclic oxidation at 650°C in air (1 h hot/30 min cold) and SEM images after
exposure (untreated fig. 3b, treated fig. 3c).