Download - Thermal oxidation behavior of an Al-Li-Cu-Mg-Zr alloy

Thermal Oxidation Behavior of an AI-Li-Cu-Mg-Zr Alloy

MAQSOOD AHMAD

The chemical composition of oxide films formed during thermal treatments of an A1-Li-Cu-Mg-Zr alloy has been studied by means of Auger electron spectroscopy and X-ray photoelectron spectroscopy. The oxide layers formed after oxidation of 2.5 minutes to 30 minutes at 530 °C in lab air have been characterized. In the early stages of oxidation the surface is composed of both the lithium rich oxides and magnesium rich oxides. However, after longer oxidation times the oxidation of lithium becomes predominant and the air/oxide interface is completely covered by lithium compounds. Oxidation products formed on the alloy surface have been studied by X-ray diffraction analysis. The following three phases, namely, Li2CO3, a-LisA104, and y-LiAIO2, were identified. During heat treatment in lab air at 530 °C and at atmospheric pressure the dominating reaction product is Li2CO3. Due to the selective oxidation of lithium a soft surface layer is developed. The width of the soft layer formed during solution heat treatments carried out in lab air and in salt bath environments has been determined by microhardness measurements. The lithium concentration profiles were calculated from a diffusion equation. The depletion of alloying elements from the near surface region during heat treatments has been investigated using energy dispersive X-ray analysis. The oxide morphology was examined using scanning electron microscopy and optical microscopy.

I. INTRODUCTION

ALUMINUM alloys containing 2 to 3 wt pct lithium offer an attractive combination of mechanical properties, particu- larly in terms of reduced density and increased stiffness. Since lithium is highly reactive, one may expect that during thermomechanical processing of A1-Li alloys, lithium may diffuse to the surface and react with the atmosphere. Lith- ium metal reacts with the non-metals except the inert gases, most of the metalloids, and many of the metals under proper conditions. It is known that when A1-Mg alloys are heat treated during their production and fabrication, magnesium is depleted from the bulk of the material with the formation of magnesium rich oxide. 1 Hine et al. 2 studied the oxidation of aluminum-magnesium alloys in the temperature range 440 to 540 °C and showed that the oxidation rate depends on the concentration of magnesium in the alloy and a magnesium depleted layer is formed next to the oxide film. Textor and Grauer 3 studied the oxide composition of technical pure aluminum by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). They found that after thermal annealing Li, Mg, and Na were strongly enriched at the surface.

In situ electron microscopic study of oxidation of an AI-1 wt pct Li alloy at 475 °C and at low partial pressure of oxygen was performed by Scamans and Butler. 4 They report formation of spinel LiAlsO8 oxide crystals. Fridlyander and his co-workers s studied the thermal oxidation of an 01420 (A1-5 wt pct Mg-2 wt pct Li) alloy in air, and Li2Co3 and MgO were reported to be the predominant crystalline phases. Field et al. 6 studied oxidation of A1-3 wt pct Li and A1-3 wt pct Li-2 wt pct Mg alloys in the temperature range 480 to 575 °C. The results of transmission electron microscopic study under dry conditions showed the initial amor- phous A1203 film is replaced by the following oxide phases: primary y-LiAIO2 (epitaxial), secondary MgO, and Li20.

MAQSOOD AHMAD is Research Assistant, Division of Engineering Materials, Department of Mechanical Engineering, Link~ping Institute of Technology, S-581 83 Linkrping, Sweden.

Manuscript submitted April 11, 1986.

Csanady et al. 7 studied the influence of Cr, Cu, and Li additions on high temperature oxidation of AI-Zn-Mg alloys. The MgO, LisA104, and LiAIsOs oxide phases were detected by X-ray diffraction for the alloy heat-treated at 500 °C for 160 minutes in air.

Previous studies of A1-Li-Cu-Mg-Zr alloys by means of microhardness measurements have shown that during heat treatment of the alloys a soft surface layer is formed. 8'9 Recently there have been several investigations dealing with the problem of oxidation of the A1-Li alloys. ~°'n'n

The aim of our work was to study the compositional and structural changes occurring during the heat treatment at the surface of the A1-Li-Cu-Mg-Zr alloys. The surface composition and concentration profiles through the oxide layer were determined by AES (Auger Electron Spectroscopy) in conjunction with argon ion sputtering. The oxide morphology of the outer oxide layer was examined using a scanning electron microscope, and solute depletion from the bulk was studied by EDX (Energy-Dispersive X-ray Analysis). The oxidation reaction products were identified by using X-ray diffraction technique. We were also interested in knowing how solution treatment environment (lab air/salt bath) affects the solute depletion from the bulk of the alloy.

II. EXPERIMENTAL PROCEDURES

An extruded bar with dimensions 51 mm x 25 mm of the 8090 alloy produced by British Alcan Aluminium was received in the peak aged condition. The composition of the alloy determined at Saab-Scania AB is given in Table I. The specimens for solute depletion studies were cut from the bulk of the bar. The specimens were of thickness 1 mm to 3 mm. Solution heat treatment was carried out for one hour in a salt bath (Durferrit As 140, NaNO2 40-50 pct, KNO3 60-50 pct) operating at 530 °C. Anotherspecimen was solution treated for one hour in an air furnace at 530 °C. After the solution heat treatment the specimens were quenched into water at 20 °C and subsequently aged for 24 hours in an oil bath at 190 °C. Following the aging treatment the speci-

METALLURGICAL TRANSACTIONS A VOLUME 18A, APRIL 1987--681

Table I. Alloy Composition (in Weight Percent)

Li Cu Mg Zr Fe Si Ti Na AI

2.5 1.4 0.65 0.14 0.07 0.04 <0.01 0.0002 balance

mens were ground and polished before measuring the hardness as a function of distance from the surface. The microhardness measurements were made using 15 g and 50 g loads for the specimens solution treated in the salt bath and in air, respectively. Each hardness value is based on the average of three values with a scatter of ---2 Vickers hardness units. Magnesium loss profiles were determined by using energy dispersive X-ray technique (EDS) linked to a scanning electron microscope (SEM).

Specimens for AES (Auger Electron Spectroscopy) and XPS (X-ray Photo-Electron Spectroscopy) analysis were cut from the bulk of the bar. They were ground, polished, and etched for one second in a solution of nitric acid and meth- anol (1:2). They were washed in ethanol before carrying out heat treatments for different periods of time in an air furnace at 530 °C.

The AES measurements were made in a scanning Auger Electron Spectrometer, Varian model 981-2001. The low kinetic energy AES spectra (25 to 525 eV) were recorded using a 3 keV primary electron beam and a CMA (Cylin- drical Mirror Analyser) modulation voltage of 2 V peak to peak. The high kinetic energy spectra covering the energy range 1000 to 1500 eV were recorded using 5 keV primary electron energy and a CMA modulation voltage of 5 V peak to peak. The instrument was equipped with an ion gun with scanning facilities. Sputter etching of the specimens was performed at an argon pressure of 5 x 10 -5 tort. The argon ions were accelerated by 3 keV, and the beam current density was 50 #A/cm 2, corresponding to a sputtering rate of approximately 20 nm per minute. The diameter of the electron beam was about 10/~m. As a measure of the relative quantities of metallic aluminum and oxidized aluminum, the A l LE3VV transitions at 51 and 66 eV were used. The amount of oxidized magnesium and oxidized lithium were evaluated using Mg KLEL2 Auger peak at 1180 eV and Li KVV Auger peak at 39 eV, respectively. As a measure of the amount of carbon and oxygen the Auger peaks at 272 eV and 510 eV were used, respectively. The Auger depth profiles were obtained by measuring the peak heights and plotting them as a function of sputtering time. In the depth profiles shown, the peak heights are normalized by putting the sum of the peak heights to 100 everywhere. The amount of any element in the depth profiles is in arbitrary units. No Auger peaks characteristic of copper were observed. This means that the amount of copper present in the alloy was lower than the detection limit. When plotting the depth profiles correction was made for the magnesium peak at 1180 eV due to the higher primary electron beam energy and higher CMA modulation voltage.

The XPS spectra were obtained using A1K~ radiation. The effective sample area in the XPS instrument is several mm 2, much larger than the AES area of analysis/zm 2.

A Jeol 25 Scanning Electron Microscope (SEM) was used to study the oxidized surfaces.

The oxidation products formed on the alloy surface were identified by using an X-ray diffractometer assembly. The diffraction patterns were recorded using CrK~ radiation and

a V filter. The operating parameters were 28 kV, 30 mA. A Guinier camera, CUKal radiation, was also used in identi- fying the reaction products. For accurate d-spacing measurements, KC1 was added and the films were measured with a microdensitometer.

III. RESULTS AND DISCUSSION

The loss of lithium to the surface was theoretically analyzed by applying a solution of Fick's second law for the case of semi-infinite solids. 13 For the experimental times of one hour employed in the present study the thickness of the specimens can be considered infinite. It was assumed that the diffusion coefficient (D) does not change with composition and the concentration of Li at the surface is equal to zero.

_- erf( )

where Co = alloy composition of Li x = distance from surface

D = diffusivity of Li in aluminum t = time

C(x, t) = concentration of Li at distance x from the surface at time t

Costas 14 has investigated the diffusion of lithium in aluminum according to the following expression for D:

kmol/ cm2/s [2] D = 4.5 exp RT

Any significant loss of lithium and of magnesium occurs during homogenizing and solution treatments. The recom- mended solution treatment temperature for the alloy is 530 °C. For the study of denuded zones, specimens were solutionized at 530 °C for one hour. Equation [2] yields a diffusion coefficient of 3.8 • 10 -9 c m 2 / s . The nominal alloy composition of lithium Co - 2.5 wt pct. The concentration of lithium C ( x , t ) calculated using Eq. [1] is plotted in Figure 1 vs the distance from the surface. Microhardness as a function of distance from the surface for both the specimen solution heat-treated in the salt bath and the specimen solution heat-treated in air was measured after they have been aged for 24 hours at 190 °C. The microhardness measurements were made on a transverse section. The hardness measurements were normalized by dividing each hardness value at distance x from the surface with the bulk hardness. Figure 2 shows that there is a soft surface layer and its width is about 80 ~m. The extent of the soft surface zone is consistent with the Knoop microhardness measurement re- sult reported by Jaensson et al.15 The two specimens solution treated in the salt bath and in laboratory air showed the same width of the soft surface layer. However, the shapes of the microhardness curves differ slightly from each other. In addition to the measured microhardness, the normalized

682--VOLUME 18A, APRIL 1987 METALLURGICAL TRANSACTIONS A

or///nal Li content ~5~[

3-

2 . 5 -

~" z. *[E-.~ /

/

o.5- s - / , / / , ' /

o o 5o 1~o 1~o 2oo

Distance from the surface (~m)

Fig. 1--Concentration profiles of lithium, calculated from a diffusion equation for 0.5 and 1 h solution treatment at 530 °(2.

1.5-

t .

0.5-

...@ ................... I . . . . . 0

~ S , solub.'on .heat-trea!ed in air

/ • hardness solution heat-treated in salt [ ................................................

1 r i

0 0 100 150 200 Distance f r o m the s u r f a c e (/~m)

Fig. 2--Normalized microhardness and calculated concentration of lithium vs the distance from the surface. The specimens for hardness measurements were solution treated in a salt bath and in an air furnace for 1 h at 530 °C, and were aged for 24 h at 190 °C.

calculated concentration of lithium (C (x, D/Co) for one hour solution treatment at 530 °C is also plotted in Figure 2. It is seen that the lithium loss curve is parallel with the microhardness curves and is slightly shifted to the fight. This slight shift may be explained by considering the way the microhardness data and lithium concentration are plotted. The surface concentration of lithium was assumed to be equal to zero, while the surface hardness should be equal to the hardness of the unalloyed aluminum, i.e., nonzero. It is obvious that depletion of lithium is controlled by the bulk diffusion. In a recent paper Papazian et al. 16 have studied the loss of lithium during heat treatment of the 8090 and 8091 alloys. They applied a nuclear reaction technique and report that in the wet or dry air, the rate of lithium loss is limited by the diffusion of lithium from the bulk to the surface. Light microscopy of specimens solution treated and aged revealed a narrow band depleted of coarse precipitates at the surface, which also indicates presence of a solute depleted surface layer.

The loss of lithium could not be analyzed with the energy dispersive X-ray technique. However, the loss of magnesium was studied by energy dispersive X-ray micro- analysis across a transverse section. The specimens cut from the bulk of the bar and solution treated in the salt bath and in the air furnace showed no significant loss of magnesium near the surface. However, study of an external surface of the bar showed that magnesium has been lost and the re- suiting normalized magnesium loss profile is shown in Figure 3. This loss of magnesium has occurred during previous thermomechanical processing, e.g., homogenizing treatments, solution treatments, and extrusion. Our results appear to be contradictory with those of Fox et al. 17 They report that both the lithium and magnesium have depleted at the same rate during solution treatments. In the present study, however, we did not find any significant loss of magnesium during the solution heat treatments. This con- tradiction is probably due to the different furnace environments. The fact that no magnesium depleted layer next to the oxide film was observed indicates that the rate of oxidation was slower than the diffusion of magnesium in the alloy. The corresponding concentration profile of copper showed no measurable change from the interior to the oxidized surface.

A. AES and XPS Analysis

AES technique was used to study the composition of oxide films formed on the samples oxidized (= annealed) in an air furnace for 2.5, 5, 10, and 30 minutes at 530 °C. AES spectra were also recorded from a specimen which was not annealed. Natural oxides present on the surface of a pure aluminum and pure magnesium samples were also studied. A typical AES spectrum obtained from the surface of the specimen oxidized 2.5 minutes is shown in Figure 4. The spectrum displays large peaks of Mg, Li, and O together with small peaks of AI and C. AES studies of lithium and some of its oxides have been reported in the literature. ~s-21 Oxidized lithium shows a large peak at 40 eV and a small peak at 33 eV. Magnesium oxide formed on the pure magnesium sample displayed LVV Auger line at 32 eV and KLL Auger line at 1205 eV. Note there is a large shift of the

1.5

8 1 -

0.5-

o 5'o ' I00 150

Distance f r o m t h e su r f ace (/zm) Fig. 3 - - Normalized magnesium content as a function of distance from the outermost surface of the bar.


"el

~+LI"

e

AI

L i

! I I I

0 100 200 300 400

Auger

A1

=p

O A

500 1100 1200 13~00

electron energy (eV) Fig. 4--Differentiated AES spectra from the surface of a specimen oxidized for 2.5 min at 530 °C in air.

I I

1,100 1500 180~

magnesium KLL Auger line toward higher kinetic energy due to the charge effect induced on the oxide surface. Thus the Auger peak at 33 eV from the surface of the specimen annealed 2.5 minutes is the sum of Mg and Li peaks. An elemental distribution-depth (sputtering time) profile for the specimen in the unannealed condition is shown in Figure 5. The surface is completely covered by A1203. The metallic aluminum peak (66 eV) becomes visible after 5 seconds sputtering. The Auger depth profiles of the oxide films on the samples oxidized for 2.5, 5, 10, and 30 minutes are shown in Figures 6(a) through (d). For the specimen annealed 2.5 minutes metallic aluminum was first detectable after 25 seconds sputtering. Low energy Auger spectra of aluminum oxide A1203 displayed a large peak at 51 eV and a small peak at 37 eV. The amount of oxidized Li was obtained using the 39 eV transition. In this there might be a small contribution from the oxidized aluminum. The re- suits (Figures 6(a) through (d)) show that both the oxidized lithium and oxidized magnesium coexist over a large pro- portion of oxide thickness on the samples annealed for short times. However, after 30 minutes annealing the outermost surface is completely covered by oxidized lithium, magnesium appearing after 10 minutes of sputtering. The AES results indicate that magnesium and lithium present at and just under the surface are oxidized in the early stages of oxidation. However, with continued oxidation more and more lithium diffuses to the surface from the bulk and is preferentially oxidized. At the same time the rate of oxidation of magnesium is slowed down. This qualitatively agrees with EDS analysis which showed no magnesium depleted layer near the surface. It also agrees favorably with the results reported in References 22 and 10. Csanady et al . 22 studied the surface composition of aluminum and of aluminum alloys by SIMS. They report that the diffusion of Li is faster than of Mg in both the metal and the oxide. Since the atomic diameter of lithium is smaller than of magnesium, one should expect that diffusion of lithium should

100

~ 50

o Z

. / O - ~ "

0

\ I I \ f " / u°xygen ]

/~ Jo m e_t__allic_-__Al_ ]

I I I I

0 1 2 3 4

Sputtering time (min) Fig. 5--Composition-sputtering time profiles for a specimen in the unannealed condition.

be faster. In a recent paper Ashton et al . 1o studied the effect of homogenization atmosphere and time on the thermal oxidation of an A1-Li-Cu-Mg alloy. They report that only lithium was severely depleted from the alloy during homogenization in dry argon. In all the concentration-depth diagrams of Figure 6, a small amount of oxidized aluminum is present throughout the oxide thickness.

XPS studies, as well as AES studies, of the specimen in the unannealed condition showed that the surface entirely consists of aluminum oxide A1203, no peaks of lithium or magnesium being observed. An XPS spectra of the surface of a sample oxidized for 5 minutes at 530 °C in the air furnace are shown in Figure 7(a). Apart from the peaks of carbon, oxygen, and magnesium, there is a lithium (Li 1 s) peak at 56 eV binding energy. One of the specimens was


8° 1 ~o~

4 0 -

2 0 -

5 10

Sputtering t ime (min)

(a)

a ox id ized-Li . . . . . . . . . . . . . . . . . . . . . . . . . .

x c a r b o n

[ ] oxygen

• oxid ized-M~

• o~id.iz~:,~.. o m e t a M c - A 1

I

15

,,,-I 2~

t

k~

8 0 -

/ O

/ /

o J 60--t ~- ,, oxidized-Li

40- ~ ,'~ o oxygen • o_xi_d 'me__d _-M_g

~\ i ~ • oxide-A!. ~.0- ~ """~, -'-'a ............ ~/ ~ o ~etallic-Al_

9 . . . . . . . ~ - - ~ ' ~ .... . . . . . . . . . . . ,,-----~_..~, 1 - ~ - . ~

I I

0 20 4 0 6 0

Sputtering t ime (min)

(b)

I

80

80 -

. ~ 6 0 -

/ o

/

t . _ o . 1 . . . /

f i ~ . ............... ~ .............. . ............... : ........ o

oxidized-Li . . . . . . . . . . . . . . . . . . . . . . . . . .

× carbon

[] oxygen

• o x i ~ - ~ . ,

o meta l l i c -Al

8 0 -

6 0 -

40 ¸

a~ ~. 20 ̧

"~, ...... / ",.... ..... ~, ....... g ~- . . . . . . . . ~ . . . . . . . .e, . . . . . . . . . . . . a

oxidized-Li . . . . . . . . . . . . . . . . . . . . .

x carbon

[] oxyRen

• o_ra_~_z~_ : ~

• oxidized-Al

o metallic-A1

0 20 40 60 80 0 o ~o 4'0 ~o 8'0 ~oo Sputtering time(min) Sputtering time(min)

(c) (d)

Fig. 6--Composition-sputtering time profiles for the specimens oxidized for (a) 2.5 min, (b) 5 min, (c) 10 min, and (d) 30 min, at 530 °C in air.

o l

x~ am z,J. zs ~ .. ~ (il,z,) . • : " o 2~

.":';'~'-:~.~,j~..). :~:

"~7. ~liE mn '1~ Im Iki 21118 0

• ~ "~,, : %

BOO ~ 400 300 200 1 ~ 0

B m c l ~ energ~ (dO

(a)

6o0

A 1 2 s

3

• " : : x v : "..,., "._ ~ o 2 e

" ; : : . . ' 3 " : . , . . ""~r-%r-r~/ : "tm ,~, "~ ~, .k

2--

". ' ~ 2 m l ~ 211

" ~'~.""-'.' ~ ~ o a,

Binding energy (eV)

(b)

Fig. 7 - - ( a ) XPS spectra of the surface of a specimen heat treated for 5 min at 530 °C in lab air; (b) same treatment as in (a) plus the sample was quenched into water.

METALLURGICAL TRANSACTIONS A VOLUME 18A, APRIL 1987--685,

quenched in water after it had been oxidized for 5 minutes. The XPS spectra, Figure 7(b), show peaks of carbon, oxygen, and magnesium, and the lithium (Li Is) peak is now missing. This indicates that oxidized lithium was washed away during quenching of the specimen into water. The peaks of oxidized aluminum are also present in the spectra, Figure 7(b). The XPS spectra obtained from the surface of a specimen oxidized 30 minutes are shown in Figure 8. There are peaks of carbon, oxygen, and lithium, while the peaks of magnesium and aluminum are absent. The XPS results confirm the AES results that after longer oxidation times the top surface is completely covered by lithium compounds.

B. X-Ray Diffraction Analysis

The oxidation products formed on the surface of the alloy were identified by using a diffractometer, without separating the oxide scale from the matrix alloy. The X-ray diffraction patterns were obtained for the samples oxidized from one minute up to 5 hours at 530 °C in lab air. Analysis of the diffraction patterns recorded from the surface of the specimens oxidized for one minute and 2.5 minutes revealed no oxide phases. At these short oxidation times the volume fraction of the oxide phases was too low to separate the characteristic diffraction peaks from the background. How- ever, after 5 minutes oxidation calculation of d values showed that the reaction product was Li2CO3. The d values matched very well with the monoclinical phase Li2CO3. The diffraction patterns obtained for the samples oxidized 10, 15, 20, 30, 60 minutes, and 5 hours showed that the intensity of diffracted peaks characteristic of Li2CO3 progressively increased. This indicates that with increased oxidation times more and more lithium carbonate (Li2CO3) was forming. A part of the diffraction patterns of the samples oxidized for 10 minutes and 5 hours is shown in Figure 9. The diffraction peaks from the Li2CO3 and aluminum are marked. There are some peaks which have not been marked; they were also present in the as-polished condition before oxidizing. There is a strong aluminum (200) peak at 68.9 deg 20. The (111) aluminum peak is missing, probably due to the texture in the alloy. Small peaks of aluminum due to the CrKt3 radiation are also present in the diffraction patterns. After having made the X-ray diffraction patterns a

.Li I s

" 0 l s 0 2 s 5 .~ .'. C.. l e ~

600 500 400 300 200 100

Binding energy (eV)

Fig. 8 - - XPS spectra of the surface of a specimen heat-treated for 30 min at 530 °C in lab air.

,F,,I

k--I

(, ~)

IlIN

s

Key: A1

' C

a ....... i

-- oR .......... ~-

I - ~- I

i

-i-

--4

-- 4 - - -- --4--

--4 . . . . . . . .

= alumin~

= Li2CO 3

lit

. . . . . . 411---

I

. . . . 4 1 - -

"1

80 70 60 5O 40 30

2O Fig. 9 - -X- ray diffraction patterns of the samples oxidized for (a) 10 min, (b) 5 h, at 530 °C in air.

couple of the oxidized samples were washed in water, then diffraction patterns were recorded once again. It showed no diffraction peaks characteristic of Li2CO3, indicating that all the lithium carbonate had been washed away.

In order to obtain better data, X-ray powder diffraction analysis was carded out using a Guinier camera with a monochrometer. The samples were heat-treated at 530 °C in lab air for different period of times, and the surface oxides were scratched off from the substrate with a knife. The X-ray powder diffraction data obtained for a sample heat treated for 1 hour are shown in Table II.

A comparison of the measured d-spacings with the corresponding literature data 23 revealed the presence of metallic a l u m i n u m , L i2CO3, t~-LisAIO4, and 7-LiA102 phases. There was excellent agreement between the d-spacings measured and the data reported for LiECO3 in the powder diffrac-


Table II. The X-Ray Powder Diffraction Data

d (/k) Phase d (/~) Phase

5.184 a 2.112 C 4.152 C 2.077 C 4.054 a 2.021 A1 4.022 a 1.926 a 3.990 y 1.892 C 3.784 C 1.862 C, 7 3.688 a 1.807 C 3.027 C 1.616 C 2.917 C 1.591 C, y 2.806 C 1.542 C 2.674 y 1.508 C, y 2.624 C 1.463 C 2.612 a 1.429 A1 2.581 y 1.388 C 2.480 C 1.370 C 2.427 C, a 1.349 C 2.335 A1 1.335 C 2.271 C 1.312 C 2.253 C

Key: AI = aluminum a = a-LisAIO4 C = Li2CO3 31 = ~-LiAIO2

tion file. 23 From the relative intensities of the diffraction peaks it was obvious that the principal reaction product was Li2CO3. The X-ray powder diffraction analysis of a sample heat-treated for 10 minutes showed only metallic aluminum, Li2CO3, and a-LisA104 phases; the y-LiA102 phase was absent. This suggests that the y-LiA1Oa phase forms after longer heat treatment times. Guinier type X-ray diffraction patterns were also recorded for a sample which had been washed in water after heat treatment in the lab air; apart from the metallic aluminum, no other phases were detected.

Previously, the AES results (Figure 6) showed that magnesium rich oxides were present near the surface after heat treatment. However, no magnesium containing phase was detected in the X-ray diffraction pattems. This suggests that either the amount of magnesium rich oxides is too small or it is present in such a finely derived form that it is un- detectable by the X-ray.

In a previous study Field et al. 24 reported that under 30 torr dry air in the 500 to 525 °C temperature range, y-LiA102 is the predominant oxide phase on the A1- 2.7 wt pct Li alloy. The present results, however, indicate that on the commercial A1-Li-Cu-Mg-Zr alloy oxidized in lab air at 530 °C and at atmospheric pressure the dominating reaction product is Li2CO3.

Field and his co-workers 24 discussed in terms of free energy changes, the various reactions leading to the formation of Li20, Li2CO3, LiA102, and LiAlsOs phases. The formation of Li2CO3 may occur in the following two-step reaction proposed by Field et al. :24

6Li + A1203 = 3Li20 + 2A1 [3]

and then

Li20 + CO2 = Li2CO3 [4]

The Gibbs free energy changes for the reactions [3] and [4] at 530 °C are - 7 3 KJ/mol of AIzO3 and - 9 7 KJ/mol of CO2, respectively. The partial pressure of CO2 in equilibrium with Li20 and Li2CO3 calculated from Eq. [5] is

4.9 • 10 -7 atm. The partial pressure of CO2 in air is 3 • 10 -4

(assuming that air contains 0.03 pct carbon dioxide on volume basis), and this value is greater than the equilibrium partial pressure of COs in contact with Li20 and Li2CO3 at 530 °C, indicating that under the present environmental conditions the formation of Li2CO3 is quite possible.

AG ° Pco2 = exp RT [5]

There are a number of thermodynamic possibilities leading to the formation of LiAIO2 phase; reaction between Li20 and A1203 to form this phase is accompanied by a Gibbs free energy change of approximately -134 KJ/mol of A1203; reaction between Li2CO3 and A1203 to form LiA102 is accompanied by a free energy change of - 3 6 KJ/mol of A1203. A direct reaction between Li and A1 with 02 to form LiA102 is also possible.

Li + A1 + O2 = LiA102 [6]

Reaction [6] occurring at 530 °C is accompanied by a Gibbs free energy change of - 1018 KJ/mol of oxygen.

The formation of a-LisA104 phase may be expressed by the following equation:

5Li + A1 + 202 = Li5A104 [7]

However, no thermodynamic data are available for the above reaction. An alternate route of the a-Li5A104 formation may be a reaction between Li and A1203. In the literature Konys and Borgstedt 25 reported that lithium metal heated in a-A1203 crucibles in vacuum to 527 °(2 reacts completely with the crucible material. The reaction products identified by means of X-ray powder diffraction analysis were Li9A14, ot/fl-LisAlO4, and LiA102.

C. Oxide Morphology

A scanning electron micrograph of a specimen annealed 1 minute at 530 °C in air is shown in Figure 10. Voids have been formed on the surface probably due to the selective removal of lithium. They seem to follow the slip lines formed during mechanical polishing. Wagner 26 explained

Fig. 1 0 - - S c a n n i n g electron micrograph showing voids formed on the surface of a specimen annealed 1 min at 530 °C in air.


the selective removal processes at elevated temperatures in terms of a slow interdiffusion step. At low points in the surface the flux of the reactive component from subsurface sites to the surface is greater than at other locations because of a shorter diffusion distance. 26'z7

A SEM micrograph of a specimen oxidized 2.5 minutes shows large and small oxide nodules (Figure 1 l(a)). EDS analysis carried out on large oxide nodules showed that they very often contained a small amount of iron. In a few cases a small amount of titanium and the impurity element cal- cium were found associated with the large oxide nodules. It seems that the large nodules may have precipitated on the coarse equilibrium particles (A16CuLi3, A17CuzFe) present in the alloy, z8 Optical microscopy of the specimens oxidized 2.5 minutes showed that grain boundaries have become visible. The oxide nodules studied with polarized light displayed blue color. The large and small oxide nodules seen with polarized light are shown in Figure ll(b). Csanady et al . 19 explained the blue color in terms of interference

effect produced by the periodicity of perpendicularly ori- ented lamella. The light waves scattered by the different lamella interfere with each other and give a bluish appear- ance. Scanning electron micrographs of the surface of the specimens oxidized 30 minutes and 5 hours are shown in Figures 12(a) and 12(b). The micrographs illustrate that a porous oxide layer is developed in the alloy. Another SEM instrument equipped with a windowless detector was also used in making energy dispersive X-ray analyses. The top oxide layer of the sample heat-treated for 5 hours revealed oxygen and carbon peaks; no peaks of aluminum or magnesium were observed. This indirectly confirms that the top-most oxide layer was completely made of Li2CO3.

IV. CONCLUSIONS

1. During thermal treatments of AI-Li-Cu-Mg-Zr alloys a soft surface layer is developed due to the selective oxidation of lithium.

(a) (a)

(b)

Fig. II --Micrographs of the surface of a specimen oxidized 2.5 min at 530 °(2 in air showing small and large oxide nodules. (a) scanning electron micrograph, (b) optical photo micrograph, polarized light.

688--VOLUME 18A, APRIL 1987

(b) Fig. 12--Scanning electron micrographs of the surface of specimens oxidized for (a)30 min and (b)5 h at 530 °(2 in air, showing porous oxide layer.

METALLURGICAL TRANSACTIONS A

2. The width of the soft layer was the same for the solution treatments carded out both in the lab air and in the salt bath.

3. AES and XPS studies show that after relatively short oxidation times in the lab air the surface is composed of both the lithium rich oxides and magnesium rich oxides. However, after longer oxidation times, the oxidation of lithium becomes predominant and the air/oxide interface is completely covered by lithium compounds.

4. X-ray diffraction analysis of the surface oxides formed during heat treatment of the alloy in the lab air revealed the presence of Li2CO3, a-LisAIO4, and y-LiA102 phases. The principal reaction product was Li2CO3.

5. The oxide layer developed on the alloy is porous and nonprotective. In a water shower the oxide layer can be washed away from the surface.

ACKNOWLEDGMENTS

The author would like to thank the National Swedish Board for Technical Development (STU) and Saab-Scania AB, Aircraft Division for financial support. Advice and support given by Professor T. Ericsson and Dr. S. Johansson is gratefully acknowledged. The help of Dr. S. E. HiSrnstriSm (AES) and Mr. Wu, Changru (XPS) is appreciated. Dis- cussions with Dr. C. Burman on various aspects of the work have been fruitful.

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