1068-1302/00/0910-0487$25.00 ©2001 Plenum Publishing Corporation 487

Powder Metallurgy and Metal Ceramics, Vol. 39, Nos. 9-10, 2000

EFFECT OF SCANDIUM AND CHROMIUM ON THE STRUCTUREAND HEAT RESISTANCE OF ALLOYS BASED ON γ-TiAl

V. E. Oliker, V. I. Trefilov, V. S. Kresanov, and T. Ya. GridasovaUDC 621.762:669’715’29

It is established that microalloying of γ-titanium aluminides with scandium provides an increase in heat

resistance, structure refinement and modification, and formation of a dispersion-strengthened structure with a

coherent bond between the strengthening and matrix phases. Proceeding from this an improvement might be

expected in strength characteristics over a wide temperature range. The effect in scandium consists in

changing the ratio of Al:Ti thermodynamic activities in the direction of forming aluminum oxide at the alloy

surface during oxidation as a result of the deoxidizing effect of scandium and the formation of fine oxide

inclusions. As a result of this aluminum does not form oxides within the alloy. The distribution of elements

within the microstructure of γ-Ti − Al with 5% Cr after oxidation at 900°C for 300 h is studied. It is

established that the surface scale layer that forms sometimes contains Cr in addition to Al and O. A diffusion

mechanism is suggested for realizing the Cr-effect according to which chromium and aluminum ions

participate in place of titanium ions in forming Al2O3 − Cr2O3 scale at the metal − air atmosphere interface.

Keywords: aerospace structural materials, gamma titanium aluminide oxidation-resistant alloys.

Comprehensive evaluation of the operating properties of one of the currently most promising structural materials for

aerospace purposes, i.e., gamma-titanium aluminide, makes it possible to separate the two main problems for their

improvement; an increase in heat resistance (900-1000°C) and strength characteristics, in particular crack resistance and

fatigue strength [1-3].

Theoretical calculations show that the required heat resistance for binary Ti − Al alloys should be achieved with an

aluminum content of about 54 at.% [4]. With this aluminum concentration at the alloy surface a continuous protective layer of

Al2O3 scale should form during oxidation in air. However, under practical conditions this normally occurs with an aluminum

content of not less than 60-70 at.% [5].One of the most notable differences between the ideal and real alloy with a capacity to affect material behavior

radically is the presence of oxygen, that as a rule is contained in commercial γ-alloys due to its very considerable affinity forboth aluminum and titanium. For example, results are given in [4] indicating that in spite of protection from oxidation(induction melting in purified argon) all of the test alloys of the system Ti − Al (the relative aluminum concentration variedfrom 0.05 to 0.9) contained oxygen in comparatively large amounts (in some cases up to several percent).

Oxygen, carried into the melt mainly from charge components, binds a certain amount of aluminum into oxide by aninternal oxidation mechanism. In addition, some part of the oxygen dissolves in the alloy crystal lattice. Thus, part of thealuminum hardly participates in forming an oxide layer in the outer surface, i.e., neither in adsorption by metal surface atomsof oxygen molecules from the gas phase in the first stage, nor in subsequent diffusion transport from the alloy to the metal −scale interface. As a consequence of this there is a change in the ratio of the thermodynamic activities for Al and Ti oxidationin the direction of forming rutile that does not have a capacity to protect the alloy from oxygen penetration in the requiredtemperature range. There is no exact description of a universal mechanism for alloy oxidation due to the complexity andvariety of the processes. However with considerable certainty it is possible to talk about some general assumptions.The chemical composition and morphology of scale on alloys is connected with the thermodynamic activities of the alloy

Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev. National TechnicalUniversity of the Ukraine “Kiev Polytechnic Institute.” Translated from Poroshkovaya Metallurgiya, Nos. 9-10(415), pp. 77-88, September-October, 2000. Original article November 24, 1999.

488

Fig. 1. Transformation of alloy microstructure in relation to annealing duration. ×500.

a) Original condition (ingot), b-d) after annealing for 3, 7, and 10 h, respectively.

components which in turn depend on the concentration of the corresponding metal in the alloy. This is valid for the cases

when metal oxides and mutually insoluble. Therefore in our opinion it is possible to consider that rutile is not observed within

γ-alloys and there are only inclusions of aluminum oxide formed by the mechanism of internal oxidation. This phenomenon

may be explained by the fact that during phase formation directly from the melt the concentration of aluminum is sufficient to

form Al2O3 inclusions. With external oxidation of the alloy the deficit in aluminum that arises as a result of the fact that it is

TABLE 1. Thermodynamic Properties of Compounds

Compound −∆H, kJ/mole Reference

Sc2O3 1720.775 [7]

Al2O3 1584.00 [8]

Cr2O3 1130.436 [7]

TiO2 862.10 [8]

TiN 308.10 [8]

TiAl 3 146.30 [9]

Ti3Al 98.23 [9]

TiAl 75.24 [9]

ScAl3 41.90 [10]

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Fig. 2. Ingot microstructure and identification of inclusions. Image in SE (a, b, c), in

characteristic ScKα radiation (d); concentration curve for the distribution of the lines ScKα

(e), TiKα (f), AlKα (g), CrKα (h). Magnification: 500 (a), 1700 (b), 3800 (c), 1700 (d).

490

Fig. 3. Alloy microstructure after annealing and identification of scandium oxide

inclusions. Image in BSE (a), in SE (b, c); concentration curve for the distribution of

lines ScKα (d), OKα (e), TiKα (f), CrKα (g), AlKα (h). Magnification: 700 (a, b);

10000 (c).

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consumed in internal oxidation is the reason for its insufficient activity. Under these conditions the fact that the rate of rutile

formation is highe by the order of magnitude than that of aluminum oxide has an effect. It follows from above that if the

aluminum does not participate in forming oxides within the alloy (consequently the number of its atoms taking part in surface

processes increases) it might be expected that there will be formation of a continuous layer of Al2O3 with a concentration of

the components (Ti, Al) close to the calculated value. This may be achieved by introducing an element into the γ-alloy that

forms oxides more actively than Al.

Our choice of basic alloy (Ti − 52 Al − 5 Cr) for further improvement is based on the results of studies in NASA,

Lewis Research Center, Cleveland, USA [6]. The considerable series of studies carried out at this center has shown that

introduction of a certain amount of chromium provides the required alloy resistance to oxidation in air (1000°C), although the

alloy is embrittled. Apparently a chromium content of about 5% is the minimum for providing the effect suggested. It was

established that in this case chromium provides the required heat resistance in dry air and it makes it possible to retain to a

considerable degree the characteristic structure for γ-alloys, which is suitable in order to maintain a satisfactory level of

ductility. However, a continuous layer of Al2O3 scale does not form at the surface of alloy of this composition during

oxidation in undried air. Comparatively large areas of a phase enriched in titanium are detected in the surface layer.

Evaluation of the thermodynamic characteristics of phases that may form in the system Ti − 52 Al − 5 Cr − Sc during

oxidation indicates a high potential for scandium to form oxides. As can be seen from Table 1, the heat of formation of

scandium oxide is at a minimum. An additional factor in favor of the choice of scandium is the fact that among rare earth

metals it has the least ionic radius (0.083 nm) [11] and this facilitates its relatively high diffusion mobility in the alloy. In

addition, introduction into the alloy of a surface-active element may promote oxide phase nucleus formation and lead to a

reduction in critical nucleus radius since between its size and change in free energy (with formation of Sc2O3 instead of

Al2O3) there is an inversely proportional relationship. It is well known that the less the average size of oxide particles

(strengthening phase) and the average distance between them, the greater is alloy heat resistance and crack resistance. Finally,

the fact that the compound γ-TiAl has a face-centered cubic lattice with insignificant tetragonality (c:a = 1.02 [2]), and Sc2O3

has a cubic lattice [7], creates a good prerequisite for providing coherent dispersion strengthening that is effective over a wide

temperature range.

Specimens of Ti − 52 Al − 5 Cr alloys with a different oxygen and scandium content were prepared by repeated

electric-arc remelting on a cooled copper substrate in an argon atmosphere. Then by analogy with [12] they were heat treated,

i.e., soaked at 1300°C in a vacuum followed by furnace cooling to room temperature.

A study of the alloy microstructure by means of an optical microscope showed that immediately after melting they

consist of coarse lamellar γ-phase dendrites genetically connected with lamellar colonies of γ + α2 arranged between them

(Fig. 1a). The effect of annealing as its duration increases involves successive transformation of the coarse irregular structure

into one consisting almost entirely of coarse equiaxed grains with alternating platelets of α2 and γ. Similar structures, typical

for γ-TiAl, were observed in the alloys Ti − 47.0 Al − 1.0 Cr − 0.91 V − 2.6 Nb [13] and Ti − 47.0 Al − 1.0 Cr − 1.0 Mn −1.5 Nb − 0.2 Si [14].

A study of alloy microstructure by means of a Camebax SX-50 (France) electron-probe microanalyzer immediately

after melting made it possible to reveal formation during melting of oxide inclusions with an average size of about 1 µm.

These inclusions contain either only Sc, or Sc and Cr (Fig. 2). In this connection it is possible to note that in the system

Sc2O3 − Cr2O3 there is typically formation of phase of variable composition based on the compound Sc3CrO6 and areas of

solid solutions containing scandium and chromium oxides [15]. It is mentioned that above 1000°C the compound

3Sc2O3⋅CrO6 was detected x-radiographically that is stable up to the melting point (2250 ± 50°C). Thus, the oxide inclusions

detected by us containing chromium are apparently scandium chromites.

The dimensions and disordered distribution of oxide inclusions in the matrix of equiaxed grains are maintained in the

alloy structure after annealing (Fig. 3). With an increase in Sc content there is an increase in the amount of oxide particles

whose average size is maintained (Fig. 4).

An increase in the amount of oxygen in the alloy above the critical level, which may be connected with scandium,

leads to formation of Al2O3 inclusions in the matrix. Their size exceeds that of scandium oxide inclusions at least by an order

of magnitude (Fig. 5). Inclusions of Al2O3 of approximately the same size were detected previously [6] in studying alloys of

492

Fig. 4. Alloy microstructure with an increased Sc content and identification of scandium

chromite inclusions. Image in SE (a, b); concentration curve for the distribution of lines

ScKα (c), CrKα (d), OKα (e), TiKα (f), AlKα (g). Magnification: 800 (a); 5000 (b).

the Ti − Al − Cr system. If there is no scandium, only Al2O3 inclusions form within the alloy structure, whose dimensions and

nature of arrangement are typical for microcomposite wear-resistant materials reinforced with hard particles.Isothermal oxidation of alloy specimens with scandium was performed at 900°C for 300 h in air without drying in

a chamber furnace. The nature of element distribution from the outer surface into the depth of the alloy makes it possible to

493

Fig. 5. Alloy microstructure containing aluminum and scandium oxide inclusions. Image

in SE (a, b); concentration curve for the distribution of lines AlKα (c), OKα (d), TiKα (e),

CrKα (f), ScKα (g). Magnification: 66 (a); 500 (b).

494

a

b

Fig. 6. Microstructure of oxidized alloy and distribution of elements into the depth from the surface in a

section of Al2O3 scale. (a) Image in SE. ×4000. (b) Concentration curve for O (1), Al (2), Ti (3), Sc (4).

conclude that from the outside of a specimen a scale layer forms containing oxygen with aluminum or with aluminum andchromium (Fig. 6).

Thus, the results of studies indicate that microalloying of gamma-titanium aluminide with scandium promotes anincrease in their heat resistance at least to 900°C, formation of a dispersion-strengthened structure with a coherent bondbetween the strengthening and matrix phases (due to which it is possible to expect higher strength properties over a widetemperature range), and provision of refining and modification of the structure (which may have a favorable effect onmaterial ductility).

Fig. 7. Distribution of elements in the depth of oxidized alloy in a

section of Al2O3 − Cr2O3 scale. Intensity coefficient: 1 (Al); 0.75 (Ti);

0.125 (Cr); 0.1 (Sc); 0.075 (O).

495

Fig. 8. Distribution of elements into the depth of oxidized alloy and arrangement of

chromium segregates. Image in SE (a). ×5000. Concentration curve for the distribution of

lines OKα (b), AlKα (c), TiKα (d), CrKα (e).

Among the immediate problems of improving materials based on gamma-titanium aluminides alloyed with scandium

is a comprehensive study of their mechanical properties over a wide temperature range.

Results obtained by us also make it possible to include the effect of chromium in the oxidation mechanism for

gamma-titanium aluminides, moreover since there is no single opinion about this question. There is quite extensive published

information about the opinion of authors [16] connected an increase in the heat resistance of Ti − Al − Cr alloys mainly with

Laves phase Ti(Cr, Al)2 present within their structure. The effect of this phase is explained by its capacity to form Al2O3 scale

with a comparatively low overall aluminum content within it (37-43 at.%), whereas in binary alloys of the Ti − Al system in

order to form a continuous Al2O3 oxide layer it is necessary to have 60-70% Al [5]. In those cases when the alloy structure is

mainly represented by the Laves phase (Ti − (37-43) Al − (26-29) Cr) its key role is logical; if it is present in the form of

sparse inclusions occupying in the gamma-phase an area of the order of several per cent, its role during formation of the

496

required continuous Al2O3 layer at the alloy surface does not seem so marked. Moreover, the aluminum content in the

gamma-base of these alloys, in particular Ti − 52 Al − 5 Cr, according to data in [6] is about 53%, which, as was noted above,

is insufficient for forming a continuous layer of Al2O3. It follows from this that the suggested mechanism does not totally

reflect the behavior of chromium in gamma-alloys.

In our opinion, the Cr-effect is realized by means of a mechanism according to which chromium atoms replacing

some of the titanium atoms in the titanium sublattice of the TiAl intermetallic participate together with aluminum atoms in

absorbing oxygen atoms at the alloy-air interface in the initial stage of oxidation, and then after ionization in crystal-chemical

formation of the oxide. Comparison of the thermodynamic characteristics indicates that the probability of forming chromium

oxide (−∆H0298 = 1130.1 kJ/mole [7]) is higher than for TiO2 (−∆H0

298 = 862.1 kJ/mole [8]). Here chromium oxide has a

capacity to form continuous Cr2O3 − Al2O3 solid solutions together with aluminum oxide. This makes it possible to suggest

that the dissociation pressure for the aluminum oxide − chromium oxide system is greater than the dissociation pressure for

titanium oxide for the titanium, chromium, and aluminum concentrations in the alloy selected. As a result of this scale not

containing titanium forms at the surface. As the oxidation duration increases the thickness of the scale layer increases due to

transport towards it of aluminum and chromium atoms. Here the diffusion coefficient for chromium in β-titanium at 900°C(1.426⋅10−13 m2/sec) is greater than that for self-diffusion of titanium at the same temperature (0.29⋅10−13 m2/sec [17]), and

the atomic radius is smaller (RCr = 0.127 nm, RTi = 0.146 nm [18]). Thus, chromium atoms outstrip titanium atoms in diffusion

by titanium vacancies towards the metal − scale interface. In addition, in the binary system Cr2O3 − Al2O3 it is possible to

form scale both in the form of a solid solution based on one of the oxides, and in the form of immiscible solid solutions [19].

In favor of this version is the fact that the diffusion coefficient for chromium ions in Cr2O3 (D = 0.32⋅10−8 m2/sec [20]) is

greater by approximately five orders of magnitude than the diffusion coefficient for chromium in titanium. This is confirmed

both by the presence of chromium in the sale formed (Fig. 7), and the fact that within the metal close to the scale − metal

interface pores are visible formed apparently due to merging of vacancies as a result of active diffusion of chromium cations

into the scale. Formation of pores playing the role of barriers for transport of metal cations into the scale reduces its growth

rate. It should be noted that chromium in the scale layer is not uniformly distributed. In certain areas the analyzer records only

Al and O, and an increased chromium concentration is observed ahead of the Al2O3 scale − alloy interface (Fig. 8). These

chromium segregates may block the transport of titanium cations into the scale assisting formation of only aluminum oxide.

Formation of pores that play the role of barriers for transport into the sale of metal cations reduces the growth rate of its

thickness.

On the other hand, one of the reasons for nonuniform chromium distribution within the scale may be the fact that

rates of chromium oxidation and evaporation rates at 950°C are approximately the same [21]. From this it is possible to

conclude that an increase in heat resistance for γ-TiAl alloys on introducing chromium into them is achieved due to a

diffusion mechanism, according to which chromium ions take part instead of titanium ions in forming the Cr2O3 − Al2O3 scale

at the metal − air atmosphere interface. The process of scale formation may be presented in the form of several stages: at first

aluminum oxide forms and chromium oxide diffusing through it then forms a Al2O3 − Cr2O3 solid solution.The authors sincerely thank Academician V. V. Skorokhod for numerous valuable discussions of the results

obtained.

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