Effect of Scandium and Chromium on the Structure and Heat Resistance of Alloys Based on γ-TiAl

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  • 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 -TiAl

    V. E. Oliker, V. I. Trefilov, V. S. Kresanov, and T. Ya. GridasovaUDC 621.762:66971529

    It is established that microalloying of -titanium aluminides with scandium provides an increase in heatresistance, structure refinement and modification, and formation of a dispersion-strengthened structure with acoherent bond between the strengthening and matrix phases. Proceeding from this an improvement might beexpected in strength characteristics over a wide temperature range. The effect in scandium consists inchanging the ratio of Al:Ti thermodynamic activities in the direction of forming aluminum oxide at the alloysurface during oxidation as a result of the deoxidizing effect of scandium and the formation of fine oxideinclusions. As a result of this aluminum does not form oxides within the alloy. The distribution of elementswithin the microstructure of -Ti Al with 5% Cr after oxidation at 900C for 300 h is studied. It isestablished that the surface scale layer that forms sometimes contains Cr in addition to Al and O. A diffusionmechanism is suggested for realizing the Cr-effect according to which chromium and aluminum ionsparticipate 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 foraerospace purposes, i.e., gamma-titanium aluminide, makes it possible to separate the two main problems for theirimprovement; an increase in heat resistance (900-1000C) and strength characteristics, in particular crack resistance andfatigue strength [1-3].

    Theoretical calculations show that the required heat resistance for binary Ti Al alloys should be achieved with analuminum content of about 54 at.% [4]. With this aluminum concentration at the alloy surface a continuous protective layer ofAl2O3 scale should form during oxidation in air. However, under practical conditions this normally occurs with an aluminumcontent 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 behaviorradically is the presence of oxygen, that as a rule is contained in commercial -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.

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    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 caseswhen metal oxides and mutually insoluble. Therefore in our opinion it is possible to consider that rutile is not observed within-alloys and there are only inclusions of aluminum oxide formed by the mechanism of internal oxidation. This phenomenonmay be explained by the fact that during phase formation directly from the melt the concentration of aluminum is sufficient toform 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]

    TiAl3 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), incharacteristic ScK radiation (d); concentration curve for the distribution of the lines ScK(e), TiK (f), AlK (g), CrK (h). Magnification: 500 (a), 1700 (b), 3800 (c), 1700 (d).

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    Fig. 3. Alloy microstructure after annealing and identification of scandium oxideinclusions. Image in BSE (a), in SE (b, c); concentration curve for the distribution oflines ScK (d), OK (e), TiK (f), CrK (g), AlK (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 rutileformation is highe by the order of magnitude than that of aluminum oxide has an effect. It follows from above that if thealuminum does not participate in forming oxides within the alloy (consequently the number of its atoms taking part in surfaceprocesses increases) it might be expected that there will be formation of a continuous layer of Al2O3 with a concentration ofthe components (Ti, Al) close to the calculated value. This may be achieved by introducing an element into the -alloy thatforms 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 thatintroduction of a certain amount of chromium provides the required alloy resistance to oxidation in air (1000C), although thealloy is embrittled. Apparently a chromium content of about 5% is the minimum for providing the effect suggested. It wasestablished that in this case chromium provides the required heat resistance in dry air and it makes it possible to retain to aconsiderable degree the characteristic structure for -alloys, which is suitable in order to maintain a satisfactory level ofductility. However, a continuous layer of Al2O3 scale does not form at the surface of alloy of this composition duringoxidation 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 duringoxidation indicates a high potential for scandium to form oxides. As can be seen from Table 1, the heat of formation ofscandium oxide is at a minimum. An additional factor in favor of the choice of scandium is the fact that among rare earthmetals it has the least ionic radius (0.083 nm) [11] and this facilitates its relatively high diffusion mobility in the alloy. Inaddition, introduction into the alloy of a surface-active element may promote oxide phase nucleus formation and lead to areduction in critical nucleus radius since between its size and change in free energy (with formation of Sc2O3 instead ofAl2O3) 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 -TiAl has a face-centered cubic lattice with insignificant tetragonality (c:a = 1.02 [2]), and Sc2O3has a cubic lattice [7], creates a good prerequisite for providing coherent dispersion strengthening that is effective over a widetemperature range.

    Specimens of Ti 52 Al 5 Cr alloys with a different oxygen and scandium content were prepared by repeatedelectric-arc remelting on a cooled copper substrate in an argon atmosphere. Then by analogy with [12] they were heat treated,i.e., soaked at 1300C 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 theyconsist of coarse lamellar -phase dendrites genetically connected with lamellar colonies of + 2 arranged between them(Fig. 1a). The effect of annealing as its duration increases involves successive transformation of the coarse irregular structureinto one consisting almost entirely of coarse equiaxed grains with alternating platelets of 2 and . Similar structures, typicalfor -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 immediatelyafter melting made it possible to reveal formation during melting of oxide inclusions with an average size of about 1 m.These inclusions contain either only Sc, or Sc and Cr (Fig. 2). In this connection it is possible to note that in the systemSc2O3 Cr2O3 there is typically formation of phase of variable composition based on the compound Sc3CrO6 and areas ofsolid solutions containing scandium and chromium oxides [15]. It is mentioned that above 1000C the compound3Sc2O3CrO6 was detected x-radiographically that is stable up to the melting point (2250 50C). Thus, the oxide inclusionsdetected 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 thealloy structure after annealing (Fig. 3). With an increase in Sc content there is an increase in the amount of oxide particleswhose 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 orderof 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 scandiumchromite inclusions. Image in SE (a, b); concentration curve for the distribution of linesScK (c), CrK (d), OK (e), TiK (f), AlK (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 andnature of arrangement are typical for microcomposite wear-resistant materials reinforced with hard particles.

    Isothermal oxidation of alloy specimens with scandium was performed at 900C for 300 h in air without drying ina chamber furnace. The nature of element distribution from the outer surface into the depth of the alloy makes it possible to

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    Fig. 5. Alloy microstructure containing aluminum and scandium oxide inclusions. Imagein SE (a, b); concentration curve for the distribution of lines AlK (c), OK (d), TiK (e),CrK (f), ScK (g). Magnification: 66 (a); 500 (b).

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    a

    b

    Fig. 6. Microstructure of oxidized alloy and distribution of elements into the depth from the surface in asection 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 900C, 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 asection of Al2O3 Cr2O3 scale. Intensity coefficient: 1 (Al); 0.75 (Ti);0.125 (Cr); 0.1 (Sc); 0.075 (O).

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    Fig. 8. Distribution of elements into the depth of oxidized alloy and arrangement ofchromium segregates. Image in SE (a). 5000. Concentration curve for the distribution oflines OK (b), AlK (c), TiK (d), CrK (e).

    Among the immediate problems of improving materials based on gamma-titanium aluminides alloyed with scandiumis 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 forgamma-titanium aluminides, moreover since there is no single opinion about this question. There is quite extensive publishedinformation about the opinion of authors [16] connected an increase in the heat resistance of Ti Al Cr alloys mainly withLaves phase Ti(Cr, Al)2 present within their structure. The effect of this phase is explained by its capacity to form Al2O3 scalewith a comparatively low overall aluminum content within it (37-43 at.%), whereas in binary alloys of the Ti Al system inorder to form a continuous Al2O3 oxide layer it is necessary to have 60-70% Al [5]. In those cases when the alloy structure ismainly represented by the Laves phase (Ti (37-43) Al (26-29) Cr) its key role is logical; if it is present in the form ofsparse inclusions occupying in the gamma-phase an area of the order of several per cent, its role during formation of the

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    required continuous Al2O3 layer at the alloy surface does not seem so marked. Moreover, the aluminum content in thegamma-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 totallyreflect the behavior of chromium in gamma-alloys.

    In our opinion, the Cr-effect is realized by means of a mechanism according to whic...

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