1986-Met Trans-Grain Refinement of Al by Ti C

Development of AI-Ti-C Grain Refiners Containing TiC

ABINASH BANERJI and WINFRIED REIF

Cast A1-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred AI-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A14C3 and Ti3A1C which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB2 commercial grain refiner. TiC particles introduced via the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals.

I. INTRODUCTION

GRAIN refinement of aluminum and its alloy castings through additions of grain refiners has been a common industrial practice. It has been known that some of the transition metals, viz . , Ti, Zr, and Nb, can grain-refine aluminum. Several authors ~2'3 have asserted that refinement by these elements, which form peritectic systems with aluminum, is due to nucleation of solid solution grains by crystals of aluminides of these elements via the peritectic reaction. As a contrast, it was also found that marked reduction in the grain size could occur at concentrations of these elements which are too small to produce their respective compounds with aluminum. Cibula 4 attributed this grain refinement obtainable through additions of transition elements in hypo- peritectic concentrations, to their interstitial carbides which could possibly arise as a result of reaction of transition elements with the traces of carbon which is almost always present in the melts, since aluminum is usually melted in graphite crucibles. The so-called "carbide theory" has been generally accepted in spite of a few contradictory results. 1.5.6

Cibula 4 made various trials to add carbon to aluminum melts in the form of graphite powder or rod, carbon tet- rachloride, carbon monoxide, acetylene, or high carbon steel. Carbon was also added together with potassium halide flux or the same picked up by melting aluminum in carbo- naceous crucibles. The work was further extended 7 to syn- thesize hardeners by reacting potassium titanofluoride with graphite. TiC powder was also mixed with aluminum powder and pressed into pellets which were subsequently stirred into aluminum melts, or alternately carbide powder was stirred into small pellets of molten flux. But all the above trials virtually failed to introduce any appreciable amount of carbon, and Cibula finally concluded that very little success was achieved in attempts to increase carbon of aIloys containing titanium and no useful results were obtained owing to the difficulties in forming and dispersing the carbides.

In the subsequent era, from time to time, though, there were some attempts directed toward introducing carbon or

ABINASH BANERJI, Scientist on leave from Regional Research Labo- ratory (CSIR), Bhopal, is Research Associate, and WINFRIED REIF is Professor, both with the Institut fiir Metallforschung-Metallkunde, Tech- nische Universit~it Berlin, Strasse des 17 Juni 135, D-1000 Berlin 12.

Manuscript submitted January 2, 1986.

TiC in A1-Ti alloys, but they either did not succeed at all or could at the best generate only too little carbide to provide any scope for practical use. These included mainly the works of Mondolfo and co-workers, 1,6 Lihl et al. ,8 Nakao et al. ,5,9 and Thury. ~o The latter reacted graphite with potassium titanofluoride salt in molten aluminum melt similar to that of Cibula and briefly outlined the possible chemical reactions, but failed to bring forward any new ideas other than what Cibula had already conveyed. Morimune et al. " reported that TiC particles are fairly stable in aluminum melts but decompose gradually during isothermal holding.

Now, examining the state of art prevailing over the last three to four decades as well as based on our own experience to increase the carbon content of A1-Ti alloy melts, we felt that there are two essential problems encountered in syn- thesizing these alloys, viz . , poor wettability of carbon with molten aluminum, and the other problem which will be dis- cussed later in the paper, that even if carbon is successfully reacted with titanium to form TiC particles in the melt at the usual melt temperatures, then also they would fail to grain-refine aluminum due to their thennodynamical in- stability at the usual melt temperatures. Therefore, by look- ing into the above problems with the help of energetics prevailing at the carbon/melt interface and relevant thermo- chemistry applied to the formation of carbides in the melt, we have now succeeded in generating substantial and con- trolled volume fractions of submicron sized TiC particles in A1-Ti alloys, which could in turn successfully grain-refine aluminum and its alloy castings. ~2 After briefly reporting the initial results, ~3 we now proceed further to demonstrate that these master alloys containing TiC particles could be viable potential additives to aluminum and its alloy melts for the nucleation of multiple grains in the cast products, thereby posing a strong contention for the existing A1-Ti-B and A1-Ti commercial grain refiners.

II. EXPERIMENTAL

A1-Ti-C grain refiners were prepared from commercial A1-Ti binary alloys containing 5 to 10 pct Ti (compositions in weight pct throughout unless otherwise specified) and graphite powder of 20/.tm average particle size. Typical compositions of some A1-Ti alloys and that of graphite are given in Table I.

METALLURGICAL TRANSACTIONS A VOLUME 17A, DECEMBER 1986--2127

Table I. l~pical Compositions of Materials Used

Elements Ti Fe Si V Ca S Others AI C

pct

AI-6 pct Ti alloy 5.81 0.14 0.07 0.26 - - - - 0.20 balance AI-10 pct Ti alloy 9.62 0.15 0.11 0.48 - - - - 0.30 balance

ppm

Graphite 25 160 130 - - 45 60 40 - - balance

A1-Ti alloys were melted using either electrical resistance or a medium frequency induction furnace. In the case of the former, 100 to 400 g batches of alloys were melted, and each batch was superheated to a temperature between 1023 and 1273 K and stirred by a mechanical stirrer, so that a deep vortex was generated. A similar stirrer was earlier employed by Banerji and Rohatgi 14'~5 to prepare cast aluminum matrix particulate composites. Details about the mechanical stirrer and a comprehensive review on cast composites can be found in Reference 14. Now the graphite powder, which was suitably preheated to expel the adsorbed moisture, was added to the vortex in small batches and stirred into the melt in succession. After complete additions, the stirring was further continued at a reduced speed till the reaction of graphite with the melt was completed. This was ascertained by stopping the stirring action so that if any graphite particles did not react, then they were found to be rejected by the melt. However, experience showed that, to obtain desired results, the entire process of incorporation and reaction of the graphite particles with the melt must be completed rapidly within an optimum time which depended somewhat upon such factors as the melt chemistry, size of the graphite particles, temperature of the melt, and the in- tensity and mode of stirring. Further details about them can be found elsewhere. ~2 Holding the melts longer than this optimum reaction time was found to "poison" the melts, and the resultant master alloys no longer possessed good grain refining quality for aluminum castings. Now, after the graphite/melt reaction was completed, the stirrer was with- drawn and melt poured into permanent molds.

In case of induction melting, in situ electromagnetic stirring was utilized to incorporate the graphite particles into the melt and also to activate reaction at the graphite/melt interface. Batches of 500 to 1000 g of AI-Ti alloys were melted and the rest of the process was quite similar to the mechanical stirring process, except that in the case of induction melting, incorporation of graphite into the melt and the subsequent reaction for the formation of TiC apparently needed more time than that in the case of mechanical stirring, because of the absence of any vortex which delayed the transfer of carbon particles across the melt/oxide interface.

This resulted in occurrence of certain undesirable reactions in the melts due to prolonged holding, which was, however, necessary to get all the graphite particles incorporated and reacted with the melts.

Therefore, these melts as well as those prepared by mechanical stirring which were held longer than the optimum reaction time, e.g., due to lack of an exact optimization and/or control of this time, were further superheated to higher temperatures (preferably between 1573 and 1673 K) and held there for about 5 to 10 minutes for the rejuvenation of active nucleants, and the melts subsequently cast.

Chemical analysis (except for carbon) of the master alloys was done with an atomic absorption spectrometer. Total carbon was determined by an automatic combustion appara- tus, wherein the sample is combusted in a stream of oxygen and the carbon of the specimen is converted into CO2, which is then fed into a measuring chamber, where the concentration peak is detected with a non-dispersive spectrometer. The linear signal is integrated and displayed digi- tally after weight compensation and blank value correction. The free carbon was estimated by wet chemical analysis. The master alloys prepared as above were subsequently tested for grain refinement of commercially pure aluminum (99.7 pct). To each 100 g melt of aluminum, different amounts (0.05 to 0.2 pct) of grain refiners were added at a melt temperature of 998 K, and the melts poured at various holding times into a water cooled steel mold. For com- parison, similar castings were also prepared with additions of A1-5 pct Ti-1 pct B and A1-6 pct Ti commercial grain refiners. The cast structures were microscopically studied and grain size determined using a line intercept method. J6 The average grain size was computed from a large number of non-overlapping measurements using a computer program.

I I I . RESULTS

A. On the Metallography of Grain Refiners

Table II summarizes the different compositions of A1- Ti-C master alloys prepared and their respective typical microstructures are shown in Figures l(a) through (e). They

Table II. Typical Ti and C Analyses of Master Alloys

Master Alloy Total

Ti Pct

Carbide Excess Total Carbide

C Pct

Free Free (Calculated) xl0 -3 X10 -6

AI-5 pct Ti-0,5 pct C 4.91 AI-6 pct Ti-l.0 pct C 5.64 A1-7 pct Ti-l.5 pct C 7.25 A1-8 pct Ti-2.0 pct C 7.88

1.632 3.278 0.41 0.408 3.244 2.396 0.82 0.811 5.712 1.538 1.44 1.428 7.204 0.676 1.82 1.801

2.0 1.43 9.0 1.96

12.0 3.06 19.0 6.95

2128--VOLUME 17A, DECEMBER 1986 METALLURGICAL TRANSACTIONS A

(a) (d)

(b) (e) Fig. 1--Typical microstructures of master alloys having compositions: (a) A1-5 pct Ti-0.2 pct C, (b) AI-5 pct Ti-0.5 pet C, (c) A1-6 pct Ti- 1.0 pet C, (d) AI-7pct Ti-l.5 pct C, and (e) AI-8 pct Ti-2.0 pct C. Mag- nification 166 times.

(c)

contained about 5 to 8 pct Ti, up to 2 pct C, and the rest aluminum. The usual impurities including mainly V, Si, and Fe did not exceed 0.7 pct.

The microstractures of master alloys exhibited primary A13Ti particles and another particulate phase which segregated generally at the cell or grain boundaries. The latter phase consisted of fine particles usually of submicron size as shown in the secondary electron image (SEI) of a selected region of segregation (Figure 2(a)). Figures 2(b) and (c) show typical X-ray mappings of Ti and C, respectively, in the region of Figure 2(a). Enrichment of both Ti and C in these particles confirmed that they are TiC. The volume fraction of TiC increased with increasing carbon content of the alloy (Figures l(a) through (e)) and though the micro- structure of the alloy showed very high concentration of TiC


(a)

(b)

at a composition of 1.8 pct C and 8 pct Ti, presence of some A13Ti needles could be still observed. Further, no systematic estimation of the volume fraction of TiC could be made as the particles were too fine to be observed indi- vidually in an optical microscope and hence the interparticle spacings could not be sufficiently resolved for their estimation.

The master alloys which were cast before all carbon reacted completely with the melt, exhibited frequently unreacted or partly reacted graphite particles as shown in Figure 3. In general also, as evident from the chemical analysis of free carbon (Table II), the microstructures of sufficiently well- reacted alloys, too, showed the presence of some graphite. The amount of free graphite eventually increased with decreasing time of processing after graphite additions. A theoretical calculation of unreacted carbon which should be in equilibrium with the excess titanium of the melt (see Appendix) yielded considerably smaller values than the actual free carbon analyses (Table II). This discrepancy might be attributed to (a) possibly much lower activity of titanium in the melt than its wt pct used in the calculation, because its actual concentrations may not strictly conform to dilute concentration, (b) insufficient holding time of the melts for attaining equilibrium conditions.

Figure 4(a) shows a reaction zone along the periphery of such a carbon particle, indicating that the carbon particle had just begun to react with titanium when the melt solidified, preventing any further reaction to proceed. On the other hand, Figure 4(b) shows that a similar carbon particle has now been penetrated by the melt and reaction proceeds further via diffusion, and in Figure 4(c) it can be seen that yet another carbon particle has completely reacted generating fine TiC particles which, however, still did not disin- tegrate and disperse into the melt. From these micrographs it is evident that carbon particles of about 20/zm average size would generate a large number of TiC particles which on disintegration from the mother compact, disperse as fine particles of submicron size. This is well illustrated in the electron micrographs showing individual TiC particles (Figures 5(a) and (b)). Figure 5(a) shows a Scanning

(c) Fig. 2--Electron probe microanalysis of master alloy: (a) SE1 of TiC segregated region, (b) Ti-X-ray image, and (c) C-X-ray image. Mag- nification 1660 times.

Fig. 3--Optical micrograph of A1-Ti-C master alloy showing unreacted graphite particles. Magnification 166 times.


(a) (a)

(b)

Fig. 5 - Electron micrographs of TiC particles: (a) SEM picture. (b) TEM picture. Magnification of (a) 2120 times and of (b) 5300 times.

(b)

(c)

Fig. 4--Optical micrographs of master alloy showing reaction sequence of the melt with graphite particle: (a) onset of reaction at the particle periphery, (b) penetration by the melt, and (c) completion of reaction. Magnification 830 times.

Electron Microscopic (SEM) picture of the alloy exhibit- ing a group of TiC particles which have well-developed crystalline morphology and angular to polygonal shapes. In the Transmission Electron Microscopic (TEM) picture (Figure 5(b)), these particles as well as the interparticle spacings could be further resolved, and the particles were found to range generally in the sizes of 300 to 1500 nm. Their interparticle spacings, however, varied considerably and could not be generalized.

B. On the Grain Refinement of Aluminum

Figure 6 shows a macrograph of commercially pure aluminum castings before and after grain refinement with additions of increasing amounts of AI-6 pct Ti rod and A1-6 pct Ti-1 pct C ingot alloys each. It can he seen that the amount of the columnar grains decreased with increasing additions of both the grain refiners; however, in the case of the TiC additions, the columnar zone diminished more rapidly with increasing additions and the size of the equiaxed grains became relatively more fine. At an addition of 0.2 pct of the carbide grain refiner, the columnar zone virtually disappeared, whereas this zone could still be observed in the case of similar binary alloy addition.

Figure 7 shows the variation of average grain size with increasing holding time of commercially pure aluminum


300-

A!-6%Ti

AI - 6%T i - 15~C

O~Z O. 05~f 0.19' 0 ,.2% Additions

Fig. 6--Macrographs of cast aluminum after grain refinement with different additions of A1-6 pct Ti and A1-6 pct Ti-1 pct C master alloys, respectively. Holding time = 5 min. Magnification 0.24 times.

castings through 0,2 pct additions of A1-6 pct Ti-1 pct C and A1-7 pct Ti-l.5 pct C as-cast alloys and A1-5 pct Ti- 1 pct B standard rod alloy each. The castings without any additions showed predominantly columnar grains whose average size exceeded 1200/~m, whereas the inoculated castings showed fine equiaxed grains. The above two A1-Ti-C

Qc.

200

- 1

I

1 1 0 0 I I I I [

2 5 10 30 60 Log f , min.

Fig. 7 - -Effect of holding time (t) of Al-melts on the average grain size of castings made after 0.2 pct additions of (1) A1-5 pct Ti-1 pct B rod, (2) AI-7 pet Ti-l.5 pct C ingot, (3) AI-6 pct Ti-1 pct C ingot.

(a) (b)

(c) (d)

Fig. 8 - - E P M A of crystallization center showing (a) SEI of the subgrain structure, (b) SEI of the nucleant, (c) Ti-X-ray image of the nucleant, and (d) C-X-ray image of the nucleant. Magnification of (a) 364 times and of (b) through (d) 7280 times.


grain refiners produced excellent grain refinement in the one-hour test, and average grain sizes of 221 and 210/xm were, respectively, obtained in the castings which compared well with the 201 /~m average grain size obtained with the A1-Ti-B rod, which is today regarded as the most efficient grain refiner.

Figures 8(a) through (d) show a typical result of Electron Probe Microanalysis (EPMA) of crystallization center in an inoculated casting. Figure 8(a) shows SEI of a subgrain structure, followed by magnified SEI of the nucleant (Figure 8(b)). Typical X-ray mappings in the region of Figure 8(b) showing enrichment of Ti (Figure 8(c)) and C (Figure 8(d)) confirmed that the nucleants are TiC particles, which were observed frequently in such other crystallization centers.

IV. DISCUSSION

It has been extensively observed that liquid aluminum does not wet carbon up to a temperature of about 1273 K. t7,~8,19 Above 1273 K, the contact angle was found to decrease with increasing temperatures and attained values below 90 deg at temperatures exceeding 1373 K. This was accompanied by an interfacial reaction resulting in the formation of ALC3 at the carbon/melt interface. Further, according to the chemical theory of wetting, the work of adhesion at interracial boundary is determined by the free energy loss (AG) through the liquid metal/wetted body interaction. Therefore, it should he possible to determine the wettability of carbon with a liquid metal by AG of formation of the metal carbide. In view of this, it was suggested 2~ that aluminum should wet carbon at temperatures beginning at its melting point, since free energy change of aluminum carbide formation is negative at the melting point of aluminum as follows:

4AI + 3C = A14C3, AG9~ = -191 kJ " mol -~

[1]

However, this does not actually occur, presumably because of the presence of aluminum oxide film in the melt which does exist even in a vacuum of 10 -3 Pa. The oxide film is believed to break only above 1273 K. Considering this theory, Naidich et a1.2~ employed a capillary method to remove the oxide film in order to wet carbon, but their results also showed that only in some cases they could at the best lower contact angle below 90 deg at a temperature of over 1223 K. It might be noted that using a similar technique earlier Kostikov et al . ~8 had reported a contact angle of 75 deg for aluminum on vitreous carbon at a temperature of 973 K; however, their results have not yet been confirmed by others. 2~ Moreover, the same authors have also reported large contact angles of aluminum on graphite up to temperatures of 1373 K.

Therefore, considering the various reports available on the wetting of carbon by aluminum, it could well be pre- sumed that graphite would not react with aluminum below 1273 K in the usual melting conditions. Further, the presence of titanium, though it increases the viscosity sharply, has no effect on the surface tension of liquid aluminum. 22 When all other conditions remain the same, the wettability is a function of the surface tension of liquid phase. There-

fore, graphite, either, is not likely to get wetted by AI-Ti alloy melts below 1273 K, unless some chemical reaction takes place. In this work, hence, the temperature for the addition of graphite was chosen below 1273 K in order to prevent any direct reaction of carbon with aluminum. At the same time, graphite particles were preheated to expel the adsorbed moisture in order to increase their surface energies so that a reaction between carbon and titanium could be promoted as follows:

T i + C = TiC [2]

A13Ti + C = TiC + 3A1 [3]

In addition, removal of moisture releases the hydrogen bonds, thereby causing debonding of the clusters of carbon particles and at the same time minimizing any gas pick-up of the melts.

Titanium has been found to react with carbon below 1273 K forming TiC, 23 and this is the only titanium carbide known with variable composition due to carbon depletion. For the formation of TiC, of course, it is necessary to agitate the melts sufficiently to get the carbon particles inside the melt surface for the above reactions to take place. Once the TiC particles form in the melt, they will be wetted by the melt as the contact angle of liquid aluminum with TiC above 973 K has been found to be below 90 deg and decrease rapidly with increasing temperatures. ~.24 Moreover, the above reported studies on wettability, even though carried under high vacuum or inert atmosphere, were performed on extemal blocks of the solid phase, whose surface energies and purity are likely to be much lower than the freshly formed TiC particles of this study which were generated insi tu in the melt and eventually in submicron sizes possess- ing very high specific surfaces. This would even cause much better wetting of the TiC particles than could be ex- pected from the earlier reports. Therefore, these fine TiC particles would get thoroughly dispersed into the melt owing now to a low solid-liquid interfacial energy (y sl) and mixed uniformly throughout the melt due to the stirring provided. However, stirring must be continued at least inter- mittently until the melt is cast, because both TiC and the remaining primary A13Ti precipitates tend to settle to the bottom of a static melt owing to their higher densities than the melt.

Wetting of the TiC particles with the melt is, however, desirable for their stable dispersion; on the other hand, this also has quite a disadvantage, because now the liquid aluminum being able to wet TiC should induce the following reactions at the particle/melt interface:

3TiC + 4A1 = A14C3 + 3Ti [4]

9Ti + A14C3 --- 3Ti3A1C + A1 [5]

thereby forming A14C3 and Ti3A1C compounds at the TiC particulate surfaces. However thin might be the presence of these phases, they would now change altogether the surface chemistry of TiC particles, rendering them unsuitable for the nucleation of aluminum crystals.

Now, the relative stabilities of A14C3 and TiC in Al-melt would depend upon their repective AG of formation. There- fore, in order to make any further prediction, first of all AG should be determined as a function of temperature for Eqs. [1] and [2], respectively.


Calculation of Free Energy Changes

The standard free energy change of a chemical reaction is given by:

t" T

A G ~ = (~/-/~98 K+298J A C e d T )

c T

-- z(ms~98 K+298 I A C p / T d T ) {61

where AH~98 K = standard enthalpy change at 298 K AS~98 K = standard entropy change at 298 K

AC e = change in the molar heat capacity For the formation of TiC, vide reaction [2], the free en-

ergy change (cal �9 mo1-1) is given by: 25

AG~- =

- 4 5 1 0 0 - 2.48T In T + 1.37 • 10-3T 2

+ 0.74 x 10ST -~ + 19.4T [7]

for the temperature range of 298 to 1150 K, and

AG~. = - 4 5 2 0 0 - 0 .23T In T + 0.11 • 10-3T 2

+ 0.74 x 10ST -1 + 4 .96T [8]

for the temperature range of 1150 to 1800 K. The free energy change of reaction [1] for the forma-

tion of A14C3 w a s calculated from the standard thermo- chemical properties of the reactants and product (Table III) as follows:

Changing Joules to calories, changes in the entropy, enthalpy, and heat capacity for formation of A14C3 after sim- plification are, respectively,

AS~98K = - 6 . 1 6 cal �9 K - l . mol -l

AH~98 K = - 5 3 4 4 0 cal �9 mol -~

and,

ace = 7.292 - 21.055 x 1 0 - 3 T - 8.958 x 10ST -2

+ 12.465 x 10 -6 T: cal �9 K -1 �9 tool -1 [9]

Substituting the values in Eq. [6] and integrating the functions:

AG} = - 5 7 7 9 4 - 7.29T In T - 2.08 • 1 0 - 6 T 3

+ 10.53 x 10-3T 2 + 4.48 x 10ST -j

+ 54.33T cal" mo1-1 [10]

The above Eq. [10] could be used to calculate the free energy change of A14C3 formation, vide Eq. [1], in the temperature range of 298 to 1800 K.

Figure 9 shows the standard free energy changes of the formations of TiC and A14C3 from their respective elements as functions of temperature in the range of 298 to 1800 K. It can be seen that TiC is unstable in aluminum melt below about 1450 K which would favor reaction [4] followed by reaction [5] at the TiC/Al-melt interface. This would lead to the formations of A14C 3 and Ti3A1C compounds enveloping the TiC particles.

What we call "poisoning" of the TiC particles was clearly evidenced in the studies carried out on the extracted particles with the help of EPMA and electron diffraction. The extraction process consisted of initially extracting the carbide particles on cellulose triacetate, followed by sputtering the replica face with carbon film of about 200 to 300 A and subsequently dissolving the plastic in acetone which, in turn, left a secondary carbon replica. Figures 10(a) through (c) provide some typical results of EPMA performed on the poisoned TiC particles which show in Figure 10(a), electron image of the carbide particles, followed by X-ray mappings of Ti and A1 in Figures 10(b) and 10(c), respectively. Distri- bution of carbon in these particles could not be determined because the preparation of extraction replicas was possible

/30

/50

,.. 170

~. 190

.L

~ 210 I

2 3 0 -

25O 0

I I I I I I I I

400 800 1200 1600 2000 TEMPERATURE, K

Fig. 9--Variat ion of standard free energy changes for the TiC and A14C3 formations with temperature.

Table llI. Thermochemicai Properties

- A H % 8 K S~98 K Substance kJ �9 mol- ' J �9 K- ' ' mol 1

Cp = 4.1868(a + 10 -3 bT + 105 cT 2 + 10-6dT2) J �9 K ~ �9 mol -~ (Temp. Range 298 to 1800 K)

b c d

A14C3 223.6 104.67 36.97 6.866 - 10.02 - - A1 0 28.34 7.4 - - - - - - Cgraphit e 0 5.69 0.026 9.307 -- 0.354 --4. 155


(a)

(b)

(c) Fig. 10--EPMA micrographs of extracted TiC particles showing (a) SEI picture, (b) Ti-X-ray image, and (c) AI-X-ray image. Magnification 3320 times.

only on carbon substrates. Presence of A1 in the extracted particles (Figure 10(c)) suggested chemical reactions of the type given in Eqs. [4] and [5], and the subsequent electron diffraction studies 26 confirmed the presence of phases, v iz . , A14C3 and Ti3A1C in addition to TiC. Formation of A14C3 at the graphite/aluminum interface is well known and was also reported earlier by Cisse et al. 27 while studying the nucleation of aluminum crystals on TiC blocks. However, its poisoning effect on the TiC nucleants was not known earlier. Further, the perovskite phase Ti3A1C was known to exist in the titanium-rich side of A1-Ti-C system, 2s which has now been found to develop also in the Al-rich side.

Now, let us go back to the disputed report of Crossley and Mondolfo, ~ who found that low carbon additions did not affect the grain size of aluminum castings and higher amounts increased the same, particularly at hyperperitectic Ti-levels. This was attributed to the eventual reduction in number of active AI3Ti particles available in the melt as a result of TiC formation. Now, based on our findings of the poisoning of TiC particles, their results could possibly be explained. If they had obtained TiC dispersions, it was most likely that these particles became poisoned during preparation of the grain refiners, because unless deliberate pre- caution is taken, they are atmost sure to get contaminated at least on the particulate surfaces, and prolonged holding might even cause the above-mentioned reactions to proceed further across the bulk of the particles through diffusion. For example, the latter could be the reason for the observed decrease in the number of TiC particles during isothermal holding of Al-melt containing TiC dispersions. II Similarly, here it may also be mentioned that, hitherto all previous attempts to disperse or generate TiC particles in A1-Ti melts didn't succeed in obtaining substantial amounts of the same in the solidified alloys, yet whatever amounts of this phase could be obtained, they were prone to poisoning below 1450 K as a result of the above-mentioned superficial reactions. Of course, this does not dispute the hitherto believed carbide theory forwarded by Cibula, according to which the TiC particles develop in the melt after the A1-Ti grain refiner has been added. Since the holding of the melts after the grain refiner additions is usually quite short, this would not provide enough time for the damaging reactions to occur, thereby preventing the TiC particles from getting poisoned.

The initial results of the grain refinement test (Figure 6) confirmed that the additions of TiC via the A1-Ti-C grain refiners caused substantially greater refinement than that obtainable through the additions of identical amounts of the AI-Ti binary commercial alloy, thereby removing any in- hibitions about the effectiveness of TiC as a grain refiner for aluminum. Further, frequent identification of TiC in the grain centers also confirmed that the above grain refinement was due to nucleation of aluminum by the TiC particles. It may be noted that the A1-Ti binary alloy was used in the rod form which showed block-like A13Ti particles. This form of the alloy is very effective at short holding periods, like the one used in this test. Whereas, in case of similar additions of an AI-Ti ingot, which showed A13Ti needles, a five- minute holding test produced practically no grain refinement. In the case of the A1-Ti-B master alloy also the use of rod form was made for similar reason. On the other hand, all the A1-Ti-C grain refiners were used in the as-cast ingot form, which generally showed the needle-like morphology of AI3Ti (Figure 1). In spite of that, these master alloys pro-


duced excellent grain refinements in the short holding periods (Figure 7). The average grain sizes obtained through them were comparable with those through TiB2 additions. However, this report provides only the introductory results of grain refinement and more comprehensive work is on the way to characterize these master alloys containing TiC in varying industrial conditions in cooperation with London & Scandinavian Metallurgical Co., before they could be suitably inducted into useful practical applications.

V. CONCLUSIONS

1. AI-Ti-C master alloys were produced by reacting carbon particles of 20/~m average size with Al-(5 to 10) pct Ti alloy melts, while the melt was stirred.

2. Typical master alloys contained 5 to 8 pct Ti and 0.4 to 2.0 pct C, and their microstructures showed TiC particles of average size < 1 /zm in addition to the AI3Ti phase. Amounts of free carbon were usually quite neg- ligible, but exceeded theoretically calculated equilibrium values.

3. TiC particles were found to become poisoned in the usual melting temperatures of < 1273 K, possibly due to their higher free energy of formation than that of A14C3, which led to the formation of A14C 3 and Ti3AIC compounds on the TiC particulate surfaces. Theoretical calculations showed that TiC is more stable than A14C3 above 1450 K and the experiments showed that the poisoned TiC particles could be rejuvenated by heat-treating the melts sufficiently above 1500 K.

4. All A1-Ti-C master alloys produced much better grain refinement of Al-castings than AI-Ti master alloys, and some of them produced grain sizes close to that obtainable through A1-5 pct Ti-1 pct B commercial rod additions.

5. TiC particles which became poisoned due to formations of A14C3 and Ti3AIC produced poor grain refinement.

6. TiC particles introduced via master alloys were found in the grain centers of Al-castings, thereby confirming that they are active nucleants.

APPENDIX

Calculation of unreacted carbon in equilibrium with titanium of the melt

The free energy change of a chemical reaction is given by:

s [A1] AG~ = - R T In aR x a ' •

where a denotes activity of the respective component shown by the subscript and the power stands for the number of moles each of the reactants A, B and products R, S.

For the formation of TiC from Ti and C present in A1- melts, the free energy change may be given by:

AG~ = - R T I n ( aTic / [A2] \aTi X ac /

Assuming the activity of TiC to be unity as it is in the pure state, and the activities of Ti and C equal to their weight percentages, the above Eq. [A2] becomes:

1 AG~ = - R T In

(wt pct Ti) x (wt pct C)

[A3] provided the concentrations are low.

Since In K = -AG~ where K = equilibrium con- stant, R = 8.31434 J �9 K -1 �9 mol -~, and AG ~ = -165.6 kJ �9 mol -~ (for the TiC formation at 1623 K), K becomes equal to 2.137 • l0 s. Since the usual superheatings of the melts were in the range of 1573 to 1673 K, the average temperature of 1623 K was used to calculate AG ~

Now, substituting the value of K in Eq. [A3],

(wt pct C) = 4.7 • 10-6/(wt pct Ti)

IA4]

Percentage of carbon which should be in equilibrium with the excess titanium of melt was calculated from Eq. [A4], and typical values for different master alloy compositions are provided in Table III as C pct (calculated).

ACKNOWLEDGMENTS

The authors wish to acknowledge a recent grant from London & Scandinavian Metallurgical Co. for further work in this area and thank RRL(CSIR) Bhopal for granting study leave to A. Banerji. This work is a part of Abinash Banerji's Ph.D. Thesis at the Technische Universit~it Berlin.

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1986-Met Trans-Grain Refinement of Al by Ti C