Figure 1 shows an SEM micrograph at an intermediate stage of dissolution. The facets of the TiB2 particles (A and B in Figure 1) remained sharp during dissolution, showing an absence of chemical attack of HC1 on TiB2 particles. Phase analysis was performed using selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED) in the STEM or by X-ray diffraction (XRD). Composition variation across TiB2 particles was determined using X-ray line profiling and EELS spot profiling in the STEM.

I l l . RESULTS AND DISCUSSION

A. As-Cast Condition

The microstructure of the as-cast material consists of grains of a2 + y lamellae with a dispersion of randomly oriented particles of TiB2. The microstructures of the Ti-43A1 alloys without and with the addition of TiB2 par- ticles are shown in Figures 2(a) and (b), respectively. The microstructures differ with respect to grain size. Grains in the TiB2-containing alloy are much smaller than in the alloy without TiB2. The TiB2 particles act as an innoculant and provide extra nucleation sites during so- lidification and, therefore, a finer grain size.

B. As-Extruded Condition

Extrusion of the alloy changes the microstructure of the alloy in various ways. The TiB2 particles have a ran- domly oriented rod-type morphology in the as-cast con- dition. After extrusion, these rods are aligned with the major axis (predominantly [0001]) along the extrusion direction (shown with an arrow in Figure 3).

Fig. 1 - - S E M micrograph of TiB2-containing alloy at an intermediate stage of dissolution of ti tanium-aluminide matrix (after chemical etch- ing with HC1 for 2 h).

Extrusion in the c~ + y temperature range allows the formation of proeutectoid 3/phase having an equiaxed grain morphology (for example, see A through C in Figure 4). Such equiaxed 3/grains were not present in the cast material, an observation consistent with those of other researchers. [5-8]

The proeutectoid y is the equilibrium phase and should be expected to form under equilibrium cooling, but since the kinetics of precipitation of y in an a matrix are ex- tremely slow, t7] no proeutectoid y was observed in the as-cast condition. Shong et al. r81 reported the absence of proeutectoid 3' when the cast material was cooled at rates as slow as 1 K/min. The formation of the proeutectoid y-phase grains observed in the as-extruded material may have been facilitated by the increased diffusivities of atoms during thermomechanical working of the composite. Using an SEM, the microstructure of the matrix was shown to consist of (a) lamellar a2 + y regions separated by chains of y-phase grains and (b) equiaxed y- and a2-phase grains. The two kinds of structures are marked as A and B, re- spectively, in Figure 5.

Transmission electron microscope micrographs of re- gions similar to A and B in Figure 5 are shown in Figures 6(a) and (b). The d E and y lamellae have the conventional orientation relationship: (0001)~2 II (111}v and (1120)a 2 II (110) w The lamellar regions are separated by chains or "necklace"-type regions of 3' phase. The y grains have roughly the same orientation as the y la- mellae in the a2 + y regions. The equiaxed 3/grains in the chains are separated by low-angle boundaries.

The matrix/TiB2 particle interfaces are faceted and sharp with no visible evidence of the presence of any reaction zone. The TiB2 particles contain a moderate number of stacking faults. Figures 7(a) and (b) show bright- and dark-field TEM images from one of the TiB2 particles. No attempt was made to analyze the nature of these stacking faults.

C. Extruded and Aged Condition

1. Microstructural changes in the matrix Significant changes in the microstructure of the matrix

were observed when the material was aged at elevated temperature. The microstructures of the alloy after aging at 1253 or 1473 K for 7 days are shown in Figures 8(a) and (b), respectively. The og 2 -~- y lamellar structure is gradually coarsened during aging. The mechanisms of the coarsening reaction have been investigated by other researchers. [7,8] Coarsening via discontinuous thickening of the a 2 and y lamellae and via discontinuous grain boundary migration has been identified as an operating mechanism of the coarsening reaction. The latter mech- anism is more effective above a critical temperature of 1173 K.E81 Kinetics of coarsening by discontinuous thick- ening of lamellae are abrupt, tT~ as indicated by a very large variation of the lamellae size (1.5 nm to several micrometers).

The y-phase regions parallel to t~2 + y lamellae grow at the expense of the lamellar structure by forming stepped or ledged interfaces (marked with arrows in Figure 9(a)) at the y/o~ 2 + y boundaries. Recrystallization by the conventional mechanism of migration of high-angle grain

1722--VOLUME 22A, AUGUST 1991 METALLURGICAL TRANSACTIONS A

(a) (b)

Fig. 2- -Opt ica l micrograph of Ti-43A1 alloy in the as-cast condition: (a) without any TiB2 particles showing large size grains; (b) grain refinement after addition of 7 vol pct TiB2 particles. Etched with Pratt and Whitney etchant (30 ml lactic acid, 30 ml nitric acid, 3 ml hydrochloric acid, and 300 ml water).

Fig. 3 - - L i g h t micrograph of Ti-43A1 alloy containing dispersion of TiB2 particles in the as-extruded condition; extrusion direction is ver- tical (polished with a colloidal solution of SiO2).

Fig. 4 - - Optical micrograph of Ti-43A1 alloy containing a dispersion of TiB2 particles in the as-extruded condition, showing proeutectoid y grains and a2 + "y lamellar structure (etched with Pratt and Whitney etchant).

METALLURGICAL TRANSACTIONS A VOLUME 22A, AUGUST 1991--1723

Fig. 5 - - S E M micrograph of Ti-43A1 alloy with TiB2 dispersoids after extrusion, showing regions of equiaxed grains of t~2 and 3' (A) and lamellar a2 + y with chains of y grains (B) (imaged using secondary electrons).

boundaries has also been observed. The boundaries of prior equiaxed y grains act as a reaction front. G r o w t h of 3, grains takes place by bulging of the reaction front into the o~ 2 -I- T grains without formation of any ledges (Figure 9(b)).

2. Changes in TiB2 particles and TiB2 / matrix interfaces Measurements of the particle size distribution before

and after 1000 hours of exposure at 1473 K show almost no change in the shape, size, or number density of the TiB2 particles. This is in agreement with the results of a previous study which reported little or no coarsening of TiBz particles even after 1000 hours at 1473 K. [91 Transmission electron microscope images of the bound- aries of TiB2 particles are shown in Figures 10(a) and (b). The boundaries are sharp with no signs of any re- action zone at the TiBE/matrix interfaces after 1 week of exposure at 1253 or 1473 K. The sharp steps on the TiB2 particles seen in Figure 10(b) are believed to be the growth steps from the melt, because similar steps were also observed in the as-cast material. Convergent beam electron diffraction patterns from the particles show that the structure is the C32-type hexagonal structure typical of TiB2. The EDS Spectrum from the TiB2 particles chemically extracted from the specimen extruded and aged at 1473 K for 7 days is shown in Figure 11. The absence of an A1 peak in the spectrum indicates that there can be only negligible diffusion of A1 into the TiB2 particles. Electron energy loss spectroscopy analysis was carried out only on TiB2 particles projecting out from the foil edge to avoid any signal from the matrix. The analysis across the TiB2 particles shows no change in the boron

(a) (b) Fig. 6 - - T E M micrograph of Ti-43A1 alloy with a dispersion of TiBz particles: (a) region similar to A in Fig. 5 and (b) region similar to b in Fig. 5.

1724--VOLUME 22A, AUGUST 1991 METALLURGICAL TRANSACTIONS A

(a) (b)

Fig. 7 - - T E M images from TiB2 particles in Ti-43A1 alloy, showing the presence of stacking faults inside the particles: (a) bright-field image and (b) dark-field image.

(a) (b) Fig. 8 - - S E M micrograph of Ti-43Al alloy containing a dispersion of TiB2 particles in extruded and aged condition: (a) aged at 1253 K for 7 days and (b) aged at 1473 K for 7 days (imaged using backscattered electrons).

METALLURGICAL TRANSACTIONS A VOLUME 22A, AUGUST 1991 - - 1725

(a) (b) Fig. 9 - - T E M micrograph of Ti-43A1 alloy containing TiB2 extruded and aged at 1253 K for 24 h showing modes of growth of ~/grains at the expense of a2 + 3' lamellar structure: (a) y grain growing parallel to lamellae and (b) 3' grain growing perpendicular to lamellae.

concentration. Electron energy loss spectroscopy spectra from regions near the edge and at the center of a TiB2 particle (about 1.5-/xm size) are shown in Figure 12. After subtraction of background intensity, the chemical com- positions calculated from the spectra are 65.5 at. pct B at the center and 64.8 at. pet B near the edge. This dif- ference in chemical composition is within the accuracy of the EELS technique.

This suggests that there is little or no diffusion of boron across the TiB2 particle/t~2 + 3' interface. No loss of boron from TiB2 particles is expected, because the XD process is known to yield a matrix saturated with boron.

No boron signal could be detected from t~2- or 3'-phase regions away from the TiB2 particles. An accurate de- termination of B concentration in the matrix adjacent to a TiB2 particle was not feasible because of the interfer- ence of the signals from the TiB2 particle. It may be noteworthy that the only phases detected using X-ray diffraction and TEM analysis, in the as-received and aged TiB2-containing alloy, were a2(Ti3A1), 7(TiA1), and TiB2. No borides of Ti or A1, except TiB 2, could be detected in the alloy.

The reactions between TiB2 and A1 (pure or in TiA1 or Ti3A1) leading to the formation of A1B2 and AIB~2 are

A1 + TiB2 = A1B2 + Ti [1]

A1 + 6TiB2 = A1B~2 + 6Ti [2]

The phase diagram t~~ for the system A1-B shows that A1B2 is stable up to 1248 K and that A1B~2 is stable at least up to 2123 K. From the available thermodynamic data t~~ for TiB2, A1B2, and A1B12, the value of the equilibrium constant K~ for Reaction [ 1 ] is calculated to

�9 be ~2 .3 • 10 -9 at 1248 K. The value of the equilibrium constant K2 for Reaction [2] at 1473 K is =2 .0 • 10 -47.

At 1473 K, A1B2 is unstable and decomposes into A1B12 and a liquid A1-B solution. The values of activities of A1 and Ti in the compounds Ti3A1 and TiA1 in the two-phase (a2 + 3') region of the Ti-A1 phase diagram can be estimated from the available Gibbs energy of for- mation data for these compounds. From these thermo- dynamic activities, one can assess the feasibility of Reactions [1] and [2] in a TiB2 + 3' + t~2 (or a) com- posite. However, it must be pointed out that there may be uncertainties in the thermodynamic data due to mea- surement errors and/or extrapolation to higher temper- atures. Therefore, the calculated values of activities may only be approximate.

In a two-phase mixture of stoichiometric TiaA1 and TiA1, the activity of Ti is fixed according to the follow- ing equilibrium:

2Ti + TiA1 = Ti3A1 [3]

Using the thermodynamic data [11-151 for TiA1 and Ti3A1 and assuming that the values of ACp for reactions be- tween Ti and AI to form TiA1 and Ti3A1 are negligibly small, the values of 0.54 and 5.17 x 10 -3 are obtained for axi and aAl (with solid Ti and liquid A1 as the stan- dard states) at 1248 K. The ratio a.n/aAl, therefore, is ~105, which is much greater than the value of K~ at 1248 K. At 1473 K, the values of aTi and aA~ are esti- mated to be 0.68 and 1.42 • 10 -2, respectively. If one takes the composition of t~ 2 at 1473 K as approximately 60 at. pet Ti, as shown by the phase diagram of the Ti-A1 system, ~1~ the ratio awi/aAl is estimated to range from

1726--VOLUME 22A, AUGUST 1991 METALLURGICAL TRANSACTIONS A

(a) (b) Fig. 1 0 - - T E M micrographs of T iB: /mat r ix interfaces after 7 days thermal exposure of the extruded material, showing the absence of any reaction zone at the dispersoid/matrix interface: (a) at 1253 K and (b) at 1473 K.

118

qm

4OO

r m

~m

&nnn l.nnn 2.nm 9.000 4.~11 ?.~11 8.000 0.0130 In mn

T

5,0110 Is 0r

ENERGY keY

Fig. 1 1 - - E D S spectrum from a TiB2 particle extracted from a spec- imen extruded and aged at 1473 K for 7 days, showing no diffusion of AI into TiB2 particles.

approximately 6 to 90 at 1248 K and f rom 3 to 50 at 1473 K. As these values are much greater than the val- ues of K~ and K2, it is concluded that the formation of A1B2 or A1B]2 as separate phases according to Reactions [1] and [2] is not feasible. However, it is pos- sible that a solid solution of A1B2 and TiB2 may form. The activity of A1B2 in this solution is estimated to be very small (-< 10 -8) in the temperature range of interest.

Another possible reaction is

3TiA1 + 4TiBz = 4TiB + Ti3A1 + 2A1Bz [4]

Assuming the activities of TiA1, TiB2, and Ti3A1 to be unity and the compounds to be stoichiometric, one ob- tains from the thermodynamic data for these compounds

a2iB "aAm2 = 5.81 • 10 -8

Thus, if aTi B ---- aAm2, aA]B~ = 3.87 x 10 -3. If one as- sumes that the solution of AIB2, TiB, and TiB2 is ideal, there will be a maximum of 0.39 mole pct of A1B2 (i .e. ,

METALLURGICAL TRANSACTIONS A VOLUME 22A, AUGUST 1991 - - 1727

W I - Z

O (J

CHEMICAL COMPOSITION (In atomic %)

Near edge: TI = 35.2, B = 64.8

At center: TI = 34.5, B = 65.5

164.00 214o00 284.00 a14.00 ~4.00 414.00 411,4.1)0 514o00 ,$64.00

E N E R G Y e V

Fig. 12- -EELS spectra from TiB2 particles in the Ti-43A1 alloy ex- truded and aged at 1473 K for 7 days: (a) near the edge and (b) at the center of the particle. The corresponding chemical compositions are given in the inset.

0.15 wt pet A1) dissolved in TiB2. The quantity of AI in A1B 2 dissolved in the TiBE-A1B2 solid solution, accord- ing to the calculations presented above, is below the de- tection limits of EDS analysis. This is in agreement with the observed absence of an A1 peak in the EDS spectrum in Figure 11.

IV. CONCLUSIONS

The TiB2 particles are found to be very effective in refining the grain size of the Ti-43A1 alloy. No detect- able change occurs in the size or the number density of the TiB2 particles due to Ostwald ripening, and the par- ticles are virtually stable at temperatures up to 1473 K. Since this temperature is considerably higher than the likely maximum service temperature for this class of al- loys, it seems reasonable to assume that the TiB2 addi- tion is thermally stable for high-temperature service.

The kinetics of two major life-limiting factors for dispersoid containing materials, i .e. , dispersoid particle/ matrix interaction and coarsening of the dispersoid, are

extremely slow in the tested alloy. This suggests that the addition of TiB2 to Ti-43 pet A1 for dispersion strength- ening may be technically feasible as far as the above- mentioned factors are concerned.

ACKNOWLEDGMENTS

The authors thank Edison Materials Technology Center and Wright State University for financial support for this project. The technical assistance and support from the Air Force Wright Research and Development Center/ MLLM at the Wright-Patterson Air Force Base, OH, is greatly appreciated.

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INTERNATIONAL, Metals Park, OH, 1987, p. 91. 11. J.L. Murray: in Phase Diagrams of Binary Titanium Alloys, J.L.

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13. I. Bat-in, O. Knacke, and O. Kubaschewski: Thermochemical. Properties of Inorganic Substances, Supplement, Springer-Verlag, New York, NY, 1977, pp. 39-40.

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