Reaction of Ti and Ti-Al alloys with alumina
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I.
INTRODUCTION
FIBER-reinforcedmetal matrix composite
systems based on Ti and Ti alloys are attracting considerable attention because of their potential for high stiffness and strength at high temperature. The alloy matrices of current interest are either the low Al-containing alloys, such as the Ti-6AI-4V* alloy, or the high M-containing alloys, which *Unless otherwise stated, all alloy compositions are in atomic percent.
include the Ti aluminides (Ti3A1 and TiA1). Nearly all of the Ti-base composite systems currently studied involve reinforcing the Ti alloys with SiC fibers. There are two major disadvantages for using silicon carbide fibers in Ti alloy matrices: first, the silicon carbide fibers react I]'2'31 with the Ti alloys; and second, the coefficients of thermal expansion (CTE) of the Ti alloys are considerably higher (nearly twice as high) than those of SiC. This large difference in CTE between the fiber and the matrix can lead to residual thermal stresses during cooling of the composite. In comparison to SiC, the CTE difference between Ti alloys and alumina is considerably less. Thus, alumina fibers would be better candidates as reinforcement materials for the Ti-base matrices. However, the chemical compatibility of A1203 with the Ti alloys needs to be addressed. Pure Ti is known to react with A1203, r e s u l t i n g in the formation of TiO and Ti3Al.t4j Recent studies by Krishnamurtny -. [3] have shown A1203 to be chemically compatible with the alloys based on the y-TiA1 phase, i.e., alloys with about 50 at. pct AI. The chemical compatibility of AI203with other Ti aluminides, such as alloys based on the a2 (Ti3A1) phase, is not known. Thermodynamically, the reaction between A1203 and Ti or a Ti-AI alloy should be similar and is likely to occur by one of the following two mechanisms. One possible mechanism would be the reaction of Ti with A l 2 0 3 to form TiO via 3Ti + AlzO3 = 3TiO + 2A1
[1]
The underscores in the above reaction denote that the elements are present at reduced activities, i.e., activities less than unity. The equilibrium constant (K~) for Reaction [ 1] is Kl = ( a A l ) 2 / ( a x i ) 3
[2]
AJAY K. MISRA, Senior Research Engineer, is with Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, OH 44135. Manuscript submitted June 4, 1990. METALLURGICAL TRANSACTIONS A
where aAl and aTi are the activities of A1 and Ti in the alloy. Reaction [1] can proceed in the forward direction if the activity ratio, {(aAO2/(ari)3}, in the alloy is less than K~. Samokhval et al. [5] have measured the A1 activities in Ti-A1 alloys at 980 K with alloy concentrations in the range of 5 to 50 at. pet. Ti activities in these alloys can be derived by Gibbs-Duhem integration of the AI activity data. Based on the activity data of Samokhval et al.,ts] it appears that Ti-A1 alloys with A1 concentrations less than 50 at. pct would react with A1203 to form TiO by Reaction [ 1]. The other possible mode of reaction between Ti-A1 alloys and A1203 would be dissolution of A1 and atomic oxygen in t
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