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[21b]

Creep Deformation of TiAI and TiAI + W Alloys

The interfacial compositions are obtained from the conditions of continuous flux and activity as: c ~/t~ =

c ~° + c ~ ° ~ / D 1 +

c ~/'~= (f"/ft3)c"/tJ

~

[22a] [22b]

where f " and f ~ denote the activity coefficients in the two parts of the weld. Disregarding the printing error in Stark's paper, pointed out by Ku~era and Str~nsk~, 7 we find that Eqs. [21] and [22] are identical to the ones derived by Stark. This approach not only allows the use of different activity coefficients but also different diffusion coefficients in the two parts of the weld. Since only exact solutions to Fick's second law are applied, the mass balance is ensured. Figure 1 shows the concentration profile in a compound material w i t h f ~ / f ~ = 1.65 calculated using Eqs. [21] and [22]. This diagram should be compared with Figures l(a) and (b) in Million's report. The dashed curve shows the relative activity, calculated from the concentration profile. It is interesting to notice that, although the activity curve is continuous, its slope is not. Of course, in view of Eq. [17], this is the only possibility for maintaining a continuous flux when Oac/OCc is discontinuous. As a concluding remark, one may add that in approximate calculations one should use Eqs. [21] and [22]. If the diffusion of the alloy elements cannot be neglected, one should try some diagonalization procedure (see Reference 6), and for the more complicated case one has to apply numerical methods and solve Eqs. [10] or [11] (see References 8 and 9).

PATRICK L. MARTIN, MADAN G. MENDIRATTA, and HARRY A. LIPSITT Intermetallic compounds based upon TiA1 and Ti3A1 exhibit attractive combinations of physical and mechanical properties for use at elevated temperatures..The main detracting feature is their lack of tensile ductility at low temperatures. TiAI (referred to as y) has the Llo structure with only a 2 pct deviation from cubic symmetry. Ti3A1 (referred to as a2) has the DOl9 superlattice of the hcp structure. Alloys containing less than 52 at. pct A1, such as those described in this paper, are two-phase mixtures of y and a2 usually coexisting in a lamellar morphology. 1.2 The slip of TiAI has been studied both in compression 1 and in tension. 3 Dislocations are observed to be the same as in fcc crystals except that two-thirds of the {111} (110) slip systems require the passage of superdislocations to maintain long-range order. Twins of the type {112} (111) have also been observed. 1'2 These features combine to give (single-phase) TiA1 a sharp brittle-to-ductile transition at about 700 °C.3 In contrast, Ti3A1 shows a continuously • increasing tensile ductility with increasing test temperature. 4 The dislocations observed in this phase are of the basal (so-called "a") type with some "c + a" type being observed at 700 °C. 5'6 The study of creep behavior in these alloys has, thus far, been restricted to a2-base alloys; 7 this study revealed that a2 alloys obey the conventional powerlaw creep equation /~min

6. 7. 8. 9.

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