Point Defect Behavior in B2-Type Intermetallic Compound FeAl
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understand the mechanism in the TRD-type alloys because a TRD-like behavior cannot be interpreted only from RVD process. In the present report, possibilities of both mechanisms of the RVD and ASAR in point defect behavior are examined from thermodynamic point of view based on the Bragg-Williams method. THERMODYNAMIC TREATMENT Model
In binary B2 alloy with composition c, AI-,Bc, there are generally four kinds of point defects, Avacancy (VA), B-vacancy (VB), A-ASD (AB) and B-ASD (BA). We denote these concentrations as xA, xB, YA and YB, respectively, and the total vacancy concentration as cv. According to the previous work [5,7], RVD process occurring due to increase in temperature may be expressed by the following reaction
0
-4 VA
(1)
+V
0 means a defect free state. When the RVD process occurs once, one A-vacancy and one B-vacancy
are simultaneously created and they are thought to randomly distribute over lattice sites. If only this process proceeds in a whole temperature region, the vacancy concentrations, xA and XB, are of course always the same. Possible processes succeeding the RVD process are an atom jumping into neighboring vacant sites. We here consider following three reactions:
(2a)
AA + VA -- AB + VA AB + VA -- AA + VB BA + VB -- BB + VA
(2b)
(2c)
The reaction combining (2a) with (1) results in a creation of one A-ASD and two A-vacancies, which just corresponds to the TRD mechanism. We call this reaction the TRD process. The reactions combining (2b) or (2c) result in a recover of antisite A or B atoms, corresponding to the ASAR mechanism. We call these reactions the A atom (B atom) recovering process, A-R process or B-R process. Note that a larger increase in the A-vacancy concentration with temperature can be brought about not only by the TRD process (2a) but also by the B-R process (2c). We here consider the hybrid state, based on the previous conclusion that the defect type in the ground state is the ASD type [7,8]. Detailed description about the treatment will be given elsewhere [9]. In the B-rich region, we suppose that some processes among all the RVD processes occurring due to increase in temperature are followed by the reactions of the TRD or the B-R. Letp and q be the fracp, q _ 0 [8,9]. First condition comes from a stability condition of B2 phase and the remaining two come from a demand that the ASD state is the ground state in any composition. We will now examine the possibilities of appearance of RVD and ASAR states. In the B-rich region, the internal energy differences between the pure states are given by U (TRD)-U(RVD)= 2cv (VBv - VAv - VAB) U(B- R)-U(RVD)= 2c1 (Vv - VAv + VAR) Note that always U(TRD) - U(B - R) = -4cvVAB > 0. If the condition of IV8vU (TRD)>U (RVD)> U (B - R)
(6a) (6b) VAvI < IVABI
is satisfied, then (7)
If a magnitude of VAR is considerably large, internal energy for the pure B-R state is much lower than that for the pure TRD state so that B-R state can be expected to appear as stable state in a wide temperature region. Further, if the condition of V8 v < VA
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