Ordering with Phase Separation in an Fe-Al-Si Alloy
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ORDERING WITH PHASE SEPARATION IN AN Fe-Al-Si ALLOY S.
MATSUMURA,
A.
SONOBE,
K. OKI AND T. EGUCHI
Department of Materials Science and Technology, Sciences, Kyushu University 39, Jasuga-shi 816,
Graduate School of Engineering Japan
ABSTRACT Two kinds of process of ordering with phase separation, B2 or DO3 -. (A2 + DO3 ), on annealing at a temperature of 873 K in an alloy of Fe-6at%Al-9 at%Si were investigated by means of transmission electron microscopy and X-ray diffraction. The decomposition of D03 phase proceeds by the development of periodic fluctuations along directions in both B2 and DO3 types of degree of order. Newly born DO3 domains take the form of rectangular prism and align along directions. In the transition from B2, on the contrary, there is a marked difference between the morphologies of the fluctuations in the two types of degree of order at the early stage of annealing. The degree of B2 type order fluctuates along directions, while the fluctuation in the degree of DO3 type order does not form the modulated structure iat the early stage of annealing. The morphology after a prolonged annealing is similar to that in the case of DO 3 -I (A2 + DO3 ). INTRODUCTION In the phase diagrams of some alloys, there are sometimes two-phase fields where an ordered phase coexists with a disordered or another ordered one. In such a two-phase field, the ordering or disordering develops accompanying phase separation. As the binary systems of iron-aluminum and ironsilicon have several two-phase fields where one of the phases is the B2 or D03 ordered phase [1-5], the systems have been frequently employed for the investigations on the ordering with phase separation [6-10]. Existence of similar two-phase fields is expected in the phase diagram of Fe-Al-Si system [10]. The iron-aluminum-silicon alloys are seemed to be a system well worth investigating to reveal the features of ordering with phase separation in iron alloys. This ternary alloy system is well known as one of the soft magnetic alloys possessing excellent performance. Experimental investigations have been largely focused on their magnetic properties. The technological interest may enhance the worth in the study of the equilibrium phases and microstructures in this alloy system. Figure 1 is a part of the phase diagram of Fe85(AlXSilIX)I5 system which we determined by means of electron microscopy and X-ray diffraction. As is expected, there exists a two-phase field of ( disordered A2 + ordered DO3 ). In the case of X=0.4 (Fe-6at%Al-9atSi), there is a single-phase region, DO3 , around 970K above the two-phase one, while the B2 ordered structure prevails above 1000K or so. In this alloy consequently occur two types of ordering with phase separation, B2 -(A2 + DO3 ) and DO3 0 (A2 + DO3 ), which differ in the initial state. The aim of the present research is to study the above two processes of ordering with phase separation at a temperatue of 873K in the alloy of Fe-6at%Al-9at%Si, by means of electron microscopy and X-ray diffraction. The occupation probabiliti
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