Formation of bulk magnetic nanostructured Fe 40 Ni 40 P 14 B 6 alloys by metastable liquid state phase separation
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ABSTRACT METGLAS alloy #2826 (Fe40Nia0Pi 4B6) is a eutectic alloy. Its melt can undergo metastable liquid state spinodal decomposition deep in the undercooling regime AT defined as AT = T, - T where T, is the liquidus of the alloy and T is the temperature at which the decomposition reaction takes place. Micrographs of undercooled Fe 4oNi 40Pl 4B6 of AT = 209, 260 and 307 K were displayed. All of them exhibit refined microstructures, esp. the one with the largest AT.
INTRODUCTION Bulk nanostructured materials are systems that comprise of grains of diameter d that falls in the range of I < d < 100 nm. They are mostly synthesized through two techniques. In the first method [1, 2) nanometer powders are compacted together, sometimes also under an elevated temperature, to produce a bulk nanostructured specimen. The main drawback is that an as-prepared specimen is filled with flaws, mostly microvoids, serving to reduce the mechanical strength of the nanostructured material [3]. Thermal annealing is very often undesirable for it causes grain growth giving rise to wide grain-size distribution [3]. In the second method [4], a bulk amorphous specimen is annealed at an elevated temperature. Nanocrystals then emerge. However, it turned out the size distribution of these nanocrystals can be quite wide that would smear out the unique properties of the nanostructured alloys. Fe based nanostructured alloy also possesses interesting magnetic properties [2, 5]. At very small grain size, d < 30 rnm, Fe based nanostructured materials start to exhibit soft magnetic properties. At d = 15 rum, the soft magnetic properties become very interesting and they are comparable to those of the amorphous alloys [5]. It is however important to keep the size of the constituent grains uniform if the attractive magnetic properties are to be kept. Recently it [6, 7, 8] was found that when a eutectic alloy melt is undercooled to a temperature way below its thermodynamic melting temperature, it undergoes metastable liquid state spinodal decomposition. Right after the decomposition reaction, the system consists of intertwining undercooled liquid networks of characteristic wavelength X. The magnitude of X depends on how far the temperature T of the decomposition reaction is taken place below T, where T, is the liquidus of the eutectic alloy. The extent below T, is called undercooling AT (AT = T1 - T.). More precisely, according to Cahn [9], X oc {(Ts - T)/Tc}"1/2 where T, is the temperature of the chemical spinodal and T, is the critical temperature of the metastable liquid miscibility gap. It turned out that when the undercooling is sufficiently large, X can enter the nanometer scale regime[10,11]. The physical dimension of an as-prepared nanostructured specimen in the shape of droplet can have a diameter > 1 cm. Furthermore, it is microvoid free and the size distribution of the constituent grains is narrow. So far the systems under investigation are non-magnetic. In this article, we report the formation of nanostructured 277
Mat. Res. Soc. Symp. Proc. V
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