A Tem Study of Inverse Melting in Nb 4 5Cr 55
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W. SINKLER*, C. MICHAELSEN AND R. BORMANN Institute for Materials Research, GKSS Forschungszentrum, 21502 Geesthacht, Germany. *[email protected] ABSTRACT Inverse melting of bcc Nb 45Cr 55 is investigated using transmission electron microscopy, highresolution TEM and electron diffraction. It is shown that the transformation to the amorphous phase initiates at the bcc grain boundaries. The transformation results in an increase in incoherence, evidenced by a loss of bend contours. Some anisotropy is found in the amorphous phase produced by inverse melting, which is associated in HRTEM with preferentially oriented but discontinuous and distorted fringes. The results are consistent with the production of an amorphous phase by inverse melting. INTRODUCTION It has recently been reported that a polymorphous crystalline to amorphous (c-to-a) transformation is induced by annealing bcc alloys of Ti-Cr [1], Nb-Cr [2, 3] and Zr-Mo [4]. This transformation is known as inverse melting [5], as it represents the "melting" of a metastable crystalline phase to an undercooled liquid or amorphous state. This process is unique among solid state amorphization transformations, in that it requires neither simultaneous energy input, as do irradiation or ball-milling, nor does it involve composition discontinuities which are present in thin film reactions [6]. The present work is a study of the inverse melting transformation in a Nb 45 Cr 55 alloy using transmission electron microscopy (TEM), high resolution TEM (HRTEM) and electron diffraction. The purpose of the study is twofold: Firstly, it is attempted to clarify the mechanism of inverse melting, i.e. whether it occurs by nucleation and growth, or by a homogeneous collapse of the bcc phase. Secondly, the work is intended to shed light on structural aspects of the inverse melted amorphous phase. This is important for removing doubt caused by previous reports of inverse melting from a bulk-quenched bcc precursor phase [7, 8], which have not been reproduced [9-11]. EXPERIMENT
Preparation of bcc Nb 45 Cr 55 by ball milling has been described elsewhere [2, 3]. The powders were heat treated in a Netzsch DSC 404 heat-flow calorimeter under flowing Argon (99.9999 at%), following repeated purging. TEM foils were prepared by pressing the powder to 3 mm disks in a cylindrical die. The disks were polished and dimpled, and final thinning was performed using argon ion milling (Gatan Duomill, 4 kV, 0.8 mA at room temperature). Preparation of a foil of co-sputtered Nb 45 Cr 55 , also by ion milling, is described elsewhere [ 12]. Conventional TEM was performed with a Philips CM200 operated at 200 kV. For highresolution imaging, a Philips CM30ST operated at 300 kV was used. RESULTS Electron Diffraction Figs. 1 a and b show electron diffraction patterns respectively from the as-milled bcc Nb 45 Cr 55 phase, and the same alloy which has been transformed to the amorphous state by heat treating at 20 K/min to a temperature of 850'C. For comparison, diffraction from a co-sputtered amorphous Nb 45Cr
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