A transmission electron microscopy investigation of inverse melting in Nb 45 Cr 55
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A transmission electron microscopy investigation of inverse melting in Nb45 Cr55 W. Sinkler,a) C. Michaelsen, and R. Bormann Institute for Materials Research, GKSS Research Center, 21502 Geesthacht, Germany (Received 19 November 1996; accepted 21 March 1997)
In inverse melting, a supersaturated crystalline phase transforms polymorphously under heat treatment to the amorphous state. Inverse melting of body-centered cubic (bcc) Nb45 Cr55 is studied using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The crystalline to amorphous transformation is heterogeneous, initiating at the bcc grain boundaries. HRTEM reveals 2–3 nm domains with medium range order (MRO) in the amorphous phase. Preferred orientation of MRO domains is found on a scale corresponding to the precursor bcc grain size. Using HRTEM and calorimetry, MRO development in cosputtered Nb45 Cr55 films is characterized and compared to that in the amorphous phase produced by inverse melting.
I. INTRODUCTION
The formation of amorphous metallic phases has been the subject of a large quantity of research, both fundamental and applied, for more than two decades. To date many processes are known which may produce an amorphous phase.1 A subset of these processes are those which start from a solid precursor, and are socalled crystalline to amorphous (c to a) transformations. Most c to a transformations fall into two general groups: In the first, energy is added to a previously homogeneous system such as an intermetallic compound, for instance by irradiation2 or by ball milling.3 This causes damage to the crystalline phase due to chemical disordering and/or the introduction of point defects. The resulting energy increase of the crystalline phase may suffice to induce a c to a transformation, which if complete can be considered polymorphous. The second class of c to a transformations are those which start from a chemically heterogeneous state, such as solid state amorphization reactions of elemental multilayers,4 or reactions of elemental powders in ball-milling.5 In alloy systems characterized by large negative heats of mixing, the elements in contact with each other represent a high-energy state. A homogeneous amorphous state will generally have a lower free energy in such a situation, and may form by interdiffusion if formation of competing compound phases is kinetically suppressed. In contrast to the homogeneous c to a transformations just described, this process is not polymorphous. In spite of some solid solution formation prior to amorphization,6,7 there are nevertheless sharp composition discontinuities at the crystal-amorphous interfaces.7 a)
Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3108.
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 7, Jul 1997
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Recently, reports have been made that bcc solid solutions of Ti–Cr,8 Nb –Cr,9,10 Zr–Mo,11 or Ta–Cr12 may be induced by anneali
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