The effect of plastic deformation at near room temperature on the solid state reactions between Ni and Sn
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I. INTRODUCTION
In the last few years mechanical alloying (MA) and cold rolling (CR) have been extensively used to investigate solid state reactions (SSR) at near room temperature. A great deal of MA experiments have been devoted to produce amorphous materials (see Ref. 1 for a comprehensive review), starting from elemental powders and/or intermetallic compounds. It is now widely accepted that for the easy formation of the amorphous phase it is necessary that the alloying elements have a large negative heat of mixing and that the nucleation of the stable intermetallic compounds be hindered by some factor, possibly of kinetic origin. Several different mechanisms have been suggested for the amorphization process induced by cold working, depending also on the different reaction paths.2 In fact, it has been shown that, while some binary systems may form an amorphous phase directly from the terminal solid solutions, other binary systems pass through the formation of intermetallic compounds. The investigation of the early stages of alloying is, therefore, of great interest, considering that the formation of these compounds, which is also influenced by the milling energy,3 may substantially alter the outcome of the process. In this regard, CR of macroscopic multilayers has proved to be a very valuable approach since the slow rate at which plastic J. Mater. Res., Vol. 6, No. 3, Mar 1991 http://journals.cambridge.org
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deformation is imparted to the sample allows an accurate monitoring of the alloying process. In addition, because of the large mass and contact surface involved in CR, good heat dissipation during this process is expected to cause a temperature increase of the reacting interface lower than that caused by MA. On the other hand, the difficulty to perform CR automatically and under a controlled atmosphere considerably limits its application. The present experiment has been undertaken with the aim of examining in detail all the stages of alloying in Ni-Sn samples obtained by means of both CR and MA. In a previous investigation of the same system by MA,4 it was possible to synthesize a single amorphous phase only at the composition Ni75Sn25, while at all other compositions ranging between 25 and 40 at. % Sn the amorphous phase coexisted with the Ni3Sn2 compound. The easy nucleation of this compound was considered responsible for the narrow range of amorphization by MA, in contrast with the obtainment by vapor quenching of amorphous Nii-^Sn* alloys in the range 0.57 < x < 0.73.5'6 Recently,7 the amorphization of the compound Ni3Sn after prolonged mechanical milling has also been reported. The compositions Ni75Sn25 and Ni60Sn40 have been selected for the present investigation and the results of CR and MA have been compared. 499
© 1991 Materials Research Society IP address: 138.251.14.35
S. Martelli, G. Mazzone, and M. Vittori-Antisari: The effect of plastic deformation at near room temperature
II. EXPERIMENTAL
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The starting materials for this experiment have been elemental foils of co
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