Nucleation and growth in surface-melted crystalline and amorphous Fe 40 Ni 40 P 14 B 6 alloys
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I. INTRODUCTION Since their discovery metallic glasses have developed from a scientifically interesting phenomenon to a commercially useful material. Their utility results from unique properties which include improved magnetic, mechanical, and corrosion behavior. The amorphous structure of metallic alloys often results in very soft magnetic properties.1 They can be exceptionally hard with very high tensile strength,2 and some amorphous alloys are exceptionally corrosion resistant.3 These desirable properties can be lost upon the slightest partial crystallization. Conversely, useful microstructures and properties that are otherwise unobtainable may be produced by partial or total crystallization of metallic glasses. Thus there is an ongoing need to study the mechanisms leading to the creation of glassy metallic alloys as well as their subsequent thermal stability in an effort to elucidate what impedes and controls formation of and crystallization in amorphous materials. One obstacle to obtaining a "unified" understanding of metallic glasses is the number and complexity of alloy systems currently under study, each tending to emphasize a particular property.4 Also, there are a variety of techniques that can be used to form amorphous materials. These techniques encompass a range of approaches from deposition (electrolytic, chemical, and vapor) to direct quenching from the liquid (splat, melt spinning).5 Most of these techniques cover limited and different "cooling" regimes leading to ambiguities and an incomplete picture of glass formation. The complexity of the problem is further highlighted when one realizes that amorphism itself is not a unique state but comJ. Mater. Res. 1 (1), Jan/Feb 1986
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prises a considerable and continuous variation at the microscopic level. One technique, directed energy surface processing based on rapid quenching from the liquid state, shows promise as a tool to both produce and study metallic glasses.6 An electron or laser beam can melt the surface of a material followed by rapid cooling under the chilling effect of the unmelted substrate. This type of processing also allows for the alteration of the surface structure without significantly affecting the underlying, possibly desirable bulk properties. The approach offers several advantages including very high maximum quench rates on the order of 1010 K/s. These arise from reduced thermal resistance at the liquid-solid interface and large temperature gradients created by the intense localized energy flux. In addition, the quench rates obtained can be varied over a wider and more controllable range than with other methods, by controlling the process parameters affecting energy density and interaction time. The intent of this work is to apply those features of directed energy surface processing to study the influence of underlying bulk material, whether crystalline or amorphous, on the glass formation process. Surface processing on already-formed amorphous material is studied to explore thermal stability. The resu
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