Laser-Generated Glassy Phases
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University of Bern,
Sidlerstr.
5,
ABSTRACT Short laser pulses have recently been discovered to be powerful means for glass formation in binary alloys as well as in some pure elements Due to relevant cooling rates in excess of 10 °K/s, glass formation is achieved in binary systems over most of the phase diagram, even in systems not previously known as glass formers. We first give a review of recent progress in laser melt quenching, and then discuss the kinetic conditions of glass formation by laser irradiation. INTRODUCTION Glassy metals have been made by mechanical quenching methods for almost a quarter of a century. The different quenching methods are usually compared on the basis of t e average cooling rates achieved; cooling rates ranging up to 10 °K/s are expected to arise with the best mechanical methods. The use of laser pulses as a means to quench materials into a glassy state has first been attempted only a few years ago and exploitation of the method has barely started. In laser quenching heat is produced in a solid sample by very short irradiation. A surface layer of a width of typically one micron or less is heated, melted and subsequently cooled by conduction into the bulk of the specimen. Owing mainly to the small width of the molten zone the heat flux during cooling after laser irradiation exceeds that achievable by mechanicT6 quenching by orders of magnitude. Relevant cooling rates reach 10 °K/s under typical conditions. Not surprisingly, the faster cooling is found to result in a drastically extended range of materials that can be obtained and studied in the glassy state. In what follows we first give a - necessarily sketchy - overview of experimental results obtained to date. In the second part we analyse the criteria of glass formation from the point of view of laser quenching. EXPERIMENTAL RESULTS Recent experimental work on laser-quenched glassy phases tends to fall into one of two categories: In the first, short pulses are used with the goal to extend previous limits of glass formation to new materials and material combinations, so far mainly to Si-metal systems. In the second, mostly continous laser beams in a scanning mode are adopted in work typically aimed at achieving surface hardening of machine alloys [i]. While there is consideMat.
Res.
Soc. Symp.
Proc.
Vol.
13 (1983)
(Elsevier
Science Publishing Co.,
Inc.
692 rable technological interest in the latter application, its results have not been shown to be qualitatively different from We shall therefore consider the first those of splat cooling. category only. Alloys Alloying of a single deposited surface layer to a Procedure. substrate by means of short laser pulses tends to produce complicated microstuctures, due to the nonuniformity of the melt comCellular growth patterns position prior to solidification 12]. with cell size down to a few tens of nm have been found as well as Quantitative investigations in such apparently amorphous zones. structures are difficult. A more controlled technique to investigate the properties of i
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