Mechanisms of the electron irradiation-induced amorphous transition in intermetallic compounds
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I. INTRODUCTION Amorphous metallic alloys have been produced by various techniques that involve rapid quenching from the melt.1>2 On the other hand, certain crystalline targets have been rendered amorphous by particle bombardment at relatively low temperatures, depending on the system and on the incident particle.3"21 The transition under heavy iori bombardment has been likened to the rapid quenching procedures on account of the high degree of atomic disorder created by the displacement spike and its short duration.22 However, there is a most remarkable difference between the two procedures that refers to the initial state. While in one case the initial state is the liquid, in the other it is the crystalline solid. This difference is further enhanced by the occurrence of amorphization under electron irradiation, where cascade effects are absent. In the case of electrons, neither a highly concentrated disorder nor the effect of implanted impurities can be invoked as basic causes whereby amorphization may take place. Knowledge of the radiation damage produced by electrons is therefore basic to understanding the mechanisms responsible for the crystalline-to-amorphous transition in irradiated intermetallic alloys. It is well known that Frenkel pairs constitute the primary defects induced by electron irradiation. The subsequent behavior of these pairs and their interaction with the defective lattice determine the microstructure that evolves or the transitions that may be promoted with increasing dose. In pure metals and in very dilute solutions, interstitials are mobile down to fairly low temperatures23; so, depending upon the temperature, either interstitials alone J. Mater. Res. 1 (3), May/Jun 1986 http://journals.cambridge.org
or both interstitials and vacancies migrate and, besides recombining with each other, reach the various available sinks. Lattice defects such as dislocations, grain boundaries, the free surface, and ihterphase interfaces are common point defect sinks that may exist in the specimen prior to irradiation. Either type of point defect might also aggregate as small clusters or collapse as dislocation loops producing self-created sinks, in addition to those already existing. The development of an irradiation-induced microstructure thus constitutes a mechanism for decreasing the free energy of the crystal. Such a mechanism may be particularly important at the lower temperatures owing to the little overall loss rates of point defects to sinks existing prior to irradiation. Another possible effect than can be ascribed to point defect annihilation in the case of ordered alloys is that of producing disorder (e.g. Ref. 9). Annihilation of free point defects during irradiation, inasmuch as it involves a free energy decrease, reflects a tendency toward equilibrium. However, if annihilation processes are prevented from occurring under given conditions, for instance of temperature and electron dose, a defect buildup might take place. Experimental results by Halbwachs et al.24'25 and by Dimitrov et al.26 indica
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