Modulated Structures in Ion Implanted Al-Fe System
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MODULATED STRUCTURES IN ION IMPLANTED Al-Fe SYSTEM K.V. JATA, D. JANOFF, AND E.A. STARKE, JR. Department of Materials Science, University of Virginia, Charlottesville, VA 22901 ABSTRACT The results of transmission electron microscopy studies of iron implantation into high purity aluminum foils are described. For both 50 and 100 + 50 keV incident ion energies, modulated structure has been detected in the as-implanted foils. Upon annealing at 793 K the modulated structure decomposes into the Al matrix and Al3Fe precipitates for the 50 keV implantation. A similar annealing treatment for the 100 + 50 keV implantation indicates that the modulated structure is more stable, although some Fe3AI precipitation occurs. INTRODUCTION Ion implantation offers the potential for developing alloys with a superior fatigue crack initiation resistance [1,2]. In structural alloy systems incoherent particles in the near surface region tend to disperse dislocations thereby homogenizing slip and improving fatigue crack initiation resistance. Aluminum alloys having solute additions that form nonshearable particles which are thermally stable are also desirable for high temperature services. Iron has a solubility limit of 0.05 wt. percent and a diffusivity of only 4.1 x 10-13 m2 /sec is a good candidate. This paper describes the microstructure resulting from ion implantation into aluminum. It is part of a program directed towards improving the fatigue resistance of aluminum and its alloys by surface modification using ion implantation. The binary aluminum-iron alloys have been investigated by several workers over the past decade [3-5]. They have been processed either by continuous casting where the cooling rate is approximately 3 K per second or by rapid quenching from the melt with cooling rates of the order of Ilb K per second. Alloys obtained by the latter method have superior mechanical properties primarily due to a refinement of the microstructure and lower segregation of embrittling elements to the grain boundaries. The rapidly quenched alloys often contain two characteristically different zones, designated Zone A and Zone B by Jones [3]. Zone A essentially consists of a supersaturated solid solution and Zone B contains both metastable and stable phases. Ion implantation offers the possibility of obtaining alloys having: (1) a highly supersaturated solid solution with extended solid solubility over that predicted by the phase diagram; (2) a solid solution with stable and metastable phases; and (3) a disordered or amorphous structure. In many instances alloy systems produced by rapid quenching have been duplicated by ion implantation [6,7] and parallels have been drawn between the two techniques. The amorphous metal-metal or metal-metalloid systems produced by ion implantation have led researchers to again suggest that the thermal spike concept [8] where local heating of a solid takes place on the impingement of an ion with energy values of the order of 50-500 ke'V may apply. Although the termperatures in the small hot zone caused
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