Formation of Submicron Metastable Phase Structures in Alloys with Focused Electron or Proton Beams
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FORMATION OF SUBMICRON METASTABLE PHASE STRUCTURES IN ALLOYS WITH FOCUSED ELECTRON OR PROTON BEAMS* N. Q. LAM, P. R. OKAMOTO AND G. K. LEAF Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439 ABSTRACT Recent theoretical and experimental studies of radiation-induced segregation in alloys under irradiation with focused charged-particle beams have shown that point-defect currents generated by axial and radial displacement-rate gradients can cause significant redistribution of the alloying elements within the irradiated zone. In the case of irradiation of thin films with highly-focused electron beams, two important features have been established experimentally: (i) the diameter of the local region in which the alloy composition and phase are modified is practically equal to the beam diameter, and (ii) the time required to produce a given change in the alloy composition in the center of the irradiated zone decreases rapidly with beam diameter. Our theoretical modeling indicates that these features will also be observed in semi-infinite alloys bombarded with focused proton beams. However, in this case, the spatially-nonuniform defect production in both the axial and radial directions renders the compositional redistribution more complex. The present work shows that the ability to locally modify the alloy composition by focused electron or proton beams may offer a new method for producing local regions of controlled composition and microstructure on a submicron scale. The results of our model calculations and experimental studies will be presented to demonstrate the feasibility of this novel technique. INTRODUCTION During the past decade, several techniques have been developed for modifying surface layers. For example, pulsed heating by laser or electron beams can melt large surface areas in very short times. The heating and cooling rates achieved with these beams are so fast that metastable alloys can be produced in the near-surface regions. The outermost micron layers of a solid can also be modified in a reproducible way by ion implantation or ion-beam mixing techniques. With these techniques the depth of the modified region can be controlled on a nanometer scale. However, because of the difficulty in focusing heavy-ion beams down to submicron sizes, these techniques only enable the modification of large surface areas. In the present work, a new method for producing submicron metastable structures in alloys is proposed. This method exploits defect-assisted segregation processes that are driven by damage-rate gradients during irradiation with highly-focused energetic electron or ion beams. Fundamental aspects of the process will be discussed, and some results of theoretical modeling and experimental observations will be presented in order to demonstrate the technique. THE RADIATION-INDUCED SEGREGATION (RIS) PROCESS Nonequilibrium point defects (interstitials and vacancies) created by energetic irradiation migrate over long distances at elevated temperatures *Work supported by the U. S. Department of Energy,
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