Grain Refinement and Improved Hardness/Corrosion Balance by Pulsed Electron Beam Surface Treatment of a FeAl Alloy

Abstract

Abstract:In the present work, it is demonstrated that significant surface modifications can be generated by a low energy, high current pulsed electron beam (LEHCPEB) treatment under the so called 'Heating mode' without surface melting. Using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) observations, it is shown that the extremely fast thermo-mechanical cycles generate grain refinement and the formation of vacancies in the surface layer which account for a significant increase of the surface microhardness of a Fe 40at%Al alloy. Comparatively, the polarisation curves measured in 0.5 M H 2 SO 4 solution indicate that the LEHCPEB treatment without melting does not change the corrosion behaviour. Keywords: Low Energy, High Current Pulsed Electron Beam; FeAl; Grain refinement; Corrosion resistanceAs is well known, the failure of industrial materials usually starts from their surface, especially when the working environment is corrosive or the applied load is moving. Therefore, it is often necessary and effective to improve the global properties of industrial materials by using surface modification techniques. During the past decades, pulsed energetic beams, such as laser, ion and electron beams, have attracted much attention in the field of materials surface modifications. Among these pulsed beam surface modification techniques, the surface treatment by Low Energy High Current Pulsed Electron Beam (LEHCPEB) has been initially developed at the Tomsk Intense Current Research Institute 1-3 . It is well established that improved surface properties (hardness, corrosion resistance) can be obtained by this treatment when the top surface is melted and rapidly solidified (10 7 K/s) 3,4 . This is essentially the result of ultra-fine grains formed from the highly undercooled melt 5 , melt surface purification 6 as well as strain hardening induced by the thermal stresses and shock waves 7 .In this work, we demonstrate the effectiveness of the LEHCPEB treatment for materials modification without melting of the top surface. This approach is of the outmost importance to get the full potential of this surface treatment technique 8 . In order to study the typical effects induced by HCPEB treatment under the so called "heating mode", a FeAl intermetallic alloy was selected in the present work. It is one of the potential candidates to replace steels for weight saving applications.The oxide dispersed strengthened (ODS) B2 Fe (40at%)Al alloy was hot extruded at 1250 o C from milled powers (with 1% Y 2 O 3 ) in the form of a rounded bar 9 . The specimens for the HCPEB treatment were cut perpendicularly to the extrusion axis into thin disks. Thus, the sample normal direction (ND) is parallel to the extrusion axis. The HCPEB treatments were carried out using a "Nadezhda-2" source 1,3 , with parameters including : accelerating voltage; 27kV, energy density; 3J/cm 2 , number of pulses; 5 and 20, pulse duration; 1.5 s, dwell time between pulses; 10s.