Effects of simultaneous boron and nitrogen implantation on microhardness and fatigue properties of Fe-13cr-15ni alloys
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INTRODUCTION
ION implantation has been shown to significantly alter the fatigue properties of metals in many cases, t~l Crack initiation on the surface is a crucial step in the fatigue process. At low strain amplitudes, this is achieved by the formation of persistent slip bands (PSB), which causes plastic strain localization and leads to an intrusion-extrusion topography at the surface. Fatigue cracks subsequently develop at the intersection of the PSB with the free surface, t2l Ion implantation has been shown to delay and suppress crack initiation by inhibiting the emergence of PSB at the surface through several mechanisms, r3-~] Ion implantation can alter and suppress slip-band formation by (1) inducing residual compressive stresses; (2) altering the stacking fault energy; and (3) forming precipitates, solid solutions, and defects, leading to inhibited dislocation movement. These factors act synergistically to alter the overall fatigue properties of the material. Most studies on the effects of ion implantation on fatigue properties of metals have utilized single ion species. However, implantations using multiple ion species, because of the additional interactions that occur between species, can result in unique microstructures and cause unusual property changes that cannot be achieved by single ion implantation. In an earlier study, tgj Lee and G.R. RAO, Postdoctoral Researcher, E.H. LEE, Senior Research Staff Member, and L.K. M A N S U R , Group Leader, are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. B.A. CHIN, Professor and Chairman, is with the Materials Engineering Department, Auburn University, Auburn, AL 36849. Manuscript submitted June 14, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
Mansur investigated the effects of simultaneous B + and N + ion implantation on the fatigue properties of four simple Fe-13Cr-15Ni alloys (designated as B-alloys), the compositions of which are given in Table I. Results from their study (Figure 1) indicated that although single ion implantation caused modest or no improvement in the fatigue properties of the alloys, dual implantation significantly improved fatigue life, up to 250 pct in the case of alloy B6. The study conducted by Lee and Mansur was limited to four simple model alloys based on the ternary Fe- 13Cr-15Ni system. The present investigation was undertaken to study the effects of multiple ion implantation on more complex engineering-type alloys of the same base composition. The effects on fatigue properties and hardness caused by the simultaneous implantation of B + and N + into eight alloys based on the Fe-13Cr-15Ni2Mo-0.2Ti-0.8Si-0.06C system were analyzed.
II.
EXPERIMENTAL PROCEDURES
A. Specimen Preparation and Composition Eight model alloys, designated as alloys E1 through E8, were used in this investigation; their compositions are given in Table II. Each alloy has an element added to or removed from the basic E1 composition. This set of alloys was prepared as part of the radiation effects program at O
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