Effects of nitrogen implantation on low cycle fatigue behavior of ferritic Fe-24Cr-4Al stainless alloy

  • PDF / 6,539,696 Bytes
  • 10 Pages / 598 x 778 pts Page_size
  • 89 Downloads / 204 Views

DOWNLOAD

REPORT


I.

INTRODUCTION

IN recent years there has been much interest in modification of the surface-sensitive properties of materials by ion implantation and its related techniques. Ion beams can be used to create a highly nonequilibrium surface alloy layer without any change in the bulk properties. Ion implantation involves bombarding the surface layer of solids with energetic ions typically in the 40 to 500 keV range. The energetic ions penetrate the solids, displace the host atoms from their positions, and eventually come to rest with an approximate Gaussian distribution about a mean projected range. Of the many applications of ion implantation, one important aspect is on the improvement of fatigue resistance of metals and alloys.V,2,3] The fatigue process generally can be divided into three partly overlapping stages: (a) fatigue hardening and/or softening taking place as a consequence of the interaction between structural defects (mainly dislocations) in the whole loaded volume; (b) initiation of fatigue cracks resulting from the formation of stress-concentrating geometry (intrusions and extrusions) at free surface where cyclic strain localization occurs in the form of persistent slip bands (PSBs); (c) propagation of fatigue cracks governed by the cyclic plastic deformation localized in the plastic zone ahead of the crack tip. t4] Of the three stages, the initiation of a microcrack at the free surface is a crucial step in the fatigue process and up to 90 pct of the fatigue life may be spent in the crack initiation stage. Ion implantation can influence the initiation of fatigue crack at the surface without affecting the crack propagation stage; therefore, it holds a greater potential for fatigue life improvement. Extensive studies have been made on the improvement of fatigue properties by ion implantation, ts-16] In these studies, the target materials ranged from pure metals and simple S.M. ZHU, Graduate Research Assistant, and S.C. TJONG, Associate Professor, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong. Manuscript submitted June 14, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A

alloys to complex engineering steels. The species implanted included metalloid interstitial ions (nitrogen, carbon, and boron) as well as metallic substitutional ions such as aluminum and chromium. Several implantation techniques were employed, including single ion implantation, multiple ion implantation, and ion beam mixing. For the most part, various degrees of improvement of fatigue life have been produced by ion implantation. It is generally accepted that the suppression of the fatigue crack initiation by ion implantation is associated with either the homogenization or the inhibition of cyclic slip at the ion implanted surface. These effects are, in turn, derived from the alteration of the stacking fault energy, the introduction of the residual compressive stress, and surface layer hardening, e.g., solution hardening, precipitation hardening, and defect hardening, etc. after

Data Loading...