Extension of High Cycle Fatigue Life by the Formation of Nano-Sized Martensite Particles at Intersections of Dislocation
- PDF / 1,793,760 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 206 Views
EXTENSION OF HIGH CYCLE FATIGUE LIFE BY THE FORMATION OF NANO-SIZED MARTENSITE PARTICLES AT INTERSECTIONS OF DISLOCATIONS IN AN AUSTENITIC STAINLESS STEEL T. Inamura, M. Shimojo, K. Takashima and Y. Higo Precision and Intelligence Laboratory, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan ABSTRACT A new fatigue strengthening method which is applicable for both the micro-sized materials and ordinary-sized materials have been proposed. Dislocations are pinned at their intersections by the formation of nano-crystals after cold work in this strengthening method. Nano-sized α’-martensite particles, the diameter of which was approximately 5 nm, were formed by a cryogenic treatment at a certain temperature above Msαb (martensite burst starting temperature) in a commercially available 316-type austenitic stainless steel. These nano-particles are considered to be formed at intersections of dislocations. Fatigue life tests using ordinary-sized specimens revealed that high cycle fatigue life of the 316-type austenitic stainless steel was extended by the cryogenic treatment and this is considered to be due to the pinning of dislocations by the formation of nano-sized α’-martensite particles. Keywords : fatigue life extension, pinning dislocations, nano-sized α’-martensite INTRODUCTION MEMS devices and/or micromachines have been intensively developed for the use in information technology, bio-medical technology and so on. The size of the components of these machines will be in the order of microns or sub-microns. Fatigue strength is one of the most important properties in micro-sized materials because the maintenance or the exchange of the damaged elements is practically impossible in such tiny machines. Thus, the development of a fatigue strengthening method for micro-sized materials is extremely beneficial. For micro-sized materials, the microstructure which is beneficial for the improvement of fatigue properties must be smaller than sub-micron, i.e. in the order of nanometer, to minimize the microstructural heterogeneity which is increased by the miniaturization of materials. Dispersion hardening using nano-crystals and work-hardening are adequate methods for the strengthening of micro-sized materials from the point of view that the size of the strengthening microstructure (nano-sized crystals, dislocation) is in the nanometer order. However, neither of them are designed for fatigue strengthening, but for an increase in tensile strength. In many cases, dislocations cut dispersed nano-crystals and then re-solution of the precipitates occurs under cyclic stresses. Fatigue softening is caused by cyclic loading in work-hardened materials due to the re-arrangement of dislocations. Therefore, neither of the methods is so effective on fatigue strengthening. We propose a new fatigue strengthening method which is a combination of nano-crystal-dispersion and cold-work. Fatigue of metallic materials is mainly due to the accumulation of irreversible motion of dislocations. Pinning dislocations at their
Data Loading...
