Internal Friction and Elastic Study on Surface Nanocrystallized 304 Stainless Steel Induced by High-

1. IntroductionMaking use of excellent properties of meta11ic nanocrys-talline materials, the traditionaI engineering materials canbe successfully modified by generation of a nanostructuredsurface layer, i.e. by means of surface nanocrystallization(SNC)<'> technic. This method may effectively improve theovera1l properties and performance behavior of materials.This kind of SNC technology on the one hand conquers somedifficulties in syllthesizing 3-D bulk nanocrystal1ine materi-als, on the other hand it combines -excellent properties ofnallocrystalline with that of the traditional engineering mate-rials. Therefore it is a feasib1e way to carry out the industrialapplication of nano-technique.Recellt investigationsI'~51 revea1ed that by using the u1tra-sonic shot peening and the high-energy shot peening (HESP)technologies, surface nanostructured layers call be success-fully obtained on different meta11ic materia1s. The mechan-ical properties of this kind of materials axe signiflcantly im-t To whom correspondence should be addressedE-mail: xwang@imr.ac.cn, Assoc. Pro f.proved. The average grain size and the thickness of the surfacenanocrystal1ine layer vary dependillg upon the shot peeningtreatment conditions. Although many microstructural obser-vations have been done on the SNC materiaIs, the investi-gations on the mechanical properties with the distributioncharacteristic and configuration of defects in SNC materialshave been relatively absent. We studied the distribution ofdefects of surface layer in HESP low-carbon steel by meansof positron lifetime .easu..m..t<6>. Previous ...uit. haveshown that the microstructure of surface layer could be char-acterized in two parts: nano-grain-sized region and transitionlayer, which is located between surfaCe nanocrystalline regionand coarse grain region. Based on the nature of microstruc-tural sensitivity of SNC materia1s, the surface nanostructured1ayer and the transition layer may p1ay key role in whole me-chanical properties of materials.Internal friction measurement is an important experi-ment technique to study the so1id defect and mechanicalproperties<'>. Its most important characteristic is to sensi-tiveIy detect the change of microstructure and defects in materials under nondestructive conditions. It has been widelvMater. Sci. Tech;uol., Vol.18 No.2, 2002l33applied on the research of convelltional polycrystalline.at..i.l.I8> and nanocrystalline .at..i.ls<9~13>.' By mutu-ally combining internal friction measurement with other mi-croscopic obserVation and analytical methods, the relation-ship between the microstructure of material and macroscopicproperties, such as mechanical properties, can be more deepl,yclarified.In the present paper) the nanostructured surfaCe layer of304 stainless steel treated by HESP was studied by using in-ternal friction measurenlent technique. The changes of mea-sured internal friction and Young's modulus were analyzed inrelation to the characteristic of micrO8tructural change alont;the depth from the top surface.2. ExperimentalThe material used in this work is the austenitic stainlesiisteel AISI 304 plate of 4 mm thick. Its chemicaI composition(mass fraction) contains: 0.049C, 18.20Cr, 8.64Ni, 1.04Mn,0.021S and Fe balance. The initial microstructure is a fC(:austenite phase in which the initial average grain size is about120~150 pm. Samples of 100 mmx 100 mm were prepared fo>.the HESP treatmellt. The main experimelltal parameters oirthe HESP process were chosell as follows: the vibration fro'quency of the chamber is 50 Hz, the diameter of spherica:tshots is 8 mm? and the processing duration is selected to bft5, 15, and 60 min respective1y.Measurements of illternal friction and Young's modulin;were carried out on a vibrating reed apparatus. The sam-ple, of which the one end was fixed and the other end wai,free, was ho1d between excitation electrode and reception elec-trode. Under the action of cyclic electrostatic fOrce that comesfrom the excitation electrode