Investigation of defects and nanoparticles with martensitic phase transformation in surface nanostructured 316L stainles
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Chunlan Zhou and Long Wei Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China (Received 26 August 2009; accepted 14 December 2009)
In this article, we investigated the defects introduced by surface mechanical attrition treatment by Doppler-broadening spectroscopy of positron annihilation radiation in surface-nanostructured 316L stainless steel. Through the measurement of different thinning layers in the samples treated for 15 min, the slope of line shape parameter S versus wing parameter W curves showed three different values with depth responding to the change of defect configuration. An unusual change of S and W parameters near the surface was mainly from the effect of quantum-dot-like state caused by the formation of nanoparticles. Based on the change of S W with depth, the martensite phase transformation induced by strain could be estimated to occur within a depth of 35 mm.
I. INTRODUCTION
Surface mechanical attrition treatment (SMAT) is a surface modification technology that makes use of repeated multidirectional impacts of flying balls on the sample surface. The treatment can result in severe plastic deformation and induce grain refinement into the nanometer regime in the surface layer of bulk materials. Surface nanocrystallization not only improves the mechanical properties of materials but also changes the configuration of defects in surface layer.1 When Type 316L stainless steel is processed by SMAT, several significant changes have taken place in the configuration of the defects. In addition to the normal, expected increase in dislocation density and vacancy concentration, there is a phase transformation in which the face-centered cubic austenite transforms to a strain-induced martensite. However, it is not easy to quantitatively describe the deformation characteristics in such surfaces by using conventional methods. To explain the influence of ultrafine grain size and phase transformation on formation of surface layer, it is important to investigate the characteristics of the defects in the surface layer by more sensitive and exact methods, to understand the formation mechanism of surface nanocrystallization layer and the influence of the deformation conditions on the defects configuration. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0074 J. Mater. Res., Vol. 25, No. 3, Mar 2010
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It is well known that the positron is a probe of vacancy-type defects in metals. Because the vacancies lack positively charged nuclei, the positrons are sensitively trapped at the defects and annihilate there with the surrounding electrons, conveying useful information on the local electronic environment around the defects. Thus, the positron is called a self-seeking probe,2,3 because the probes used in the usual methods have no “site selectivity” because it is difficult to unambiguously extract the useful (weak) interaction signals of the embedded particles from a much
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