13th International Conference on Fracture June 16–21, 2013,Beijing, China -5- logarithmic strain, for tension is smaller than the engineering strain, whereas the value of the compressive true strain is generally larger than that of the linear strain. It results in different martensite content for the same deformation grade (engineering strain) in martensite evolution since the martensite evolution law is expressed by the true strain, Eq. (3). Experimental observations reveal that SS304 is a metastable austenitic steel [16-18]. Figure 2 confirms development of crystallographic pictures of the SS304 after different deformations. The figure demonstrates clearly that the density of martensite needles increases with strains. In pictures dark lines denote martensite phase in the needle form. The pictures reveal that the grain size is ca. 100µm and remains constant even after severe plastic deformations. However, martensite seems proportional increasing with deformations, under both tension and compression. After -45% compression deformations, a large part of austenitic contents in material has been transformed into dark martensite phases. The phase transformation changes and disorders the microstructure of grains, thus arises resistance of the dislocation motions. The material with martensite phase obtains higher strength and less ductility. Experiments do not show significant difference in development of martensite phase under tension from that under compression. (a) Base material (b) After compression, eng. strain -19% (c) After compession, eng. strain -45% (d) After tension, eng. strain 40% Figure 2: Crystallographic pictures of the SS304 after different deformations. The dark phases denote the martensite content. 3.3 Experimental measurement of martensite phase evolution A ferritescope[19-22] is used to measure the martensite content in the plate specimens. The method measure the martensite content based on changes of the linear relationship between the output
RkJQdWJsaXNoZXIy MjM0NDE=