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In ultra low-carbon Fe-18%Ni alloy with a lath martensitic structure, the effects of microstructure and dislocation density on the yielding and deformation behavior were investigated. The microstructure of the matrix became finer with decreasing austenite grain size but no difference was found in the yielding behavior up to 2% deformation. Dislocation density was constant independent of the microstructure and was estimated to be about 1.5 x 1015/m². The dislocations that had been introduced by martensitic transformation formed a tangled dislocation cell structure within a lath and the amount of dislocation strengthening was determined by a critical stress that was required to make the tangled dislocations bow out. As a result, it was confirmed that the 2% proof stress of the steel used can be evaluated by adding the amount of dislocation strengthening to the friction stress. [ABSTRACT FROM AUTHOR]

The time-temperature-precipitation (TTP) and corresponding mechanical properties in high-nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N steel (all in weight percent) were investigated using electron microscopy and ambient tensile testing. The precipitation reactions can be categorized into three stages: (1) high-temperature region (above 950 °C)--mainly coarse grain-boundary (intergranular) Cr2N; (2) nose-temperature region--intergranular Cr2N → cellular Cr2N → intragranular Cr2N + sigma (σ); and (3) low-temperature region (below 750 °C)--intergranular Cr2N → cellular Cr2N → intragranular Cr2N + σ + chi (X) + M7C3 carbide. After cellular Cr2N precipitation became dominant above 800 °C, yield and tensile strength gradually decreased, whereas elongation abruptly deteriorated with aging time. On the contrary, prolonged aging in the low-temperature regime increased tensile strength, caused by the precipitation of fine X and M7C3 within grains. Based on the analyses of selected area diffraction (SAD) patterns, the crystallographic features of the second phases were analyzed. [ABSTRACT FROM AUTHOR]

The ordered structure of Cr2N precipitates in high-nitrogen austenitic steel was investigated utilizing high-resolution neutron powder diffractometry (HRPD). On the basis of the Rietveld refinement of neutron diffraction patterns, the ordered Cr2N superstructure was confirmed to be trigonal (space group ), with lattice parameters a = 4.800 (4) and c = 4.472 (5) Å, as suggested in previous transmission electron microscopy studies [Lee, Oh, Han, Lee, Kim & Takaki (2005). Acta Cryst. B 61, 137–144; Lee, Kim & Takaki (2006). Acta Cryst. B 62, 190–196]. The occupancies of the N atoms in four crystallographic sites [1( a), 1( b), 2( d) and 2( c) Wyckoff sites] were determined to be 1.00 (5), 0.0, 0.74 (9) and 0.12 (3), respectively, reflecting a partial disordering of N atoms along the c axis. The position of the metal atom was specified to be x = 0.346 (8) and z = 0.244 (6), corresponding to a deviation from the ideal position ( x = 0.333 and z = 0.250). This deviation caused the -type superlattice reflection to appear. A comparison between the ideal and measured crystal structures of Cr2N was performed using a computer simulation of selected-area diffraction patterns. [ABSTRACT FROM AUTHOR]

Sharp needle-like specimens for atom-probe field-ion microscope (APFIM) analysis have been prepared successfully from mechanically milled pearlitic steel powders. Mechanically milled powders were packed in a stainless steel tube with carbonyl steel powders and then consolidated by sheath cold-rolling to obtain a bulk composite. A small square rod was cut out from the bulk and electropolished to a blunt needle. It was then fabricated to a nanometre-sized needle by a focused ion beam technique using an annular gallium ion beam. The tip radius of the specimen was <50 nm, which is suitable for use in APFIM. [ABSTRACT FROM AUTHOR]