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In this paper, we proposed an inverse gradient nanograined structure (IGNS) to improve the mechanical behavior of an austenitic stainless steel. Using high strain cold rolling and subsequent ultra-high-frequency electromagnetic induction heating, we successfully fabricated an IGNS layer on AISI 316L stainless steel, within which the grain size exponentially decreases from the micrometer scale in the surface to the nanometer scale in the inner center. Microstructure evolution was systematically characterized by transmission electron microscopy. The uniaxial tensile test result shows a superior strength–ductility synergy in the IGNS sample. It obtained a yield strength of 842 MPa and a uniform elongation of 16%. A linear relationship between yield strength and uniform elongation was exhibited in IGNS samples with different volume fractions. Effects of the IGNS layer on strength and ductility were examined. Additionally, the effect of recrystallized grain distribution on mechanical properties was discussed.

The methods of measuring the fractal dimension of a Sierpinski carpet are summarized. The Sierpinski fractal dimension of a metallograph, consisting of regions of martensite and retained austenite in a Fe-29 wt% Ni-0.16 wt% C alloy by cooling to - 125[ddot]C, is determined and its physical significance illustrated.

acad sinica,inst met res,fatigue & fracture mat lab,shenyang 110015,peoples r china.;long, qy (reprint author), acad sinica,int ctr mat phys,72 wenhua rd,shenyang 110015,peoples r china

ACAD SINICA,INT CTR MAT PHYS,SHENYANG 110015,PEOPLES R CHINA.;ZHU, ZM (reprint author), ACAD SINICA,INST MET RES,STATE KEY LAB FATIGUE & FRACTURE MAT,72 WENHUA RD,SHENYANG 110015,PEOPLES R CHINA

ACAD SINICA,INT CTR MAT PHYS,SHENYANG 110015,PEOPLES R CHINA.;LONG, QY (reprint author), ACAD SINICA,INST MET RES,STATE KEY LAB FATIGUE & FRACTURE MAT,72 WENHUA RD,SHENYANG 110015,PEOPLES R CHINA

Fracture as one of the mechanical properties of materials is structurally dependent. Defects, defect assemblies, grain boundaries and sub-boundaries materials, modify the local stress intensity factors intensively. Brittle fracture prefers to confine itself to the grain boundary where the specific surface energy is lower than that in the grain. Again, transgranular cracking may occur on the crystal cleavage plane or planes where the local toughness is lowered by dislocation interaction and motion. This paper shows the complexity of the fractal dimension or roughness index of fractured surfaces in materials with metallographic structures or in inhomogeneous media. (C) 2000 Elsevier Science B.V. All rights reserved.

ACAD SINICA,INST MET RES,SHENYANG 110015,PEOPLES R CHINA.;FU, R (reprint author), ACAD SINICA,INT CTR MAT PHYS,WENHUA RD 72,SHENYANG 110015,PEOPLES R CHINA

The effects of processing parameters on austenite retention in intercritically annealed HSLA steels have been investigated. Low-temperature tempering was found to be necessary for successful tint etch of high-carbon martensite in dual-phase structure with sodium metabisulfite. Cold working before the intercritical anneal increases the nucleation sites for transformation to austenite and, thereby, the amount of retained austenite is generally increased. The microstructural evolution during postanneal cooling is detrimental to austenite retention. Both the particle size effect and the chemical stabilization by carbon are responsible for the suppression of martensitic transformation.

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.

1. (1) Grain boundaries in iron provide short-circuit paths for hydrogen diffusion. 2. (2) DIGM is observed after cathodic charging at room temperature. 3. (3) Dislocations are not significant short-circuit diffusion paths for hydrogen. 4. (4) Dislocation activity has not been detected by cathodic charging alone for current densities up to 100 mA/cm 2 , except in the vicinity of blisters.