Minimum Segregation Research Articles

Statistical analysis of phosphorus segregation on intergranular fracture facets in pressure vessel steels

A533B and C–Mn steels, widely used as nuclear pressure vessel steels, have been aged at 520°C after tempering at 650°C for various periods of time to produce different levels of embrittlement resulting from the segregation of P to grain boundaries. Metallographic observation and tensile test results showed that the embrittlement heat treatment did not have significant influence on the microstructures or tensile properties of the steels. P segregation at grain boundaries and on intergranular facets was investigated using field emission gun transmission electron microscopy and Auger electron spectroscopy. After such treatment, enhanced segregation was found to be a linear function of the square root of embrittling time. Statistical analysis of the AES measurements indicated that there is a minimum segregation level for intergranular fracture to occur.

Further statistical analysis of the composition of a grain boundary in a Mo(Re) alloy studied by atom-probe field-ion microscopy

We have recently performed a combined transmission electron microscope CIEM) and atom-probe field-ion microscope (APFIM) investigation of Re segregation to a single grain boundary (GB) that is within ---0.4 of a E = 9 coincident site lattice (CSL) orientation in a Mo-5.58_+0.31 at.% Re alloy [ 1,2]; specifically the misorientation is 0 =38.5 about the c 1 = (1/'-,/2)[ 1,1,0l i direction, and the plane of the GB is -(-1,-1,4); this GB is mainly a tilt GB with a twist component. The primary conclusion of our paper [1,2] with respect to GB segregation is that this specific GB studied has an Re concentration which is = 1.27 or 27% greater than the Re concentration of the matrix -this value represents the minimum segregation enhancement factor, 3 as it includes some solute atoms from the matrix.[3,4] In this letter we recapitulate briefly our experimental procedure, the results obtained, and we then consider in detail the question of the statistical uncertainties involved. This detailed statistical analysis confirms the original conclusions made in reference 1 with respect to the uncorrected Re concentration at the GB, as well as the corrected Re concentration.

The degree of mixing in continuous flow systems

The concepts of D anckwerts about the degrees of mixing and segregation are extended to the case of a continuous flow system with an arbitrary but known residence time distribution. For this purpose a life-expectation distribution is defined in addition to the age distribution. Further, a condition of maximum mixedness (minimum segregation) is defined for such a system. This condition, and the condition of complete segregation introduced by D anckwerts, are two opposite extremes. When the system is a reactor in which a chemical reaction of an arbitrary order takes place, the conversion can be calculated for both cases; thus two limits are obtained between which the conversion must lie.