Non-equilibrium Grain Boundary Segregation Research Articles

Comment on “Kinetics of non-equilibrium grain-boundary segregation induced by applied tensile stress and its computer simulation”

Non-equilibrium solute grain boundary segregation, a model for which was proposed by Xu, is discussed with particular focus on the influences of atomic diffusion and microstructure on grain boundary segregation.

Creating and destroying vacancies in solids and non-equilibrium grain-boundary segregation

Determining how the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known to be sources and sinks for bulk vacancies, but the exchange that will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. At the same time, this physical process will produce solute non-equilibrium grain-boundary segregation and dilution. A set of kinetic equations is established to describe this physical process. Additionally an attempt has been made to simulate the experimental data with the kinetic equations to justify the physical process.

Experimental study on non-equilibrium grain-boundary segregation kinetics of phosphorus in an industrial steel

The non-equilibrium grain-boundary segregation level of phosphorus in an industrial steel, 12Cr1MoV, was measured. A non-equilibrium grain-boundary segregation kinetics curve of phosphorus is given, which provides direct evidence of the non-equilibrium grain-boundary segregation kinetic model proposed by Xu Tingdong.

Kinetics of non-equilibrium grain-boundary segregation induced by applied tensile stress and its computer simulation

A set of kinetic equations is offered for non-equilibrium grain-boundary segregation induced by applied tensile stress. The simulation using the equations is in good accordance with the observation of sulfur grain-boundary segregation in steel.

Importance of Molybdenum on Irradiation-Assisted Stress Corrosion Cracking in Austenitic Stainless Steels

Constant extension rate tensile (CERT) specimens were irradiated in the core of a commercial operating boiling-water reactor (BWR). Subsequent to irradiation, CERT testing was performed in a test loop attached to the reactor water cleanup system in the same BWR, using reactor water at a high flow rate. Testing was performed in BWR normal water chemistry (NWC) and hydrogen water chemistry (HWC). Type 316 (UNS S31600) stainless steel (SS) was much less susceptible to irradiation-assisted stress corrosion cracking (IASCC) than type 304 SS (UNS S30400) in NWC and HWC. High-purity (HP) type 304 SS was more susceptible to IASCC than commercial purity (CP) type 304 SS in NWC at intermediate fluence (∼ 1 × 1021 n/cm2, E > 1 MeV). Results suggested chromium depletion resulting from radiation-induced segregation (RIS) was an operating mechanism in NWC and may have been the primary radiation effect at fluences < 3 × 1021 n/cm2 (E > 1 MeV). At higher fluences, another mechanism supervenes. Detailed microchemical analysis was conducted on unirradiated archive material and irradiated material. Two complementary techniques were used: field emission gun scanning transmission electron microscopy (FEGSTEM) and Auger electron spectroscopy (AES). The unirradiated materials showed nonequilibrium grain-boundary segregation of chromium, molybdenum, and phosphorus. Irradiation to intermediate fluences (∼ 1 × 1021 n/cm2, E > 1 MeV) resulted in further enhancements of silicon and phosphorus at the grain boundaries. In the CP materials, molybdenum and chromium remained enhanced at the grain boundaries, with chromium depletion on either side of the grain boundary. Chromium was depleted significantly in the HP material. At higher fluences (∼ 1 × 1022 n/cm2, E > 1 MeV) chromium was depleted significantly in the CP type 304 SS. The presence of molybdenum correlated with retention of the chromium level at the grain boundary during irradiation up to a certain fluence. For the HP type 304 SS, which contained virtually no molybdenum, the chromium level was lower than for the CP heats.

Nonequilibrium grain-boundary segregation and ductile-brittle-ductile transition in Fe-Mn-Ni-Ti age-hardening alloy

Nonequilibrium segregation kinetics of alloying elements and a ductile-brittle-ductile transition behavior have been investigated in an Fe-8.4Mn-7.4Ni-l.7Ti alloy. The alloy experienced a ductilebrittle-ductile (DBD) transition during isothermal aging. In the brittle region, the alloy showed a decrease in intergranular fracture strength and a subsequent increase with aging time. This is due to the segregation of titanium to the grain boundaries and its desegregation into the matrix. The intergranular fracture strength was higher on the zero tensile elongation finish curve than on the start curve. This is because the grain-boundary segregation level of titanium is relatively lower on the finish curve. The lowest intergranular fracture strength increased with increasing aging temperature, which was attributed to a lower grain-boundary segregation level of titanium at higher temperature. Manganese caused an overall reduction in intergranular fracture strength and, as a result, the delayed occurrence of the zero tensile elongation (ZTE) finish curve in a temperature and log-time plot.

Two types of boron segregation at austenite grain boundaries and their mutual relation

The distribution of boron in 10 p.p.m. B-doped Fe-30 wt% Ni austenitic alloy was investigated by means of particle tracking autoradiography (PTA). It was shown that when quenched from a certain temperature between 750° C and 1220° C with medium cooling rates, both equilibrium and non-equilibrium grain boundary segregation of boron appeared in the alloy. The degree of segregation represented the sum of the two kinds of segregation of boron. A transition temperature, of approximately 950° C, was found below which the equilibrium grain-boundary segregation was dominant and above which the non-equilibrium grainboundary segregation was dominant. For the degree of segregation achieved by these two types of segregation of boron there exists a temperature from 1000° C to 1050° C at which a minimum amount of segregation is obtained. It was also shown that non-equilibrium segregation of boron can considerably promote the precipitation of boride at grain boundaries.

A kinetic model of non-equilibrium grain-boundary segregation

An isothermal kinetics of non-equilibrium grain-boundary segregation was developed both for segregation processes and for desegregation processes within a phenomenological theory. An effective time concept of a cooling process was discussed. On these bases, a simple and accurate method for evaluation of the levels of non-equilibrium grain-boundary segregation during cooling was proposed. According to the method, we have calculated the levels of non-equilibrium segregation to austenite or prior austenite grain boundaries for boron in Fe-30%Ni alloy, aluminium in Inconel 600, chromium in 2.25%Cr1%Mo steel and tin in 2.25%Cr1%Mo0.08%Sn steel in different experimental conditions respectively. Results calculated from the kinetic model in this paper are in satisfactory agreement with the observed data of experimental measurements for all the above samples.

Determination of critical time and critical cooling rate of non-equilibrium grain-boundary segregation

A new method of determining critical time and critical cooling rates of non-equilibrium grain-boundary segregation experimentally has been developed. Accordingly, the critical time at different temperatures, t c , and the critical cooling rate, θ c, as cooled from different starting temperatures for boron non-equilibrium grain-boundary segregation in a B-doped Fe-30 wt% Ni austenitic alloy were determined experimentally by means of Particle Tracking Autoradiography (PTA).

Non-equilibrium grain-boundary segregation kinetics

A new model is proposed in this article concerning non-equilibrium grain-boundary segregation kinetics. The model not only includes the isothermal non-equilibrium grain-boundary segregation kinetics but also provides a formula for calculating the non-equilibrium segregation level under continuous cooling. Results calculated from the non-equilibrium grain-boundary segregation model for boron in steel grade Type 316 and chromium in steel grade $$2\tfrac{1}{4}$$ % Cr-1 % Mo are in satisfactory agreement with the observed data for experimental measurements.

Non-equilibrium grain-boundary segregation in austenitic alloys

The theory of non-equilibrium grain-boundary segregation is discussed with particular reference to recent ideas and data relating to boron grain-boundary segregation in Type 316 austenitic steel. The kinetics of the non-equilibrium grain-boundary segregation process are considered in depth and a model is developed which, it is hoped, will more realistically describe the magnitude and extent of the process. Reasonable agreement is found between the predictions of the model and experimental evidence for non-equilibrium boron, aluminium and titanium segregation to grain boundaries in austenitic steels. The model predicts, generally, that elements with large misfits with the matrix atoms will segregate most. Larger grain sizes lead to greater grain-boundary segregation. Also, the two critical heat-treatment parameters in non-equilibrium segregation are the solution-treatment temperature and the cooling rate from the solution-treatment temperature. Predictions of the worst combinations of these parameters for maximum non-equilibrium segregation to grain boundaries in austenitic steels are presented.

The segregation of boron to grain boundaries in solution-treated Type 316 austenitic stainless steel

The distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900–1350°C, has been determined using an optical microscope autoradiographic technique. Segregation of the boron to the grain-boundary regions was detected in the specimens cooled at the slower rate from the higher solution-treatment temperatures, the extent of the intergranular segregation diminishing with decreasing solution-treatment temperature; grain-boundary segregation of the boron was not detected in the specimens cooled at 500 degC/s irrespective of the solution-treatment temperature. A theoretical model is proposed to account for this non-equilibrium grain-boundary segregation of boron during cooling, based on the existence of mobile vacancy-boron complexes. The predictions of the model are in good agreement with the experimental observations if realistic values for the vacancy-boron binding energy and complex diffusion rate are assumed.