Bulk Phosphorus Content Research Articles

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:Pmelt>Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

Intergranular Cracking Susceptibility of 2.25Cr Heat-Resistant Steels Depending on Alloying Elements and Impurities

The intergranular cracking susceptibility of 2.25Cr heat-resistant steels increases with increasing bulk phosphorus content. This is due to the increase in phosphorus segregation concentration of prior austenite grain boundaries (PAGBs) and the prior austenite grain boundary/carbide interfaces (GCIs) with increasing bulk phosphorus content. Moreover, the susceptibility is higher in tungsten-added steels than the molybdenum-added steel. This is attributed to the higher driving force for carbide formation of tungsten which causes more active carbide formation in the tungsten-added steel, the consequent absence of the repulsive segregation between carbon and phosphorus, and the final higher phosphorus segregation concentration at the PAGBs and the GCIs. Additionally, the absence of sulfur segregation at the PAGBs and the GCIs of the molybdenum-added steel, which arises from the repulsive segregation between carbon and sulfur, acts as an additional factor which lowers the intergranular cracking susceptibility.

Grain Boundary Segregation of Phosphorus and Columnar Grain Growth during Decarburization in Plain Carbon Steels

During decarburization of plain carbon steels, the grain boundary segregation concentration of phosphorus increased with increasing bulk phosphorus content and with decreasing decarburization temperature. The grain growth kinetics decreased with increasing bulk phosphorus content which is due to the grain boundary pinning effect of highly segregated phosphorus. After decarburization at 973 K for 24 h, the columnar grain growth following the abnormal grain growth was observed in the steel containing a low bulk phosphorus content, while the steel containing a high bulk phosphorus content showed only the abnormal grain growth behavior. Such grain growth behaviors can be understood in the light of the abnormal grain growth driven by the grain boundary carbides and the solute drag effect of highly segregated phosphorus on moving grain boundaries. During decarburization at 1173 K, only the normal grain growth was observed due to the absence of grain boundary carbides, regardless of the bulk phosphorus content. The decarburization reaction in the present study can be expressed by the parabolic relationship x = k(Dt)1⁄2 where x is the decarburization depth, k the reaction coefficient, D the diffusivity of carbon and t the decarburization time.

Effect of Grain Boundary Segregation on Mechanical Properties of P-Doped Fe-Si Base Alloys

Abstract The grain boundary segregation in two Fe-Si base alloys with the different bulk phosphorus contents was investigated. The phosphorus grain boundary concentrations determined with the use of Auger electron spectroscopy were related to the microstructure, fracture mode, and mechanical properties of the alloys. Transgranular cleavage was found to be the dominant fracture mode of the samples tested by impact at room temperature. The correlations between the phosphorus grain boundary concentration, the portion of intergranular fracture, and the dynamic fracture toughness were presented, which fulfil the theoretical expectation. Two possibilities were proposed to explain a discrepancy in the temperature dependence of the phosphorus grain boundary concentration for the alloy with the higher bulk phosphorus content. Introduction Impurity segregation and precipitation of secondary phases at the grain boundaries are the fundamental diffusion-controlled processes contributing to the intergranular embrittlement of commercial steels [1]. With intention to quantify the particular effects of the above processes, simple alloys have been investigated. In this respect the P-doped Fe-Si alloys, mostly free of precipitates, are suitable for the segregation studies [2,3]. As was described by McLean [4], the equilibrium grain boundary concentration of the impurity depends directly on its bulk concentration and the annealing temperature. The segregation kinetics can also be influenced by other factors as are grain size, type of the microstructure etc. [5]. It is generally known that small changes in the grain boundary chemistry can provide significant changes in mechanical properties of the alloy steels. This process was excellently characterised on the phenomenological level [6]. However, the precise quantification of the relationship between the grain boundary composition and the energy of intergranular fracture is still missing [7].

Non‐equilibrium intergranular segregation and embrittlement in neutron‐irradiated ferritic alloys

AbstractThis study describes intergranular segregation and embrittlement in several model ferritic alloys doped with Mn, P, S and/or Cu subjected to neutron irradiation, irradiation‐equivalent thermal ageing (ETA) and post‐irradiation annealing (PIA). Neutron irradiation produced a larger amount of intergranular phosphorus segregation than sulfur segregation. Irradiation‐induced phosphorus segregation became more prominent with decreasing bulk phosphorus content. Intergranular carbon segregation remained small in all the as‐irradiated alloys, which was enhanced by the ETA except for the high phosphorus‐containing alloy. A PIA study has shown that the phosphorus segregation in phosphorus‐doped alloys subjected to lower temperature PIA proceeds via mobile phosphorus–interstitial complexes, whereas the sulfur segregation, which occurs over a wide PIA temperature range, is controlled by vacancy‐enhanced diffusion. The mechanisms of non‐equilibrium intergranular segregation induced by neutron irradiation are discussed in light of coupled fluxes of point defects and impurities and changes in the segregation capacity of grain boundaries. Small punch tests demonstrated how the impurity segregation or desegregation and hardening or softening induced by the irradiation, ETA and PIA influence intergranular embrittlement in the various ferritic alloys. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Effect of Bulk Phosphorus Content on Hardening, Non-Equilibrium Segregation and Embrittlement in Neutron Irradiated Iron-Based Alloys

ABSTRACTThe effect of bulk P contents on hardening, non-equilibrium intergranular segregation and embrittlement has been studied in Mn-doped ferritic alloys subjected to neutron irradiation (E&gt;0.1MeV: fluence of 1 × 1025 n/m2 at 711K for 2120 h) or irradiation-equivalent thermal aging. Neutron irradiation-induced intergranular P segregation became more prominent with decreasing bulk P content. Thermal aging slightly enhanced the amount of segregated P independent of the bulk P content. Intergranular C segregation in all the alloys was suppressed by the irradiation. An alloy with low bulk P content showed only moderate irradiation-induced hardening. The ductile-brittle transition temperature (DBTT) in alloys with low and intermediate amounts of P increased by the same shift during the irradiation but not at all during the thermal aging. Doping high bulk P led to a high DBTT in the as-heat-treated alloy while the irradiation decreased the DBTT. The irradiation effect on the DBTT in the model ferritic alloys containing the different levels of P is discussed in light of embrittling or toughening effects caused by the changes in the P or C segregation, and hardness.

Embrittlement observed in Cr–Mo turbine bolts after service

The present paper describes a study aimed at investigating the extent of service related embrittlement suffered by a series of Cr–Mo steel turbine bolts after over 200,000 h at 450°C. A small section of material was removed from a non-critical location of all the 51 bolts. From this section, the chemical composition, average hardness and average prior austenite grain size were measured. The toughness of the bolts was measured by Charpy impact testing and/or Auger electron spectroscopy. From the various parameters investigated, it was established that grain size and phosphorus level were the only factors which consistently identified whether a bolt was embrittled or non-embrittled. Indeed it was established that bolt embrittlement could be predicted using the simple product of microstructure grain size d (μm's) and bulk phosphorus content. Finally, it was observed that bolt embrittlement could be predicted using the simple product of microstructure grain size and bulk phosphorus level and that such a trend indicated the importance of grain boundary area available for service induced phosphorus segregation.

Material Property Studies of Two High-Pressure Turbine Rotors for Remaining Life Assessment

A remaining life assessment study was performed on two no-bore HP rotors after 169,000 hours of operation. These rotors were manufactured from 1CrMoV (ASTM A470, Class 8) steel. Southwest Research Institute conducted a materials evaluation program to generate pertinent material property data necessary to perform a SAFER analysis by Lehigh University. Two ring samples were removed from periphery locations of a rotor. One of the rings was from a packing location where the operating metal temperature was 860°F (460°C), and the other ring from the cold coupling end. Evaluation tests included chemical analysis, metallography, Auger analysis, tension and hardness, Charpy impact, fracture toughness, and stress rupture. A small sliver sample was available from the second rotor. Limited metallurgical analysis was conducted on this sample. Results indicated that the hot end embrittled in service with a shift in FATT of 240°F (116°C) when compared to the cold end. Fracture toughness versus temperature data were developed using various correlations available from the literature. Good correlation was found among the various procedures and lower bound curves were established for the hot and cold ends. Hot end toughness properties were significantly lower than those of the cold end. Auger results indicated segregation of phosphorus on the grain boundaries at the hot location. After service FATT can be predicted using the bulk phosphorus content and correlations available from the literature.

Some observations concerning reverse temper embrittlement and creep damage in a series of CrMoV steel turbine bolts after 120 000 h of service

This paper describes a detailed investigation which was specifically aimed at providing a simple practical assessment procedure which identified reverse temper embrittlement (RTE) in a series of high-pressure (HP) and intermediate-pressure (IP) CrMoV steel turbine bolts from two separate 120 MW sister units which had been subjected to 120 000 h of service. A small section of material was removed from a non-critical location from all 182 bolts. From this section the chemical composition, average hardness and prior austenite grain size were assessed. The toughness of selected bolts was measured by either (i) Charpy impact testing or (ii) Auger electron spectroscopy. It was established that grain size and bulk phosphorus content were the primary factors which consistently correlated with the embrittled condition. An embrittlement estimative diagram was established by plotting grain size d versus percentage phosphorus. This portrayed two distinct regions, namely an embrittled region and a non-embrittled regio...

An appraisal of certain factors which influence toughness restoration (de-embrittlement) in some temper embrittled low alloy steels

This paper describes and assesses the various factors which affect the degree of de-embrittlement, ΔFATT, of a selection of embrittled low alloy steels after a simple tempering heat treatment. It has been shown that the degree of de-embrittlement was variable and could be divided into two separate sets of data depending upon the embrittled fracture mode characteristics. The steels which failed in a wholly intergranular manner, predominantly the nickel-containing steels, exhibited large ΔFATT values of between 150 and 250°C. However, the non-nickel-containing steels, viz. the Cr-Mo and Cr-Mo-V steels, fractured in a mixed mode fashion, which comprised intergranular failure and transgranular cleavage, and exhibited much lower levels of toughness rejuvenation. When the raw data were corrected for grain size effects it was clear that the bulk phosphorus content of the steels significantly influenced the extent of de-embrittlement. Also when the de-embrittlement data were portrayed in terms of the embrittled condition toughness, FATT EMB, and corrected to a constant composition and grain size, it was observed that the degree of toughness restoration, ΔFATT, was insensitive to FATT EMB.

The effect of phosphorus and oxygen on the nitriding of α-Fe

The kinetics of nitriding of iron and iron-phosphorus alloys in ammonia-hydrogen gas mixtures have been investigated at 580°C. The rate of nitriding of pure iron was found to be consistent with that predicted assuming nitrogen diffusion in the solid to be the rate limiting step and this condition was also observed over a range of oxygen potentials under the same nitriding conditions. The presence of phosphorus in the iron was found to reduce nitriding rates. The rate limiting reaction in the nitriding of Fe-P alloys in ammonia-hydrogen gas mixtures at one atmosphere total pressure is determined to be NH 2(adsorbed) → NH(adsorbed) + H(adsorbed). A first order reaction rate equation is developed which predicts the nitriding rate to be inversely proportional to the bulk phosphorus content of the alloy at a given hydrogen partial pressure.