C. L. Briant

The role of chemical bonding in grain boundary embrittlement

Abstract Brittle intergranular fracture occurs when impurity elements segregate to the grain boundaries of a material and lower their cohesive strength. Although many embrittling elements have been identified by experimental studies, the reason why these elements cause embrittlement has remained elusive. This paper presents results of fully quantum mechanical cluster calculations which address this question. It will be shown that strong embrittling elements draw charge from the neighboring metal atoms onto themselves. They thus remove charge from the metal-metal bonds which hold the grain boundary together and weaken them. Cohesive enhancers do not draw charge off the metal atoms and thus do not weaken the metal-metal bond network. In addition, they form rather homopolar bonds with the metal atoms and thus provide an added increment of bonding in the grain boundary.

Molecular dynamics study of the structure and thermodynamic properties of argon microclusters

Microclusters of 2–100 argon atoms were studied using molecular dynamics. The microclusters were ordered solids at low temperatures and energies and disordered liquids at high temperatures and energies. The melting transition occurred considerably below the bulk melting temperature. Radial density functions, interference functions, diffusion coefficients, and surface energies were calculated for both the solid and liquid phases. The surface energies of the microclusters could be expressed as a function of the form Ai2/3+Bi1/3 where i is the number of atoms in the cluster. For 0 °K solid clusters A and B have the values 0.26×10−12 and −0.12×10−12 erg, respectively; for liquid clusters at 40 °K, A and B have the values 0.17×10−12 and −0.045×10−12 erg, respectively. Clusters containing 7 or more atoms melted with the occurence of a first‐order‐like transition. This transition was studied further through Monte Carlo calculations. A possible model for the first‐order‐like transition is proposed.

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M23C6 carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M23C6 carbides.

Hydrogen assisted cracking of type 304 stainless steel

This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the grain boundaries. With martensite present intergranular impurities such as phosphorus play a secondary role. As martensite becomes more difficult to form, the importance of impurities increases.

Embrittlement of a 5 Pct Nickel high strength steel by impurities and their effects on hydrogen-induced cracking

Temper embrittlement induced by isothermal aging at 480°C (753 K) in a commercially produced heat of HY 130 steel is shown by Auger electron spectroscopy to be due mainly to intergranular segregation of Si, with additional contributions by P, N, and Sn. Studies of rates of crack growth in gaseous hydrogen at controlled temperature and pressure in specimens aged various amounts showed that, as the impurity concentration increased, the stress intensity for crack growth Kth dropped precipitously, the cracking mode changed from cleavage to intergranular, and the crack growth rate atK > Kth increased. Hydrogen-induced crack growth occurred in a step-wise, rather than continuous, fashion. The impurity-plus-hydrogen effect is discussed in terms of a model adapted from an earlier theory of Oriani.

Mechanism of adhesion of alumina on MCrAlY alloys

X-ray diffraction has been used to measure stains/ stresses generated in oxide films formed under isothermal conditions at 1150–1225° C and cooled to room temperature. High compressive strains, of the order of 1%, were measured in alumina films formed on FeCrAlY. However, little or no strain was measured in oxide films on NiCrAlY and NiCoCrAlY samples, even when there was no scale spallation. Auger Electron Spectroscopy (AES) experiments have also been conducted to evaluate the role of segregation on scale adhesion. Our studies suggest that the adhesion mechanism might depend on the alloy composition. On iron-based alloys, the scale spallation might be prevented by mechanisms that involve strong bonding at the interface. On the other hand, the scale spallation on nickel-based alloys might be prevented by a mechanism that relieves stresses. Yttrium segregation might help in this process.

The preparation of “1223” Tl-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-Ba-Cu-Oxide with thallium oxide vapor

Abstract A process is described for the preparation of superconducting films of “1223” TlxCa2Ba2Cu3Oy (0.65 10 000 A/cm2 was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographic c-axis of the film.

Mechanical properties of high-temperature alloys of AlRu

A number of alloys based on the binary CsCl (B2) compound AIRu are tough and also are ductile in compression at room temperature. Alloys that are substoichiometric in Ru tend toward intergranular brittleness, are enriched in Al at the grain boundaries, and are improved by the addition of 0.5 at. pct B. Al47Ru53 + 0.5 at. pct B underwent 33.6 pct true strain in compression at 23°C and work hardened to 3.1 GPa true stress. Dislocations of slip vectors 〈100〉, 〈110〉, and 〈111〉 have been identified by transmission electron microscopy (TEM) study of plastically deformed samples, so that ample slip systems appear to be present for compatible deformation of polycrystalline samples.

Sensitization of Austenitic Stainless Steels, I. Controlled Purity Alloys

Abstract This paper presents a study of the effects of carbon, nitrogen, molybdenum, and manganese on the sensitization of high-purity austenitic stainless steels of composition similar to Type 304. The modified Strauss test (ASTM-A262-E), the oxalic acid etch test (ASTM-A262A) and analytical electron microscopy were used to determine the degree and nature of sensitization in the steels. The alloy compositions are considered in terms of effective chromium content, and from plots of this parameter versus sensitization time a strong effect of carbon content is seen. Additions of nitrogen, molybdenum, and manganese are found to delay sensitization at any given carbon concentration. In the case of nitrogen, the amount of improvement depends on both the carbon content and the sensitization temperature. Strong evidence is presented that nitrogen acts to retard the nucleation and/or growth of carbides at grain boundaries and hence increase the time necessary for sensitization. Molybdenum appears to increase the ...

Tempered martensite embrittlement in phosphorus doped steels

In this paper the effect of phosphorus on tempered martensite embrittlement of Ni−Cr steels is reported. It is shown that the measured degree of embrittlement depends on the phosphorus concentration, test temperature, grain size, and austenitizing temperature. Although reducing the prior austenite grain size tends to reduce the observed embrittlement, this can be offset by the fact that the low austenitizing temperatures used to produce the fine grain size cause an increased amount of impurity segregation. It is further shown that bulk phosphorus concentrations below 100 wppm may be required to avoid embrittlement of this type in ultra-high strength steels.