Corrosion of nickel-base and iron-base alloys in a simulated fluidized-bed coal-combustion environment

The modes of initiation and propagation of corrosion attack on a series of high-temperature alloys were studied in synthetic gas mixtures at 900°C. The gas mixtures were intended to simulate the oxygen and sulfur partial pressures experienced in reducing zones in a coal-fired fluidized-bed combustor and comprised mixtures of CO, CO₂, and SO₂. The alloys studied were candidates for in-bed heat exchanger tubing for an air-heater cycle operating at 843°C and 300–500 psig and so ranged from type 300-series stainless steels to nickel-base alloys. With the exception of two FeCrAlY alloys and types 304 and 347 stainless steels, it was found that sulfidation corrosion could be initiated on all the alloys within 0.25 hr; the rate of propagation of the corrosive attack depended on the flux of SO₂ in the environment and on the nickel content of the alloys. The presence of iron in the alloys appeared to slow the initiation of sulfidation, by forming a continuous iron oxide layer. The effects of various alloying additions are discussed, and a schematic model for the initiation of sulfidation is proposed.     

Warm compacted NbC particulate reinforced iron-base composite(Ⅱ)--Microstructure and properties

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｂｉｇｇｅｓｔｐｒｏｂｌｅｍｓｉｎｔｈｅｆａｂｒｉｃａｔｉｏｎｏｆｐａｒｔｉｃ ｕｌａｔｅｒｅｉｎｆｏｒｃｅｄｍｅｔａｌｍａｔｒｉｘｃｏｍｐｏｓｉｔｅｓ (ＭＭＣｓ)ａｒｅｔｈｅｅｖｅｎｄｉｓｔｒｉｂｕｔｉｏｎｓｏｆｒｅｉｎｆ     

Creep-induced local lattice parameter changes in a monocrystalline nickel-base superalloy

The γ-γ lattice mismatch of specimens of the monocrystalline nickel-base superalloy SRR 99 has been measured by a high-resolution X-ray diffraction technique for the undeformed state and after high-tem-perature creep deformation. During creep deformation beyond the minimum creep rate (total strain =0.5%), the lattice mismatches, measured in and perpendicular to the [001] stress axis, respectively, un-dergo changes in opposite directions. This reflects the buildup of a complex deformation-induced triaxial state of internal stress in the phases y and γ. The overall resolved shear stresses that act in γ and y due to the combined action of the external and internal stresses are estimated, and the conditions under which cutting of the γ phase by dislocations should occur are discussed.     

Synthesis of MWNT-based composite materials with inorganic coating

Multiwalled carbon nanotube (MWNT) based metal oxide composites were prepared by an impregnation method using organometallic compounds as precursor. Aluminium isopropoxide (AlIP), tetraethyl orthosilicate (TEOS), and tetraethyl orthotitanate (TEOTi) were used as inorganic sources and decomposed by hydrolysis on the surface of carbon nanotubes. The composites were subsequently investigated by transmission electron microscopy and their coverage was compared. A direct, solvent-free impregnation technique turned out to be the most successful for all organometallic compounds and provided homogeneous inorganic cover layer on the surface of purified MWNTs.     

TCP-phases in nickel-base alloys with elevated chromium content

Phase reactions in three high-temperature nickel-base alloys with 16–22% Cr are studied in the process of long-term laboratory aging after different kinds of heat treatment and after operation. The conditions of formation, the composition, and the amount of the σ-phase and its influence on the properties of the alloys are determined.     

Stability of composite materials NiAl-refractory metal with cellular structure

The structure and mechanical properties of NiAl-Wand NiAl-W-Mo composite materials (CM) obtained by sintering from powders are studied. Comparative analysis of the effect of hot compressive deformation of a compact material at 1000–1300°C on the integrity of the microspecimens and of the tungsten shells on NiAl granules in CM with cellular structure is performed. The thermokinetic stability of the grain structure of unalloyed nickel aluminide NiAl and of a NiAl-W composite material with cellular structure is investigated. The temperature of the beginning of recrystallization of the NiAl intermetallic is determined. A map of structural states is plotted in the “temperature-operating time” coordinates for CM with cellular structure. The local chemical composition of the “NiAl-refractory metal” phase boundary is studied in CM with cellular structure and without it. The effect of the structural state of CM on the yield strength in compressive tests at 1000°C is determined. The oxidation resistance at 1000–1300°C is studied and a treatment approaching the oxidation resistance of CM with cellular structure at 1000–1300°C to the level of high-temperature strength of unalloyed NiAl and of its alloy with 4 wt.% Hf is suggested. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 25–30, June, 2006.     

Structure and mechanical properties of Ni-Cu-Ti-Zr composite materials with amorphous phase

The structure of specimens of Ni-Cu-Ti-Zr alloys with an amorphous phase has been examined by X-ray diffraction, as well as by transmission and scanning electron microscopy. Mechanical characteristics of the alloys have been determined using universal testing machines. Transformation-induced plasticity has been found to exist. The specimens demonstrate a good combination of strength and plasticity owing to both the composite effect of a heterophase structure and the dynamic martensitic transformation that develops during deformation.     

Damping characteristics of composite materials and structures

This paper presents an overview of experimental and analytical approaches that have been used to characterize damping properties of composite materials and structures, with emphasis on polymer composites. A discussion of damping mechanisms operating in composites is followed by a review of several experimental methods for measuring damping. A summary of analytical models for predicting damping at both the micromechanical and macromechanical levels is presented and comments on ways of improving and optimizing damping in composites are offered.     

Crystallization and decomposition of metastable phases in iron-base amorphous alloys

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 23–24, March, 1990.     

Structural changes associated with strain-induced grain boundary migration in Si-Fe

Strain-induced grain boundary migration was observed in several bicrystal sheet specimens of Si-Fe (3¼ pct Si) which were cold rolled 2 to 12 pct and then annealed at temperatures up to 1200°C. A chrome-acetic acid electroetching method was used to reveal the dislocation sites before and after grain boundary migration. Recovery effects were noted in the microstructure prior to boundary motion. Consequently, the residual strain energy in neighboring grains may determine if boundary migration with resulting increase of area occurs, and its direction of movement. Microstructural data indicate that the region initially traversed by the moving grain boundary has many structural defects in the form of low-angle boundaries and random dislocations of relatively high density. With increased distance of grain boundary migration, the density of these imperfections was found to decrease. Continued annealing at 1200°C, after boundary migration, lowered the density of random dislocations in the swept region to a limiting value of about 2×10⁶ lines per sq cm.     

Constitutive relation of particulate-reinforced viscoelastic composite materials with debonded microvoids

In this paper, the constitutive relation of particulate-reinforced viscoelastic composite materials is studied by considering the evolution of microvoids. Owing to the difference in mechanical properties between matrix and second phase particles, debonding of particle-matrix interface may occur under the condition of high stress triaxiality. This kind of damage will lead to the nucleation and growth of microvoids. The reinforcing effect due to rigid particles and the weakening effect due to microvoids caused by debonding on the overall mechanical properties of particulate-reinforced composites are investigated. Using the energy criterion of interfacial debonding, an expression for the nucleation rate of microvoids is suggested. Then, the growth of these voids is studied by means of Eshelby’s equivalent inclusion method. It is shown that the strain of the microvoids depends not only on the remote strain history but also on their nucleation time. By considering the effect of nucleation time, a new definition of an average strain of microvoids is given. According to the Mori–Tanaka scheme, a macroscopic constitutive relation of the composite material is finally derived. The results show that macroscopic strain rate, particle-size dispersity, relaxation time of matrix, and interface adhesive strength all play key roles in the overall mechanical properties of particulate-reinforced composites.     

Fracture resistant super hard materials and hardmetals composite with functionally designed microstructure

Polycrystalline diamond and other hard materials are widely used in earth boring, mining, and construction tool applications. Chipping and fracture resistance is often improved by various means at the expense of hardness and wear resistance. This trade-off between wear resistance and chipping resistance hinders the development of hard and super hard materials for many industrial applications. A new approach, characterized as “hard materials composites with functionally designed microstructure” including polycrystalline diamond and cemented tungsten carbide, is discussed. The functionally designed microstructure offers enhanced chipping resistance and toughness without significantly sacrificing wear resistance.     

Mapping single-crystal dendritic microstructure and defects in nickel-base superalloys with synchrotron radiation

Solidification of single-crystal nickel-base superalloys introduces large-scale segregation of constituent elements and defects such as dislocations and mosaicity. By exploiting the energy tunability and interference capabilities of high-brilliance X-ray radiation, key structural features of the dendritic single crystals were mapped over large areas. Interference and diffraction of synchrotron X-rays revealed significant misorientations between individual dendrites in the as-solidified state. For the first time this mosaic structure was quantified for an array of dendrites and correlated with the density of “grown-in” dislocations whose density ranged from 10⁷ to 10⁸ cm⁻². Absorption contrast permitted simultaneous mapping of the distribution of refractory metal additives (e.g. rhenium and tungsten), which segregated preferentially to the dendrite cores with a linear composition gradient toward the interdendritic regions. The results demonstrate that synchrotron X-ray imaging is promising for in situ studies of single-crystal structure and defects in nickel-base superalloys.