A. Raman

Crystal structures of AB3 and A2B7 rare earth-nickel phases

Abstract The structural data of several nickel-rich rare earth-nickel phases are reported. Excepting CeNi3, the other LnNi3-phases crystallize with the PuNi3-type structure. Both the Ce2Ni7- and Gd2Co7-type structures occur in several Ln2Ni7-phases. Ce2Ni7 possesses only the Ce2Ni7-type structure. Three modifications are encountered for Nd2Ni7. The low-temperature form is of the Ce2Ni7-type. The high-temperature forms are found to possess large periodicities along the c0-direction. Nd2Ni17 has the Th2Ni17-type structure at high temperatures.

Crystal structures of some Ln3Rh, Ln7Rh3 and LnRh3 Phases☆

Abstract The Fe3C-type structure occurs in the phases Nd4Rh, Gd3Rh, Y3Rh and Er3Rh. The atomic positional parameters in the Pnma space group are: 4Er in 4(c) 0.021, 0.250, 0.138 B 1 = 2.0 A 2 8Er in 8(d) 0.180, 0.061, 0.667 B 2 = 2.0 A 2 4Rh in 4(c) 0.389, 0.250, 0.937 B 3 = 0.5 A 2 Phases with the Ln7Rh3 stoichiometry in the GdRh, YRh and ErRh systems possess the Th7Fe3-type structure. The same structure occurs in the alloys of the light rare earths, La, Ce and Nd at the Ln3Rh stoichiometry. Unlike LaRh3 and NdRh3, CeRh3 crystallizes in the Cu3Au-type structure. It is shown that electronic factors rather than the size factor play prominent roles in stabilizing these structures in the rare-earth alloys.

Tensile strengthening in the nickel-base superalloy IN738LC

The tensile properties of superalloy IN738LC with different precipitate microstructures are evaluated at room temperature, 650 °C, 750 °C, and 85 °C at two different strain rates. The properties can be presented in two groups based on the comparable closeness of the values obtained—those of microstructures C and M, with coarse and medium size precipitates, and those of microstructures F and D, with fine and duplex size (medium + fine) precipitates. Preferred orientations, lattice parameters, and metallography are used to characterize the microstructure and tensile testing to determine the yield strength, tensile strength, and strain hardening coefficients. An anomalous increase in yield strength is observed, which occurs at temperatures about 100 °C higher with higher strain rate than with lower strain rate applied. The experimental results show that the yield strength is influenced by preferred orientations and precipitate size, while the tensile strength is effected by the size and morphology of precipitates.

Fracture behavior of superalloy IN738LC with various precipitate microstructures

Abstract Fracture studies on differently aged specimens of IN738LC, tensile tested to failure at different temperatures with two different strain rates, were carried out using a scanning electron microscope. Specimens with fine-size γ′ precipitates (F) and duplex-size γ′ precipitates (D) exhibited generally more cleavage type fracture than those with medium (M) and coarse (C) size γ′ precipitates. {100}-faceted cleavage-type fracture was dominant at room temperature, while dimple fracture was dominant at 650°C. A different type of cleavage fracture was observed in the specimens broken at 750°C, which is attributed to secondary strengthening and change in flow mechanism at this temperature under slow strain rate conditions. At 850°C, F and D showed wider faceted cleavage fracture, while M and C showed dimple-ductile fracture. Low tensile fracture toughness correlates with cleavage fracture, while specimens exhibiting dimple fracture have higher ductility and toughness. Presence of few (Ta, Ti)C-type carbide particles along the grain boundaries probably enables easy intergranular crack propagation. Dimples, however, are postulated to be initiated by decohesion at the interface between the γ′ precipitate and the matrix. Notably such decohesion is initiated only on coarsened particles, not on the fine ones.

Crystal structure of Ce5Rh4 and analogous phases

Abstract Single-crystal studies were used to establish the crystal structure of Ce5Rh4. It is orthorhombic, Pnma, with lattice constants a = 7.434(5), b = 14.86(1), and c = 7.604(5) A , and is isotypic with Gd5Si4. Lattice parameters of analogous isotypic phases La5Rh4, Nd5Rh4, Sm5Rh4, Gd5Rh4 and Ce5Ir4 were determined from powder X-ray diffraction data.

Formation and Transformation of Magnetite (Fe3O4) on Steel Surfaces Under Continuous and Cyclic Water Fog Testing

Abstract Formation and transformation of magnetite on two selected low-alloy structural steels were studied using cyclic and continuous water fog tests. It is shown that continuous wetting of steel...

Characteristics of the γ′ precipitates at high temperatures in Ni-base polycrystalline superalloy IN738LC

IN738LC is a Ni-base cast superalloy used in land-base gas turbinesand aerospace applications. As in other superalloys γ′ precipitates contribute to strengthening of this alloy at hightemperatures. In this study, the authors investigate thecharacteristics and mechanisms of precipitate dissolution into thematrix solid solution. The precipitates grow in cuboidal shape up to1130°C, above that a duplex-size precipitate microstructure sets inupon quenching from the temperature range 1140–1150°C. Theduplex-size precipitate microstructure consists of two very distinctsizes of precipitates (fine and coarse). Holding for longer times inthe temperature range 1140–1150°C does not coarsen the fineprecipitates of the duplex microstructure. The source for theformation of the fine precipitates in the duplex microstructure isthe dissolution of the newly grown smaller-sized precipitates whenthe agings start from fine size precipitates and the “corner dissolution” of coarse precipitates when the startingmicrostructure consists of the maximum-sized cuboidal precipitates.At and above 1160°C, the duplex as well as the coarse precipitatemicrostructures dissolve to form a single-size fine precipitatemicrostructure upon quenching from any temperature up to 1225°C. A single-phase solid solution with no precipitates is obtained onlyupon quenching from 1235°C or above. The dissolution of coarseprecipitates and formation of the fine ones are found to be very fastprocesses in the corresponding temperature ranges. The fineprecipitates are postulated to form during quenching from thetemperature range 1140–1225°C and are considered to be of the “cooling” type.

The magnetic properties of Gd7Rh3

Abstract Measurements of the magnetic properties of Gd7Rh3 by the Faraday technique between 100 and 800 K on samples of a weak, fibrous texture showed orientation-dependent antiferromagnetic ordering at 142 ± 1 K. Above 470 K, normal Curie-Weiss behaviour was observed corresponding to the effective moment of tripositive gadolinium, with θp = 175 ± 7 K.

Characteristics of the Mn5Si3-, CaZn5- and CeNi3-type phases

Abstract The Mn5Si3-type structure occurs in the high temperature modification of the heavy rare earth-rhodium alloys of composition Ln5Rh3. It is shown that in the Mn5Si3-type phases the axial ratio increases with increasing values of r a r b or decreasing values of the free electron concentration in the phases. Based on the general trend in the variations, anomalous behavior of either the A- or the B-component atoms is postulated. The CaZn5-type structure is encountered at high temperature ranges in the LnRh5 phases of heavy rare earths. The axial ratio decreases with an increase in r a r b unlike in the Mn5Si3-type phases. Yet the behavior is similar in both cases when variations with respect to free electron concentration are considered. The CeNi3-type structure occurs in the LnRh3 phases of light rare earths (La to Gd, and Y, except Ce) and is favoured when r A r B lies between 1.32 and 1.39. In these phases the axial ratio decreases slightly with increasing values of radius ratio. Based on the variations in the axial ratio, Ce is inferred to be tetravalent in CeNi3, The PuNi3-type prevails at higher values of radius ratio, between 1.39 and 1.50. Cu3Au-type phases have radius ratios ranging between 1.27 and 1.35. The ranges of radius ratio for PuAg3-, TiCu3-, and YZn3-type structures are given, and the occurrence of structures at Ln5X3, LnX5 and LnX3 stoichiometries is reviewed.

Impulse excitation study of elasticity of different precipitated microstructures in IN738LC at high temperatures

The elastic modulus of the cast superalloy IN738LC in various heat-treated conditions was determined with multiple specimens for each microstructure using the impulse excitation technique and the resonant frequencies while heating and cooling. Whereas the second and higher order harmonics were also excited in the high temperature range 700–1000 °C in 50 mm long specimens during controlled heating, analogous specimens 35 mm in length, impacted in similar fashion, did not excite the higher harmonics. Also, the 50 mm long specimens became excited and stayed in the second harmonic over broader temperature ranges during uncontrolled cooling inside the closed furnace. All precipitated conditions had nearly similar elastic data, varying from about 200 to 115 GPa, with small deviations, within 5%, found among multiple specimens of similar microstructures tested. Specimens with fine nano-size precipitates had a distinctly smaller rate of decrease in elastic modulus with increasing temperature, in contrast to a somewhat larger and nearly similar rate of decrease in specimens with coarse or medium-sized precipitates. This behavior is indicative of a larger average cohesive strength between the atoms and/or between the matrix and the precipitate particles in the former microstructure. The duplex size precipitate microstructure seemed to have both small and large drops in different specimens.