Ni-Cr Solid Solution Research Articles

Simultaneous Synthesis and Joining of a Nicraly Layer to a Superalloy Substrate by Self-Propagating High-Temperature Synthesis

Self-propagating high-temperature synthesis (SHS) is the process in which initial reagents, when ignited, spontaneously transform into products due to the exothermic heat of reaction. The aim of this study is to propose a new method to produce corrosion-resistant alloys NiCrAlY on a nickel-base superalloy substrate by SHS. This article deals with the study of combustion synthesis of NiAl and NiCrAlY compounds. Thus, using the heat released by the combustion synthesis, a nickel-base superalloy substrate can be built up by simultaneous synthesis and joining of a NiCrAlY layer. The NiCrAlY layer is composed of β-NiAl precipitates (with a fraction of dissolved chromium) dispersed in a matrix of (NiCr) solid solution. Microhardness measures are close to those of usual NiCrAlY coatings. A convenient mechanical resistance is thus expected.

Synthesis and characterization of nanostructured Cr 3C 2NiCr

Pre-alloyed Cr 3C 2-25 (Ni20Cr) powder was synthesized by mechanical ball milling in Hexane [H 3(CH 2) 4CH 3]and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average powder size drastically decreased with time during the first four hours of milling; then decreased slightly as milling continued up to 20 hours. For milling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subjected to heavier deformation than the chromium carbide powder. During the initial stages of milling, the brittle chromium carbide powders are fractured into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for times up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and composite powders. Milling times of up to 20 hours led to the formation of a poly crystal nanocomposite powder system in which chromium carbides, with average size of 15 nm, were uniformly distributed in NiCr matrix.

Atomistic simulation of an f.c.c./b.c.c. interface in NiCr alloys

The embedded atom method is applied to study the atomic structure and energy of an f.c.c./b.c.c. interface in NiCr. The two phases are oriented in a Kurdjumov-Sachs orientation relationship, and the interface considered is the (1 2̄ 1) f habit plane adopted by precipitate laths of the b.c.c. phase. The interfacial energy and coherent strain energy at 0 K are calculated for boundaries between an f.c.c. NiCr solid solution and b.c.c. Cr. The calculated interfacial energy varies from 216 mJ/m 2 when the f.c.c. phase is pure Ni to 200 mJ/m 2 when the f.c.c. phase is Ni50 at. % Cr. Atomic relaxations appear limited to atoms in contact with the interphase boundary. Most of the interfacial energy is attributed to the structural difference across the f.c.c./b.c.c. boundary, and the chemical contribution to the energy is estimated to be less than 20% of the total energy. The value of the calculated energies and the widespread occurrence of the (1 2̄ 1) f habit plane in a variety of alloy systems indicate this boundary orientation has a relatively low interfacial energy.

The effects of yttrium ion implantation on the oxidation of nickel-chromium alloys. I. The microstructures of yttrium implanted nickel-chromium alloys

Yttrium ions of 150 keV energy were implanted into the alloys Ni-20Cr, Ni-4Cr, and into nickel. The microstructures were then characterized using transmission electron microscopy, selected area channeling patterns and back-scattered electron images. Low yttrium fluences between 1×1014 and 5× 1015 Y+/cm2 did not alter the microstructures of Ni-20Cr. However, fluences of 1×1016, 5×1016, and 7.5×1016 caused the crystalline structures of the alloy to be replaced by an amorphous phase. Fluences of 7.5×1016 Y+/cm2 also rendered Ni-4Cr and nickel amorphous. Self-ion implantation experiments on Ni-20Cr did not cause the amorphous phase to form. The depth distribution of elements in Ni-20Cr following yttrium ion implantation (7.5× 1016 Y+/cm2) was determined by Auger electron spectroscopy. This showed in addition to the added yttrium a surface depletion in nickel concentration and a simultaneous enrichment in chromium concentration. At approximately 500 A, the chromium concentration is approximately 32 at.%. This depletion/enrichment zone extends throughout the implanted layer. Annealing the Ni-20Cr implanted with 7.5×1016 Y+/cm2 in vacuum for one hour at 600°C resulted in the recrystallization of Ni-Cr solid solution and the formation of very fine grains of Y2O3. Annealing at 800°C for 5 minutes showed recrystallized Ni-Cr, Y2O3, and an additional phase or phases.

Reactions of Fe-Cr and Ni-Cr alloys in CO/CO2 gases at 850 and 950 °C

A series of Fe-Cr and Ni-Cr solid solution alloys was reacted at 850 and 950 °C in CO/CO2 gas mixtures in which FeO and NiO were unstable. The compctitive tendencies toward the carburization and oxidation of the chromium solute, as compared to a graphical thermodynamic "metastability" criterion, were tested experimentally. Relatively good agreement was found between predictions and experiments for the occurrence of Cr carburization beneath Cr2O3 internal oxides or external scales. The chromium contents required for the transition from internal oxidation of Cr to the formation of Cr2O3 external scales in CO/CO2 gas mixtures were established for Fe-Cr and Ni-Cr alloys. The Cr2O3 external scales formed on Fe-Cr alloys were found to be relatively impervious to carbon penetration for short (12-hour) experiments. No carburization was observed in the Ni-Cr alloys, but the only alloys that were predicted to carburize were the ones that formed external scales.

New determination of solubility limit of chromium in Ni-Cr solid solution

New determination of solubility limit of chromium in Ni-Cr solid solution

The development of a pseudoeutectic nickel-based composite alloy

This paper gives a detailed account of the development of a Ni-Cr-Be composite alloy. Its cast structure is that of a duplex eutectic with hard nickel beryllide fibers embedded in a softer Ni-Cr solid solution matrix. A partial liquidus projection of the Ni-Cr-Be system was determined in order to define the solidification conditions and optimum composition range for the alloy. Alloying with aluminum, tungsten, and molybdenum was used to enhance the mechanical properties of the material. Heat treatment conditions, mechanical properties, and oxidation behavior are presented.

Cellular precipitation in Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys

The phases present in directionally solidified Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys are identified. These are the nickel-and chromium-rich lamellae, a Widmanstatten precipitation of nickel in the chromium-rich lamellae, grain boundary precipitation of chromium, and scattered discrete particles of chromium oxide. Upon annealing, the amount of the nickel precipitates increases drastically and a well defined cellular precipitation appears in the nickel-rich lamellae. The cellular precipitation conforms in almost every way to typical cellular reaction such as in the Pb-Sn system. Cellular (linear) growth rate, G, and interlamellar spacing, S, were measured on specimens annealed for times ranging from 1/2 to 100 hr at 625°, 700°, 760°, and 850°C. G increased from (average for both Ni-Cr alloys) 2.3 × 10−8 cm per sec at 625°C to 7.7 × 10−7 cm per sec at 760°C and decreased again at 850°C. S varied from 2 to 10 × 10−5 cm as the annealing temperature was increased. The calculated grain boundary diffusivity, DB, representing the diffusion of chromium in fcc Ni-Cr solid solution, increased from 6.7 × 10−11 sq cm per sec at 625°C to 8.6 × 10−8 sq cm per sec at 850°C. The activation energy 64 kcal per mole, is of the order of that obtained for self-diffusion.